The Simulation Of HCCI Combustion With Phase Controllable On-Board Fuel Reformation

Since the problems of energy crisis and environment pollution are becoming more andmore serious, Homogeneous Charge Compression Ignition （HCCI）, as a new combustionstyle of internal combustion engine, has received extensive concern and been regarded as anessential method to meet the requirements about fuel economy and clean exhaust of people onpower devices in future. For the HCCI engine, the premixed charge gets into cylinder andthen be compressed to igniting by piston. The properties of low temperature combustion,homogeneous combustion and low throttling loss for the HCCI engine guarantee a highthermal efficiency and low exhaust emissions especially low NOx and PM emissions. In spiteof all outstanding advantages of HCCI engine, it has to face the challenges of combustionphase directly controlling and operational range remarkably extending.In order to control combustion phase and extend operation range of HCCI engine, amethod of phase controllable on-board fuel reformation is studied in this paper. The point ofview that HCCI combustion process is dominated by chemical kinetics is already acceptedwidely, and H₂, H, OH, H₂O2, and O are generally regarded as the key radicals in controllingthe HCCI combustion process. The attempt of the method is to try to change the control ofchemical kinetics into the operations of the on-board fuel reforming system for HCCIcombustion. The system includes an independent reformation chamber with an injector and acontrollable valve that connects the reformation chamber and main chamber. The valve isclosed at a phase in expansion stroke, which makes the reformation chamber form into anindependent space. In the space, reforming chemical reactions will start spontaneously afterinjecting some reformation fuel, ethanol, and then a high pressure gas rich in H₂and CO canbe got when the reactions finished. The reformed gas enters into the cylinder to influence theHCCI combustion process during the compression stroke when the valve is opened in nextcycle. All the valve timing and lift of the original HCCI engine will be maintained while thereformation system is added.Based on the idea of the method of on-board fuel reformation, a model to simulate theHCCI combustion with fuel reformation is established by using the reactors of IC （InternalCombustion Engine）, CHR （Closed Homogeneous Reactor） and GM （Gas Mixer） ofcalculation software. The engine parameters are set based on a four-stroke diesel, CT2100Q,and both main fuel and reformation fuel are chose as ethanol while running the model in thispaper. By analysis the calculated results, the findings as followed can be found. The materials of H₂and CO in reformed gas are capable to impact the HCCI combustion, and the effect ofH₂is more significant than that of CO, so H₂can be considered as the target product forrefored gas. The components of reformed gas are hardly influenced by initial pressure butsignificantly influenced by initial temperature and reforming mixture concentration and themaximum fraction of H₂in reformed gas can be reached by optimizing the trapping timingand reforming mixture concentration. The optimized reformed gas does have the ability toinfluence the HCCI combustion process after introduced into cylinder, and the start ofcombustion retards with the postponement of reforming inlet timing. The mixture will notburn immediately after the reformed gas enters into the cylinder, but there is a reformationignition delay between the time of reforming inlet timing and start of combustion, whichprolongs with the retard of reforming inlet timing and shortens with the increase of enginespeed and decrease of intake temperature. The volume of reformation chamber has anoptimized value, while both too big and too small are bad for getting a high-indicated thermalefficiency for HCCI engine. With the help of the phase controllable on-board fuel reformationsystem, the combustion process can be controlled precisely by adjusting parameters needed,and the HCCI engine is allowed to operate under lower intake temperature and higher speedcondition while keeping a high indicated mean pressure and indicated thermal efficiency.