Investigation On Combustion And Emission Characteristics Of Hydrogen-Assisted Spark Ignition Methanol Engine

Methanol has the characteristics of high octane number and high oxygen content,and can be produced in large quantities from coal,natural gas and biomass.Therefore,methanol is considered to be one of the most promising alternative fuels for automotive engines in China.However,since methanol is a single-component fuel,the lower vapor pressure and high latent heat of vaporization make it more difficult for gas mixtures of methanol engines to form than that of gasoline engines,resulting in problems such as difficult cold start,large cyclic variations,and high emissions of unburned methanol and formaldehyde in methanol engines.Hydrogen has the characteristics of strong diffusion ability,low ignition energy,and fast combustion speed,thus is a good combustion promoter to improve the combustion characteristics of methanol engines.Aiming at the two methanol injection modes of port injection and in-cylinder direct injection,this thesis studied the effects of port injection hydrogen-assisted combustion on the combustion and emission characteristics of methanol spark-ignition engines under the two different injection modes through engine bench tests and three-dimensional CFD numerical simulation.The potential of hydrogen-assisted methanol engines to improve combustion and emissions performance in lean combustion conditions was also investigated.Main research work and conclusions are as follows:?1?The combustion characteristics of a hydrogen-assisted methanol spark-ignition engine under two different combined injection modes of hydrogen port injection+methanol port injection?HPI+MPI?and hydrogen port injection+methanol direct injection?HPI+MDI?were studied.The influence of different combined injection modes on the combustion characteristics of the engine under lean combustion conditions was compared and analyzed.Affected by different combined injection modes,air intake charge of the engine in HPI+MPI mode was slightly reduced,resulting in a slight reduction in combustion performance of the engine in the HPI+MPI mode.Under the condition of homogeneous charge,the hydrogen-blended fuel combustion did not greatly improve combustion stability.The hydrogen-assisted combustion accelerated the combustion process of the engine in the HPI+MDI mode.The cylinder pressure,indicated mean effective pressure,and heat release rate were all higher than those of pure methanol combustion.The combustion support of hydrogen blending significantly improved the combustion stability of the engine under the HPI+MDI mode.The lean burn limit increased from an excess air coefficient of 1.6 in MPI mode to 2.2 in MDI mode with 6%hydrogen blending ratio.The hydrogen-assisted combustion deteriorated combustion performance of the engine under the HPI+MPI mode,but it can make the engine in the HPI+MDI mode obtain better combustion characteristics and combustion stability.?2?The emission characteristics of methanol engines in the HPI+MPI/HPI+MDI modes were studied.The specific emissions of CO,HC and NO_x are mainly determined by the excess air coefficient,and have no obvious relationship with the injection mode.The hydrogen-assisted combustion reduced CO and HC emissions and increased NO_x emissions.However,CO and HC emissions increased slightly under the condition of extra lean mixture.The effect of hydrogen blending on Soot generation was not obvious.?3?The lean combustion characteristics of the engine in the HPI+MDI mode were studied.The stratified charge was formed in the cylinder by late methanol injection,and the characteristics of hydrogen-assisted combustion were used to improve the lean combustion capability of the engine.The combustion characteristics depended on the coupling effect of excess air coefficient,ignition timing and hydrogen blending ratio.As the mixture became leaner,the optimal ignition timing advanced,and range of the ignition angle became narrower.The hydrogen-assisted combustion shortened the ignition time of methanol,accelerated the combustion process,delayed the optimal ignition advance angle,increased the range of ignition angle,and expanded lean burn conditions.Under the coordinated control of the hydrogen blending ratio and the ignition timing,the lean burn limit of the MDI engine can be effectively raised to an excess air coefficient of 3.0.The emissions of CO,HC and Soot were mainly affected by the excess air coefficient and hydrogen blending ratio,while the emissions of NO_x were mainly affected by the ignition timing.?4?The combustion phase control strategy of the engine in HPI+MDI mode was studied.Different combustion phase control modes were analyzed by the maximum cylinder pressure angle(?_Pmax)and combustion heat release center angle(?_CA50).When using the?_pmax ignition timing control method for hydrogen-assisted methanol engines,there was less impact on engine output,and the emissions of CO,HC,NO_x and Soot were lower.The?_Pmax ignition timing control method can obtain better performance especially under the lean mixture conditions.When adopting the?_Pmaxmax ignition timing control method,and taking both power output and emission control into consideration,the optimal working area of the engine is running at the hydrogen blending ratio?=3%⁶%,and excess air coefficient?=1.2¹.8.?5?The three-dimensional CFD of combustion in the engine cylinder was simulated in HPI+MDI mode.The mechanism of hydrogen-assisted combustion and non-regulated emission generation characteristics were studied.The study found that hydrogen blending has an extremely important effect on promoting the growth of free radical pools in the cylinder at ignition time,which can trigger the"chain reaction"of methanol oxidation,make methanol oxidation earlier and more violent,and make flame propagation faster.There was a linear correlation between the growth rate of the fire nucleus and the hydrogen blending ratio in the initial stage of combustion in HPI+MDI mode engine.HPI+MDI mode engine can effectively achieve stratified charge in the engine cylinder.The impact of ignition timing on unburned methanol and formaldehyde was relatively small.The hydrogen blending ratio was the most important factor affecting the mass fraction of OH radicals,unburned methanol and formaldehyde.When the excess air coefficient of the HPI+MDI mode engine was 1.4,there was less unburned methanol and formaldehyde.Increasing the hydrogen blending ratio can keep the unburned methanol and formaldehyde production low in the leaner gas mixture range.Under the condition of optimal ignition timing through combustion phase control by?_Pmaxmax the methanol injection timing was optimal at 635°CA?85°CA BTDC?.