Study On The Effects Of Fuel Properties And Combustion Modes On Low Temperature Combustion In Diesel Engines

Low temperature combustion (LTC) is an advanced combustion concept. Fuelproperty plays an important role in the combustion, performance, emissions, andhigh-efficiency and clean combustion region of LTC. On the other hand, it is also animportant technique to accomplish the high-efficiency and clean combustion over thefull operating range by adopting the compound combustion mode that is acombination of a variety of combustion modes. Therefore, the research on fuelproperties and combustion modes has very important theoretical and practicalsignificances. In the current work, experiments, numerical simulations andthermodynamic analysis method were combined to explore the effects of fuelproperties and combustion modes on LTC in diesel engines.Gasoline LTC has been investigated in the first stage, mainly on the effects ofcombustion control parameters and injection strategies on it. The results indicate that,simultaneous reductions of NOXand soot relative to that of diesel, as well ascomparable thermal efficiencies with those of conventional diesel engine can beachieved by gasoline LTC with higher maximum pressure rise rate (MPRR) andcyclical variation. Comparable carbon monoxide (CO) with that of diesel and slightlyhigher hydrocarbon (HC) emissions relative to that of diesel can also be achieved bygasoline LTC. Emissions of gasoline LTC can be effectively reduced by increasing theintake pressure. Expansion of gasoline LTC to the high load can be attained byadopting the double injection strategy, with increasing soot, CO, and HC emissions atthe high loads and slight declining combustion efficiency and indicated thermalefficiency, however.For the issue of high soot emissions at higher loads with gasoline, the effect ofn-butanol addition into gasoline on LTC has been investigated then. The resultsindicate that, the addition of n-butanol can effectively reduce soot emissions ofgasoline. The addition of n-butanol may also reduce CO and HC emissions at highexhaust gas recirculation (EGR) regions without substantially changing NOXemissions. Increasing n-butanol ratio in the fuel blend will not change the ratio of NOto NO2in NOXsubstantially, but will reduce the ratio of aromatic hydrocarbons tototal HC and increase the ratio of formaldehyde and acetaldehyde to total HC.The high octane number of the above-mentioned fuels favors the achievement ofLTC at high loads, but the low cetane number of the fuels may cause misfire readily. To expand LTC to the low load, LTC performance of gasoline/diesel blend has beeninvestigated. The results indicate that, with a fixed CA50, increasing gasoline ratiowill increase the heat release rate, MPRR, and cylinder pressure, with little effect onthe indicated thermal efficiency. Soot emissions will be reduced with the increasedgasoline ratio, but NOXemissions will be increased. However, NOXemissions can bereduced by adopting even higher EGR. Increasing injection pressure enablessignificant reduction of soot emissions of the fuel blend with diminishing effects ofthe excessive pressures. Increasing diesel ratio in the fuel blend favors the expandingof the stable combustion to the even lower load limit with compromising of highersoot emissions.Comparisons between the modes of highly premixed charge combustion (HPCC)and blended fuel LTC fuelled with gasoline and diesel has been made to explore thecombustion characteristics of different combustion modes and their effects onperformance and emissions with the same fuel. The results show that, the in-cylinderdistributions of both mixture concentration and chemical reactivity of the HPCCmode with the early injection strategy (E-HPCC) are relatively homogeneous.Mixture stratifications in the HPCC mode with late injection strategy (L-HPCC) andLTC mode persist to various degrees. E-HPCC closely resembles the dual-fuelhomogeneous charge compression ignition (HCCI) in that the combustion process isdetermined by the low temperature reactions of the diesel. L-HPCC is a kind ofQuasi-HCCI (QHCCI) with premixed gasoline and air mixture being multi-pointtriggered by the compression-ignited diesel, delivering the lowest heat release rateconsequently. The fuel stratification in LTC mode leads to a fast heat release and ahigher MPRR, and the simultaneous reactions of gasoline and diesel in regions ofhigher mixture concentration. The reduced NOXand soot emissions of E-HPCC canbe attributed to the homogeneous charge. The unburned HC emissions mainlyoriginate from the gasoline trapped in the crevice volume. With a fixed CA50, theheat transfer loss of LTC is the highest, the exhaust heat loss of L-HPCC is the highest,and the combustion loss of E-HPCC and L-HPCC are all higher than that of LTC,with the indicated thermal efficiencies of the three combustion modes being similar.