Final Report Summary - LIBRALATO (Libralato Engine Prototype)
Executive Summary:
The Libralato Engine is a novel rotary engine design that is intended to overcome a number of issues associated with the more established Wankel design. The principle of the Libralato engine is that of two connected rotors which create in succession the volumes required for air intake and compression, combustion, expansion and exhaust. Like the Wankel engine the Libralato engine implements the four stroke cycle in a single revolution of the engine lending the efficiency and emissions benefits of this type of cycle with a power stroke for each engine revolution. The Libralato engine has, as an integral aspect of the engine design, an extended expansion stroke that leads to efficient operation. The assembly of the engine, which allows the tip seals in the Wankel rotor to be replaced by a series of long tapering passages supplemented by seals in the rotor lips.
An early engine design aimed at a rated power of 25 kW was supplied to the team by the engine inventor, Mr Ruggero Libralato. This design is referred to as the Mark 1. An early realisation of this design was essential to the progress project, but was hampered by the weakness of the design model and the need to include a new manufacturing partner in the project. Once the new partner was formally appointed the existing design was adapted to the machining processes and accuracy capabilities of the new partner. This turned out to be a\ long and tortuous process leaving the project in a difficult position.
While confirming the proper function of the engine, early analysis revealed a number of significant mechanical design issues. The use of a slider to connect the two rotors led to very high reaction torques that would lead to yield stress in rotor components even at modest speeds. With the existing design, the rated speed and power could never be realised. The relatively large diameter of the following rotor bearing led to extremely high friction torque. It was these fundamental mechanical properties of the engine that led to the early decision to move as quickly as possible to a reduced diameter rotor that would halve the working volumes. The reduction in internal forces would be significant and lead to a manageable implementation. The consequence of this decision is that a 50 kW engine would be composed of three of the smaller rotors connected in a phase relationship that would substantially balance their operation.
The design of the Mark 2 was initiated as soon as the analysis results indicated the difficulties with the Mark 1 design. As the Mark 1 engine was implemented a number of construction difficulties appeared such as the tendency of the rotor to lose its alignment. It was information like this that led to the adoption of a range of new features. The absence of a viable lubricating system on the Mark 1 engine led to the adoption of an ad-hoc system of pipework providing feed and return to bearings. For the Mark 2 this meant to an adoption of sealed rolling element bearings that could be used without a lubrication supply making the measurement of engine emissions possible.
The Mark 1 design was accompanied by the design of a special purpose test rig. The Libralato engine requires phase adjustment of the exhaust valve, and the synchronised operation of the high pressure fuel pump. The combustion system can only possibly work in a direct injection mode and so the complexity of a direct injection system needed to be managed. From the start of the project, the control solution was oriented to direct injection which ultimately proved successful. The test rig included the capability to run the engine, exhaust valve and high pressure pump in strict synchronisation.
Tests of the Mark 1 engine demonstrated the very high torque values that had been predicted in the analysis phase of the work. Those torque values showed some reduction as the engine was operated indicating that the source of at least some of the friction was misalignment in the engine. In low speed tests of the engine, combustion pressure is evident well before the secondary rotor begins the expansion process and essentially delivers a negative torque. The analysis work demonstrated otherwise and the implication is that the combustion phasing is too advanced. The two results suggest that the performance of the engine critically depends on the phasing of combustion. The resolution of this question is the essential next step in the development of the engine concept and may require both a mechanical design change to ensure the later phasing of the peak pressure as well as the control of a later combustion event. Test facilities and the manufacturing of the Mark 2 engine are substantially complete.
Project Context and Objectives:
New engine designs are required which meet the demands of Hybrid Electric Vehicles (HEV) and Plug In Hybrid Electric Vehicles (PHEV) :–
• aggressively downsized and integrated within series/ parallel or dual mode hybrid electric vehicles;
• designed for ‘steady state’ operation exclusively within their BSFC peak efficiency zone;
• exceeding Euro 6 emissions standards;
• with reduced production and maintenance costs,
• with exceptionally low NVH behaviour;
• with compact proportions that can be easily integrated within vehicles.
The Libralato rotary engine is a potential breakthrough technology with a thermodynamic cycle that particularly suits fuel efficient operation. . Patents are pending - WO2004020791 A1 for the engine design and PCT/EP 2009 / 006807 for the engine’s thermodynamic cycle.
The Libralato engine was announced as one of six winners of the UK Low Carbon Vehicle Partnership’s Technology Challenge in Dec 09, awarded by Neville Jackson, Ricardo UK Ltd. Group Technology Director and Richard Parry-Jones, UK Automotive Council Chair. A physical proof of concept engine mechanism has been produced and has been tested at speeds up to 1500 rpm – representing a sensible mid-load operating speed. A CREO/Pro-E CAD model of the engine has been produced and this model will provide the basis for simulations, studies and construction of high grade prototypes.
The Libralato engine has only four principal moving parts: two rotors fixed by their own bearings, connected by a sliding vane and a rotating exhaust. The rotors have different (overlapping) diameters of circumference and their motion forms and reforms three separate chambers within the engine each revolution. The leading rotor is engaged with the expansion of the air/ fuel mixture and a first stage compression of intake air. The following rotor is engaged with aspiration and a second stage compression. Air enters via a port at the centre of the engine and at one point also via the exhaust port.
The Libralato engine design is very different from the Wankel engine and avoids the reported difficulties associated with the Wankel engine by: large sealing surface areas of the rotors (equivalent in size to pistons), short flame paths acting against an acute angle working surface area, good thermal dispersion via fresh air scavenge phase and low bearing wear due to dynamic balance.
The engine cycle mechanically creates aspiration, first stage (low) compression, exhaust of scavenge gases, second stage (high) compression and expansion phases in parallel each full rotation of the engine, giving it twice the power density of a 4 stroke reciprocating engine with equivalent expansion volume. Due to the different diameters of the rotors, compression and expansion volumes are asymmetrical, allowing more complete (efficient) combustion of the air/ fuel mixture. The combination of more complete combustion and more complete scavenge of the exhaust gases using pressures crated during the first stage compression means that emissions, exhaust gas temperatures and noise can be controlled.
The project objectives as presented in the Technical Proposal are listed – and against each a summary of the achievements of the project are listed.
1) To construct and test prototypes of the Libralato engine – respectively 25 kW; <60 kW
A 25 kW prototype was re-designed, constructed and tested; a second prototype was designed, based on the observations and analysis from the Mark 1 evaluation and tests and the parts manufactured.
2) To construct a 3D CFD model, test and optimise scenarios of the compression and expansion ratios, heat transfer rates and leakage flows of the engine.
A 3D simulation model was assembled from the engine design files. An overset mesh method was prepared and used to show air flow in the second phase of compression.
3) To assess, refine and optimise the performance of the hot prototypes, determining fuel efficiency over a range of operating conditions. Target BSFC = <210g/kWh (39% efficient)
Some limited test work was completed to demonstrate that combustion could be supported. IN particular the assertion that this engine technology required the deployment of direct injection technology was supported by test results. Using liquid fuel a direct injection combustion process could be initiated and run.
4) To assess, refine and optimise the performance of the hot prototypes, determining emissions over a range of operating conditions in relation to Euro 6 standards : –
Targets - THC < 0.1 g/km; CO < 1.00g/km; NOx < 0.06 g/km; PM < 0.005 g/km
This objective could not be met owing to the lubrication management process used in the Mark 1 engine. A re-design of the lubrication system and the adoption of selected rolling element bearings was to be the basis for emissions tests on the Mark 2 design , which unfortunately could not be completed.
5) To conduct design/ cost analysis of the Libralato engine. Target production costs for 50kW = $500
Based on the design and analysis work conducted for the Mark 1 and Mark 2 engines, a cost analysis has been conducted and indicates a competitive manufacturing cost at €15/kW.
6) To assess NVH behaviour. Target NVH behaviour = 50% comparative reduction
NVH analysis was not possible with the Mark 1 engine. Some vibration information could be taken with accelerometers but the data simply indicated that there were significant out of balance forces due to the rotating masses of the engine. The Mark 2 engine was intended to be the basis for a more extensive investigation of vibration information.
7) To assess critical package and weight boundaries. Target critical package and weight boundaries = 50% comparative reduction
A preliminary assessment of packaging was completed and suggests that a packaging efficiency of 50% can be achieved making the engine particularly appropriate of parallel hybrid assembly.
8) To conduct a limited market assessment for the Libralato engine
The market assessment suggests a substantial opportunity in the hybrid vehicle sector.
9) To apply for patent protection for the updated Libralato engine design and thermodynamic cycle
Two patent applications have been developed as a result of the work.
10) To undertake dissemination activities via academic and industrial networks
Dissemination activity was based on the design analysis work that was conducted during the design work on the Mark 1 engine.
Project Results:
The results and conclusions are organised according to the main technical activity of the project. In the next section we review the design process for the engine. There is a detailed description of the Mark 1 design – the first design of the engine which was developed from an existing but incomplete design. The Mark 2 design represents a significant step forward and utilises the results of an extensive analysis into the dynamic behaviour of the Mark 1 engine.
The analysis is presented to demonstrate the reasoning that led to the adoption of the Mark 2 design features. The analysis continues with the development of the understanding of air flow in the engine, and concludes with the simulation and prediction of the engine performance.
The control system implementation was critical to the testing of the engine, and the control system solution in the form of an electronic implementation is presented followed by a brief description of the software.
Although the team prepared a test facility for the engine, only limited testing could be done because of the relatively fragile state of the Mark 1 engine However the combustion process was demonstrated and an understanding of internal gas pressures developed that could act as a source of information about the continuing development of the engine concept.
1 The Design of the Libralato Engine
This section summarises the design process by which the engine emerged.
1.1 The Mark 1 Design
The engine design is presented as a computer aided model created using the tool historically known as Pro/Engineer (Pro/E) and now sold as CREO Elements/Pro. Pro/E is maintained by Parametric Technologies and is widely used in manufacturing, notably by Caterpillar for engine and machine design. The design work for this deliverable has been performed using version CREO 2.0.
The original design work for the Libralato engine was done using traditional sketching and drawing techniques which were used as the basis for the manufacturing of a model using mild steel and which became the basis for some concept tests and demonstration of assembly techniques. Libralato Ltd, owners of the design, commissioned the preparation of design files using Pro/E. However those files were based a set of manufactured features that were inappropriate to the machine tools to be used for initial manufacture.
The first task to be initiated in Work package 6 was the reworking of the model to incorporate new features that better matched the capabilities of the machining centres used by Techmachine, the member of the project consortium responsible for manufacture of the engine.
The original design model that was made available at the start of the project was not a completed design. Firstly, it did not include some important information that is essential to the manufacturing process, such as tolerances and materials. Secondly, some requirements are not properly considered in the design making certain functions unachievable. Thirdly, certain of the design features included in the initial design are not readily manufacturable. Although the engine functions are realised, their manufacturing is very difficult and the associated manufacturing cost correspondingly high. Consequently, the new CAD model was improved in all these aspects. The new model conveys the information, such as materials, processes, dimensions, and tolerances, which was not included in the original design. Techmachine built the first prototype engine based on this new updated model using their NC machining centres. This new model was also imported to finite element analysis software (ABAQUS & ANSYS) and CFD software (STAR-CCM) to build the analysis models and perform FE analysis. The model was also imported to dynamic analysis software (ADAMS) to investigate the dynamic performance of the engine.
1.2 The Mark 2 Design
The Mark 1 design was by no means complete. Firstly, certain of the design solutions were not fully implemented simply because there was insufficient information available, and that particular information needed to come from practical test. Consequently the Mark 1 engine needed to fulfil a role where it would support certain tests whose purpose was to provide design information for later versions of the engine. . The original design of the engine that had formed the starting point for the project was much less well understood than we had expected.
