Research On The Effect Of Valve Timing On Combustion And Emission Performance Of Gasoline Engine

In recent years,with the increasing attention paid to energy and environmental issues and against the background of " carbon peaking and carbon neutrality ",the need to achieve higher thermal efficiency and lower emissions for internal combustion engines has become imminent.Variable valve technology is an effective means to improve fuel economy and reduce emissions.However,there are few studies on variable exhaust valve timing and variable valve timing coupled dilution combustion.Therefore,this paper designed and developed a new type of mechanical motor controlled variable valve timing mechanism,and studied the impact of valve timing on engine economy and emission characteristics during stoichiometric combustion.At the same time,combined with GT-Power simulation,the impact of valve timing on engine thermal power conversion process and BSFC under different operating conditions was studied.Further explore the potential of variable valve timing combined with lean combustion to improve fuel economy and emission characteristics of gasoline engines under different combustion modes,providing reference for the application of variable valve timing technology in future engines.The specific research content and conclusions of this article are as follows:1.Based on the mechanical-electro-hydraulic variable valve lift mechanism that has been developed by the research group,a variable valve timing mechanism has been designed and developed by ourselves,and a test bench has been built,verifying the stability and accuracy of the valve mechanism.2.Changes in the timing of the intake and exhaust valves will cause different changes in the amount of exhaust gas in the cylinder,which will have different effects on the combustion process of the engine.Overall,the forward movement of the intake valve opening time causes less exhaust gas in the cylinder than the forward movement of the exhaust valve opening time.When the intake and exhaust valve timings are 20 ° CA BTDC and 200 ° CA BTDC respectively,the lowest BSFC under current operating conditions can be obtained.When the intake and exhaust valve timings are 20 ° CA BTDC and 230 ° CA BTDC,CO and NOx are improved by 46.5% and 58.14%,respectively,compared to the intake and exhaust valve timings of 30 ° CA ATDC and 200 ° CA BTDC.3.Through GT-Power simulation,the influence of intake and exhaust valve timing on thermal power conversion process and BSFC under different operating conditions was simulated.When the exhaust valve opening time is 200 ° CA BTDC,as the intake valve timing moves forward,the exhaust loss decreases,indicating an increase in thermal efficiency.When the intake valve opening time is 10 ° CA BTDC,as the exhaust valve timing moves forward,the heat transfer loss first increases,then decreases,and then increases,indicating that the thermal efficiency first increases,then decreases.Under different operating conditions,the overall trend of the impact of intake and exhaust valve timing on the thermal power conversion process is consistent,which can improve fuel economy.However,as the load increases,the effect of intake and exhaust valve timing on BSFC gradually decreases.When the exhaust valve opening time is 200 ° CA BTDC,the BSFC at low load improves by 22.9% as the intake valve timing moves forward.The improvement degree of BSFC under heavy load is 10.59%.4.When the intake and exhaust valves are opened at 20 ° CA BTDC and 200 ° CA BTDC,it is found that with the increase of lambda,PMEP decreases,and the combustion duration and ignition delay period first shorten and then extend.When lambda=1.4,BSFC improves by 7.69%compared to equivalent ratio combustion.When lambda rises to 1.1,CO emissions decrease and tend to stabilize,while NOx emissions continue to decrease.At lambda=1.4,NOx decreases by62% compared to lambda=1.5.With the increase in excess air coefficient,the PMEP under different intake and exhaust valve timings has been reduced compared to the stoichiometric ratio,and the economy and emission characteristics of the engine have also been improved to varying degrees.In contrast,the improvement range of lean burn coupled to intake valve timing on engine economy becomes larger;Exhaust valve timing coupled lean burn has a greater improvement in NOx emissions.6.Using the existing variable valve lift mechanism,explore the CAI combustion range that can be achieved through joint control of lean burn and variable valve technology.It was found that within the achievable CAI load range,when the exhaust valve closing time is higher and the intake valve opening time is lower,the dependency of achieving CAI combustion on excess air coefficient is lower,and the peak pressure rise rate and heat release rate are lower.The achieved IMEP range is larger,the ignition start point is lower and the combustion duration is longer,and the BSFC and NOx emissions are lower.