Investigation On Combustion And Emissions Characteristics Of A Hydrogen-Dimethyl Ether Fuelled Internal Combustion Engine Under Different Combustion Modes

It is an inevitable trend for internal combustion engine development to realize carbon neutralization,high efficiency and clean combustion.Traditional spark ignition internal combustion engines have low volume,high power-to-weight ratio and widely used.Compared with using single fuel and single combustion mode,a traditional spark ignition engine can use homogeneous charge compression ignition(HCCI)combustion mode to obtain higher thermal efficiency by fuelling the renewable fuels,zero-carbon hydrogen and carbon-neutral dimethyl ether.Facing the narrow stable operation range of HCCI combustion mode,the engine still could be running in traditional sparkignition and spark-assisted controlled-auto-ignition(SACAI)combustion modes.In this study,the laminar flame and ignition characteristics of hydrogen-dimethyl ether fuels are analyzed by numerical simulation;the effects of fuel injection and spark ignition strategies on combustion and emissions characteristics of a hydrogen-dimethyl ether engine under traditional spark-ignition,HCCI and SACAI combustion modes are investigated by experimental methods.Based on chemical reaction kinetics,the laminar flame and ignition characteristics of hydrogen-dimethyl ether fuels were studied.The results showed that the adiabatic flame temperature of hydrogen-dimethyl ether-air mixtures was decreased with the increase of excess air ratio or dimethyl ether mole fraction,while the laminar flame velocity was slow down.The ignition delay of the mixtures was extended with the increase of excess air ratio or hydrogen mole fraction.It meant that hydrogen could inhibit the auto-ignition and combustion process of dimethyl ether.The rise of initial pressure raised reaction rate of the mixtures.With the increase of initial temperature,the duration of low temperature oxidation and heat release process of dimethyl ether were shortened.The consumption of dimethyl ether and the reaction rate in the negative temperature coefficient range were decreased.Then,the effects of fuel injection and spark ignition strategies on combustion and emissions characteristics were experimentally investigated under traditional sparkignition combustion mode.The results showed that blending dimethyl ether into a hydrogen engine could improve its power output.Indicated mean effective pressure was increased with the rise of dimethyl ether volume fraction.When air excess ratio was1.2,blending 3.4% dimethyl ether could improve the indicated mean effective pressure by 18.3%.The power output could be further improved by adjusting spark timing or using hydrogen split-injection strategies.In addition,NOx emissions could be reduced by dimethyl ether blended,due to lower combustion temperature of dimethyl ether.The reduction influence of NOx emissions was the most obvious when excess air ratio was in the range of 1.1 to 1.3,and the maximum value of NOx emissions reduction was63.3%.However,indicated thermal efficiency was reduced after dimethyl ether blended,yet it was increased when dimethyl ether volume fraction was higher than 3.0%.Then,to improve thermal efficiency of the hydrogen-dimethyl ether engine,effects of the engine control parameters on combustion and emissions properties were investigated under HCCI combustion mode.The experimental results showed that maximum indicated thermal efficiency under HCCI combustion mode was improved by 10.6% compared with the one under traditional spark-ignition combustion mode.Hydrogen could control the HCCI combustion process of dimethyl ether engines.The ignition timing and combustion phases of HCCI could be controlled by adjusting hydrogen and dimethyl enter volume fractions,using adopting hydrogen split-injection strategies,and adjusting the coolant temperature.Higher concentration of dimethyl ether could obtain stronger combustion intensity and increase ringing intensity and combustion noise level.Ether over low dimethyl ether concentration or over high hydrogen concentration could raise the cyclic variation,due to the deterioration of combustion process.Besides,blending hydrogen could reduce HC and CO emissions.NOx emissions were increased due to the higher combustion temperature,yet they were normally not over 200 ppm.Finally,to improve the lean combustion variation under HCCI combustion mode and achieve the transition between traditional spark-ignition and HCCI combustion modes,effects of fuels injection and spark ignition strategies on engine performance under SACAI combustion mode were investigated.The experimental results showed that the engine had highest apparent heat release rate and shortest combustion duration when spark plug ignited the mixtures during the initial section of low temperature oxidation reactions.Thus,the engine could obtain higher power output and thermal efficiency.Indicated thermal efficiency could has 11.2% improved than traditional spark-ignition combustion mode under the same indicated mean effective pressure.The variation of dimethyl ether concentration had a great influence on the combustion intensity.The variation of hydrogen concentration had a great influence on the combustion stability.Furthermore,HC and CO emissions could be reduced with the improvement of hydrogen volume fraction.NOx emissions could be increased with the concentrations rase of hydrogen and dimethyl ether,due to the increase of combustion temperature.Under the same engine speed and manifolds absolute pressure,traditional sparkignition combustion mode had wide adjusting range of indicated mean effective pressure by changing the parameters of fuels injection and spark ignition.The lower limit of the indicated mean effective pressure of HCCI combustion mode could be widened after using SACAI combustion mode.The maximum indicated thermal efficiency of SACAI combustion mode was lower than it under HCCI combustion mode,yet they were significantly higher than the one under traditional ignition combustion mode.