Some design features originally proposed for Mark 1 proved to be difficult to manufacture. Although these features could be made, their manufacturing would be very difficult and the cost is consequently high. During the tests of Mark 1, more design weaknesses have been identified, which mainly include the uncertainty of the friction sources, the severe inertia force at normal engine speeds, an intake behaviour that was very difficult to characterise, and difficulties with sealing the rotors against the fixed part of the engine. All of these discoveries have revealed important information about the mechanisms of Libralato engine.
In order to improve the performance of the engine and identify solutions for the new Mark 2 engine design, our team visited Techmachine on several occasions to review the test and analysis results with our partners. In the meetings, the new design solutions for Mark 2 design have been reviewed and discussed.
Therefore, the new Mark 2 model has been improved in all these aspects. Compared with the Mark 1 design, the new Mark 2 design demonstrates improved solutions in all aspects of the engine function. The main design changes are summarized in the following Table.
1.2.1 Summary of design changes
This section summary all the main design changes in Mark 2 design.
Mark 1 feature Issue Change required
1 Engine size The diameter of the main rotor in Mark 1 engine is 270 mm, which causes high inertia force at high running velocity.
The most effective way to reduce this inertia force is reducing the size of the engine.
2 Lubrication System The lubrication passages have not been properly sealed, which leads to flows of oil into the expansion chamber and the exhaust system Closed oil path for slider and rolling element bearings for rotors have been applied, which can guarantee the oil only flows in the specified oil passages.
3 The main rotor The main shaft is connected to the main rotor independently, which affect the strength of the connection and can cause the misalignment of the main shaft. The main shaft should be made as one piece and fixed on the main rotor during assembly.
4 The following rotor The position of three pieces of the following rotor can be moved during the working cycle, which can cause misalignment of the main shaft. Also can cause severe friction at some local surface. The three pieces of the following rotor should be made as a single piece.
5 The main rotor bearing
& The following rotor bearings The bearings are plain bearing. Considering the running loads the inertia force, the plain bearing is not a good option for this application. The needle roller bearing can provide much stronger supporting for the rotors, which can guarantee the rotor on the right position during rotating and is more suitable for this engine design.
6 The side plate The side plate not only provides the mounting for the bearings, but also needs to make sure the bearings being sealed properly.
Considering the roller bearings adapted in Mark 2 design, the main dimensions and structure need to be modified.
7 The exhaust valve In the original design, the exhaust valve not only works as the exhaust valve, but also works as the intake port to let the air enter the engine. However, in the Mark 1 test, the intake on the exhaust port didn’t work properly. The air doesn’t flow into the engine during the working cycle.
In the Mark 2 design, the exhaust valve is redesigned to only work for exhaust system.
8 The seals for the bearings The two rotor axes are very close, also the intake port and the main rotor bearing need to be inside the internal ring of the following bearing. The limited space on the side plate is a big challenge for the seal solution. The grease can be used to lubricate the needle roller bearing.
9 The seals for the slider The slider needs to withstand a large torque from the rotor and have to keep moving in the cylinder. In order to reduce the friction and keep the temperature on the slider within a specified limit, the proper lubrication must be guaranteed. At the same time, the seals must be provided to make sure there is no oil leaking into the chambers. The special passage need to be designed on the main shaft and the cylinder to guarantee the lubricant supply. Also, two seals need to mount on the cylinder to keep the oil inside the cylinder.
10 The intake system The intake port on the exhaust valve doesn’t really let the air enter the engine. Most of air into the engine is coming from the intake port at the centre of the engine. The air intake will only use the port on the centre of the engine,
11 The engine block When the engine size is decreased, the manufacturing for some parts becomes more challenging. For the engine block, it has to be made as two pieces, which can guarantee the cutter can reach some local manufacturing positions.
1.3 The Mark 3 Design
During the Mark 2 design process, we have already identified where even the new design could be improved. For example, in Mark 2 design, the air flow into the engine may be not sufficient to support combustion. Therefore a major design change could be made where air is introduced directly in to the compression chamber. Alternatively pressurised air could be introduced into the existing intake in the Mark 3 design. The combustion process needs to be properly considered in the new design with a more ordered pattern of air motion – with a degree of control over the motion air about a vertical axis. Combustion phasing remains a significant concern that would require tests woith the Mark 2 engine to identify changes needed for Mark 3.
The slider remains a risk – subject to high forces and as a reciprocating device, difficult to lubricate. For the Mark 3 design a rolling element bearing for the slider is needed that further simplifies the lubrication function (although lubricant will still be needed for cooling).
The discussion of Mark 3 suggests that this design is used as a development benchmark – allowing emissions and friction to be brought to acceptably low levels. The step to Mark 4 is then one of controlling costs and making design changes to keep costs to a target level while retaining the fuel economy and emissions performance.
2 Design Analysis
We considered the design analysis in three aspects –
• The mechanical design analysis with considerations of kinematics, dynamics and the behaviour of materials
• The thermodynamic analysis consisting particularly of the analysis of mass and energy transport on the engine and how these transport processes are simulated.
• The analysis of internal gas flows using computational fluid dynamics calculations to understand the conditions at the start of combustion.
2.1 Mechanical aspects of the engine – stress analysis
Finite element (FE) stress analysis has been carried out using ANSYS, to ascertain the stresses within engine components as a result of engine loading/combustion. In addition, thermos-elastic stresses were predicted as a result of engine constrained expansion due to heating.
The main findings are as follows:
1) Up to 180 MPa combined stress predicted for main shaft
2) Up to 200 MPa thermos-elastic stress predicted for mounted engine
Further stress analysis is required, taking into account fatigue behaviour of the materials, as well as dynamic loading.
Multi-body work was carried out using three distinct approaches, namely: a) an ADAMS model, b) a multi-body model programmed in MATLAB® from first principle and c) a Simulink model, set up using the Simmechanics® toolbox. The Simmechanics® model is also a high-fidelity non-linear model that is developed to offer a link with the GT-Power® engine model and also to benefit from all the functionality of MATLAB, including the optimisation toolbox. The materials used were pure cast iron and a blend of cast iron and aluminium.
In an effort to further improve balancing, an automated optimisation study was carried using a Nelder-Mead optimisation algorithm to find the optimum position of the centres of mass of the leading rotor, following rotor and slider. The centre-of-mass positions were constrained within physical limits imposed by the geometry of the engine. The method resulted in reduction of peak loads by about 60% in the vertical direction and 90% in the longitudinal direction of the main rotor pivot. However, the remaining loads are still high and further work is required in this area.
In light of the sub-optimal balancing of the engine, consideration has been given to a multi-rotor-pair engine design. A pilot study using a three-rotor-pair engine whereby three leading rotors are connected via a common shaft at a phase angle of 120o between them was carried out using Simmechanics®.
Further analytical and numerical work was carried out in order to understand the mechanism of load generation at the bearings and the slider. It was found that significant exchange of momentum takes place between the leading and following rotor during a full cycle of rotation. This can be observed as a significant periodic fluctuation in the speed of the leading/following rotors, when an initial speed is imposed to the engine and it is then left to rotate in the absence of friction and combustion forces. The torque on the slider is primarily related to this momentum exchange and cannot be improved by balancing. It is therefore an inherent characteristic of the Libralato mechanism. On the other hand, the loads at the bearings can be reduced by improving the balancing of the mechanism, as already illustrated. This observation brings forward the torque on the slider as one of the main design concerns. Analytical work has resulted in the following dependency of slider torque on the change in the size and speed of the engine:
T = k x^4 y (gamma) (1)
Where T is the torque, k is a constant of proportionality, x is the variation factor of the radial dimension, y of the axial dimension and γ (gamma) is the variation factor of angular velocity. It is assumed that material density is kept constant. The torque increases with the 5th power of engine size (if a uniform expansion is considered) and with the 2nd power of speed. Importantly, the radial dimension of the engine has a 4th power influence on slider torque. These results have been verified by simulation and together with lubrication concerns have fundamentally influenced the design of the Mark 2 engine, as explained below.
2.2 Mechanical aspects of the engine - engine size and sealing
The design refinement work reported in this section has been based on a range of simulation work. At the time of conducting the analysis it was not possible to gather any experimental evidence due to the delays in manufacture of the Mark 1 engine. However, significant knowledge has been gained from simulation studies.
With regards to overall engine design, the findings from the multi-body and lubrication analyses have led to the adoption of smaller rotors. In particular it has been decided that the Mark 1 engine is reduced uniformly in size by a factor of 0.79 resulting in a reduction in volume by a factor of 0.5. To maintain power output and in an effort to improve balancing, it has been suggested that future design, including the Mark 2 design, will consist of multiple rotors, In the same figure, the implementation of water cooling is shown, to address high temperatures in the engine. Finally, external balancing of the rotors is suggested at least for pre-production versions, in order to eliminate loads at bearing and vibration related concerns.
In the absence of experimental results and in light of manufacturing delays/difficulties, a subset of the changes proposed above will be implemented in the Mark 2 engine, as follows:
Two rotor-pairs sharing a common shaft between main rotors
Engine remains air-cooled
Use of external balancing to eliminate loads on bearings
Basic design of each rotor-pair remains the same as in Mark 1 engine, but downsized in order to reduce size-related problems
The sealing system required significant re-design for the Mark 1 engine. The size of the original seals was inappropriately small for production or smooth operation. In addition, various radii of curvature had to be changed where rotors met seals, to reduce impact loads and prevent seal/rotor damage. In summary, the Mark 1 sealing system was improved by:
Calculation of all clearances between rotors/block, including the effect of thermal expansion and bearing clearances
Re-definition of tolerances
Re-sizing and general re-design of side- and face-seals
Specification and sourcing of appropriate springs to work with the seals
2.3 Summary of conclusions from Mechanical Analysis
Large engine size (especially radially) causes extreme loads on bearings and slider. Future engines should feature compact rotors.
Possibility of near perfect balancing using a multi-rotor-pair design with external balancing.
Relatively high stresses are predicted for some components – potentially destructive levels if temperature and fatigue behaviour are taken into account.
Very high temperatures are predicted – water cooling seems essential in production versions.
Extreme friction losses at Mark 1 following-rotor bearings. Again, large size is the culprit here.
2.4 Thermodynamic performance
The unique feature of the Libralato concept is the transient nature of its working volumes, characterized by the separation and merging of volumes as the cycle proceeds. This is in contrast with the conventional piston engine, where the cylinders form the working volume and the inlet/exhaust ports/manifold are fixed. The mathematical formulation and the results are presented in this report to assist in the validation of the program against the engine test results as they become available. A new Libralato Engine-specific simulation software (“LECS”) was developed
The current version of the LECS simulation software is based on a single zone combustion model of the Libralato engine and has already been used to simulate the performance of the engine at 25 KW brake power at 1500 rpm. The simulation indicates that the compression pressure will be around 45-48 bar and peak pressure of 70-73 bar, with a mechanical efficiency of 90-92%. The rate of volume changes versus ignition timing and gas exchange timing have been derived from the initial CAD model, as explained earlier.
The most innovative aspect of the simulation is the way in which it processes the merging of the central volume during the expansion phase, a process which can lead to instability and which requires adjustment to pressure and temperature to provide the final solution. This modelling strategy for the central volume will be reviewed at a later stage for the complete Libralato cycle, including the gas exchange process between the expansion and compression volumes, particularly if the test results do not adequately validate the assumptions made in this first model.
