Optimization Of Combustion System Of Common Rail Diesel Engine Based On EGR Technology

Due to good fuel economy, power performance and reliability, diesel engine has been widely used, but the higher particulate matter (PM) and NOx emissions cause serious pollution to the environment. Exhaust gas recirculation (EGR) technology is one of the most effective measures to reduce NOx, but burning with EGR in diesel engine lead to the lower thermal efficiency. Therefore, how to reduce Soot and NOx emissions and achieve high combustion thermal efficiency is the modern diesel engine development the important challenge.Basing on D19 high pressure common rail diesel engine as the research model, application one dimensional thermodynamic simulation model GT-Power and AVL Fire three-dimensional CFD simulation model, to study the effect of structural parameters of different combustion chamber on diesel engine combustion process, the effects of gas flow in cylinder and exhaust emissions, and under different combustion boundary conditions examines the adaptability of the combustion chamber. Finally, coupling EGR and fuel injection strategy, study on the influence law of diesel engine combustion, performance and emissions.The study of the combustion chamber structure shows that, vortex maintaining of the larger reducing rate (plan A) or the path depth ratio (plan C) combustion chamber is stable; Small reducing rate (plan B) or big diameter deep than (plan D) is benefial to the mixture of oil and gas rapidly at the beginning of burning. Path depth ratio reduction (plan C) for late inverse extrusion flow combustion forms, and reduce Soot. With no EGR and small EGR rate, Soot emissions decrease with compression ratio inceasing, EGR rate was 37%, Soot emission increases with the compression ratio.The study of combustion chamber adaptation to boundary conditions show that, when the EGR rate was 37%, different combustion chamber of NO tend to be "zero emissions". Shorten the injection duration or improve the injection pressure, equivalent ratio in cylinder is more homogeneous, so as to shorten the ignition delay and combustion duration; Shorten the injection duration can weaken the influence combustion chamber structure parameters on the gas flow in cylinder, reduce Soot emissions, but increase NO emissions. Eddy ratio of 1.5, combustion velocity field enhance in cylinder, the plan A equivalent is the most uniform distribution; With the increasing of eddy ratio, the Soot of plan A reduct and plan B, C Soot linear increase, plan D Soot emissions decrease before they rise.Under different combustion boundary conditions, the plan C achieve minimum NO and Soot emissions at the same time, show its better adaptability to different combustion boundary, reasonable structure design.The study of injection parameter coupling EGR indicated that, with the main injection timing delay, BMEP reduce after rising firstly, BSFC contrary trend. The delay of main injection time with the larger EGR rate, the BMEP and BSFC changes more significant; With high EGR rate or the main injection timing delay, NOx emission reduce linear, Soot is first reduced rise again. Improving EGR rate can weaken the main injection timing in advance to promote the formation of NOx; with the main injection timing delay, effective thermal efficiency higher at first and lowered later; exhaust loss rate increases, heat transfer loss and friction loss decrease. The main injection quantity enlarge, the BMEP enlarge; NOx emissions reduced with the decreasing of the fuel injection quantity. Medium load thermal efficiency increases while the exhaust loss rate is reduced, heavy load, the lower thermal efficiency is mainly because the exhaust loss rate is higher; The main injection timing delay, BMEP reduce rapidly with RPM increase, a significant rise in BSFC, but NOx reduce; low speed condition, main injection time too early can cause NOx and heat loss rate increased at the same time;High speed condition main injection timing too delay will reduce engine power capability, exhaust loss increase rapidly in the thermal efficiency rate significantly reduced.The study of injection pressure coupling EGR showed that:when the main fuel injection quantity or injection pressure increase, BSFC, Soot reduce significantly while NOx emissions; NOx is sensitive to the change of the injection pressure. The larger fuel injection quantity lead to the small Soot emission; With the increasing of injection pressure, the effective thermal efficiency and exhaust gas loss rate decreases, heat transfer and friction loss rate increases. The larger main fuel injection quantity lead to the greater thermal efficiency; Under high speed condition, the increasing of injection pressure resulting in the lower BSFC more significantly, but reduce NOx.With high speed condition, injection pressure is benifial to the thermal efficiency, exhaust gas loss rate reduced significantly.The study of post injection parameter coupling EGR showed that, the delayed post injection parameter and enlarge fuel injection quantity, BSFC rises; Under the same injection quantity, with the post injection time delayed, NOx emissions rise, after injection parameter of 12°CA ATDC, the larger fuel injection quantity lead to the small NOx, when post injection continue to delay, NOx increases with the rising of fuel injection quantity; Then post injection parameter delays and fuel injection quantity increases, the effective thermal efficiency and heat transfer and friction loss reduce, but the exhaust loss rate increase.