Experimental Study On Stratified Combustion Mode Of High Pressure Direct Injection Natural Gas Engine

High pressure direct injection technology is one of the most advanced natural gas engine techniques,which has the advantages of low emissions and high efficiency.Therefore,it is being regarded as one of the most advanced techniques of natural gas engines.However,this technique still has the potential for further improvement of its thermal efficiency,because its combustion rate is limited by the diffusion rate of the natural gas.Direct injection stratified combustion technique could control the fuel injection,reduce pump gas loss,and significantly improve the compression ratio and lean burn limit.It is an important method to improve the thermal efficiency of gasoline engine under medium and low load,but the application of this technology on natural gas engine is still limited.This paper combines in-cylinder high-pressure direct injection technology with stratified combustion technology,and proposes a new combustion concept,which named pilot ignited high-pressure direct-injection natural gas engine stratified combustion mode.The paper chose a six-cylinder,turbocharged natural gas engine as the research object,and discussed the combustion process and emission characteristics of the new mode.Then,the article analyzed the effect of three parameters on engine combustion and emissions:natural gas injection timing,diesel advanced angle and the mass of pilot diesel.Finally,the injection interval between diesel and natural gas is optimized.The main work and achievements are as follows:The test bench for high pressure direct injection natural gas engine was built.The basic information of the engine,the measurement system and the overall arrangement of the bench were briefly introduced.Then,the paper designed a detailed test scheme and explained the definition and calculation method of combustion parameters.The stratified combustion mode was compared with the high pressure direct injection mode from the aspects of the in-cylinder pressure,instantaneous heat release rate,thermal efficiency and emissions.The results showed that in the stratified combustion mode,the maximum burst pressure and maximum heat release rate increased,the combustion phase angles were advanced and the thermal efficiency was significantly improved.The improvement effect is more significant at 1000 N·m with the growth of 3.38%.Due to the uncertainty of natural gas diffusion movement,the cyclic variation of maximum pressure and indicated mean efficient pressure increased,but the coefficient of variation was independent of the engine load.In terms of emissions,NOx and THC emissions increased,especially in 600 N·m,unburned hydrocarbon gas emissions were serious.Because of the difference in the adequacy of natural gas combustion,CO emissions showed an opposite trend under medium and low load.The effects of three injection parameters on the stratified combustion mode were studied by the control variable method.The results showed that injection timing of natural gas was the key factor affecting the in-cylinder concentration distribution of the mixture.At the medium load of 1000 N·m,the stratified combustion near TDC could be realized at the time of 32°BTDC.At this time,the combustion process was stable and gentle,and the thermal efficiency was as high as 44.81%.Under the low load of 600 N·m,the combustion process was affected by the time of natural gas injection and the amount of gas injection,the change of in-cylinder pressure and heat release rate was more complex,the improvement effect of thermal efficiency was not significant,and THC emission was deteriorated.Diesel advanced angle determined the combustion start time.With the increase of advanced angle,MBF10%and MBF50%phase angles advanced,and the maximum pressure and maximum heat release rate increased.The MBF10-90%got declined first,and after that,the curve showed a rising and then decreasing trend.Concerning emissions,advanced diesel injection could reduce THC and CO emissions,but the rise of combustion temperature would lead to the increase of NOx emissions.Pilot diesel played the role of ignition source,and the increase of diesel mass had little influence on cylinder pressure and heat release rate.Under medium and low load,the diesel mass of 3.5 and 6.5 mg/cycle had met the ignition energy demand of natural gas,and excessive diesel injection would reduce fuel economy instead.Under the experimental conditions of 1300r/min and 1000 N·m,the influence of diesel/natural gas injection interval on the stratified combustion mode was studied by adjusting the natural gas injection timing and diesel advanced angle.The following results were obtained:when the engine speed was 1300 r/min,the optimal injection interval between diesel and natural gas was 15℃A.Under this interval,the diffusion time of natural gas in the cylinder was appropriate,which made it easier to form a mixture with good concentration stratification,and the thermal efficiency reached the highest.In the range of 11～17℃A interval,the larger the diesel advanced angle,the higher the maximum pressure and maximum heat release rate.The 10%and 50%combustion phase angles were advanced,the 10～90%combustion duration was between 12～14 degrees,NOx emission increased,and THC and CO emission decreased.The injection interval of 17℃A was the turning point for the change of combustion quality.If the injection interval fuel continues to increase,the combustion quality would significantly deteriorate,and the higher the diesel advanced angle,the more serious the deterioration will be.