Numerical Study On The Effect Of Diluted Mixture Distribution In The Cylinder On The Combustion Characteristics In A Two-stroke Gasoline Engine At High Loads

Poppet-valve two-stroke gasoline engines can increase the specific power of their counterparts with the same displacement and hence decrease fuel consumption.However,knock may occur in the cylinder at high loads.To solve the knock in the two-stroke engines,the combustion with stratified lean mixture was proposed to decrease knocking tendency and improve combustion stability.Based on the distribution characteristics of the mixture in the cylinder of the two-stroke engines,a simplified n-heptane and isooctane gasoline surrogate chemical reaction mechanism including 45 species and 57 elementary reactions was built.The reaction paths of n-heptane and isooctane in exhaust-diluted conditions at high temperature were analyzed.On this basis,this simplified mechanism was coupled with a three-dimensional simulation software.Finally,the effect of intake pressure,ratio of the second injection and intake valve timing on the combustion characteristics and nitrogen oxide emissions of a poppet-valve two-stroke engine was numerically simulated.The results can be drawn as follows:In the case of the mixture highly-diluted by exhaust gases,high temperature consumption rate of isooctane is lower than n-heptane.The effect of residual gas on H atom abstraction in isooctane is slightly lower than that in n-heptane.With the increase of residual gas fraction,the pyrolysis reactions of isooctane increase while it slightly changes for n-heptane.With the increase of the second injection ratio at fixed intake pressure,ignition timing slightly delays first and then advances.In the meanwhile,combustion duration first increases and then decreases.But the fuel-air equivalent ratio around spark plug first slightly decreases and then rises.At the second injection ratio of 70%,the fuel distribution in the cylinder promotes flame propagation.With the increase of intake pressure at a given second injection ratio,ignition timing advances,combustion duration slightly decreases first and then increases while the maximum pressure rise rate first rises and then drops.High intake pressure can prevent the occurrence of auto-ignition and decrease the tendency to knock through decreased fuel-air equivalent ratio around the cylinder.With the delay of intake valve closing timing(IVC),the inhomogeneity of the mixture in the cylinder first decreases and then increases.And the inhomogeneity of temperature and residual gases in the cylinder rises gradually.The temperature of the rich mixture with low residual gas rate increases gradually.As the IVC is retarded,the flame propagation speed first reduces and then increases.When IVC timing is at 90°CA before top dead center,the flame propagation speed is the fastest.In stratified lean combustion mode,nitrogen oxide distributes at lean mixture region where the equivalent ratio is less than 1.