Validation of the LECS program and of its Libralato engine model against the measured results from the test bed is particularly important since the Libralato cycle is completely unlike the conventional cycle of either a two-stroke or a four-stroke reciprocating engine. In the conventional cycle, working volumes are linked only through fixed volumes (i.e. the inlet and exhaust ports and manifolds). This is not the case in the Libralato cycle, where the working volumes can exchange mass directly between them.
Results of TEC engine performance simulations have indicated that, compared with a conventional petrol engine, the Libralato engine concept is capable of excellent thermal efficiency and will deliver very low exhaust emissions. Initial test results may not confirm this claim, but once the design is optimized, there is no thermodynamic reason why the Libralato will not reach the levels of performance shown by the LECS simulations.
The analysis of the engine integrity suggests that the torque developed cannot be contained locally on the slider in this Mark 1 design. Although this version of the engine was designed for 50 KW at 3000 RPM, the mechanical analysis shows that this engine will not operate satisfactorily at above 1000 RPM, due to the excessive torque developed by the slider. Thermodynamic analysis also suggests that the performance at 3000 RPM is going to be an issue for the air compression temperature and some cooling system changes will be required to achieve this engine speed. All the predictions made from the LECS code have been done at 1500 RPM to reach 50% of the design power output. This is a representative output at about 50% of rated conditions.
2.5 CFD Analysis of engine airflow
This section of the analysis includes the findings of a CFD study. The aim of the study was to obtain flow field visualisation in the 2nd compression stage of the engine cycle. This provides crucial data required in determining suitable positioning of fuel injectors, particularly important for early liquid fluid injection. This report gives details on the CAD simplification, meshing process and finally flow field results that were obtained.
The visualisation shows how the motion of the engine components forces high velocity flow into the combustion chamber. There is evidence of this early in the compression stage. Due to the position of the leading rotor and combustion chamber design the flow enters the chamber with a large amount of swirl. As this flow reaches the top wall of the chamber it splits forming counter rotating eddies. The strengths of these eddies is increased as the compression stage progresses and pressure in the chamber increases. The high intensity of the eddies suggests that a re-design of the combustion chamber should be considered with an objective to create a specific pattern of air motion to induce stoichiometric air fuel ratios in the vicinity of the spark plug, while avoiding excessive heat transfer
It is also interesting to see the leakage of flow between the slider and two rotors. This behaves as expected with leakage velocity values increasing as the compression progresses and hence pressure in the combustion chamber and around the link head increases. In simulation we just consider leakage between main and trailing volumes however these results show some leakage in the central intake. These results will provide good boundary conditions for future simulations that consider additional leakage areas.
The next step would be to introduce turbulence modelling through higher order models such as Large Eddy Simulation (LES). There is also the potential to study multiphase flows, important for analysing the mixing of fuel with air in the combustion chamber for maximum efficiency or in order to achieve specified combustion goals.
3 Control System Implementation
The control system implementation consists of two components – respectively a host and “AddOn” ECU module that provides the foundation controls for the engine. The host provides interfaces with respectively: (a) an external computer for programming purposes and (b) with instrumentation needed for assessment of the engine condition and calculation of control requirements. The host was originally proposed to be the Infineon PSK, a computer system developed for motorsport applications and development work. Later in the programme, the functions were transferred to a National Instruments cDAQ system.
The main issues of the implementation of the control system are as follows
:
Electronic hardware – the design requirements and the resulting electronic hardware that was implemented to support the investigation of controls requirements.
System Description – how the hardware and software elements are integrated to give the complete controls solution.
Systems software – the low level software elements needed to support the basic functions of the control system
Interfaces – the connections to sensors, actuators and external systems that are required for the test and development processes.
Calibration tool – the interfaces that have been implemented to allow a calibration support tool to be used.
The development path that may be followed to continue to development of this system.
The Infineon Powertrain Starter Kit (PSK) allows 12 standard electro-magnetic, low-current (1A) fuel injectors to be driven using conventional low-side drives. To support current Libralato engine development and prolong the life of the starter kit, it was necessary to be able to drive dual coil, high voltage solenoid injectors suitable for Gasoline Direct Injection (GDI) applications. It was therefore proposed to design an add-on module to allow such injectors to be interfaced to the existing kit. The add-on will be known as the “PSKInjAddOn ”.
3.1 Function of AddOn module
The Add-On ECU module incorporates the following three main drivers implemented to provide full control of the Libralato engine:
1) Gasoline Direct Injection Driver (GDI)
GDI driver is designed to generate control signals for quad low side injector driver TLE6270R especially suited for Gasoline Direct Injection systems in automotive applications. The device controls the external High Side Transistors, implemented on the Add-On ECU module, to supply the injectors alternating with battery voltage and a boosted high voltage according to the requirements of the applied injectors. The TLE6270R device incorporates the Low Side driver Transistors for four Injector channels.
2) Ignition Driver (IGN)
IGN driver generates control signals for high voltage ignition coil driver VB525SP-E assembled on an external stand-alone module attached to the Add-On ECU module.
The VB525SP-E is mainly intended as a high voltage power switch device driven by a logic level input and interfaces directly to a high energy electronic ignition coil. The input of the VB525SP-E is fed from a low power signal IGN_PULSE generated by the Add-On ECU IGN driver.
3) High Pressure Fuel Pump Driver (HPP)
HPP driver is designed to generate control signals for the Flow Control Valve (MSV5) located and fixed to the High Pressure Pump HDP5. The HDP5 is a demand controlled high-pressure fuel pump. It works according to the principle of a cam-driven single-cylinder pump. The Flow Control Valve (MSV5) itself is a magnetic actuator, which controls the inlet valve of the HPP.
2.2 Integration of Hardware
This section describes technical progress achieved during integration of the Add-On ECU hardware module with sensors. It provides brief description of mandatory sensors integrated into the system during final phase of the Libralato engine testing at Loughborough University.
2.2.1 Crank Sensor
A crank sensor is an electronic device used in an internal combustion engine to monitor the position of the crankshaft. It is also commonly used as the primary source for the measurement of engine rotational speed in revolutions per minute (RPM).
Common mounting locations include the main crank pulley, the flywheel, the camshaft or on the crankshaft itself depending on the engine type. Commonly a Hall-Effect sensor is used, which is placed adjacent to a spinning steel disk.
Engine Control Units (ECU) use the information transmitted by the crank sensor to control parameters such as ignition timing and fuel injection timing.
This sensor is the most important sensor in modern day engines. When it fails, there is a chance the engine will not start, or cut out while running.
A Hall-Effect speed sensor HA-D 90 has been utilized as a Libralato engine position sensor. This sensor is designed for incremental measurement of rotational speed (e.g. camshaft, crankshaft or wheel speed).
Due to the rotation of a ferromagnetic target wheel in front of the HA-D, the magnetic field is modulated at the place of the Hall probe. The HA-D 90 is no true-power-on sensor. It needs the falling edge of two teeth for correct working. After a time of 0.68 s without rotation of the detected wheel it needs again the falling edge of two teeth.
The main feature and benefit of this sensor is a very good detection of the falling edge, due to a differential measuring method.
For more details on HA-D 90 sensor including technical specification please refer to the Hall-Effect Speed Sensor HA-D 90 Data Sheet from Bosch.
During Libralato engine testing the HA-D 90 is placed adjacent to the wheel, with 36-1 teeth, assembled to the Libralato engine drive shaft. Profile 36-1 describes the wheel with 35 teeth plus one missing tooth called “gap” tooth.
For the proper operation the HA-D 90 sensor requires power supply in a range 5 to 18 V. Battery power supply has been connected to the sensor.
Sensor signal designated as a CRANK_POS has been connected to the Add-On ECU. This signal is used by Engine Position Driver to determine engine position and engine rotational speed given in RPM.
2.2.2 Fuel Pressure Sensor
The Bosch Pressure Sensor Fluid PSS-250R has been utilized for a fuel pressure measurement.
This sensor is designed to measure the pressure of media in relation to the ambient pressure (e.g. gasoline, diesel, water, engine oil, transmission oil or air). The sensor is available for two different supply voltage ranges 4.75 V to 5.25 V and 8 V to 30 V. The sensor uses stainless steel measuring cells with piezo-resistive measuring bridges in thin layer technique, which are hermetically welded together with stainless steel pressure ports. This guarantees a complete media compatibility. The sensor has a protection for over voltage, reverse polarity and short circuit. It is not recommended to fix the sensor directly to the engine block in order to avoid undesired strong vibrations.
During Libralato engine testing the sensor was assembled to the high pressure fuel pump driven separately and locked to the engine rotation.
PSS-250R sensor has a maximum pressure of 500 bar and generates an analog voltage output of 16 mV per bar on a ratio metric basis.
The sensor has an independent 5 V regulated power supply, derived from the TLF7368 voltage regulator Q_T1 output which is intended as a stable reference supply for analog sensors.
The 5V sensor supply has a feedback line to the ADC to allow the accuracy of the voltage to be monitored. By reading the 5V supply to the sensor via a separate channel, the output could be corrected using the following equation:
Pressure (kPa) = (5V * ADCreading) / (0.016 * ADCsupply)
Fuel pressure reading provided by the sensor is used in closed loop control algorithm to calculate HPP control wave parameters. In particular PWM duty cycle for holding phase of the HPP driver waveform is controlled to maintain fuel pressure at the expected operating pressure of 150 bar.
2.3 Application Layer
The application layer consists of three software drivers implemented to provide control signals for three major functional units of the Add-On ECU hardware module used to control Libralato engine.
Drivers and corresponding functional units are:
1 Gasoline Direct Injection (GDI)
2 Ignition (IGN)
3 High Pressure Fuel Pump (HPP)
Gasoline Direct Injection Driver
The GDI driver is designed to generate control signals for quad low side injector driver TLE6270R especially suited for Gasoline Direct Injection systems in automotive applications. The device controls the external High Side Transistors to supply the injectors alternating with battery voltage and a boosted high voltage according to the requirements of the applied injectors. The TLE6270R device incorporates the Low Side driver Transistors for four Injector channels.
Although in a case of Libralato engine only one injector is used, the GDI driver is implemented to support control of up to four solenoid injectors, allowing for the possible future expansion.
Starting, default values are assigned to all parameters during GDI driver initialization in “Task_gdi_wave” function implemented in “TASKS.c” source code file. Depending on the GDI control algorithm, users can choose which parameters they want to modify dynamically in order to control GDI waveforms. Also, some parameters can be fine-tuned, during development stage, and then they can be set to the fixed values in the initialization stage.
Ignition Driver
IGN driver generates control signals for high voltage ignition coil driver VB525SP-E.
The VB525SP-E is mainly intended as a high voltage power switch device driven by a logic level input and interfaces directly to a high energy electronic ignition coil.
The input VIN of the VB525SP-E is fed from a low power signal generated by AURIX™ microcontroller that determines both dwell time and ignition point. During VIN high (≥ 4 V) the VB525SP-E increases current in the coil to the desired, internally set current level.
After reaching this level, the coil current remains constant until the ignition point. This corresponds to the transition of VIN from high to low level.
HPP driver is designed to generate control signals for the Flow Control Valve (MSV5) located and fixed to the High Pressure Pump HDP5. The HDP5 is a demand controlled high-pressure fuel pump. It works according to the principle of a cam-driven single-cylinder pump. The Flow Control Valve (MSV5) itself is a magnetic actuator, which controls the inlet valve of the HPP.
In order to regulate the delivered volumetric flow, the MSV5 is activated at a certain angle before top dead center position (TDC). The starting point and the end point of the MSV5 control signal has therefore to be calculated depending on the desired amount of fuel, the engine speed, the battery voltage and the temperature of the MSV5.
