Knock Suppression And Performance Improvement Strategy Based On Miller Cycle

Knock is one of the main obstacles to the improvement of the thermal efficiency of gasoline engines.The traditional knock suppression strategies usually sacrifice the power and economy.Miller cycle has the effect of improving thermal efficiency and reducing the tendency of knock,and boost can make up for its loss of power.To explore the strategies of knock suppression and gasoline engine power and economic improvement,the paper conducted different knock suppression strategies based on Miller cycle.In this paper,a direct injection ignited single cylinder engine was applied.The working conditions were all at 1600 rpm,full load,and fuel-air ratio of 1.Based on Converge numerical simulation,the effect of suppressing Miller cycle and double injection knock were explored.The cylinder temperature can be reduced and the fuel distribution can be improved by Miller cycle.Double injection can increase the turbulent kinetic energy,flame propagation speed.Both strategies inhibit knock,and the knock suppression of Miller cycle is better that of double injection.Therefore,the tests of knock suppression strategies based on Miller cycle were conducted.Firstly,at the compression ratio of 10,based on the best fuel consumption and torque output,the baseline injection timing was determined to be 240 CAD BTDC.The Miller cycle with injection timing 240 CAD BTDC tests were conducted.The results show that the effect of late intake valve close timing on knock suppression is better than that of advanced intake valve closing,and the knock suppression tendency increases with delaying angle.After that,the double injection tests were conducted.The results showed that knock suppression increased with the secondary injection ratio and the secondary increased injection ratio,but the cyclic fluctuation increased.Considering power and economic characteristic,the injection ratio of 4: 1,the injection time of 100 CAD TBDC was the best injection strategy in all double tests.It can be seen in the test that the torque output of LIVC obviously dropped.To make up for the loss of power,the boost and LIVC tests were conducted.The results show that combined LIVC and boost,the torque can be increased and knock can be suppressed.When the boost pressure is 1.4 bar and LIVC = 60 CAD,the thermal efficiency of the engine was increased by 2.5%.The tests that combined the optimal double injection strategy and the pressurized LIVC were conducted.The results show that the boost and Miller cycle combined with the double injection strategy can further improve the engine power and economy.In order to improve the performance of gasoline engine,this paper improved CR to 12,and explored the best injection strategy of CR12.The tests of boost LIVC strategy combined with the best injection strategy were conducted at CR12 operating conditions.The results show that the torque and fuel consumption of optimum injection strategy of CR12 combined with boost pressure 1.8 bar,LIVC = 85 CAD was better than that of the best condition of CR10.At the part load,thermal efficiency,torque output and fuel consumption of CR12 were better than the CR10.In summary,the Miller cycle,the double injection,booster Miller cycle and booster Miller cycle combined with the double injection strategy can effectively suppress the knock,and improve engine performance.With the compression ratio increased from 10 to 12,the Miller cycle combined with the double injection and boost can reduce fuel consumption and ensure that the power output at full load conditions.In addition,the torque and efficiency of CR12 was better than that of CR10 at the partial load.