Simulation Study On Isobaric Combustion Of Diesel Engine

Nowadays,as global oil resources are becoming increasingly scarce,emissions regulations are becoming more stringent,and environmental protection and energy conservation awareness are becoming stronger,only by continuously improving combustion technology can internal combustion engines not be eliminated.The current development trend of diesel combustion technology is to continuously increase the proportion of constant volume combustion.The method to increase the proportion of constant volume combustion is to continuously increase the injection pressure,but unilaterally increase the injection pressure without reducing the injector nozzle hole diameter(currently the minimum diameter of the nozzle hole can reach 0.1mm,the processing technology can not make the diameter of the nozzle hole smaller)has no effect on the average particle diameter of Sutter(SMD,which is an index for evaluating the particle diameter of the spray),because increase injection pressure without reducing the nozzle hole diameter will cause the fuel injection amount to increase during the ignition delay period,resulting in rough work and increased emissions,which will seriously affect the performance of the diesel engine.On the other hand,we have noticed that the brake thermal efficiency of large marine diesel engines has exceeded 50%.Compared with automotive diesel engines,the cycle of near-isobaric combustion generated by marine diesel engines shows very low mechanical and thermal losses.This shows that finding the balance between cycle thermal efficiency and various losses may be more effective than blindly improving the theoretical thermal efficiency.Compared with constant volume combustion,although isobaric combustion has a lower theoretical thermal efficiency,due to the lower pressure and temperature in the cylinder,the heat loss and mechanical loss of the cycle are smaller,and NO_x emissions are also lower,especially the improvement of diesel engine performance is obvious under heavy load conditions.Therefore,it is necessary to study the isobaric combustion technology of diesel engines to provide a theoretical basis for finding the equilibrium point between the contradiction between thermal efficiency and energy loss and emissions of diesel engines.To this end,this research conducts a simulation study on the isobaric combustion of diesel engines to explore the potential of this combustion method in improving diesel engine performance.First,the original model of the single-cylinder diesel engine was built with GT-Power software and the model verification was completed.Then the related parameters of the original model were modified,and the two-stage fuel injection strategy was used to achieve approximate isobaric combustion of the diesel engine.Based on the isobaric combustion model with brake thermal efficiency not lower than that of the original engine after preliminary optimization,the characteristic operating points of the isobaric combustion with different pre-explosive pressures were selected to analyze the impact of diesel engines on the combustion process,economy,and emissions.On this basis,the influence of isobaric combustion at different speeds and different loads on the performance of diesel engines was studied.The main conclusions are as follows:1.Isobaric combustion via two-stage injection can effectively shorten the ignition delay of diesel engines and extend the combustion duration of diesel engines to ease the combustion process,which is beneficial to reduce the peak value of the heat release rate of the diesel engine and thus causing a decrease in the peak combustion temperature and an increase in the combustion temperature in the later stage.2.Isobaric combustion is beneficial to reduce heat transfer loss of diesel engines,but it will increase the diesel engine exhaust loss,and the effect on the heat transfer loss is less than the effect on the exhaust loss,so the isobaric combustion will cause the diesel engine’s indicated thermal efficiency to decrease.In addition,isobaric combustion is beneficial to reduce the friction loss of the diesel engine but will cause a slight increase in pumping loss,and the effect of isobaric combustion on the friction loss reduction is greater than the increase on the pumping loss,so isobaric combustion is beneficial to improve the mechanical efficiency of the diesel engine.Combining the effects of indicated thermal efficiency and mechanical efficiency,the use of isobaric combustion can achieve the goals of improving the brake thermal efficiency of diesel engines and reducing the fuel consumption rate.3.Isobaric combustion is conducive to reducing NO_x emissions due to the lower peak combustion temperature.However,as isobaric combustion uses a two-stage or multistage injection strategy,the amount of diffusion combustion in the cylinder increases,so isobaric combustion will cause a certain degree of deterioration in particulate emissions.4.Isobaric combustion at different speeds can improve the brake thermal efficiency by adjusting the maximum combustion pressure,and the use of isobaric combustion at the appropriate explosion pressure can reduce the NO_x emissions of diesel engines,but due to the change in injection strategy,the effect of isobaric combustion on particulate emissions at different speeds is the same,that is,the particulate emission of isobaric combustion is higher than that of the original engine.This is due to the increase in diffusion combustion.5.Isobaric combustion under different loads can increase the brake thermal efficiency of the diesel engine by adjusting the maximum combustion pressure,thereby reducing the brake fuel consumption rate of the diesel engine.However,if the maximum combustion pressure of the isobaric combustion is too large or too small,the brake thermal efficiency will be reduced.And isobaric combustion under different loads is beneficial to improve the NO_x emissions of diesel engines but will cause the particulate emissions of diesel engines to deteriorate.6.Isobaric combustion can reduce the mechanical load of the diesel engine while maintaining or even improving the economy of the diesel engine.Therefore,isobaric combustion can increase the power of the diesel engine within the limit of the mechanical load of the engine,and achieve the expansion of the diesel engine’s power.