2.3 System Software
2.3.1 Operating System
MicroC/OS-II real time operating system from Micrium has been integrated into Libralato application source code. It is provided in source form (ANSI C standard) and in-depth documentation for free evaluation.
The features of µC/OS-II include:
• Preemptive multitasking real-time kernel
• Up to 254 application tasks (1 task per priority level), and unlimited number of kernel objects
• Semaphores with timeouts on pending calls to prevent deadlocks
• Mutual-exclusion semaphores that eliminate unbounded priority inversions
• Event flags
• Message mailboxes and queues
• Time and timer management
• Fixed sized memory block management.
In order to integrate µC/OS-II into Libralato application code AURIX™ System Timer STM0 has been configured to generate 1ms Interrupt for the OS task handling. AURIX™ specific implementation of µC/OS-II and hook functions has been defined in TriCore_os_cpu_port.c source code file.
2.4 Interfaces
The AURIX™ MultiCAN+ module provides a communication interface which is fully compliant with CAN specification V2.0B (active) and to CAN FD ISO11898-1 FDIS version 2013, providing communication speed of up to 1 Mbit/s in standard CAN (ISO 11898-1:2003(E) mode.
For Libralato project two CAN nodes have been utilized:
• CAN node 1 for communication with Compact DAQ
• CAN node 2 for communication with calibration tool
CAN Interface to Compact DAQ
For all analogue sensor readings Compact DAQ (cDAQ) from National Instruments has been intended. CAN node 1 is configured as a communication interface between Add-On ECU and cDAQ. CAN message objects are configured for frame reception from cDAQ for each sensor individually. Sensors are organized in three groups and data structures are defined for each group.
CAN Interface to Calibration Tool
Vector Informatik GmbH Universal Measurement and Calibration Protocol (XCP) device driver has been integrated into Libralato application source code, to enable calibration and fine tuning of selected parameters with Vector CANape tool.
3.4 Development Path
A Simulink® model has been integrated into Libralato application source code and some basic functionality has been tested to verify integration strategy.
In future development, full engine control should be implemented using Simulink model. Analogue sensor measurements, acquired from cDAQ, and calculated engine speed should be used as an input parameters. Simulink model should then calculate all angular and timing parameters required by GDI, IGN and HPP drivers to generate the corresponding waveforms.
4 Engine Test
4.1 Introduction
The main purposes of this phase of experimental work was to investigate the behaviour of the Libralato engine, to achieve a better understanding of the main functions of the engine including lubrication, sealing and mechanical behaviour.
Tests consisted of running the engine with throttle open across a speed range starting up to 600 rpm. Above 600 rpm the out-of-balance forces were simply too great to risk damage to the mechanical structure of the test facility. Based on this experience the engine has been modified with a balancer shaft arrangement which at the conclusion of the project was awaiting further testing.
Test planning was focussed on the need to provide data for evaluation of models and simulation codes and to provide data to help validate the dynamics modelling work conducted early in the programme.
The objectives of the cold test were respectively:
Evaluate the engine cycle, acquiring compression pressure and torques.
Measure and characterise the torque noting any changes that result with engine operating time
Observe and explain air flow to the engine
The objectives for the hot test were then:
Fire the engine in a direct injection mode
Observe torque and cylinder pressure during the fired operation so as to be able to make predictions.
The cold tests consisted of a series of engine runs at different speeds with the throttle wide open, so there was no significant loss of pressure in the inlet system.
Hot tests were also conducted under wide open throttle conditions and consisted of running the engine at a modest operating speed (between 200 and 600 rpm) and then seeking the best combination of spark and injection timing to sustain combustion. Measurements of torque and cylinder pressure were made and used primarily by the modelling team at TEC to support their cycle simulation work.
4.2 Torque Comparison
The purpose of this comparison of torques was to illustrate the change in friction that has taken place as the engine parts have worn and as modifications were made to reduce sources of friction. This represents an important understanding of engine friction which does appear to be dominated in Mark 1 by surface to surface friction. At 200 rpm for example, the average torque falls from a value of 45 Nm to about 20 Nm. At 600 rpm – the highest speed for tests, the reduction is from 70 Nm to 35 Nm. The later values are still very high for a small engine and indicate that there remain substantial issues to resolve. For an engine of this size in the early stage of development, the friction and pumping load should be between 5 Nm and 10 Nm. The transition to the Mark 2 design is expected to make a significant reduction, but in the absence of tests we are unable to confirm a number.
4.3 Analysis of engine test data
Cylinder pressure was measured at the ignition chamber and recorded against engine angular rotation.
For a number of reasons, the engine on test was unable to deliver the project rating of 25 kW (brake output) at 3000 r/min. As previously reported, many of the difficulties arose in the earlier stages of manufacturing and, as a consequence, the test programme could not be held to schedule. These delays, together with structural issues with the slider, meant that it was possible to test the engine at speeds up to 600 r/min.
Given the limited time available the analysis time was focussed on simulating performance at the lowest speed test because experience suggested that 200 r/min was the most sensitive engine speed. If operation at this speed, where the engine is most sensitive in terms of performance, could be simulated satisfactorily, it is anticipated that modelling higher speed performance would present fewer problems.
Since the behaviour of the Libralato engine as tested was successfully modelled at 200 r/min by the new LECS program, using input data from CFD, both the LECS software and the CFD model can reasonably be considered to have been validated to a considerable degree and can be used further to investigate increases in power output for the Libralato engine or for other designs.
5 Conclusions and Observations
The Libralato Engine remains at an early stage of its development and remains unproven in terms of fuel economy and emissions. However there have been a number of significant steps taken in this project that that represent the acquisition of new knowledge or the establishment of important aspects of the Libralato concept:
The engine design has been captured in the context of a modern CAE representation from which manufacture is possible and in which tolerancing appropriate to an engine concept is included.
The Mark 1 concept has been demonstrated to have a number of flaws which have been largely corrected by a reduction in the engine size and the inclusion of rolling element bearings. The resulting Mark 2 design would be implemented as a three rotor solution.
A test facility for the Mark 1 engine proved to capable to support the basic operation of the Mark 1 design including a complex operating mode which required phasing of the exhaust valve and the high pressure fuel pump. This test design is unique in being to support the continuing development of the Libralato engine. Air flow measurement and emissions measurement for the exhaust flow were both in place for testing – but the status of the Mark 1 design was not sufficiently advanced.
Test of the Mark 1 engine demonstrated that combustion supported by high pressure direct fuel injection of liquid fuel was possible. The control system is essential to maintain the coordination of fuel supply pressure and engine events. Combustion pressure measurements indicated that a reliable and stable combustion could be maintained even at low engine speeds – and this aspect of the engine’s operation is most encouraging.
The phasing of combustion remains uncertain. The dynamic analysis based on the CAE model and the ADAMS dynamics analysis code suggests a positive torque following a normal combustion cycle. However during tests the developed torque was found to be negative. It appears that the development of torque is particularly sensitive to the phasing of combustion. This situation is analogous to what is found in a conventional internal combustion engine and will require a detailed investigation of phasing with a view to changing the combustion process timing and potentially the design of the combustion chamber.
The Mark 2 design is complete and the manufacturing of parts had begun at the formal conclusion of the project. All other aspects of the project were prepared. The test rig had been designed to be able to accommodate the new engine, and the control system was in a state to run the engine with only minor adaptation of the control parameters required.
A CFD process for understanding air motion in the engine during the compression process was identified and demonstrated high speed air flow in the combustion chamber. The intensity of flow is such that a review of the combustion chamber design is warranted.
One important lesson concerned the premise on which project was initiated. At the start of the project the inventor presented a CAE model that the team had confidently expected as the source of important design information. While the model was helpful it proved to be unable to support manufacturing because it was based on a number of manufacturing features that were not well matched to the manufacturing partner’s machine tools.
At the start of the project too much time was spent on the manufacture of the first prototype and discussions about topics that were at that stage of the project unresolvable. It needed to first prototype manufactured in order to make the sensible decisions about where to proceed with the later designs.
Potential Impact:
Compared to a global manufacturing benchmark cost of $16.62/kW the Libralato engine is estimated to cost $10.50/ kW (including $1.57/kW for direct injection). This represents a 36% cost reduction.
This estimate is corroborated by AEA Ricardo research for the Service Report 4, concerning the cost of CO2 reductions in connection with European Commission 2021 CO2 Legislation for Light Duty Vehicles.
The Libralato cycle offers the potential for 14% CO2 reduction at a cost reduction of €363. No other CO2 reduction technology also offers a cost reduction. The Libralato engine presents a particularly effective solution.
Clear business opportunity - Driven by global megatrends such as CO2 legislation, emissions legislation and diminishing oil reserves, virtually all of the major vehicle manufacturers are developing low carbon and hybrid electric models. To date, with the exception of the Toyota Prius, these vehicles have not yet entered the mainstream. The IEA forecasts that by 2035, approximately 60% of all new cars will be some form of hybrid. The small car (segment A/B) market accounts for 40% and c.4m passenger cars sales in Europe pa. Customer usage profiles show that these mainly urban cars with average daily driving distances of well under 20 miles are prime candidates for full electrification, consistent with the European Roadmap for the electrification of road transport.
Libralato intends to target small cars since small car buyers are most likely to consider fuel efficiency and low CO2 as high priorities. To date, no vehicle manufacturer has even produced a concept vehicle of a plug-in hybrid segment A/B model, due to cost, packaging, range, re-charging infrastructure and performance constraints.
Disruptive market entry – Application of the Libralato engine for hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) overcomes these constraints by delivering the greatest cost-benefit to the average customer and society as a whole. It is urban traffic that must be electrified, not extra urban traffic, but customers cannot be expected to buy a second car just for this. The future of terrestrial surface transport is electrification but the foreseeable future must involve a cost-effective compromise, using electricity for ‘town’ driving and oil (including biofuel and synthetic) for ‘country’ driving.
Real world, low power urban drive cycles can be delivered affordably by low voltage (non-lethal) small battery capacity electric systems. This will reduce fuel consumption and CO2 by an order of 60% (50 gCO2/km) and radically reduce urban air pollution. The Libralato engine can deliver direct drive, diesel equivalent efficiency on the open road, enabling all the hybrid components to fit within standard engine bays and subsidizing the cost of the other components; so that any car could become a PHEV for a marginal cost repaid by the fuel savings in less than 2 years without subsidy.
Fully aligned with OEM needs – European OEMs are at the forefront of vehicle CO2 reduction technology but they must have appropriate support from policymakers and the best technology to be successful in this highly competitive landscape. By comparison, the United States programmes for automotive technologies invest more than $500m (~€370m) per year, in addition to the ongoing $2.8bn (~€2.1bn) initiative to support industry in the crisis package introduced in 2009‐10. China has recently announced an RMB100bn (~€11bn) package of investment over ten years for new energy vehicles.
EU legislation sets mandatory emission reduction targets for new cars to improve fuel economy and improve air quality. The fleet average to be achieved by all cars is 130gCO2/Km by 2015 (Euro 6) and 95g/Km by 2021 (Euro 7). The proportion of diesel engine passenger vehicles in Europe has reached over 50% in the last decade. However given the unsafe levels of diesel particulates in Europe and the fact that diesel particulates are now known to definitely cause cancer, this may change policy approaches and customer perceptions towards diesel engine vehicles. Diesel fuel could lose its current tax advantage in most of continental Europe and the Euro 6 and 7 emissions regulations are likely to require more expensive exhaust after-treatment technology. For these reasons, it is expected by the end of the decade diesel engines will begin to lose their dominance.
