Study On The Effects Of Combustion Boundaries On The Low Temperature Combustion Of Low Reactivity Fuels

As the worldwide demand for energy saving and emission reduction in internal combustion engines (ICE) is ever increasing,the new combustion technology with high efficiency clean ICE has always been given significant attention all over the world.As the research on the new generation combustion technology of ICE has been made a big progress,which represented by homogeneous charge compression ignition (HCCI) and low temperature combustion (LTC),it has been found that the LTC by burning high volatility and low reactivity(high-octane)fuel can achieve high thermal efficiency and low emissions simultaneously.In this research work,a systematic experimental study on the LTC technology burned with the low reactivity fuel under different combustion boundary conditions was carried out.The exhaust gas recirculation (EGR) system,boost system and intake cooling system are the important factors to determine the combustion boundary conditions of the low reactivity fuel LTC,so the advanced air system is the basis for the high efficiency clean combustion.The design and control parameters of the two-stage turbocharging system and the split strategy of dual loop exhaust gas recirculation (DL-EGR) system were studied first in this paper.Hence a new design and matching principle of the advanced air system based on a variable geometry two-stage turbocharger (VGT+FGT) and DL-EGR has been proposed in this work.The research shows that the inter-stage cooling intensity,the isentropic efficiency and the pressure ratio distribution (PRD) of the two-stage turbocharger system are the main factors affecting the pumping loss and the thermal efficiency.The inter-stage cooler (ISC) can significantly improve the overall isentropic efficiency,the boost pressure and the fuel economy.The PRD should be designed and adjusted according to the inter-stage cooling intensity and the isentropic efficiency of each stage.The VGT+FGT turbocharger can optimize the exhaust enthalpy and PRD under different operating conditions and improve the brake thermal efficiency (BTE).The research on EGR strategy shows that it is related to the engine operating conditions,the engine-out NOx emission targets and turbocharger matching.High pressure (HP) EGR is suitable for high-speed operation and low pressure (LP) EGR is suitable for low-speed,high-load operation.The DL-EGR should be applied on medium-speed conditions or under the conditions with low engine-out NOx emission targets.The research on the control strategy coupled EGR with the variable geometry supercharger (VGT) shows that the HP EGR valve should be wide open for the conditions with high pumping loss,then the VGT closure and LP EGR valve should be optimized to achieve the high fuel efficiency.While the LP EGR loop performs the best for the conditions with low pumping loss and the optimization of VGT closure can improve the turbocharger efficiency.Therefore,an advanced air system has been proposed,which is consisted of a variable two-stage turbocharger,an ISC and a DL-EGR system.This air system was also applied on the engine fueled with the low reactivity fuel as discussed below.The experimental study on the gasoline compression ignition (GCI) control strategy was carried out under the overall operating conditions,and the influence of the engine-out NOx emission targets on the GCI control strategy of high efficiency and clean combustion was discussed.The results show that the GCI low-speed operation offers good fuel economy,and its harmful emissions are significantly lower than that of diesel LTC.Therefore,down-speeding the GCI engine is a technical route for applying GCI technology on heavy duty commercial engines.With the increase of engine load and/or engine speed,the high volatility and the low reactivity characteristics of gasoline have less effect on the air-fuel mixing,combustion and emissions.At the high-speed high-load conditions,an early fuel injection timing and a high fuel injection pressure are beneficial to improve the fuel economy,and can greatly reduce the harmful emissions when compared with diesel fuel.Applying small pilot injection can effectively reduce the specific fuel consumption,harmful emissions and maximum pressure rise rate (MPRR),and improve the combustion efficiency and stability under the medium and low load conditions.When relaxing the engine-out NOx limitations,a double injection strategy combined with a lower fuel injection pressure and an earlier combustion phasing (CA50 timing) is suitable for GCI operation,and the best brake thermal efficiency of 44%can be achieved at the engine-out NOx emission of 5 g/kW-h.Finally,the influence of physical and chemical fuel property on the LTC engine performance and emissions was investigated,and the influence of the combustion boundaries on the high efficiency and clean LTC combustion strategy of low reactivity oxygenated fuel was also discussed.The results show that the soot reduction effect of fuel molecule dilution effect increases with the increase of dilution ratio and decreases with the increase of engine load.At the low and medium load conditions,the low fuel reactivity has a greater effect on soot reduction than the dilution effect,while at the high-load conditions,the dilution effect is more significant.Increasing the injector flow rate for the DB80 fuel(blending 80%n-butanol into diesel by volume)can improve the heat release rate,reduce the soot emissions and the fuel consumption.A good fuel economy can be maintained even if the engine-out emissions are reduced to meet Euro VI limits,and the DB80 can achieve a maximum fuel consumption improvement of 2%compared with diesel fuel.The fuel economy improvement of low reactivity oxygenated fuel compared with diesel fuel is more obvious as the engine-out NOx emission decreases.The optimum n-butanol blending ratio for diesel/n-butanol blends to achieve high efficiency and clean combustion is closely related to the engine operating conditions and the engine-out NOx emission targets.At the full-load condition,the engine-out NOx and soot emissions of DB80 can be reduced to 0.5 g/kW-h and 0.01 g/kW-h,respectively.Thus,the SCR conversion efficiency of 95%can meet the future ultra-low NOx emission limit (0.027g/kwh),which means the low reactivity oxygenated fuel can greatly reduce the difficulty in meeting the future ultra-low emission regulations.