Study On Premixed Low-Temperature Combustion Mode On Compression Ignition Engines Based On Fuel Design Concept

Conventional low temperature combustion modes in diesel engines utilize a largeamount of exhaust gas recirculation (EGR) to dramatically reduce the local in-cylindercombustion temperatures, thus successfully solving the tradeoff between NOx andsoot emissions. However, the heavy usage of EGR causes increased incompletecombustion products such as HC and CO emissions, reducing the combustion qualityand combustion efficiency. In this dissertation, fuel design concept is employed tomodulate the in-cylinder mixture formation process, increasing the mixing qualitybetween fuel and air, so strong reliance on EGR in low temperature combustion modesis reduced, and a high-efficiency and clean premixed low temperature combustionmode can be achieved at a relatively broad operational range.The spray characteristics of diesel and gasoline blends are investigated on aconstant-volume vessel. Compared to pure diesel, the viscosity and surface tension ofdiesel and gasoline blends are changed, causing the changes in the spraycharacteristics such as spray tip penetration and spray cone angle. Further, with theincrease of gasoline proportion in diesel and gasoline blends, the sauter mean diameter(SMD) of spray droplets dramtically decreases, and the percentage of small droplets indroplet distribution increases, indicating that blending diesel with gasoline couldproduce more uniform spray field, contributing to the fuel and air mixing process.To understand the effect of fuel design concept on engine mixture ignitionprocess, a rapid compression machine is used to study the ignition characteristics ofgasoline and diesel-like fuels (n-heptane). With changed fuel composition,compression ratio, fuel/air equivalence ratio and CO2concentration, the trends ofpeak cylinder pressure, pressure rising rate and ignition delay of different fuels areinvestigated. It is found that adding gasoline into n-heptane can effectively extend themixture ignition delay and reduce the pressure rising rate in combustion processes.Furthermore, a single cylinder compression ignition engine is used to study theignition, combustion and emissions characteristics of this low temperature combustionmode based on fuel design concept. The effect of EGR on simultaneous reduction ofNOx and soot emissions is the focus as different gasoline and diesel fuels are used. It is found that the low temperature combustion mode based on fuel design concept canachieve the simultaneous reduction in NOx and soot emissions at relatively low EGRrates, validating the effectiveness of this novel combustion mode. When40%gasolineand60%diesel blended fuel is used, NOx and soot emissions are simultaneouslyreduced with13.5%intake oxygen concentration, avoiding the heavy usage of EGR.Besides, the characteristics of HC and CO emissions are investigated with changedparameters such as intake pressure, fuel injection pressure and EGR rate. It is foundthat HC emissions mainly depend on the length of ignition delay and CO emissionsare strongly influenced by in-cylinder fuel/air equivalence ratio.The effects of injection timing on ignition timing, combustion phasing andemissions of different gasoline and diesel blends are investigated. It is found that thegaseous emissions show strong denpendency on combustion phasing, while changedfuel composition and fuel injection timing do not obviously influence gaseousemissions but have apparent impacts on soot emissions.To solve the problem that conventional low temperature combustion can be onlyapplied to partial loads, this dissertation also strives to broaden the operational rangeof this premixed low temperature combustion mode by combining the intake boost andEGR strategies. Increased intake pressure shifts the high-efficiency and cleanoperational range to higher loads. Meanwhile, relatively increased EGR rate canreduce NOx emissions, extending the operational range to lower loads conditions. Atthe tested load range, engine combustion efficiency is always maintained at relativelyhigh level, avoiding the deteoriorated combustion quality in conventional lowtemperature combustion modes.