Research On Combustion And Emission Characteristics Of A Natural Gas Engine

Natural gas,as an important alternative fuel,has been widely used in internal combustion engines.However,natural gas engines for vehicle should meet the stringent emission standards.In order to protect ecological environment,the China VI emission standards for vehicle engines will soon be implemented.Compared with China V emission standards,nitrogen oxides(NO_X)emission limit in the China VI emission standards will be reduced by about 80%.To meet the engine emission regulations in different stages,natural gas engines can select two different technology routes,i.e.,lean-burn and stoichiometric mixture.Therefore,the combustion and emissions characteristics of a natural gas engine were studied on the basis of simulation and experiments.In this study,the models for a lean burn/stoichiometric natural gas engine were established and validated.Then,the combustion processes in the cylinder and the generation of NO_X were simulated and analyzed to find the potential for the improvement of the natural gas engine's performances in the lean-burn/stoichiometric conditions and possible technical solutions for the development of a natural gas engine.Meanwhile,the emission characteristics and fuel consumption of the gas engine at lean-burn and stoichiometric operating conditions were experimentally investigated.The results show that,With the advance of ignition timing,the gas consumption rate and exhaust gas temperature decrease while NO_X and unburned hydrocarbon(HC)emissions increase.Advanced ignition timing leads to increased carbon monoxide(CO)emissions at lean-burn conditions while it has slight effect on CO emissions at stoichiometry.Excess air coefficient is an important parameter influencing the emissions of the lean-burn natural gas engine.Increased excess air coefficient can effectively reduce NO_X emissions and exhaust gas temperature,while it leads to increased HC and CO emissions.There is an optimal excess air coefficient at each operating condition for the lowest gas consumption.Exhaust gas recirculation(EGR)is an important factor for the natural gas engine with stoichiometric mixture.EGR can effectively reduce NO_X emissions and exhaust gas temperature while it slightly reduces CO emissions and almost does not change HC emissions.In addition,the Miller cycle is effective in reducing NO_X and HC emissions due to lengthened combustion duration.In order to obtain optimal emissions and efficiency of the gas engine,ignition timing and excess air coefficient need to be carefully matched.To further improve the gas engine performances,the selection of Miller cycle or EGR is dependent on the gas engine with lean or stoichiometric mixture.