Simulation of fuel-air interaction in a four stroke four valve Direct injected spark Ignition (DISI) engine

Of late Direct Injection Spark Ignition (DISI) engines are replacing the carburetted SI engines due to certain inherent advantages like uniform distribution of fuel-air mixture in all cylinders in multi cylinder engines. However the homogeneity of the mixture depends on the time of injection as well as the type of fuel injector. It is expected that late in the compression stroke the fuel-air mixture near the spark plug should be a combustible mixture. In order to achieve this, proper air motion during induction and compression is a must. Further the interaction of fuel and air from the start of injection is equally important. This paper addresses these issues. For this a CFD study has been carried out. The injection timings selected are 90, 180 and 2700 aTDC, the idea being to understand the effects of early or late injection on fuel air mixing. The appropriate governing equations are solved using finite volume method. RNG k-ε turbulence model is used for physical modelling. Two speeds namely 750 rpm and 1500 rpm are considered. A typical geometry for which experimental results are available in the literature is chosen so that the prediction can be validated. From the present study it is seen that the present geometry with swirl type injector is not able to provide satisfactory equivalence ratio near the spark plug location at the time of ignition. As expected the lower speed viz. 750 rpm with the above injection timings provides comparatively better equivalence ratio than the higher speeds. This study suggests that the fuel spray and in-cylinder air motion are coupled and it is possible to satisfactorily predict the fuel vapourisation, mixing and charge distribution inside the cylinder using the CFD code STARCD. Copyright © 2007 SAE International.