Study On High-efficiency And Low-emissions Combustion Mode On Compression Ignition Engines Based On Injection Strategy And Fuel Design

Under the influence of the severer energy crisis and stricter environmental regulations,study on high-efficiency and low-emissions of low temperature combustion mode wasinvestigated experimentally and computationally. In this study, the low temperaturecombustion （LTC） was achieved in a common rail diesel engine using the means of exhaustgase recirculation （EGR）, injection strategy and fuel design concept. The combustion andemissions characteristics of LTC were investigated. Then the fuel design concept was used toextend the operation range of LTC and improve the thermal efficiency. The combination ofLTC and the injection strategy was also investigated. Finally, the time evolution and spatialdistributions of the in-cylinder temperature and several spieces were studied using the3Dsimulation software, and the factors of lower thermal efficiency for were analyzed. The maincontent is as follow:（1） The HCCI-DI combustion was achieved using multiple injection strategies. Theeffects of pilot injection timing, pilot injection quantity and main injection timing oncombustion and emissions of HCCI-DI combustion were investigated. The results show thatthe heat release rate of HCCI-DI combustion consists of the cool flame stage, the thermalflame stage of HCCI combustion and diffusion stage of CIDI combustion. The smokeemissions and the brake specific fuel consumption of HCCI-DI combustion are kept almostconstant, and NO_Xemissions can be reduced greatly. But NOx emissions is still in a highlevel that need to be reduced in the high load. （2） In order to further reduce the NO_Xemissions, the low temperature combustion usingmoderate level of EGR and the prolonged injection timing was achieved in low and moderateloads. The results of LTC combustion were compared with those obtained from CIDIcombustion. The effects of EGR rate, start of injection and injection pressure on emissionsand combustion characteristics of LTC combustion were investigated. The results show thatthe in-cylinder pressure of LTC is greatly reduced and the start of combustion is obviouslyretarded. The prolonged ignition delay of LTC is ideal for fuel-air mixing. The NO_Xemissionsdecrease monotonically, and smoke emissions increase firstly and then decrease as theinjection timing is retarded. The NO_Xand smoke emissions of LTC are reducedsimultaneously in low and moderate loads. The NO_Xemissions increase, but smoke, HC andCO emissions decrease with the increase of injection pressure.（3） Based on the fuel design concept, the effects of the oxygenated fuel and multipleinjection strategy on combustion and emissions of LTC were investigated. The effects of fuelblends, pilot injection timing and pilot injection quantity on combustion and emissions ofLTC were also investigated. The results show that the combustion of LTC is improvedbecause of the oxygen content of biodiesel, leading to the reduction of smoke emissions. TheLTC combustion fueled with biodiesel fuel blends is achieved over a wide operating range.The brake thermal efficiency of biodiesel fuel blends is slightly higher than that of diesel fuel.The NO_Xemissions of LTC could be further reduced by employing pilot injection strategy,but the smoke emissions increase. The pressure rise rate and cycle variation of LTC is reducedwhen the appropriate pilot quantity is applied.（4） In order to optimize the LTC further, the effects of ethanol-biodiesel-diesel fuelblends on emissions and combustion of LTC were investigated, and the operating range ofLTC combustion was discussed. The results show that the lower smoke emissions are theresult of the lower cetane numbers of fuel blends that lead to longer ignition delay and greaterfuel-air mixing time. Moreover, the higher oxygen content in ethanol is effective for the reduction of smoke emissions. The effects of ethanol on the reduction of smoke emissions areobvious at the high loads. In this way, the upper load limit of LTC is extended. The brakethermal efficiency of BDE20（20%ethanol in blends） is improved because of the advancedinjection timing. The high-efficiency and low-emissions of LTC are achieved withethanol-biodiesel-diesel fuel blends.（5） Combustion process, n-heptane, CH₂O, OH, NO and CO distributions of both CIDIand LTC combustion were investigated using a CFD software. Factors governing the thermalefficiency were also investigated. It is found that the simulated in-cylinder pressures are ingood agreement with the measured results for both combustion modes. The maximumtemperature of LTC is lower than those of CIDI combustion. The low temperature oxidationprocess of LTC is longer than that of CIDI combustion. The results show that the combustionphasing is the most important factor, followed by combustion efficiency. The effects ofcombustion duration and the shape of heat release rate are minimal. From the means of LTCcombustion, the effect of the injection timing on brake thermal efficiency is greater than thatof EGR.