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In diesel engines, valve avoiding pit (VAP) is often designed on the top of the piston in order to avoid the interference between the valves and the piston during the engine operation. With the continued application of the downsized or high power density diesel engines, the depth of VAP has to be further deepened due to increased valve lift for more air flow into and out of the cylinder and decreased piston top clearance for less HC/CO and soot emissions. The more and more deepening of VAP changes the combustion chamber geometry, the top clearance height and the injector relative position to the piston crown. In this paper, a 3-D in-cylinder combustion model was used for a heavy duty diesel engine to investigate the effects of the depth of VAP on combustion process and emissions. Five depths of VAP were designed in this study. In order to eliminate the influence of compression ratio, the piston clearance height was adjusted for each VAP depth to keep the same compression ratio.

Computational Fluid Dynamics (CFD), as an effective analytical tool, has been applied at China North Engine Research Institute (CNERI) for combustion chamber design and combustion system optimization on a V8 heavy -duty diesel engine in order to meet increasingly stringent emission targets. The design of combustion system involves great number of parameteric optimizations such as the number of nozzle holes, the spray angle, the swirl ratio and the piston bowl shape. 3-D CFD was a convenient and cheap tool to explore the effects of all these parameters to the engine performance, compared with extensive hardware testing. 1-D modeling was used to set up boundary conditions at intake valve closure for 3-D CFD modeling during the closed-cycle. AVL FIRE software with a widely used combustion model, ECFM-3Z model, was used for 3-D simulation. Two sets of nozzle holes, four spray angles and three swirl levels were utilized and optimized under rated power.