Influence Of Post-injection Strategy On The Performance And Exhaust Emissions Of Diesel Engines

Post injection is currently one of the effective approaches to reduce the exhaust emissions of diesel engines,which significantly influences the engine performance and exhaust emissions.In the present study,a diesel engine with high pressure common-rail system was used to explore the effect of post injection strategy on the engine performance and exhaust emissions.The fuels used were neat diesel,biodiesel and F-T diesel,respectively.The results obtained possess not only theoretical significance to understand the nature and formation mechanism of diesel particulates under post injection conditions,but also potential to guide the purification technology applications.The major work and achievements of this dissertation are listed as follows:Post injection has significant effect on the in-cylinder combustion process.The heat release curves are unimodal for the single injection conditions,while the heat release curves are bimodal for the post injection conditions.When the rest control parameters of post injection are held constant,increasing the main-post interval?MPI?retards the appearance of the heat release peaks of post combustion,and the peak value increases with the post injection mass?PIM?.Due to the introduction of post injection,the accumulated heat release curves show an obvious sidestep shape,and the in-cylinder gas mean temperature increases markedly during post injection combustion period.When the rest control parameters of post injection are held constant,for all the tested fuels,the increase in the MPI brings about a gradual reduction of combustion thermal efficiency.The increase in the PIM improves the combustion thermal efficiency under some specific engine operating conditions.Under the same main/post injection conditions,the combustion thermal efficiency of post injection fuel followed the sequence:neat diesel<biodiesel<F-T diesel.When the rest control parameters of post injection are held constant,for all tested fuels,increasing the MPI results in an increase in the CO and THC emissions and a decrease in NOx emissions.The CO,THC and NOx emissions gradually increase with the PIM.When neat diesel,biodiesel and F-T diesel are used under the same engine operating conditions,the CO and THC emissions follow the sequence of neat diesel>biodiesel>F-T diesel,and the NOx emissions are in the sequence of F-T diesel>biodiesel>neat diesel.When the rest control parameters of post injection are held constant,the PM emissions for the three fuels first decrease and then increase with the MPI,and increase with the PIM.Under the same main/post injection conditions,the PM emissions for all three tested fuels are in the sequence:neat diesel>biodiesel>F-T diesel.There is an optimal post injection control strategy,under which the post injection combustion efficiency and the exhaust emissions reduction are improved.The strategies aforementioned are 10¹6%post injection rate?ratio of post injection mass to the total injection mass?and 12°CA of MPI for neat diesel,10¹6%post injection rate and 13°CA of MPI for biodiesel and 10¹6%post injection rate and15°CA of MPI for F-T diesel.The soot quantity generated from post injection combustion is related to the unburnt gas temperature(T_{ug EoI})and the ratio of the duration of post injection to the apparent combustion time?DoI/ACT?.There are soot thresholds for the post injection combustion.That is,the post injection combustion cannot generate soot when the T_{ug EoI} or DoI/ACT is lower than the threshold.The thresholds for T_{ug EoI} and DoI/ACT show marginal change for different test fuels.Moreover,the soot quantity generated from post injection combustion can be calculated by an empirical correlation of T_{ug EoI} and DoI/ACT.When the rest control parameters of post injection are held constant,the amount of aliphatic C–H surface functional groups?SFGs?decreases and then increases with the MPI,while it increases with the PIM.The equivalent ratios of C-H SFGs are in the range of 0.202⁰.520.The O/C ratio,the amount of hydroxyl SFGs and the sp3/sp2 hybridization ratio decrease first then increase with the MPI,and they increase with the PIM.The values of these three soot parameters are in the sequence of biodiesel>neat diesel>F-T diesel under the same engine operating conditions.For all three fuels,the light-off temperatures,burn-out temperatures and apparent activation energies of soot particles are in a range of 439⁵26.9?,592.5⁶33.5?and 153.6¹69.4 kJ/mol,respectively.For the constant post injection parameters,the soot oxidation reactivity decreases first and then increases with the MPI and decreases with the PIM.Under the same engine operating conditions,the sequence of soot oxidation reactivity for the fuels is biodiesel>neat diesel>F-T diesel.