Simulation on the optimization of the combustion system of a large-bore diesel engine
To improve the fuel economy of a marine diesel engine with a cylindrical diameter of 200 mm,in this study,a scheme for upgrading the piston combustion chamber and the fuel injection system was designed,and a simulative optimization was conducted.The upgrade scheme raised the compression ratio and fuel injection pressure and employed a shallow ω combustion chamber with a large diameter-to-depth ratio and a fuel nozzle with a 158° spray-included angle.We performed the computational fluid dynamics simulation for the working process of the en-gine inside the cylinder,calculated the high-pressure indicated work and the heat release rate phases,and analyzed the temperature inside the cylinder,reaction excess air coefficient,speed distribution,and evolution.The simula-tion results showed that the upgraded scheme could improve the thermal efficiency of the engine,increase the num-ber of nozzle orifices,and reduce the bore diameter;the high-pressure indicated work could be improved by 4.5%,and soot emission could be reduced by about 60%while keeping the NOxemission unchanged.By combining a"flattop"shallow (ω) combustion chamber and 158°fuel-spray-included angle,the fuel and air mixture flows into the piston's top clearance quickly and forms a counterclockwise vortex that can quicken the mix of fuel and air and the combustion process,thereby improving the thermal efficiency.
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