Experimental Study On An HCCI Engine Fueled With N-Heptane

With the growing surge of car ownership, demands for energy are becoming more andmore. To deal with the two major challenges of oil energy lacking and environment pollution,people are forced to continuously seek suitable alternative energy and develop newcombustion technology of internal combustion engine. However, conventional gasolineengine and diesel engine almost have reached the limit in further improving fuel efficiencyand reducing harmful exhaust emissions, in order to break through this limit and meet theneed of energy-saving, an ideal combustion mode named Homogeneous Charge CompressionIgnition (HCCI), which is based on its superiority of high thermal efficiency and ultra-lowNOXand PM emissions, wins more widespread attention in internal combustion engine field.An experimental study on HCCI of n-heptane fuel is carried out on a modified dieselengine to study the HCCI combustion and emission characteristics and engine operation rangeof high cetane number fuel, and the experimental data are analyzed combined with n-heptaneoxidation mechanism. The main conclusions in this paper are listed as follows.The HCCI combustion of n-heptane is characterized by a two-stage heat release process.With the increase of intake temperature and mixture concentration, the peak cylinder pressureand the peak heat release rate are increased obviously, combustion timing is advanced,combustion duration is shortened, the emissions of HC and CO are decreased, and thecycle-to-cycle variations are the minimum when the intake temperature is50℃and theexcessive air ratio is1.9. The increase of engine speed also makes combustion timingadvanced, combustion duration shortened and the emission of CO decreased, but the emissionof HC is increased, the cycle-to-cycle variation is the minimum at the speed of1500r/min.The change rules of the indicated mean effective pressure (IMEP) and thermal efficiencyare basically the same with intake temperature and excessive air ratio. There is an optimalintake temperature for the maximum IMEP and thermal efficiency in each condition ofexcessive air ratio, moreover, the optimal intake temperatur gradually increases with theincrease of the excessive air ratio. In addition, the range of IMEP is widened with the increaseof intake temperature, especially the range of upper limit is widened between30℃and60℃. With the increase of excessive air ratio, the maximum and minimum IMEP both are increasedfirst and decreased then, the range of IMEP is widest when the excessive air ratio is2.5.Increasing the intake temperature has a significant influence on extending the upper limitrange of excessive air ratio, but makes the lower limit range go up. The range of excessive airratio form1.9to3at the temperature of30℃and60℃is the most widely based on therange of intake temperature. This is mainly because increasing intake temperature mayprovide lean mixture enough energy for stable combustion, but it makes rich mixture prone toknock.