Numerical Simulation Study On Effect Of Intake Conditions On Diesel Low Temperature Combustion

Both NOxand soot formation mainly depend on the local combustion temperatureand local equivalence ratio. In the conventional diesel engine with high localcombustion temperature, NOxand soot generation regions are difficult to besimultaneously avoided by only altering equivalence ratio, so low temperaturecombustion (LTC) strategies are proposed. In LTC strategies, NOxand soot emissionsare both minimized irrespective of the local equivalence ratio because of very lowcombustion temperature. LTC strategies are generally realized by using exhaust gasrecirculation (EGR).In this paper, a three-dimensional numerical simulation model was built by usingKIVA-3V software based on a six-cylinder diesel engine. Using this model, the effectof EGR levels on in-cylinder combustion and emissions was investigated. Threeintake gas compositions with different heat capacity which represented low capacity,medium capacity and high capacity were designed in order to investigate the EGR’sdilution effect and thermal effect separately. The oxygen concentrations of each intakecomposition changed from8%to18%. In addition, the effects of EGR temperatureand intake pressure on low temperature combustion were also studied.The results showed that the in-cylinder peak temperature decreased with theincrease of EGR levels. NOxemissions can be reduced significantly by increasingEGR levels. As EGR levels increased, the soot emissions increased first and thenreduced (Soot-Bump). Lower soot emissions for the lower EGR levels were due tohigher soot oxidation rates, and lower soot emissions for very high EGR levels weredue to lower soot formation rates. The EGR’s thermal effect changed combustionprocess mainly through its effect on ignition delay. The main reason that EGR canrealize low combustion temperature was EGR’s dilution effect. Because of thepresence of EGR’s thermal effect, the Soot-Bump moved toward lower EGR levels.As the EGR temperature increased, the intake gas temperature increased which madethe in-cylinder temperature at the time of injection increase. The effect of thermalthrottling was significant and increased as EGR temperautre increased to higher levels.Increasing EGR temperature could make a reduction in ignition delay especially forvery high EGR levels. Because the effect of EGR temperaure on the peak combustiontemperaure was limited, the difference of NOxemissions when using different EGR temperaure was very little. The Soot-Bump moved toward higher EGR levels with theincrease of EGR temperature, which made it difficult to realize low temperaturecombustion. Higher boost pressure raised the ambient in-cylinder density during theinjection and combustion processes, which brought the increase of in-cylinder oxygenamount. Intake pressure could significantly affect the ignition delay especially forvery high EGR levels. The increase of intake pressure would result in less premixingat the time of igntion. The peak combustion temperature decreased as the intakepressure increased at low EGR levels, however, the peak combustion temperatureincreased as the intake pressure incerased at medium and high EGR levels. Thechanges of NOxemissions corresponded with the peak combustion temperature. TheSoot-Bump moved toward higher EGR levels when the intake pressure increased.