Numerical Analysis Of Mixture Formation And Combustion Process Of Two-stroke CNG Direct Injection Engine For Extended-Range Electric Vehicles

In recent years,with the continuous growth of traditional fuel vehicle production and possession,the energy crisis and environmental issues have become the most concerned global issues.In order to alleviate the various social problems brought about by automobiles,governments and enterprises all over the world have begun to develop new energy vehicles led by electric vehicles.Although pure electric vehicle technology has made great breakthroughs and progress,and has been mass-produced,the bottlenecks such as short cruising range,low energy density of power battery,long battery charging time and imperfect charging equipment are urgently needed to be solved.Electric vehicles have become one of the important ways to make up for the shortage of electric vehicles.In this study,the two-stroke CNG direct-injection engine for light and extended-range electric vehicles was used as the research object to analyze the formation and combustion process of the mixture under different working conditions.The research is based on a two-stroke CNG direct-injection engine with a power parameter of a light electric vehicle in China.The engine adopts a “wall-guided” combustion system to achieve energy saving and emission reduction under the premise of satisfying the power.According to the selected engine related structural parameters and technical parameters,the engine solid model was built by using the 3D graphics software Solidworks.Then the AVL FIRE 2011 software was used to mesh the calculation area and establish the dynamic mesh of the calculation area.The optical smear experiment of the engine cylinder was used to verify the correctness of the spray model used in the numerical simulation,and it was compared with the simulated spray development process of the free spray experiment to determine the various spray sub-models used in the study.After the spray model is selected,the correctness of the turbulence model is verified by comparing the experimental results of the engine cylinder visualization with the numerical analysis results of the in-cylinder spray development process.The selected calculation model is used to analyze the formation process of the mixture in the engine cylinder under different generator sets.The calculation results show that the cold start-warm condition(1800r/min,20% load)and the heavy load conditions(5000r/min,90% load,6000r/min,80% load)under extended range mode should use homogeneous theoretical mixture.The CNG is injected at a certain moment when the piston is upstream and the exhaust passage has not closed,the mixture gas formation time is increased to form a combustible homogeneous theoretical mixture gas,and no “fuel short circuit” phenomenon occurs;Partial load conditions(5000r/min,60% load)under extended range mode should be stratified lean burn,the calculation results show that it is better to inject CNG at 60~70°CA BTDC and inject fuel in the middle of compression stroke.Because the two-stroke engine speed and cylinder pressure are very high,the diffusion time of CNG is not sufficient and is limited to a certain area.At the same time,under the joint action of the turbulent flow in the cylinder and the top of the curved piston,a good combustible stratified mixture can be formed near the spark plug at the ignition timing.Based on the optimal CNG injection timing,the in-cylinder combustion and discharge process under different working conditions is numerically analyzed.The calculation results show that in the cold start-warm condition(1800r/min,20% load)when the engine and the exhaust gas treatment device are preheated,the engine ignition timing is 20°CA BTDC,so the mixed gas combustion process stability and the internal temperature of cylinder rises quickly,which is helpful for accelerating the preheating of the engine and the exhaust aftertreatment device,at the same time,the fuel is completely burned and the harmful emissions are low;in the extended mode partial ignition condition(5000r/min,60% load),when the ignition timing is 25°CA BTDC,the maximum pressure in the engine cylinder appears at 10~15°CA ATDC,the cycle thermal efficiency is high,and the economy and emission characteristics are good;In the extended mode 5000r/min,90% load condition,the ignition timing is 25°CA BTDC.The engine cycle has high thermal efficiency,meets the power demand of large-scale generator sets,generates less harmful emissions,and has good emission characteristics;In the extended mode 6000r/min,80% load condition,the ignition timing is 25~30°CA BTDC.During the ignition timing,engine has high cycle thermal efficiency,low fuel consumption rate and low harmful emissions.The overall performance of the engine is optimal and the power can meet the large load capacity of the generator set.