Research On Combustion And Emission Mechanism Of Diesel Pilot Ignited Natural Gas Engine Based On CFD Coupled With The Chemical Kinetics

Smog,as one of the most widely affected environmental pollution way in China,has seriously affected the health of our people.At the same time,the growing energy demand has made our country facing a severe energy crisis.The internal combustion engine is not only one of the main sources of smog,but also a major tool of energy consumption.Therefore,finding efficient and clean alternative fuels of internal combustion engine and organizing reasonable combustion methods are of great significance for solving the air pollution and energy crisis of China.Natural gas,as a clean fuel with abundant reserves,is generally used in internal combustion engines in two ways:spark ignition and diesel ignition,both of which have attracted extensive attention from scholars.Only using spark ignition mode will cause knocking and burning easily due to the slow flame propagation speed.Compared with the spark ignition method,the combustion method of Diesel Pilot Ignited Natural Gas(DPI NG)can effectively solve the above problems.At present,this combustion method is widely used in heavy natural gas vehicles.In this study,a DPI NG dual fuel engine test under various operating conditions was carried out.Based on the test data of the trace DPI NG,a three-dimensional CFD simulation model couple with the chemical reaction kinetic mechanism was established,and the validity of the model was verified by the test results.Based on the chemical reaction mechanism,Chemkin was used to study the effects of initial temperature,initial pressure,equivalence ratio,and mixing ratio on the ignition delay of CH4/NC7H16 mixed fuel.Then,through the combination of test data and GT-Power simulation results,the influence of fuel injection timing on the engine combustion process and emissions was studied.Finally,based on validated CFD model,the effects of fuel injection angle,excess air coefficient,substitution ratio,swirl ratio,and combustion chamber structure on in-cylinder combustion process,emissions formation characteristic and heat-power conversion process were studied.The thesis research results show that:(1)In the low temperature range of 800 K to 1100 K,the ignition delay is affected by temperature,pressure and methane molar coefficient.Therefore,under the higher methane molar fraction,increasing the equivalence ratio or the initial temperature can also achieve the effect of shortening the ignition delay.Under different equivalence ratios,R98 is the most important reason for methane consumption,and R342 is an important reaction that has a positive effect on temperature.In order to accelerate the combustion,it is necessary to strengthen the conditions conducive to the reaction of R98 and R342.(2)The influence of the spray angle on the cylinder pressure,heat release rate,combustion efficiency and indication efficiency shows that the spray angle and the shape of the combustion chamber must be optimized at the same time to obtain the best combustion efficiency and the highest indicated work under the same spray angle.To obtain a more ideal combustion process and balance NO_x,unburned methane and CO emissions,the optimal spray angle should be between 120° and 160°.(3)Although increasing the advanced pilot injection degree can effectively increase the maximum in-cylinder pressure and maximum HRR,and at the same time,it will cause a rapid increase in NO_x and a slight increase in HC,the brake thermal efficiency increases firstly and then decreases.In addition,the advanced pilot injection degree has a greater impact on the 50-90% combustion duration than the 10-50%combustion duration.As the NG energy fraction rises from 50%to 90%,NO_x and unburned methane emissions increase,and CO emissions increase firstly and then decrease.(4)At 50% load,when ? increases from 1.0 to 1.5,the maximum in-cylinder pressure increases and its position advances,and the position where HRR rapidly increases also appears earlier.At the same time,as ? increases from 1.1 to 1.5,the duration of the heat release will become longer and the maximum HRR will decrease.With the change of ?,NO_x and HC show a "trade-off" relationship,and when ? remains above 1.2,the CO emissions will remain at a low level.(5)When the swirl ratio increases,the maximum in-cylinder pressure and heat release rate increase firstly and then decrease.As the size of the piston opening becomes larger and the depth of the groove becomes shallower,both the peak pressure and the peak heat release rate become smaller,the positions of SOC and CA50 are delayed in both low-speed high-load and high-speed low-load.After the swirl ratio exceeds 2,the effect of continuing to increase the swirl ratio on the initial combustion stage becomes very small,and to obtain the optimal CA50 position,the swirl ratio needs to be maintained at about 1.In the trace DPI NG engine,the size of the piston opening and the depth of the groove still need to be similar with that of the diesel engine,which is beneficial to the combustion in the cylinder.Through the CFD simulation coupling with the chemical reaction kinetics reveals the deep-seated causes of important parameters for the combustion process and the formation mechanism of emissions in DPI NG engine,and obtained the optimal value of various parameters under various working conditions.For further improving the power and emissions of the trace DPI NG engines provides theoretical guidance.