Optimization of Piston Bowl Geometry for a Low Emission Heavy-Duty Diesel Engine

A computational fluid dynamics (CFD) guided design optimization was conducted for the piston bowl geometry for a heavy-duty diesel engine. The optimization goal was to minimize engine-out NO_x emissions without sacrificing engine peak power and thermal efficiency. The CFD model was validated with experiments and the combustion system optimization was conducted under three selected operating conditions representing low speed, maximum torque, and rated power. A hundred piston bowl shapes were generated, of which 32 shapes with 3 spray angles for each shape were numerically analyzed and one optimized design of piston bowl geometry with spray angle was selected. On average, the optimized combustion system decreased nitrogen oxide (NO_x) emissions by 17% and soot emissions by 41% without compromising maximum engine power and fuel economy. The NO_x and soot emissions of the optimized system were lower than the baseline case under both low and high Exhaust Gas Recirculation (EGR) conditions, and less sensitive to injection timing, which is beneficial to engine calibration. To reveal the key design factors of piston bowl geometry affecting emissions, three piston bowls with the geometry similar to the optimized bowl were simulated under the low speed condition. The results show that decreasing the oxygen-rich areas surrounded by high temperature mixture would reduce NO_x formation, while organizing combustion in the center of combustion chamber would boost the engine power and decrease soot emissions.