Optimization Study Of The Low Temperature Combustion Coupled With Advanced Control Strategies Based On Heavy-Duty Compression Ignition Engines

Compression-ignition(CI)engines are widely used due to their high thermal efficiency and strong dynamic performance.However,conventional diesel combustion mode can lead to high levels of NO,and soot emisisons.Nowadays,due to energy shortage and environmental pollution,the develplment of high-efficiency and low-emission engines is required.In order to solve these problems fundamentally,the low temperature combustion modes are proposed,which significantly improve the performance of conventional CI engines.However,it is still challenging when LTC modes are applied in wide load ranges.Moreover,the controllability and stability of the LTC modes need further improvement.In this study,based on two tipical LTC modes,i.e.,premixed charge compression ignition(PCCI)and reactivity controlled compression ignition(RCCI),the potential of advaced control strategies for improving the LTC performance under wide load ranges is investigated.The advanced control strategies include variable valve timing(VVT),variable compression ratio(VCR),and multiple injection.By performing optimization calculation with genetic algorithm(GA).high-efficiency and low-emission control strategies for LTC are proposed under wide load ranges.The effects of key operating parameters,as well as the conbined effects of them are systematically investigated.Based on the first and second law of thermodynamics,the influences of the parameters on the energy utilization are further revealed.Moreover,the advanced control strategies are systematically compared and estimated based on LTC modes under different load conditions.(1)In order to simultaneously minimize indicated specific fuel consumption(ISFC),nitrogen oxides(NO,),and soot emissions,a multi-dimensional computational fluid dynamics(CFD)code,i.e.KIVA,is coupled with genetic algorithm(GA)for optimizing the PCCI combustion with VVT strategy under a wide load range.The optimization results indicate that the range of the operating parameters narrows considerably with increasing load.At low load,the intake valve closing(ⅣC)timing can be chosen in a wide range.At mid and high loads,the IVC timing is fixed around-110 ℃A ATDC.As for the fuel injection timing,relatively early start of injection(SOI)timing should be used at low load for improving fuel efficiency at low and mid load,whereas a late SOI is needed to avoid overly high in-cylinder peak pressure(pmax)and peak pressure rise rate(PPRR)at high load.Moreover,the intake pressure should be increased with increasing load,while the EGR rate should be reduced at high load.This is for providing sufficient oxygen for the fuel combustion.As for the combustion mode,the combustion process can be recognized as premixed(or partially premixed)compression ignition at low and mid load.At high load,the combustion mode shifts to late-injection low temperature combustion.(2)Based on the above optimization results,the effects of operating parameters on the combustion precess.emissions and fuel consumption are investigated under wide load ranges.Meanwhile.the combined effects of IVC timing and intake pressure,as well as SO]timing and EGR rate are further investigated.The results indicate that when the LIVC strategy is applied,boosted intake pressure is needed to improve the thermal efficiency and reduce the emissions.At low and mid load,with advanced fuel injection,high EGR rate is required to reduce the nitrogen oxides(NOx)emissions.At high load,retarded fuel injection is needed for controlling engine knock.This reduces the need of EGR for reducing NO,emissions to some extent.Moreover,both advanced and retarded SOI timing result in lower net work and higher ISFC due to the higher transfer loss or exhaust loss.Consistently,both lower and higher EGR rate lead to lower net work owing to the higher transfer loss or significant incomplete combustion.Furthermore,the exergy analysis indicates that retarding IVC timing leads to increasing exergy destruction as the global equivalence ratio remains constant.The exergy destruction at mid load is the lowest owing to the highest combustion temperature.(3)The application of VVT in RCCI combustion is further investigated.By performing optimization calculation,it is found that with the utilization of VVT,the Euro 6 NOx limit can be met while maintaining ultra-low soot emissions at low and mid load.However,at high load.at least one aftertreatment device is required to further eliminate the NOx or soot emissions.As for the fuel economy,under the whole load range,the fuel consumption can meet EPA standard.By comparing the optimal strategies,it can be concluded that the strategy with higher PR and earlier IVC timing exhibits better engine performance on thermal efficiency and soot emissions.while the strategy with lower premix ratio and later IVC timing is superior in ringing intensity.Moreover,the later IVC is beneficial for the NOx reduction under wide load ranges.In order to solve the high-load problems of RCCI combustion coupled with VVT strategy,double direct-inj ection strategy is introduced.By optimizing the fuel injection parameters,the superiority of double injection strategy over the single injection strategy is demonstrated at high load.First,the fuel/air mixing process can be improved with double injection,which is beneficial for reducing soot emissions.Second,the combustion phasing and duration can be effectively controlled,which are beneficial for increasing fuel efficiency.Moreover,the evolutions of objectives and variables indicate that the strategies with more separated fuel injections realized by an earlier SOI1 and later SOI2 are preferred.In these strategies,more sufficient fuel/air premxing can be realized be advancing SOI1,which is beneficial for reducing soot emissions.However,the amount of the premixed diesel fuel in the first injection is limited by the PPRR constraint.Meanwhile,with retarded SOI2,the combustion phasing can be well controlled.and high NOx emissions can be avoided.(4)Systematic evaluation of VVT and VCR strategies is conducted based on RCCI engine in terms of combustion process control,fuel efficiency,and emission characteristics.By comparison,it is found that more significant reduction of pmsx can be achieved by the VCR strategy.At low and mid loads,high effective compression ratio,large premix ratio,and early fuel injection can be utilized to realize Euro 6 NOx limit with ultra-low soot emissions and low fuel consumption for both VCR and VVT strategies.At high load,the ECR should be reduced for controlling PPRR and pmax.Compared to VVT strategy,a further decreased ECR can be utilized for VCR strategy at high load,which allows early fuel injection,leading to the improvements of fuel efficiency and soot emissions while the NOx emissions are not sacrificed.For the VVT strategy,overly low ECR realized by retarding ⅣC timing results in decreasing volumetric efficiency and misfire.In contrast,for VCR strategy,relatively lower ECR(decreasing GCR)can be utilized at high load,while the volumetric efficiency is not deteriorated.The lower ECR enlarges the satisfactory operating region that meets the PPRR and pmax limits for VCR strategy at high load.