Skip to main content
European Commission logo print header

Diesel - injection for small engines and low emissions

Deliverables

To analyse small engines equipped with new generation Common Rail Injection systems a transparent engine was used to collect pictures of the injection and combustion phenomena. The visualisations were performed with different injection strategies: the standard one (pilot and main) and the multiple one with different values of the After Dwell Time (490, 1090 and 1350µs), that is the time between the end of the main injection and the start of after injection. Pilot and pre-injections show similar spray pattern: - Relatively low tip penetration avoiding wall impingement. - A high vaporisation rate and a long ignition delay that leads to a completely premixed combustion without visible flame (soot). Also the main injection characteristics are very similar. In this case the spray plumes hit the chamber wall. For both injection strategies, the frames show that soot appears generally in zones located at the combustion chamber centre on the spray plume edge downstream of the swirl motion. After the start of combustion, the flame rapidly tends to propagate towards the bowl wall, thus increasing the interaction with swirl motion. This stage of the combustion process is an important key in controlling exhaust NOx-soot trade-off: with the standard strategy, a high swirl velocity during late expansion could significantly improve the soot oxidation process. With the presence of the after injection, an interaction between the after injection's spray plumes and the spot flames of the main combustion is evident. No or very short ignition delay seems to characterise the after injection. During the first phase of after combustion, the flames are located in the bowl central area where air is still available. This is a first factor that contributes to improve air-fuel mixing with multiple injections. Subsequently, also the unburned fuel of the after injection propagates towards the bowl periphery, interacting with the flame spots of the previous main injection. This is another critical factor that controls the local over-rich fuel/air zone in a swirl supported combustion system.
The CRIP-MI is a common rail based fuel injection system for passenger cars. In comparison with the state of the art injectors the MI (multi injection) injector is capable of up to five injections per cycle with a minimum dwell time of 150µs. Furthermore the injector features an almost linear characteristic injection map quantity versus energizing time. This feature not only permits to adopt the quantity of the pre injections more freely but also the characteristic map can be easier mathematical described and therefore enhanced controlling strategies used to larger extend. Examples therefore would be the "Injector Quantity Adjustment" to compensate the sample-to-sample production tolerances or the "Zero Fuel Calibration" for a accurate control of the pilot injection and compensation of the drift over life time. Another result clearly showed the potential of such an enhanced temporal control strategy to reduce the emissions. The funding for this project offered just a small fraction of the total development costs of this MI technology. The focus of this project was more research orientated: development and the special needs of a small engine. The hydraulic stability is essential for such a fuel injection system: The deviation of the quantity for small changes of the control parameters or the boundary conditions (e.g. small deviations of the dwell time, energising time or viscosity of the fluid) must not be high - otherwise, big deviations in terms of quantity in a series application with different injectors under real life conditions would occur. As a result of the poor control of the injected quantities no clean and efficient combustion could be achieved. The hydraulic stability is the reason why the actual Common Rail systems of 1st generation are limited to a dwell time of about 1800µs and the 2nd generation of about 900µs. This goal of reproducible injected quantities and a linear injection map quantity vs. energising time was achieved by: - First, introducing an overlift for the armature for a more defined movement of the armature and, hence, a better control of the pull-in force of the solenoid. - Second, increased nozzle needle lift for a ballistic behaviour and for a linear injection map for medium and big quantities as well as a reduced needle throttling at full load. - Third, the hydraulic as well as the solenoid behaviour were adjusted for a linear injection map for small and medium quantities. The design phase of this injector is closed and prototypes are available.
A complete database of macroscopic and microscopic spray characteristics of Diesel fuel injection systems has been provided to the partners inside the project. Different nozzle geometries for small diesel engines have been used. Different orifice shapes, cylindrical, convergent and divergent nozzles. Two types of orifice seats were presented as well: Valve covered orifice and microsac seat. As expected, the VCO nozzle presents higher hole-to-hole dispersion than the equivalent microsac nozzle. In any case double-guided bosch nozzle present a substantial reduction in hole to hole spray dispersion than previous generation of injectors. Convergent orifices provide a higher penetration due to the higher exit velocity.
It is well known that it is essential for a clean and efficient combustion in a DI diesel engine to inject the fuel optimal in the combustion chamber. To assess the behaviour of various nozzles it is crucial to be able to characterise these sprays. A proposing technology is the measurement of the spray macroscopic characteristics of the spray by means of optical techniques (photography with digital cameras). A complete methodology to obtain Diesel sprays macroscopic characteristics for real multi hole nozzles has been developed. Previously developed software for only one spray has been adapted to use in real multi hole nozzles with multi hole configuration (5, 6 or higher number of sprays). Commercial diesel fuel is used, so that measurements can be performed under real engine conditions. The developed methodology considers all the correction and calibration procedures to perform accurate measurements on the basis of the images taken, which have been implemented into a purpose-made software. The main results obtained from the visualisation are: - Spray penetration. - Spray cone angle. - Air entrainment. - Hole to hole dispersion. - Shot to shot dispersion.
A new high-pressure nitrogen chamber has been constructed. The chamber operates with nitrogen in the range of 1 to 80 bar and 15 to 40ºC so that evaporation is not taking place. Three windows allow proceeding with different optical techniques with Diesel injectors. The chamber itself is constructed with a modular approach allowing the use of different configuration, shadowgraphy, side illumination, two hole or multi-hole nozzles, etc. The chamber is located in a close loop facility filled with nitrogen; two parallel compressors allow to reach the desired chamber pressure before injection starts. There are two filters to eliminate the fuel from previous injections. The installation could be used for any type of injectors, not only diesel ones, it maybe possible to visualize plain orifice atomizers or gasoline injectors.
A complete methodology to obtain the spray microscopic characteristics of Diesel sprays has been developed. Previous knowledge of the CMT Motores Térmicos group allows the use of PDPA technique on isolated diesel sprays. The difficulty arises when there are five or more simultaneous sprays affecting all the optics of the laser. A new masking device that allows to isolate one spray was developed together with Robert Bosch. This masking device together with good optical access allowed measuring the droplets diameter and velocity under equivalent engine conditions. The methodology developed considers all the correction and calibration procedures to perform accurate measurements on the basis of the data taken. The more relevant results from the PDPA installation are: - Sauter Mean Diameter. - Droplet velocity.

Searching for OpenAIRE data...

There was an error trying to search data from OpenAIRE

No results available