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Improving engine performance and efficiency by minimisation of knock probability (MINKNOCK)

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On the experimental side Ford's responsibility was the investigation of the knock behaviour of a production engine under different operating conditions and with different fuels. 12 special test fuels covering a large range of RON/MON were tested. In addition Shell provided a primary reference fuel with fully known chemistry. For comparison and for dyno setup and check Ford also used Ford standard test fuel (94.8 RON/ 84.4 MON). The evaluation of the data showed that all the fuels behaved differently with regard to their wide-open throttle performance. This can be attributed to the differences in heat value, evaporation phenomena, stoichiometric air-fuel ratio and combustion characteristics. All effects were analysed separately. The influence of stoich AFR and heat value was discussed in detail. The effects of spark variation are shown in terms of indicated mean effective pressure, volumetric efficiency and combustion data (10% burn duration, 50% burn duration). The correlation of knock sensitivity with RON and MON number respectively shows well known tendencies. A linear relationship between spark advance and RON number corresponds to well known tendencies. This correlation could be demonstrated for all engine speeds, however the correlation factor deteriorates with increasing engine speed. For the highest engine speed, which was investigated, the MON number shows a better correlation with regard to spark advance than the RON number. The evaluation of the Octane Index as proposed by Prof Kalghatgi/Shell shows a potential improvement to characterize fuel with regard to knock sensitivity. This index is based on a combination of RON and MON. The computational part of the project included reference case computations. In a first step meshing was performed for further analysis with STAR-CD, since this commercial code offers a highly sophisticated combustion model. Three operating points were chosen for reference simulation of the combustion process: 1500 rpm part load, 1500 rpm full load and 5000 rpm full load. An assessment of the performance of the Star-CD ECFM combustion model was performed. The analysis of the above reference test cases showed that the combustion model parameters have to be adjusted individually for each operating point. If only the recommended model parameters were used this resulted in unacceptable differences between measured cylinder pressure and mass fraction burned curves. In a second step meshing of the ports and the combustion chamber was also completed for subsequent analysis with FIRE. Similar operating points were chosen for reference simulation of the combustion process and the knock onset: 1500 rpm, 2000 rpm and 5500 rpm, all at full load. In a large range of CFD simulations different approaches for wall and heat transfer, turbulence modelling and combustion simulation were tried out. As a summary it can be stated that the results with k-x-f model look more physically and reliable during gas exchange phase compared to k-e. For the simulation of the combustion period an Eddy-Break-Up model was compared with the AVL ECFM approach. Comparison with measurements was based on mean in cylinder data such as cylinder pressure, mass fraction burned and in cylinder temperature. The simulation showed correct trends for all operating points, however changing model parameters are required for accurate prediction of the combustion period. In addition a comparison of calculated temperature data with measured radical concentrations was done. The measured data were obtained within the MinKnock project at the optical engine at CNR, Naples. The correlation was very good. This is indicating that the calculated flame front propagation correlates well with real engine operation. Finally knock simulation was performed using the Shell Auto-Ignition model. A criterion was found providing a good correlation between calculated radical mass fraction and measured knock onset. This seems to be a promising approach to predict knock probability in the future.
The work developed by this partner within the Fundamental Experiments workpackage was related with the detailed experimental analysis of a flame-wall interaction. A flame-wall interaction process is controlled by a competition between thermo-diffusive and aerodynamic processes which may lead to premature flame extinction, resulting in a deficient or non-burned fuel, or rapid consumption of the fuel trapped between flame and wall with consequences on engine efficiency. Due to the flame-flow complex process inside an IC combustion chamber, and in order to gain physical insight into the process, experiments were designed to study the freely propagating flame approaching a wall, inside a combustion chamber filled with a propane-air mixture. Experiments were conducted with a planar and inclined walls and a PIV with temporal resolution was developed in this work. The main objective in this sub-task was to quantify experimentally the flow field that develops in between the flame and the wall, as the flame is approaching, in the presence of irregularities on the flame front. Post processing data software was also developed in this task and includes the determination of flame velocity in the flame reference (Sd) and flame stretch factor (K). Three types of flames were studied: hemispherical, symmetrical and non-symmetrical flames. It is shown generically that the flow field in between the flame and the wall is not stagnant and, contrary to what is present in literature, there are free stagnation points in the flow field. In general three distinct flame patterns were found which generates different unburned flow topology. The undisturbed-concave flame generates a central stagnation point and the flow diverges to the sides. The symmetric-concave deformed flame traps the unburned gas and induces a stagnation point in the meddle of the flow field. This stagnation point move upwards and at the final instant the flow velocity points upwards. For the asymmetric flame propagation the flow generated in the unburned gas region has a topology close to a tangential wall flow. Further analysis of the flame front structure showed that there is a high correlation between the flame stretch factor and the resultant flame velocity. The most important result is that the flame can survive to very high negative and positive stretch without extinguishing during the time propagation.
The CFD code FIRE® is a multi purpose simulation software which can be used for different kind of applications. For the simulation of engines FIRE® provides special modules for the handling of the mesh movement as well as a number of mathematical models for the treatment of effects such as fuel injection or combustion, respectively. The solution algorithm is based on the well-known SIMPLE (Semi IMplicit Pressure Linked Equations) procedure. The code can handle computational cells with an arbitrary topology. Within the actual version of the program new submodels from the ongoing project have been implemented. These submodels make it possible to simulate the knock onset on a higher level of accuracy then it has been done up to now. The chemistry for knock prediction is based on a n-heptane/iso-octane reaction mechanism (POSM= phase optimised skeleton mechanism) coupled with a t-PDF code (transported probability density function).
The main objective of the work assigned to Istituto Motori in the project within WP2 (Advanced Experimental. Analysis of Auto Ignition) was the provision of comprehensive experimental data in order to improve understanding of how the changes of engine parameters and fuels affected knock phenomena. Non-intrusive diagnostic techniques based on broadband ultraviolet (UV) to visible spectroscopy were applied to optical accessible spark ignition engine. The optical spark ignition engine was realized by using a Ford Zetec SE Sigma 16V engine head and ported fuel injection (PFI) system. The simultaneous use of broadband flame spectroscopy from UV to visible and 2D chemiluminescence. at selected wavelengths, permitted the evaluation of knocking locations and concentration of radical species involved in the process with high spatial resolution around 100 um and time resolution around 0.1° crank angle. Several engine operating conditions and fuels were investigated. The knock location and radical species involved were not strongly influenced by engine operating conditions meanwhile the radicals concentration and their time evolution depended on fuel properties. The measurements carried out confirmed the capability of the presented optical technique to characterise knock formation and evolution in experimental and real spark ignition engine under realistic engine conditions.

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