Investigation On Combustion Technology Of Low-emission HD Diesel Engines With EGR Technology

Energy and environment are the two main themes of social development, andenergy saving and emission reduction are two common goals for all countries in thewhole world. With increasingly stringent emission regulations, it is necessary todevelop modern clean diesel engine with high thermal efficiency, which puts forwardnew challenges to internal combustion engine researchers and engine enterprises.In this paper, the combustion technology of a low-emissions heavy-duty (HD)engine with exhaust gas recirculation (EGR) technology, and the effects ofcombustion control parameters on diesel combustion, performance and emissionswere investigated. The combustion and emission control strategies based on EGRtechnology to meet China Stage IV were studied and a HD diesel engine meeting Euro6was successfully developed. In addition, the impacts of EGR introduction way andlow cetane number (CN) oxygenated fuels on diesel combustion, performance andemissions were investigated, the aim of which was to explore the efficiently technicalways to reduce soot and enhance thermal efficiency for diesel engine.The results of boost system optimization show that, compared to single-stageturbocharger, two-stage turbocharger (2TC) can obviously improve EGR recyclabilityand the trade-off of NOx-soot and NOx-BSFC at low speed and high load conditions,meanwhile, the exhaust temperature is much closer to the temperature region whereSCR has high conversion efficiency. The combustion chamber with reducedcompression ratio was designed and the optimization matching of injector wasperformed. The results show that, soot, BSFC and the maximum in-cylinder pressurecan be lowered by properly reducing compression ratio, optimizing structureparameters of combustion chamber (reentrant diameter and bowl depth, etc.) andusing Bosch injector with eight taper holes.The results of fuel injection control strategy show that, with increased EGR rate,NOx, soot and the maximum rate of pressure rise can be efficiently reducedsimultaneously at low loads by reducing injection pressure and postponing injectiontiming with single injection strategy. At low speed and high load conditions, soot andBSFC can also be improved by employing the same control strategy used at low loads.However, it is suitable to employ higher injection pressure and post injection toreduce soot and BSFC at high speed conditions.The experimental study on China Stage IV of diesel engine based on EGR technology was performed under different liter power conditions. The results showthat, the scheme of turbocharger and after-treatment should be optimally selectedaccording to emission control target and practical liter power of diesel engine. ESCcycle test results show that, all emissions can completely meet the limits of ChinaStage IV for the power section of29.8kW/L by using2TC without anyafter-treatment device, and the weighted BSFC is only higher3%compared with thatof the original engine. This means that2TC has the potential to further reduce dieselemissions and meet future more stringent emission regulations.Based on the combustion technology used above, a commercial diesel enginemeeting Euro6was developed successfully. ESC cycle test results of Euro6showthat, NOx weighted emissions can be reduced to the level of Euro5by using2TC andmedium EGR (<40%) with a low level of PM and a good engine performance. Allemissions can completely meet the limits of Euro6by the combined use of DOC,DPF and SCR. At the same time, the average conversion efficiencies of DPF and SCRare as high as90%and85%respectively, and the urea consumption is reduced toabout one fourth to one fifth of the original level through the optimal combinationcontrol of raw NOx emissions and urea/NOx equivalence ratio.Compared to HP-EGR, diesel engine can run over a wider region of EGR rate byusing LP-EGR, which can enhance the efficiency of turbocharging system, and reducesoot and BSFC greatly at low speed and high load conditions. When using LP-EGR,BSFC can be reduced efficiently at high speed by adjusting exhaust energy betweenhigh pressure turbine and low pressure turbine. The study on effects of fuel propertieson engine illustrates that, soot can be reduced dramatically via blending2,5-dimethylfuran (DMF), n-butanol and gasoline into diesel fuel. Extended ignitiondelay and fuel oxygen are two key factors to reduce soot emissions, and ignition delayhas greater effects on soot reduction compared to fuel oxygen. Overall, using low CNoxygenated fuel combined with medium EGR (<40%) is an efficient way for dieselengine to achieve clean diesel low-temperature combustion with high thermalefficiency under the condition of simplified injection strategy and after-treatmentsystem.