Analysis of the effect of combustion bowl geometry of a DI diesel engine on efficiency and emissions

In an effort to optimise the thermodynamic efficiency and emissions levels of a current production common rail DI Diesel engine, an experimental campaign was undertaken accompanied closely by analytical work. The objective of the analytical CFD work was to study the processes of fuel spray/wall interaction, flame propagation and interaction with the piston surface and the most relevant mechanisms of soot formation and oxidation in order to guide the experimental work. Given the project's objectives, the only parameters which could be modified regarded the piston-bowl geometry leaving the rest of the combustion system architecture intact. The paper describes only two extreme cases which, in the authors' opinion, demonstrate the interaction processes between combustion and combustion chamber walls. The analysis is limited in two engine operating conditions: one concerns maximum power operation and quantifies the effect of combustion chamber geometry on efficiency while the second is representative of the emissions test cycle. The numerical results indicate that, regarding performance, a larger diameter combustion chamber leads to higher combustion efficiency due to a weaker interaction of the fuel vapour and flame with the bowl walls. For the other operating condition, the soot model developed by Borghi and implemented in the SPEED code has been used. The numerical results showed that the larger diameter combustion bowl exhibits a lower efficiency in terms of soot oxidation due to the lower swirl velocities pertaining in the bowl during expansion. These trends have been confirmed by the experimental work. Based on the indications of the above analysis, an alternative combustion chamber has been designed and examined analytically. The main feature of this last design is a slightly smaller bowl diameter which leads to higher swirl levels during expansion accompanied by more soot oxidation and slightly lower combustion efficiency.