Experimental And Numerical Investigation Of Homogeneous Charge Induced Ignition Combustion On A Heavy-Duty Engine

The process involved in Homogeneous Charge Induced Ignition(HCII)combustion includes a port injection of high-volatile fuel(e.g.gasoline)to form a homogeneous charge and direct injections of high ignitable fuel(e.g.diesel)near the top dead center to trigger combustion.Compared to conventional spark ignition engines,higher thermal efficiency can be achieved in HCII combustion.The NOx and soot emissions are much lower in HCII combustion compared to conventional compression ignition engines.Therefore,HCII combustion tends to be a controllable,low-emission and high-efficient strategy for heavy-duty engines.In this work,the control strategy for HCII combustion over the whole operating map was optimized by bench experiments,while the HCII combustion process and emission formation were studied by numerical simulation.The effects of engine control parameters including the gasoline ratio,the diesel injection timing/pressure and the Exhaust Gas Recirculation(EGR)ratio on the combustion and emission characteristics of HCII combustion were studied on a heavy-duty multi-cylinder engine.It was found that the HCII combustion phasing and mode were directly controlled by the diesel injection timing.At low loads,the gasoline ratio and the EGR ratio should be kept at a relatively high level to achieve ultra-low NOx and soot emissions and high efficiency.At medium and high loads,the gasoline homogeneous charge tended to auto-ignite,resulting in excessively rapid heat release and limited gasoline ratios.Based on the optimization results with different compression ratios,the injection and control strategy for HCII combustion over the whole operating map was proposed.It was found that the switch between different fueling strategies mainly depended on the engine load.By lowering the compression ratio,the gasoline auto-ignition at medium and high loads were suppressed and the gasoline ratio was obviously extended;thus the NOx and soot emissions could be further reduced.In this way,the cycle-averaged regular emissions of HCII combustion could meet the Euro V emission standard simultaneously in the European Stationary Cycle test with a simple after-treatment device(Diesel Oxidation Catalyst,DOC).The effects of blended oxygenated fuels on HCII combustion were also studied.By blending ethanol into gasoline,the reactivity of the port-injection fuel became much lower.Thus,the gasoline auto-ignition at high loads could be suppressed.E10 turned out to be a reasonable blend ratio.By blending Polyoxymethylene Dimethyl Ethers(PODE_n)into diesel,the cetane number of the direct-injection fuel increased.Therefore,the NOx emissions slightly increased,while the soot,THC and CO emissions are significantly reduced.In order to develop a better understanding of the HCII combustion process,a reduced11-component(n-heptane,iso-octane,toluene,ethanol,methanol,n-decane,n-dodecane,n-hexadecane,diisobutylene,cyclohexane and methyl-cyclohexane)chemical mechanism consisting of 178 species and 758 reactions was proposed for combustion and soot formation predictions of the gasoline and diesel fuels.Furthermore,by iterating different mechanism reduction methods,a reducd PODE_n chemical mechanism was built(47 species and 139 reactions).The multi-component mechanism and the PODE_n mechanism were extensively validated against experimental data of ignition delay times,laminar flame speeds,species concentration profiles and engine HCCI combustion experiments.The numerical investigation of HCII combustion was conducted with a three-dimensional Computational Fluid Dynamics(CFD)model built in CONVERGE.It was found that,compared to simple surrogate fuels,better simulation results could be achieved with multi-component surrogate fuels.Then,the combustion process and emission formation of HCII were numerically studied in detailed.Moreover,the effects of engine parameters such as the diesel pilot injection timing,the EGR ratio,the intake temperature and the compression ratio on the HCII two-stage heat release were studied.In addition,numerical investigation of the combustion and emission characteristics of PODE_n blended HCII combustion was performed.