List of Websites:
http://www.libralato.co.uk
Libralato. Holdings Ltd. & Libralato Engines Ltd
153 Royce Road
Hulme
Manchester
M15 5TJ
UK
Email: info@libralato.co.uk
The Libralato Engine is a novel rotary engine design that is intended to overcome a number of issues associated with the more established Wankel design. The principle of the Libralato engine is that of two connected rotors which create in succession the volumes required for air intake and compression, combustion, expansion and exhaust. Like the Wankel engine the Libralato engine implements the four stroke cycle in a single revolution of the engine lending the efficiency and emissions benefits of this type of cycle with a power stroke for each engine revolution. The Libralato engine has, as an integral aspect of the engine design, an extended expansion stroke that leads to efficient operation. The assembly of the engine, which allows the tip seals in the Wankel rotor to be replaced by a series of long tapering passages supplemented by seals in the rotor lips.
An early engine design aimed at a rated power of 25 kW was supplied to the team by the engine inventor, Mr Ruggero Libralato. This design is referred to as the Mark 1. An early realisation of this design was essential to the progress project, but was hampered by the weakness of the design model and the need to include a new manufacturing partner in the project. Once the new partner was formally appointed the existing design was adapted to the machining processes and accuracy capabilities of the new partner. This turned out to be a\ long and tortuous process leaving the project in a difficult position.
While confirming the proper function of the engine, early analysis revealed a number of significant mechanical design issues. The use of a slider to connect the two rotors led to very high reaction torques that would lead to yield stress in rotor components even at modest speeds. With the existing design, the rated speed and power could never be realised. The relatively large diameter of the following rotor bearing led to extremely high friction torque. It was these fundamental mechanical properties of the engine that led to the early decision to move as quickly as possible to a reduced diameter rotor that would halve the working volumes. The reduction in internal forces would be significant and lead to a manageable implementation. The consequence of this decision is that a 50 kW engine would be composed of three of the smaller rotors connected in a phase relationship that would substantially balance their operation.
The design of the Mark 2 was initiated as soon as the analysis results indicated the difficulties with the Mark 1 design. As the Mark 1 engine was implemented a number of construction difficulties appeared such as the tendency of the rotor to lose its alignment. It was information like this that led to the adoption of a range of new features. The absence of a viable lubricating system on the Mark 1 engine led to the adoption of an ad-hoc system of pipework providing feed and return to bearings. For the Mark 2 this meant to an adoption of sealed rolling element bearings that could be used without a lubrication supply making the measurement of engine emissions possible.
The Mark 1 design was accompanied by the design of a special purpose test rig. The Libralato engine requires phase adjustment of the exhaust valve, and the synchronised operation of the high pressure fuel pump. The combustion system can only possibly work in a direct injection mode and so the complexity of a direct injection system needed to be managed. From the start of the project, the control solution was oriented to direct injection which ultimately proved successful. The test rig included the capability to run the engine, exhaust valve and high pressure pump in strict synchronisation.
Tests of the Mark 1 engine demonstrated the very high torque values that had been predicted in the analysis phase of the work. Those torque values showed some reduction as the engine was operated indicating that the source of at least some of the friction was misalignment in the engine. In low speed tests of the engine, combustion pressure is evident well before the secondary rotor begins the expansion process and essentially delivers a negative torque. The analysis work demonstrated otherwise and the implication is that the combustion phasing is too advanced. The two results suggest that the performance of the engine critically depends on the phasing of combustion. The resolution of this question is the essential next step in the development of the engine concept and may require both a mechanical design change to ensure the later phasing of the peak pressure as well as the control of a later combustion event. Test facilities and the manufacturing of the Mark 2 engine are substantially complete.
Project Context and Objectives:
New engine designs are required which meet the demands of Hybrid Electric Vehicles (HEV) and Plug In Hybrid Electric Vehicles (PHEV) :–
• aggressively downsized and integrated within series/ parallel or dual mode hybrid electric vehicles;
• designed for ‘steady state’ operation exclusively within their BSFC peak efficiency zone;
• exceeding Euro 6 emissions standards;
• with reduced production and maintenance costs,
• with exceptionally low NVH behaviour;
• with compact proportions that can be easily integrated within vehicles.
The Libralato rotary engine is a potential breakthrough technology with a thermodynamic cycle that particularly suits fuel efficient operation. . Patents are pending - WO2004020791 A1 for the engine design and PCT/EP 2009 / 006807 for the engine’s thermodynamic cycle.
The Libralato engine was announced as one of six winners of the UK Low Carbon Vehicle Partnership’s Technology Challenge in Dec 09, awarded by Neville Jackson, Ricardo UK Ltd. Group Technology Director and Richard Parry-Jones, UK Automotive Council Chair. A physical proof of concept engine mechanism has been produced and has been tested at speeds up to 1500 rpm – representing a sensible mid-load operating speed. A CREO/Pro-E CAD model of the engine has been produced and this model will provide the basis for simulations, studies and construction of high grade prototypes.
The Libralato engine has only four principal moving parts: two rotors fixed by their own bearings, connected by a sliding vane and a rotating exhaust. The rotors have different (overlapping) diameters of circumference and their motion forms and reforms three separate chambers within the engine each revolution. The leading rotor is engaged with the expansion of the air/ fuel mixture and a first stage compression of intake air. The following rotor is engaged with aspiration and a second stage compression. Air enters via a port at the centre of the engine and at one point also via the exhaust port.
The Libralato engine design is very different from the Wankel engine and avoids the reported difficulties associated with the Wankel engine by: large sealing surface areas of the rotors (equivalent in size to pistons), short flame paths acting against an acute angle working surface area, good thermal dispersion via fresh air scavenge phase and low bearing wear due to dynamic balance.
The engine cycle mechanically creates aspiration, first stage (low) compression, exhaust of scavenge gases, second stage (high) compression and expansion phases in parallel each full rotation of the engine, giving it twice the power density of a 4 stroke reciprocating engine with equivalent expansion volume. Due to the different diameters of the rotors, compression and expansion volumes are asymmetrical, allowing more complete (efficient) combustion of the air/ fuel mixture. The combination of more complete combustion and more complete scavenge of the exhaust gases using pressures crated during the first stage compression means that emissions, exhaust gas temperatures and noise can be controlled.
The project objectives as presented in the Technical Proposal are listed – and against each a summary of the achievements of the project are listed.
1) To construct and test prototypes of the Libralato engine – respectively 25 kW; <60 kW
A 25 kW prototype was re-designed, constructed and tested; a second prototype was designed, based on the observations and analysis from the Mark 1 evaluation and tests and the parts manufactured.
2) To construct a 3D CFD model, test and optimise scenarios of the compression and expansion ratios, heat transfer rates and leakage flows of the engine.
A 3D simulation model was assembled from the engine design files. An overset mesh method was prepared and used to show air flow in the second phase of compression.
3) To assess, refine and optimise the performance of the hot prototypes, determining fuel efficiency over a range of operating conditions. Target BSFC = <210g/kWh (39% efficient)
Some limited test work was completed to demonstrate that combustion could be supported. IN particular the assertion that this engine technology required the deployment of direct injection technology was supported by test results. Using liquid fuel a direct injection combustion process could be initiated and run.
4) To assess, refine and optimise the performance of the hot prototypes, determining emissions over a range of operating conditions in relation to Euro 6 standards : –
Targets - THC < 0.1 g/km; CO < 1.00g/km; NOx < 0.06 g/km; PM < 0.005 g/km
This objective could not be met owing to the lubrication management process used in the Mark 1 engine. A re-design of the lubrication system and the adoption of selected rolling element bearings was to be the basis for emissions tests on the Mark 2 design , which unfortunately could not be completed.
5) To conduct design/ cost analysis of the Libralato engine. Target production costs for 50kW = $500
Based on the design and analysis work conducted for the Mark 1 and Mark 2 engines, a cost analysis has been conducted and indicates a competitive manufacturing cost at €15/kW.
6) To assess NVH behaviour. Target NVH behaviour = 50% comparative reduction
NVH analysis was not possible with the Mark 1 engine. Some vibration information could be taken with accelerometers but the data simply indicated that there were significant out of balance forces due to the rotating masses of the engine. The Mark 2 engine was intended to be the basis for a more extensive investigation of vibration information.
7) To assess critical package and weight boundaries. Target critical package and weight boundaries = 50% comparative reduction
A preliminary assessment of packaging was completed and suggests that a packaging efficiency of 50% can be achieved making the engine particularly appropriate of parallel hybrid assembly.
8) To conduct a limited market assessment for the Libralato engine
The market assessment suggests a substantial opportunity in the hybrid vehicle sector.
9) To apply for patent protection for the updated Libralato engine design and thermodynamic cycle
Two patent applications have been developed as a result of the work.
10) To undertake dissemination activities via academic and industrial networks
Dissemination activity was based on the design analysis work that was conducted during the design work on the Mark 1 engine.
Project Results:
The results and conclusions are organised according to the main technical activity of the project. In the next section we review the design process for the engine. There is a detailed description of the Mark 1 design – the first design of the engine which was developed from an existing but incomplete design. The Mark 2 design represents a significant step forward and utilises the results of an extensive analysis into the dynamic behaviour of the Mark 1 engine.
The analysis is presented to demonstrate the reasoning that led to the adoption of the Mark 2 design features. The analysis continues with the development of the understanding of air flow in the engine, and concludes with the simulation and prediction of the engine performance.
The control system implementation was critical to the testing of the engine, and the control system solution in the form of an electronic implementation is presented followed by a brief description of the software.
Although the team prepared a test facility for the engine, only limited testing could be done because of the relatively fragile state of the Mark 1 engine However the combustion process was demonstrated and an understanding of internal gas pressures developed that could act as a source of information about the continuing development of the engine concept.
1 The Design of the Libralato Engine
This section summarises the design process by which the engine emerged.
1.1 The Mark 1 Design
The engine design is presented as a computer aided model created using the tool historically known as Pro/Engineer (Pro/E) and now sold as CREO Elements/Pro. Pro/E is maintained by Parametric Technologies and is widely used in manufacturing, notably by Caterpillar for engine and machine design. The design work for this deliverable has been performed using version CREO 2.0.
The original design work for the Libralato engine was done using traditional sketching and drawing techniques which were used as the basis for the manufacturing of a model using mild steel and which became the basis for some concept tests and demonstration of assembly techniques. Libralato Ltd, owners of the design, commissioned the preparation of design files using Pro/E. However those files were based a set of manufactured features that were inappropriate to the machine tools to be used for initial manufacture.
The first task to be initiated in Work package 6 was the reworking of the model to incorporate new features that better matched the capabilities of the machining centres used by Techmachine, the member of the project consortium responsible for manufacture of the engine.
The original design model that was made available at the start of the project was not a completed design. Firstly, it did not include some important information that is essential to the manufacturing process, such as tolerances and materials. Secondly, some requirements are not properly considered in the design making certain functions unachievable. Thirdly, certain of the design features included in the initial design are not readily manufacturable. Although the engine functions are realised, their manufacturing is very difficult and the associated manufacturing cost correspondingly high. Consequently, the new CAD model was improved in all these aspects. The new model conveys the information, such as materials, processes, dimensions, and tolerances, which was not included in the original design. Techmachine built the first prototype engine based on this new updated model using their NC machining centres. This new model was also imported to finite element analysis software (ABAQUS & ANSYS) and CFD software (STAR-CCM) to build the analysis models and perform FE analysis. The model was also imported to dynamic analysis software (ADAMS) to investigate the dynamic performance of the engine.
1.2 The Mark 2 Design
The Mark 1 design was by no means complete. Firstly, certain of the design solutions were not fully implemented simply because there was insufficient information available, and that particular information needed to come from practical test. Consequently the Mark 1 engine needed to fulfil a role where it would support certain tests whose purpose was to provide design information for later versions of the engine. . The original design of the engine that had formed the starting point for the project was much less well understood than we had expected.
