Numerical Analysis Of Mixture Formation And Combustion Process For A Stratified Lean-burn CNG Direct Injection Engine

In recent years,with the contradiction between economic development and environmental issues,more and more countries are aware of the need to take effective measures to protect the environment.The development of the automotive industry and the increase in the number of automobiles have also greatly aggravated environmental problems.Some laws and regulations to alleviate these problems have been taken in our country.One development trend is to ban traditional engines and use natural gas engine.At present,there are many researches on inlet CNG injection to form a homogeneous theoretical gas mixture.For this reason,the in-cylinder direct injection and stratified combustion technologies were integrated into CNG engines to study the formation process of stratified gas mixtures and combustion process.First,in order to verify the correctness of the selected spray model,a Schlieren experiment of CNG injection was performed.The suitable spray model was selected and the CNG free spray was simulated by AVL Fire software.And the results show that the spray development process of the experiment and simulation are basically consistent.Thus it can verify the correctness of the spray model selected in this paper.On the premise of verifying the correctness of the spray model,three-dimension modeling software was used to establish the three different engine cylinder solid model.These three different models were meshed separately,and a dynamic mesh was created and initial conditions and boundary conditions were set up to calculate the three models respectively.Under the condition of partial load conditions(2000 rpm),the stratified gas mixtures in three different models were numerical simulated,and the injection timings of three different models were determined and compared.The first pit structure was selected and the homogenous mixture was simulated to determine the best injection timing.The process of stratified lean combustion in the CNG direct injection engine of the first model was simulated in the wall guide mode.The analytical results show that the combustion process at the ignition timings of 10° CA BTDC and 15° CA BTDC is late,and the combustion heat release rate is also relatively low;the combustion process at the ignition timings of 25° CA BTDC and 30° CA BTDC is fast,but the combustion heat release rate is relatively low;at the ignition timing of 20° CA BTDC,the combustion speed is faster,and the combustion heat release rate reaches the highest,and the pressure chart once again validates that the combustion performance is good at the ignition timing of 20° CA BTDC.The CO and NO emissions at different ignition moments were analyzed,and the low-temperature and anoxic conditions for producing CO were explained,and the high-temperature oxygen-rich conditions for NO production were described.The homogeneous combustion process of the CNG direct injection engine of the first model is numerical simulated.Analytical results show that the combustion process at late ignition timings of 20°C BTDC and 25°C BTDC lags behind and the combustion heat release rate is relatively low;at the earlier ignition timing of 35°C BTDC,the combustion is relatively fast,but the heat release rate is relatively low;the combustion rate is faster at the ignition timing of 30° CA BTDC,and the combustion heat release rate is the highest.The CO and NO emissions during the combustion process of the homogeneous theoretical mixture were analyzed and compared with the combustion process of the stratified gas mixture.