Evaluation And Analysis Of Combustion Process Of Diesel Pilot Natural Gas Dual Fuel Engine Based On Cylinder Pressure

Global climate change is a common challenge facing mankind,and achieving net zero carbon emissions is the way to meet this challenge.Zero carbon emission will have a profound impact on every level of our society and will drive deep changes of future technological development in every field.In the future,the engine will still be the main driving force of the national economy and national defense construction.To realize net zero carbon emission of engine is a major demand of national "double carbon" goal and national energy security.As a low carbon alternative fuel,natural gas has attracted much attention in the field of heavy truck transportation.Diesel ignition natural gas engine has the characteristics of high compression ratio,which ensures its potential to further improve the effective thermal efficiency.However,the research and understanding of the influence mechanism of dual fuel engine is relatively weak.In order to better understand the combustion process of diesel ignition natural gas engine,this study intends to explore the evaluation method and system suitable for evaluating the combustion process of dual fuel engine.The influence law of injection time and Engine Replacement Rate(PES)on engine working process were evaluated based on cylinder pressure variation,and the combustion process of the two fuels in the dual fuel engine is holographically decoupled.The research can provide theoretical guidance for the combustion process analysis of diesel ignition natural gas engine and dual fuel engine.In this study,a test and control platform and a numerical simulation platform are built for a diesel ignition natural gas multi-cylinder engine.An innovative evaluation method suitable for dual-fuel engine combustion process is proposed.Based on this method,the influence law of sensitive combustion boundary conditions on combustion and emission performance of the engine under different working conditions is analyzed.Based on the experiment and simulation platform,holographic decoupling of the combustion process of two fuels under different working conditions was carried out.The main research contents and conclusions are as follows:1)A new diagnosis method for dual-fuel engine combustion process is presented.The analysis results show that due to the different thermal and physical properties of dual fuel and the uncertainty of fuel combustion ratio concurrently,the accuracy of the calculation formula of heat release rate based on the first law of thermodynamics decreases,and it might be unfit for the evaluation of dual fuel engine combustion process.Based on the idea that the new physical and chemical process will bring about the increase of cyclic variation,this study statistically analyzed a large number of cylinder combustion pressure data shown in the indicator diagrams,and calculated the derivative of the standard deviation of cylinder pressure to determine the start of jump and the end of jump based on the change of cylinder pressure,between which was defined as the step duration and corresponds to the dual fuel combustion process.The heat release rate curve obtained by this method is compared with that in pure diesel mode.It is found that both of them have a good agreement in terms of morphology and phase.In addition,it is shown that the optimum sampling number for engine combustion cycle variation analysis is 140.2)The influence law of injection time on engine working process is evaluated based on cylinder pressure variation.On the diesel ignition natural gas engine research platform,two operating conditions of A25(1335r/min,375 N.m)and C25(1335r/min,375 N.m)are studied in depth.The results show that there is a mapping relationship between the standard deviation curve of cylinder pressure variation and the effective thermal efficiency.With the decrease of the number of peak values of the standard deviation curve of cylinder pressure variation,the effective thermal efficiency of the engine shows an increasing trend.Taking A25 as an example,the effective thermal efficiency increases from 28.8% corresponding to three peaks to 33.7% with one peak.In addition,the earlier the start of jump and end of jump of the standard deviation of cylinder pressure variation,that is,the more advanced the step phase,corresponds to a higher effective thermal efficiency.For high speed,such as C25,the phase of the third peak value in the standard deviation curve of cylinder pressure variation has a good correspondence with the effective thermal efficiency.As the phase of the third peak value moves forward,the effective thermal efficiency increases from 11% to about 27%.It can be seen that rationally controlling the phase of the third peak of the standard deviation of cylinder pressure variation at high rotational speed can effectively improve the effective thermal efficiency of the engine.There is also an obvious correlation between the standard deviation curve of cylinder pressure variation and engine emission performance.For low speed conditions,NOx,CO and THC emissions are significantly reduced with the decrease in the number of peak values of cylinder pressure variation.Taking A25 condition as an example,the reduction ranges are 97%,33% and 37%,respectively.For high speed conditions,NOx,CO and THC emissions also show a significant decline trend with the advance of the third peak phase.Taking C25 as an example,the reduction amplitude is 97%,42% and 70%,respectively.3)Evaluation of the effect of cylinder pressure variation on the replacement rate on the engine working process.This part is based on the diesel ignition natural gas engine research platform,the six working conditions A25,B25,C25,A50,B50,C50(A25:1335r/min,25% load.B25:1656r/min,25% load.C25:1978r/min,25% load.A50:1335r/min,50% load.B50:1656r/min,50% load.C50:1978r/min,50% load)were carried out in-depth research.With the increase of PES,the standard deviation of cylinder pressure variation changes from three peaks to two peaks at low rotational speed,and the highest peak value increases.Under high rotational speed,the standard deviation of cylinder pressure variation mostly has two peaks,and the highest peak value of the standard deviation of cylinder pressure variation(the second peak)shows a downward trend as PES increases.Under low rotational speed conditions,with the increase of PES,the jump duration of cylinder pressure variation standard deviation presents a lengthening trend,the effective thermal efficiency presents a decreasing trend,NOx emission is significantly reduced,but CO and THC emission presents an increasing trend.Under the condition of high rotating speed,when PES increases,the jump duration of cylinder pressure variation standard deviation shows a shortening trend,and the effective thermal efficiency,NOx,CO and THC emissions are the same as those under the condition of low rotating speed.4)Holographic decoupling of different fuel combustion processes in dual fuel engines.This part,based on the test and numerical simulation platform of diesel ignition natural gas engine,carried out in-depth research on A25,B25 and C25(A25:1335r/min,25% load.B25:1656r/min,25% load.C25:1978r/min,25% load).The chemical reaction at the initial stage of natural gas combustion is synchronized with the combustion process of igniting diesel,and the end time of igniting diesel combustion is prior to the end time of natural gas combustion,and two combustion processes have overlapping periods.The combustion process of natural gas is determined by the spatial distribution in the cylinder at the time of ignition of diesel.With the advance of the ignition diesel’s injection timing,the ignition mode of diesel gradually changes from the traditional premix-diffusion combustion mode to the homogeneous compression combustion mode,which makes the natural gas gradually change from the ignition around the fuel beam to the ignition in the local spatial range of the cylinder.In the process of this transformation,all the indicators of the engine are developing in a better direction,but with the further advance of the injection time,the amount of ignition diesel clinging to the cylinder wall increases,which makes the combustion process and emission performance of the engine deteriorate.Under the same working condition,the shorter the combustion duration of natural gas is,the higher the effective thermal efficiency of the engine is.In A25(1335r/min,25% load)conditions,when the fuel injection time advance from 5°CA BTDC to 15°CA BTDC,the start of jump of cylinder pressure variation standard deviation presents a synchronous forward trend,and the phase of the step peak is consistent with the end phase of overlapping combustion of the two fuels(the end of ignition diesel combustion).When the injection time changes from 20°CA BTDC to 35°CA BTDC,the start of jump of the standard deviation of cylinder pressure variation is around 14°CA ATDC,and the two peaks of the standard deviation curve gradually approach each other until they become one same peak.Still under A25(1335r/min,375N·m)condition,when the natural gas energy replacement rate rises from 85% to 95%,the natural gas combustion duration extends from 20°CA to 30°CA,the first peak value of cylinder pressure variation standard shows a downward trend,and the second peak value(the main peak)shows an increasing and backward trend.When PES is 95%,the second phase lag of the peak at the end of two overlapping fuel combustion phase(the end of ignition diesel combustion)about 5°CA.