Some design features originally proposed for Mark 1 proved to be difficult to manufacture. Although these features could be made, their manufacturing would be very difficult and the cost is consequently high. During the tests of Mark 1, more design weaknesses have been identified, which mainly include the uncertainty of the friction sources, the severe inertia force at normal engine speeds, an intake behaviour that was very difficult to characterise, and difficulties with sealing the rotors against the fixed part of the engine. All of these discoveries have revealed important information about the mechanisms of Libralato engine.
In order to improve the performance of the engine and identify solutions for the new Mark 2 engine design, our team visited Techmachine on several occasions to review the test and analysis results with our partners. In the meetings, the new design solutions for Mark 2 design have been reviewed and discussed.
Therefore, the new Mark 2 model has been improved in all these aspects. Compared with the Mark 1 design, the new Mark 2 design demonstrates improved solutions in all aspects of the engine function. The main design changes are summarized in the following Table.
1.2.1 Summary of design changes
This section summary all the main design changes in Mark 2 design.
Mark 1 feature Issue Change required
1 Engine size The diameter of the main rotor in Mark 1 engine is 270 mm, which causes high inertia force at high running velocity.
The most effective way to reduce this inertia force is reducing the size of the engine.
2 Lubrication System The lubrication passages have not been properly sealed, which leads to flows of oil into the expansion chamber and the exhaust system Closed oil path for slider and rolling element bearings for rotors have been applied, which can guarantee the oil only flows in the specified oil passages.
3 The main rotor The main shaft is connected to the main rotor independently, which affect the strength of the connection and can cause the misalignment of the main shaft. The main shaft should be made as one piece and fixed on the main rotor during assembly.
4 The following rotor The position of three pieces of the following rotor can be moved during the working cycle, which can cause misalignment of the main shaft. Also can cause severe friction at some local surface. The three pieces of the following rotor should be made as a single piece.
5 The main rotor bearing
& The following rotor bearings The bearings are plain bearing. Considering the running loads the inertia force, the plain bearing is not a good option for this application. The needle roller bearing can provide much stronger supporting for the rotors, which can guarantee the rotor on the right position during rotating and is more suitable for this engine design.
6 The side plate The side plate not only provides the mounting for the bearings, but also needs to make sure the bearings being sealed properly.
Considering the roller bearings adapted in Mark 2 design, the main dimensions and structure need to be modified.
7 The exhaust valve In the original design, the exhaust valve not only works as the exhaust valve, but also works as the intake port to let the air enter the engine. However, in the Mark 1 test, the intake on the exhaust port didn’t work properly. The air doesn’t flow into the engine during the working cycle.
In the Mark 2 design, the exhaust valve is redesigned to only work for exhaust system.
8 The seals for the bearings The two rotor axes are very close, also the intake port and the main rotor bearing need to be inside the internal ring of the following bearing. The limited space on the side plate is a big challenge for the seal solution. The grease can be used to lubricate the needle roller bearing.
9 The seals for the slider The slider needs to withstand a large torque from the rotor and have to keep moving in the cylinder. In order to reduce the friction and keep the temperature on the slider within a specified limit, the proper lubrication must be guaranteed. At the same time, the seals must be provided to make sure there is no oil leaking into the chambers. The special passage need to be designed on the main shaft and the cylinder to guarantee the lubricant supply. Also, two seals need to mount on the cylinder to keep the oil inside the cylinder.
10 The intake system The intake port on the exhaust valve doesn’t really let the air enter the engine. Most of air into the engine is coming from the intake port at the centre of the engine. The air intake will only use the port on the centre of the engine,
11 The engine block When the engine size is decreased, the manufacturing for some parts becomes more challenging. For the engine block, it has to be made as two pieces, which can guarantee the cutter can reach some local manufacturing positions.
1.3 The Mark 3 Design
During the Mark 2 design process, we have already identified where even the new design could be improved. For example, in Mark 2 design, the air flow into the engine may be not sufficient to support combustion. Therefore a major design change could be made where air is introduced directly in to the compression chamber. Alternatively pressurised air could be introduced into the existing intake in the Mark 3 design. The combustion process needs to be properly considered in the new design with a more ordered pattern of air motion – with a degree of control over the motion air about a vertical axis. Combustion phasing remains a significant concern that would require tests woith the Mark 2 engine to identify changes needed for Mark 3.
The slider remains a risk – subject to high forces and as a reciprocating device, difficult to lubricate. For the Mark 3 design a rolling element bearing for the slider is needed that further simplifies the lubrication function (although lubricant will still be needed for cooling).
The discussion of Mark 3 suggests that this design is used as a development benchmark – allowing emissions and friction to be brought to acceptably low levels. The step to Mark 4 is then one of controlling costs and making design changes to keep costs to a target level while retaining the fuel economy and emissions performance.
2 Design Analysis
We considered the design analysis in three aspects –
• The mechanical design analysis with considerations of kinematics, dynamics and the behaviour of materials
• The thermodynamic analysis consisting particularly of the analysis of mass and energy transport on the engine and how these transport processes are simulated.
• The analysis of internal gas flows using computational fluid dynamics calculations to understand the conditions at the start of combustion.
2.1 Mechanical aspects of the engine – stress analysis
Finite element (FE) stress analysis has been carried out using ANSYS, to ascertain the stresses within engine components as a result of engine loading/combustion. In addition, thermos-elastic stresses were predicted as a result of engine constrained expansion due to heating.
The main findings are as follows:
1) Up to 180 MPa combined stress predicted for main shaft
2) Up to 200 MPa thermos-elastic stress predicted for mounted engine
Further stress analysis is required, taking into account fatigue behaviour of the materials, as well as dynamic loading.
Multi-body work was carried out using three distinct approaches, namely: a) an ADAMS model, b) a multi-body model programmed in MATLAB® from first principle and c) a Simulink model, set up using the Simmechanics® toolbox. The Simmechanics® model is also a high-fidelity non-linear model that is developed to offer a link with the GT-Power® engine model and also to benefit from all the functionality of MATLAB, including the optimisation toolbox. The materials used were pure cast iron and a blend of cast iron and aluminium.
In an effort to further improve balancing, an automated optimisation study was carried using a Nelder-Mead optimisation algorithm to find the optimum position of the centres of mass of the leading rotor, following rotor and slider. The centre-of-mass positions were constrained within physical limits imposed by the geometry of the engine. The method resulted in reduction of peak loads by about 60% in the vertical direction and 90% in the longitudinal direction of the main rotor pivot. However, the remaining loads are still high and further work is required in this area.
In light of the sub-optimal balancing of the engine, consideration has been given to a multi-rotor-pair engine design. A pilot study using a three-rotor-pair engine whereby three leading rotors are connected via a common shaft at a phase angle of 120o between them was carried out using Simmechanics®.
Further analytical and numerical work was carried out in order to understand the mechanism of load generation at the bearings and the slider. It was found that significant exchange of momentum takes place between the leading and following rotor during a full cycle of rotation. This can be observed as a significant periodic fluctuation in the speed of the leading/following rotors, when an initial speed is imposed to the engine and it is then left to rotate in the absence of friction and combustion forces. The torque on the slider is primarily related to this momentum exchange and cannot be improved by balancing. It is therefore an inherent characteristic of the Libralato mechanism. On the other hand, the loads at the bearings can be reduced by improving the balancing of the mechanism, as already illustrated. This observation brings forward the torque on the slider as one of the main design concerns. Analytical work has resulted in the following dependency of slider torque on the change in the size and speed of the engine:
T = k x^4 y (gamma) (1)
Where T is the torque, k is a constant of proportionality, x is the variation factor of the radial dimension, y of the axial dimension and γ (gamma) is the variation factor of angular velocity. It is assumed that material density is kept constant. The torque increases with the 5th power of engine size (if a uniform expansion is considered) and with the 2nd power of speed. Importantly, the radial dimension of the engine has a 4th power influence on slider torque. These results have been verified by simulation and together with lubrication concerns have fundamentally influenced the design of the Mark 2 engine, as explained below.
2.2 Mechanical aspects of the engine - engine size and sealing
The design refinement work reported in this section has been based on a range of simulation work. At the time of conducting the analysis it was not possible to gather any experimental evidence due to the delays in manufacture of the Mark 1 engine. However, significant knowledge has been gained from simulation studies.
With regards to overall engine design, the findings from the multi-body and lubrication analyses have led to the adoption of smaller rotors. In particular it has been decided that the Mark 1 engine is reduced uniformly in size by a factor of 0.79 resulting in a reduction in volume by a factor of 0.5. To maintain power output and in an effort to improve balancing, it has been suggested that future design, including the Mark 2 design, will consist of multiple rotors, In the same figure, the implementation of water cooling is shown, to address high temperatures in the engine. Finally, external balancing of the rotors is suggested at least for pre-production versions, in order to eliminate loads at bearing and vibration related concerns.
In the absence of experimental results and in light of manufacturing delays/difficulties, a subset of the changes proposed above will be implemented in the Mark 2 engine, as follows:
Two rotor-pairs sharing a common shaft between main rotors
Engine remains air-cooled
Use of external balancing to eliminate loads on bearings
Basic design of each rotor-pair remains the same as in Mark 1 engine, but downsized in order to reduce size-related problems
The sealing system required significant re-design for the Mark 1 engine. The size of the original seals was inappropriately small for production or smooth operation. In addition, various radii of curvature had to be changed where rotors met seals, to reduce impact loads and prevent seal/rotor damage. In summary, the Mark 1 sealing system was improved by:
Calculation of all clearances between rotors/block, including the effect of thermal expansion and bearing clearances
Re-definition of tolerances
Re-sizing and general re-design of side- and face-seals
Specification and sourcing of appropriate springs to work with the seals
2.3 Summary of conclusions from Mechanical Analysis
Large engine size (especially radially) causes extreme loads on bearings and slider. Future engines should feature compact rotors.
Possibility of near perfect balancing using a multi-rotor-pair design with external balancing.
Relatively high stresses are predicted for some components – potentially destructive levels if temperature and fatigue behaviour are taken into account.
Very high temperatures are predicted – water cooling seems essential in production versions.
Extreme friction losses at Mark 1 following-rotor bearings. Again, large size is the culprit here.
2.4 Thermodynamic performance
The unique feature of the Libralato concept is the transient nature of its working volumes, characterized by the separation and merging of volumes as the cycle proceeds. This is in contrast with the conventional piston engine, where the cylinders form the working volume and the inlet/exhaust ports/manifold are fixed. The mathematical formulation and the results are presented in this report to assist in the validation of the program against the engine test results as they become available. A new Libralato Engine-specific simulation software (“LECS”) was developed
The current version of the LECS simulation software is based on a single zone combustion model of the Libralato engine and has already been used to simulate the performance of the engine at 25 KW brake power at 1500 rpm. The simulation indicates that the compression pressure will be around 45-48 bar and peak pressure of 70-73 bar, with a mechanical efficiency of 90-92%. The rate of volume changes versus ignition timing and gas exchange timing have been derived from the initial CAD model, as explained earlier.
The most innovative aspect of the simulation is the way in which it processes the merging of the central volume during the expansion phase, a process which can lead to instability and which requires adjustment to pressure and temperature to provide the final solution. This modelling strategy for the central volume will be reviewed at a later stage for the complete Libralato cycle, including the gas exchange process between the expansion and compression volumes, particularly if the test results do not adequately validate the assumptions made in this first model.
