Research On The Influence Of Miller Cycle On The High-strength Diesel Engine Performance

Miller cycle can reduce the temperature and pressure at the end of compression,so that the combustion process is soft,the combustion temperature and pressure in the cylinder is reduced.On the one hand,it is conducive to reduce NO_Xemissions.On the other hand,it reduces the thermal and mechanical load of the diesel engine.With the development of supercharging and variable valve technology,the application of Miller cycle in high-strength diesel engine is very important.Based on a high-strength single-cylinder diesel engine,this thesis establishes one-dimensional and three-dimensional simulation models of the engine.The models for simulation are verified with the assistant of the test data.The Miller cycle is achieved by designing and optimizing intake valve lift curves of early intake valve closing timing(EIVC)and late intake valve closing timing(LIVC).The impacts of the Miller cycle on the combustion and emissions of the high-strength diesel engine,in the 3D model where the inlet and exhaust pressure and temperature of the 1D model are used as the initial conditions,is simulated under the rating conditions(speed 3600 r/min,indicated average pressure 2.5MPa)and with the air flow and the indicated output power fixed.The results show that:Under the rating conditions and keeping the indicated output power and intake air flow constant,as the intake valve closing time advances(EIVC)or postpones(LIVC),the effective compression ratio of the high-strength diesel engine decreases,and varying degrees of reduction occurs in the compression top dead center temperature and the pressure,the maximum combustion temperature in the cylinder and the maximum pressure increase rise rate,etc.The combustion of EIVC operating conditions are softer than LIVC,in which the roughness of combustion process is weaker,the suppression effect on the maximum combustion pressure and maximum temperature of high-strength diesel engine is better.In addition,the EIVC operating conditions can reduce the thermal and mechanical loads on high-strength diesel engines effectively.As the pumping loss,cylinder temperature and exhaust temperature of EIVC operating conditions are significantly reduced,the heat transfer loss and heat exhaust loss in the cylinder are suppressed results in a significantly improvement in the fuel consumption of the high-strength diesel engine.In the EIVC operating conditions,the intake valve closing angle is advanced to-30°CA ABDC,the average peak temperature in the cylinder is reduced by 150 K,the exhaust temperature is reduced by 64 K,and the indicated fuel consumption rate is reduced by 9%.As the intake valve closing timing is postponed,soot emissions slightly increase,but the trade-off relationship between NO_Xand soot emissions weakens.With the intake valve closing timing advanced,the ignition-delay period increases,the oil and gas mixture in the cylinder gradually becomes uniform,the combustion temperature decreases,NO_Xand soot emissions gradually decrease.The trade-off relationship between NO_Xand soot emissions in EIVC conditions is broken.Therefore,EIVC is more effective than LIVC in reducing the mechanical and thermal load and NO_Xand soot emissions of high-strength diesel engine.