Numerical Study of the Combustion Chamber Shape for Common Rail H.S.D.I. Diesel Engines

The Common-rail injection system has allowed achieving a more flexible fuel injection control in DI-diesel engines by permitting a free mapping of the start of injection, injection pressure, rate of injection. All these benefits have been gained by installing this device in combustion chambers born to work with the conventional distributor and in-line-pump injection systems. Their design was aimed to improve air-fuel mixing and therefore they were characterized by the adoption of high-swirl ports and re-entrant bowls. Experiments have shown that the high injection velocities induced by common rail systems determine an enhancement of the air fuel mixing. By contrast, they cause a strong wall impingement too. The present paper aims to exploit a new configuration of the combustion chamber more suited to CR injection systems and characterized by low-swirl ports and larger bowl diameter in order to reduce the wall impingement. The goal is to achieve a higher air flow rate during induction as well as to reduce the fuel vapor wall impingement without compromising air-fuel mixing efficiency. This new combustion chamber configuration has been tested numerically and its performances have been compared to those of a H.S.D.I four valve diesel engine conventional combustion chamber. The analysis has been carried out by using a customized version of the CFD code KIVA3. Experimental results of the conventional combustion chamber have been used to validate the numerical models. The influence of the injection system configuration (i.e, hole numbers, inclination of the spray axis with respect cylinder head) on pressure cycle and NO_x and soot engine-out emissions has been analyzed too. Computational results seem to indicate that the new combustion system concept may provide relevant benefits with respect to engine-out emissions without reducing engine performance.