Validation of the LECS program and of its Libralato engine model against the measured results from the test bed is particularly important since the Libralato cycle is completely unlike the conventional cycle of either a two-stroke or a four-stroke reciprocating engine. In the conventional cycle, working volumes are linked only through fixed volumes (i.e. the inlet and exhaust ports and manifolds). This is not the case in the Libralato cycle, where the working volumes can exchange mass directly between them.
Results of TEC engine performance simulations have indicated that, compared with a conventional petrol engine, the Libralato engine concept is capable of excellent thermal efficiency and will deliver very low exhaust emissions. Initial test results may not confirm this claim, but once the design is optimized, there is no thermodynamic reason why the Libralato will not reach the levels of performance shown by the LECS simulations.
The analysis of the engine integrity suggests that the torque developed cannot be contained locally on the slider in this Mark 1 design. Although this version of the engine was designed for 50 KW at 3000 RPM, the mechanical analysis shows that this engine will not operate satisfactorily at above 1000 RPM, due to the excessive torque developed by the slider. Thermodynamic analysis also suggests that the performance at 3000 RPM is going to be an issue for the air compression temperature and some cooling system changes will be required to achieve this engine speed. All the predictions made from the LECS code have been done at 1500 RPM to reach 50% of the design power output. This is a representative output at about 50% of rated conditions.
2.5 CFD Analysis of engine airflow
This section of the analysis includes the findings of a CFD study. The aim of the study was to obtain flow field visualisation in the 2nd compression stage of the engine cycle. This provides crucial data required in determining suitable positioning of fuel injectors, particularly important for early liquid fluid injection. This report gives details on the CAD simplification, meshing process and finally flow field results that were obtained.
The visualisation shows how the motion of the engine components forces high velocity flow into the combustion chamber. There is evidence of this early in the compression stage. Due to the position of the leading rotor and combustion chamber design the flow enters the chamber with a large amount of swirl. As this flow reaches the top wall of the chamber it splits forming counter rotating eddies. The strengths of these eddies is increased as the compression stage progresses and pressure in the chamber increases. The high intensity of the eddies suggests that a re-design of the combustion chamber should be considered with an objective to create a specific pattern of air motion to induce stoichiometric air fuel ratios in the vicinity of the spark plug, while avoiding excessive heat transfer
It is also interesting to see the leakage of flow between the slider and two rotors. This behaves as expected with leakage velocity values increasing as the compression progresses and hence pressure in the combustion chamber and around the link head increases. In simulation we just consider leakage between main and trailing volumes however these results show some leakage in the central intake. These results will provide good boundary conditions for future simulations that consider additional leakage areas.
The next step would be to introduce turbulence modelling through higher order models such as Large Eddy Simulation (LES). There is also the potential to study multiphase flows, important for analysing the mixing of fuel with air in the combustion chamber for maximum efficiency or in order to achieve specified combustion goals.
3 Control System Implementation
The control system implementation consists of two components – respectively a host and “AddOn” ECU module that provides the foundation controls for the engine. The host provides interfaces with respectively: (a) an external computer for programming purposes and (b) with instrumentation needed for assessment of the engine condition and calculation of control requirements. The host was originally proposed to be the Infineon PSK, a computer system developed for motorsport applications and development work. Later in the programme, the functions were transferred to a National Instruments cDAQ system.
The main issues of the implementation of the control system are as follows
:
Electronic hardware – the design requirements and the resulting electronic hardware that was implemented to support the investigation of controls requirements.
System Description – how the hardware and software elements are integrated to give the complete controls solution.
Systems software – the low level software elements needed to support the basic functions of the control system
Interfaces – the connections to sensors, actuators and external systems that are required for the test and development processes.
Calibration tool – the interfaces that have been implemented to allow a calibration support tool to be used.
The development path that may be followed to continue to development of this system.
The Infineon Powertrain Starter Kit (PSK) allows 12 standard electro-magnetic, low-current (1A) fuel injectors to be driven using conventional low-side drives. To support current Libralato engine development and prolong the life of the starter kit, it was necessary to be able to drive dual coil, high voltage solenoid injectors suitable for Gasoline Direct Injection (GDI) applications. It was therefore proposed to design an add-on module to allow such injectors to be interfaced to the existing kit. The add-on will be known as the “PSKInjAddOn ”.
3.1 Function of AddOn module
The Add-On ECU module incorporates the following three main drivers implemented to provide full control of the Libralato engine:
1) Gasoline Direct Injection Driver (GDI)
GDI driver is designed to generate control signals for quad low side injector driver TLE6270R especially suited for Gasoline Direct Injection systems in automotive applications. The device controls the external High Side Transistors, implemented on the Add-On ECU module, to supply the injectors alternating with battery voltage and a boosted high voltage according to the requirements of the applied injectors. The TLE6270R device incorporates the Low Side driver Transistors for four Injector channels.
2) Ignition Driver (IGN)
IGN driver generates control signals for high voltage ignition coil driver VB525SP-E assembled on an external stand-alone module attached to the Add-On ECU module.
The VB525SP-E is mainly intended as a high voltage power switch device driven by a logic level input and interfaces directly to a high energy electronic ignition coil. The input of the VB525SP-E is fed from a low power signal IGN_PULSE generated by the Add-On ECU IGN driver.
3) High Pressure Fuel Pump Driver (HPP)
HPP driver is designed to generate control signals for the Flow Control Valve (MSV5) located and fixed to the High Pressure Pump HDP5. The HDP5 is a demand controlled high-pressure fuel pump. It works according to the principle of a cam-driven single-cylinder pump. The Flow Control Valve (MSV5) itself is a magnetic actuator, which controls the inlet valve of the HPP.
2.2 Integration of Hardware
This section describes technical progress achieved during integration of the Add-On ECU hardware module with sensors. It provides brief description of mandatory sensors integrated into the system during final phase of the Libralato engine testing at Loughborough University.
2.2.1 Crank Sensor
A crank sensor is an electronic device used in an internal combustion engine to monitor the position of the crankshaft. It is also commonly used as the primary source for the measurement of engine rotational speed in revolutions per minute (RPM).
Common mounting locations include the main crank pulley, the flywheel, the camshaft or on the crankshaft itself depending on the engine type. Commonly a Hall-Effect sensor is used, which is placed adjacent to a spinning steel disk.
Engine Control Units (ECU) use the information transmitted by the crank sensor to control parameters such as ignition timing and fuel injection timing.
This sensor is the most important sensor in modern day engines. When it fails, there is a chance the engine will not start, or cut out while running.
A Hall-Effect speed sensor HA-D 90 has been utilized as a Libralato engine position sensor. This sensor is designed for incremental measurement of rotational speed (e.g. camshaft, crankshaft or wheel speed).
Due to the rotation of a ferromagnetic target wheel in front of the HA-D, the magnetic field is modulated at the place of the Hall probe. The HA-D 90 is no true-power-on sensor. It needs the falling edge of two teeth for correct working. After a time of 0.68 s without rotation of the detected wheel it needs again the falling edge of two teeth.
The main feature and benefit of this sensor is a very good detection of the falling edge, due to a differential measuring method.
For more details on HA-D 90 sensor including technical specification please refer to the Hall-Effect Speed Sensor HA-D 90 Data Sheet from Bosch.
During Libralato engine testing the HA-D 90 is placed adjacent to the wheel, with 36-1 teeth, assembled to the Libralato engine drive shaft. Profile 36-1 describes the wheel with 35 teeth plus one missing tooth called “gap” tooth.
For the proper operation the HA-D 90 sensor requires power supply in a range 5 to 18 V. Battery power supply has been connected to the sensor.
Sensor signal designated as a CRANK_POS has been connected to the Add-On ECU. This signal is used by Engine Position Driver to determine engine position and engine rotational speed given in RPM.
2.2.2 Fuel Pressure Sensor
The Bosch Pressure Sensor Fluid PSS-250R has been utilized for a fuel pressure measurement.
This sensor is designed to measure the pressure of media in relation to the ambient pressure (e.g. gasoline, diesel, water, engine oil, transmission oil or air). The sensor is available for two different supply voltage ranges 4.75 V to 5.25 V and 8 V to 30 V. The sensor uses stainless steel measuring cells with piezo-resistive measuring bridges in thin layer technique, which are hermetically welded together with stainless steel pressure ports. This guarantees a complete media compatibility. The sensor has a protection for over voltage, reverse polarity and short circuit. It is not recommended to fix the sensor directly to the engine block in order to avoid undesired strong vibrations.
During Libralato engine testing the sensor was assembled to the high pressure fuel pump driven separately and locked to the engine rotation.
PSS-250R sensor has a maximum pressure of 500 bar and generates an analog voltage output of 16 mV per bar on a ratio metric basis.
The sensor has an independent 5 V regulated power supply, derived from the TLF7368 voltage regulator Q_T1 output which is intended as a stable reference supply for analog sensors.
The 5V sensor supply has a feedback line to the ADC to allow the accuracy of the voltage to be monitored. By reading the 5V supply to the sensor via a separate channel, the output could be corrected using the following equation:
Pressure (kPa) = (5V * ADCreading) / (0.016 * ADCsupply)
Fuel pressure reading provided by the sensor is used in closed loop control algorithm to calculate HPP control wave parameters. In particular PWM duty cycle for holding phase of the HPP driver waveform is controlled to maintain fuel pressure at the expected operating pressure of 150 bar.
2.3 Application Layer
The application layer consists of three software drivers implemented to provide control signals for three major functional units of the Add-On ECU hardware module used to control Libralato engine.
Drivers and corresponding functional units are:
1 Gasoline Direct Injection (GDI)
2 Ignition (IGN)
3 High Pressure Fuel Pump (HPP)
Gasoline Direct Injection Driver
The GDI driver is designed to generate control signals for quad low side injector driver TLE6270R especially suited for Gasoline Direct Injection systems in automotive applications. The device controls the external High Side Transistors to supply the injectors alternating with battery voltage and a boosted high voltage according to the requirements of the applied injectors. The TLE6270R device incorporates the Low Side driver Transistors for four Injector channels.
Although in a case of Libralato engine only one injector is used, the GDI driver is implemented to support control of up to four solenoid injectors, allowing for the possible future expansion.
Starting, default values are assigned to all parameters during GDI driver initialization in “Task_gdi_wave” function implemented in “TASKS.c” source code file. Depending on the GDI control algorithm, users can choose which parameters they want to modify dynamically in order to control GDI waveforms. Also, some parameters can be fine-tuned, during development stage, and then they can be set to the fixed values in the initialization stage.
Ignition Driver
IGN driver generates control signals for high voltage ignition coil driver VB525SP-E.
The VB525SP-E is mainly intended as a high voltage power switch device driven by a logic level input and interfaces directly to a high energy electronic ignition coil.
The input VIN of the VB525SP-E is fed from a low power signal generated by AURIX™ microcontroller that determines both dwell time and ignition point. During VIN high (≥ 4 V) the VB525SP-E increases current in the coil to the desired, internally set current level.
After reaching this level, the coil current remains constant until the ignition point. This corresponds to the transition of VIN from high to low level.
HPP driver is designed to generate control signals for the Flow Control Valve (MSV5) located and fixed to the High Pressure Pump HDP5. The HDP5 is a demand controlled high-pressure fuel pump. It works according to the principle of a cam-driven single-cylinder pump. The Flow Control Valve (MSV5) itself is a magnetic actuator, which controls the inlet valve of the HPP.
In order to regulate the delivered volumetric flow, the MSV5 is activated at a certain angle before top dead center position (TDC). The starting point and the end point of the MSV5 control signal has therefore to be calculated depending on the desired amount of fuel, the engine speed, the battery voltage and the temperature of the MSV5.
2.3 System Software
2.3.1 Operating System
MicroC/OS-II real time operating system from Micrium has been integrated into Libralato application source code. It is provided in source form (ANSI C standard) and in-depth documentation for free evaluation.
