Achieving Low Emissions Of Diesel Engine By EGR Combined With Diesel Oxidation Catalyst (DOC)

The increasingly stringent emission legislations have put more serious technical challenges on I.C. engines. One of the major issues diesel engine facing is how to reduce pollutant emissions while still maintaining high thermal efficiency. To reduce the emissions, systematical research for meeting China stage IV emissions by EGR combined with diesel oxidation catalyst (DOC) was carried out on a common rail diesel engine. The preliminary exploring of the China stage V emissions as well as the effects of oxygenated additive also had been carried out in this paper.A high pressure cooled EGR system based on VNT was designed, and the effects of EGR on the performance and emissions of the engine were studied. Results show that the EGR rate can be effectively regulated by adjusting VNT vane position while still ensuring sufficient fresh intake air mass flow. The trade-off between NOx and smoke was improved by increasing the injection pressure. Through the optimization of EGR rate and injection parameters, soot can be reduced by as much as 60% compared to its original result of China stage III test, while keeping NOx meeting China IV regulations, and fuel economy also can be improved significantly.The results of multiple injections show that, the pilot injection reduced the ignition delay and premixed combustion of the main injection. Relative earlier pilot injection is better for the trade-off between NOx and smoke. However, pilot injection parameters should be carefully calibrated according to the engine load and control requirement. The influences of post injection on the performance and emissions are mainly due to its effects on the latter parts of combustion. Smoke can be effectively reduced by applying appropriate main-post injection interval. And in these cases, smoke emissions can be further reduced through increasing the mass of post injection reasonably. Compared to single injection without EGR, NOx can be reduced by 50%, and smoke emission can be effectively improved with the combination of EGR and multiple injections at the high load.The combustion chamber with reduced compression ratio was designed, and the results show that the trade-off between NOx and smoke can be effectively improved by reducing compression ratio of the combustion chamber. Based on the emissions control strategy of this research, a multi-objective function for the emission optimization was selected. The multi-objective function optimization results show that, under the promise of NOx meeting China stage IV regulations, the optimized soot emissions could get a reduction of 30% compared to the result without optimization. A better soot emission and fuel economy could be obtained with the CR16.0 (2) combustion chamber. This indicates that multi-objective function method is suitable for the current research. The emissions test with DOC shows China stage IV emission standards under ESC test cycle was met through the optimization of VNT/EGR, injection strategies, and combustion chamber geometry. This confirmed that it is feasible to meet China stage IV emission regulations by EGR combined with DOC.Test results at B75 of ESC test cycle show that, through the combination of EGR and multiple injections, controlling NOx emission at constant value of 2.0 g/kW.h (limited value of China stage V standards), specific soot emission can be reduced to lower than 0.01 g/kW.h. This means that current emission control method has the potential to meet China stage V emission standards with simplified PM aftertreatment devices. The investigation on oxygenate additive (n-butanol) indicates that under single injection strategy, soot can be significantly reduced by blending n-butanol into diesel fuel. However, the reduction effect of soot is reduced under post injection strategy when n-butanol is used. It indicates that it is possible to simplify injection strategy or reduce the dependence of the aftertreatment efficiency by blending n-butanol into diesel.