The features of µC/OS-II include:
• Preemptive multitasking real-time kernel
• Up to 254 application tasks (1 task per priority level), and unlimited number of kernel objects
• Semaphores with timeouts on pending calls to prevent deadlocks
• Mutual-exclusion semaphores that eliminate unbounded priority inversions
• Event flags
• Message mailboxes and queues
• Time and timer management
• Fixed sized memory block management.
In order to integrate µC/OS-II into Libralato application code AURIX™ System Timer STM0 has been configured to generate 1ms Interrupt for the OS task handling. AURIX™ specific implementation of µC/OS-II and hook functions has been defined in TriCore_os_cpu_port.c source code file.
2.4 Interfaces
The AURIX™ MultiCAN+ module provides a communication interface which is fully compliant with CAN specification V2.0B (active) and to CAN FD ISO11898-1 FDIS version 2013, providing communication speed of up to 1 Mbit/s in standard CAN (ISO 11898-1:2003(E) mode.
For Libralato project two CAN nodes have been utilized:
• CAN node 1 for communication with Compact DAQ
• CAN node 2 for communication with calibration tool
CAN Interface to Compact DAQ
For all analogue sensor readings Compact DAQ (cDAQ) from National Instruments has been intended. CAN node 1 is configured as a communication interface between Add-On ECU and cDAQ. CAN message objects are configured for frame reception from cDAQ for each sensor individually. Sensors are organized in three groups and data structures are defined for each group.
CAN Interface to Calibration Tool
Vector Informatik GmbH Universal Measurement and Calibration Protocol (XCP) device driver has been integrated into Libralato application source code, to enable calibration and fine tuning of selected parameters with Vector CANape tool.
3.4 Development Path
A Simulink® model has been integrated into Libralato application source code and some basic functionality has been tested to verify integration strategy.
In future development, full engine control should be implemented using Simulink model. Analogue sensor measurements, acquired from cDAQ, and calculated engine speed should be used as an input parameters. Simulink model should then calculate all angular and timing parameters required by GDI, IGN and HPP drivers to generate the corresponding waveforms.
4 Engine Test
4.1 Introduction
The main purposes of this phase of experimental work was to investigate the behaviour of the Libralato engine, to achieve a better understanding of the main functions of the engine including lubrication, sealing and mechanical behaviour.
Tests consisted of running the engine with throttle open across a speed range starting up to 600 rpm. Above 600 rpm the out-of-balance forces were simply too great to risk damage to the mechanical structure of the test facility. Based on this experience the engine has been modified with a balancer shaft arrangement which at the conclusion of the project was awaiting further testing.
Test planning was focussed on the need to provide data for evaluation of models and simulation codes and to provide data to help validate the dynamics modelling work conducted early in the programme.
The objectives of the cold test were respectively:
Evaluate the engine cycle, acquiring compression pressure and torques.
Measure and characterise the torque noting any changes that result with engine operating time
Observe and explain air flow to the engine
The objectives for the hot test were then:
Fire the engine in a direct injection mode
Observe torque and cylinder pressure during the fired operation so as to be able to make predictions.
The cold tests consisted of a series of engine runs at different speeds with the throttle wide open, so there was no significant loss of pressure in the inlet system.
Hot tests were also conducted under wide open throttle conditions and consisted of running the engine at a modest operating speed (between 200 and 600 rpm) and then seeking the best combination of spark and injection timing to sustain combustion. Measurements of torque and cylinder pressure were made and used primarily by the modelling team at TEC to support their cycle simulation work.
4.2 Torque Comparison
The purpose of this comparison of torques was to illustrate the change in friction that has taken place as the engine parts have worn and as modifications were made to reduce sources of friction. This represents an important understanding of engine friction which does appear to be dominated in Mark 1 by surface to surface friction. At 200 rpm for example, the average torque falls from a value of 45 Nm to about 20 Nm. At 600 rpm – the highest speed for tests, the reduction is from 70 Nm to 35 Nm. The later values are still very high for a small engine and indicate that there remain substantial issues to resolve. For an engine of this size in the early stage of development, the friction and pumping load should be between 5 Nm and 10 Nm. The transition to the Mark 2 design is expected to make a significant reduction, but in the absence of tests we are unable to confirm a number.
4.3 Analysis of engine test data
Cylinder pressure was measured at the ignition chamber and recorded against engine angular rotation.
For a number of reasons, the engine on test was unable to deliver the project rating of 25 kW (brake output) at 3000 r/min. As previously reported, many of the difficulties arose in the earlier stages of manufacturing and, as a consequence, the test programme could not be held to schedule. These delays, together with structural issues with the slider, meant that it was possible to test the engine at speeds up to 600 r/min.
Given the limited time available the analysis time was focussed on simulating performance at the lowest speed test because experience suggested that 200 r/min was the most sensitive engine speed. If operation at this speed, where the engine is most sensitive in terms of performance, could be simulated satisfactorily, it is anticipated that modelling higher speed performance would present fewer problems.
Since the behaviour of the Libralato engine as tested was successfully modelled at 200 r/min by the new LECS program, using input data from CFD, both the LECS software and the CFD model can reasonably be considered to have been validated to a considerable degree and can be used further to investigate increases in power output for the Libralato engine or for other designs.
5 Conclusions and Observations
The Libralato Engine remains at an early stage of its development and remains unproven in terms of fuel economy and emissions. However there have been a number of significant steps taken in this project that that represent the acquisition of new knowledge or the establishment of important aspects of the Libralato concept:
The engine design has been captured in the context of a modern CAE representation from which manufacture is possible and in which tolerancing appropriate to an engine concept is included.
The Mark 1 concept has been demonstrated to have a number of flaws which have been largely corrected by a reduction in the engine size and the inclusion of rolling element bearings. The resulting Mark 2 design would be implemented as a three rotor solution.
A test facility for the Mark 1 engine proved to capable to support the basic operation of the Mark 1 design including a complex operating mode which required phasing of the exhaust valve and the high pressure fuel pump. This test design is unique in being to support the continuing development of the Libralato engine. Air flow measurement and emissions measurement for the exhaust flow were both in place for testing – but the status of the Mark 1 design was not sufficiently advanced.
Test of the Mark 1 engine demonstrated that combustion supported by high pressure direct fuel injection of liquid fuel was possible. The control system is essential to maintain the coordination of fuel supply pressure and engine events. Combustion pressure measurements indicated that a reliable and stable combustion could be maintained even at low engine speeds – and this aspect of the engine’s operation is most encouraging.
The phasing of combustion remains uncertain. The dynamic analysis based on the CAE model and the ADAMS dynamics analysis code suggests a positive torque following a normal combustion cycle. However during tests the developed torque was found to be negative. It appears that the development of torque is particularly sensitive to the phasing of combustion. This situation is analogous to what is found in a conventional internal combustion engine and will require a detailed investigation of phasing with a view to changing the combustion process timing and potentially the design of the combustion chamber.
The Mark 2 design is complete and the manufacturing of parts had begun at the formal conclusion of the project. All other aspects of the project were prepared. The test rig had been designed to be able to accommodate the new engine, and the control system was in a state to run the engine with only minor adaptation of the control parameters required.
A CFD process for understanding air motion in the engine during the compression process was identified and demonstrated high speed air flow in the combustion chamber. The intensity of flow is such that a review of the combustion chamber design is warranted.
One important lesson concerned the premise on which project was initiated. At the start of the project the inventor presented a CAE model that the team had confidently expected as the source of important design information. While the model was helpful it proved to be unable to support manufacturing because it was based on a number of manufacturing features that were not well matched to the manufacturing partner’s machine tools.
At the start of the project too much time was spent on the manufacture of the first prototype and discussions about topics that were at that stage of the project unresolvable. It needed to first prototype manufactured in order to make the sensible decisions about where to proceed with the later designs.
Potential Impact:
Compared to a global manufacturing benchmark cost of $16.62/kW the Libralato engine is estimated to cost $10.50/ kW (including $1.57/kW for direct injection). This represents a 36% cost reduction.
This estimate is corroborated by AEA Ricardo research for the Service Report 4, concerning the cost of CO2 reductions in connection with European Commission 2021 CO2 Legislation for Light Duty Vehicles.
The Libralato cycle offers the potential for 14% CO2 reduction at a cost reduction of €363. No other CO2 reduction technology also offers a cost reduction. The Libralato engine presents a particularly effective solution.
Clear business opportunity - Driven by global megatrends such as CO2 legislation, emissions legislation and diminishing oil reserves, virtually all of the major vehicle manufacturers are developing low carbon and hybrid electric models. To date, with the exception of the Toyota Prius, these vehicles have not yet entered the mainstream. The IEA forecasts that by 2035, approximately 60% of all new cars will be some form of hybrid. The small car (segment A/B) market accounts for 40% and c.4m passenger cars sales in Europe pa. Customer usage profiles show that these mainly urban cars with average daily driving distances of well under 20 miles are prime candidates for full electrification, consistent with the European Roadmap for the electrification of road transport.
Libralato intends to target small cars since small car buyers are most likely to consider fuel efficiency and low CO2 as high priorities. To date, no vehicle manufacturer has even produced a concept vehicle of a plug-in hybrid segment A/B model, due to cost, packaging, range, re-charging infrastructure and performance constraints.
Disruptive market entry – Application of the Libralato engine for hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) overcomes these constraints by delivering the greatest cost-benefit to the average customer and society as a whole. It is urban traffic that must be electrified, not extra urban traffic, but customers cannot be expected to buy a second car just for this. The future of terrestrial surface transport is electrification but the foreseeable future must involve a cost-effective compromise, using electricity for ‘town’ driving and oil (including biofuel and synthetic) for ‘country’ driving.
Real world, low power urban drive cycles can be delivered affordably by low voltage (non-lethal) small battery capacity electric systems. This will reduce fuel consumption and CO2 by an order of 60% (50 gCO2/km) and radically reduce urban air pollution. The Libralato engine can deliver direct drive, diesel equivalent efficiency on the open road, enabling all the hybrid components to fit within standard engine bays and subsidizing the cost of the other components; so that any car could become a PHEV for a marginal cost repaid by the fuel savings in less than 2 years without subsidy.
Fully aligned with OEM needs – European OEMs are at the forefront of vehicle CO2 reduction technology but they must have appropriate support from policymakers and the best technology to be successful in this highly competitive landscape. By comparison, the United States programmes for automotive technologies invest more than $500m (~€370m) per year, in addition to the ongoing $2.8bn (~€2.1bn) initiative to support industry in the crisis package introduced in 2009‐10. China has recently announced an RMB100bn (~€11bn) package of investment over ten years for new energy vehicles.
EU legislation sets mandatory emission reduction targets for new cars to improve fuel economy and improve air quality. The fleet average to be achieved by all cars is 130gCO2/Km by 2015 (Euro 6) and 95g/Km by 2021 (Euro 7). The proportion of diesel engine passenger vehicles in Europe has reached over 50% in the last decade. However given the unsafe levels of diesel particulates in Europe and the fact that diesel particulates are now known to definitely cause cancer, this may change policy approaches and customer perceptions towards diesel engine vehicles. Diesel fuel could lose its current tax advantage in most of continental Europe and the Euro 6 and 7 emissions regulations are likely to require more expensive exhaust after-treatment technology. For these reasons, it is expected by the end of the decade diesel engines will begin to lose their dominance.
List of Websites:
http://www.libralato.co.uk
Libralato. Holdings Ltd. & Libralato Engines Ltd
153 Royce Road
Hulme
Manchester
M15 5TJ
UK
Email: info@libralato.co.uk
