| With the comprehensive implementation of China's "dual carbon" strategy,the problem of vehicle fuel consumption and exhaust emissions has become increasingly serious as vehicle ownership rises.The application of clean alternative fuels has become a significant direction of engine technology research.Among alternative fuels,ethanol fuels have some major advantages including compatibility with conventional engines and the potential to limit greenhouse(GH)emissions,which can be rapidly rolled out using existing industrial systems and has gained extensive attention as a carbon-neutral renewable fuel.However,the high latent heat of vaporization and low saturation vapor of ethanol can lead to difficulties in the formation of mixture incylinder.Also the power performance of the vehicle will be partly affected due to the low heat value of ethanol.In order to solve the inherent issues of ethanol fuel above,this thesis proposes using hydrogen by combined injection method.The hydrogen's remarkable physical and chemical properties can promote effective ignition as well as stable,rapid and full combustion of the engine.In this thesis,a local hydrogen enrichment area near the spark plug can be formed through direct injection,which can be conducive to improving engine performance while avoiding the problems such as inlet backfire.To systematically analyze the characteristics of ethanol/hydrogen combined injection engine,an experimental platform of combined injection engine with hydrogen direct injection and ethanol port injection is designed and established firstly.And a rapid control prototype is independently developed to realize the online open-loop adjustment of parameters.Secondly,through simulation research,the flow field distribution and combustion state in the cylinder are explored and the stratification mechanism of the mixture is explored.Thirdly,based on the simulation results,the engine dynamometer test is carried out to verify the influence of key control parameters on engine combustion and emission performance with the principle of single variable.Finally,an external EGR system is installed to solve the problem of increasing nitrogen oxide caused by hydrogen addition,as well as to find the cooperative optimization strategy of EGR rate and hydrogen blend ratio under different operating conditions.The main research work and conclusions are as follows:1.A CFD simulation model was established to explore the influence of different direct injection strategies and ignition strategies on the in-cylinder mixture distribution and combustion state.The results show that the injection strategy has a greater impact on the stratified state of the mixture in the cylinder.For ethanol/hydrogen engines,the ideal hydrogen distribution state should be local enrichment near the spark plug,which can be conducive to improving combustion through the low ignition energy and fast flame propagation speed of hydrogen.Moreover,the direct injection timing from 60°CA BTDC to 75°CA BTDC is beneficial to the formation of the local concentrated area of mixture.2.The effects of hydrogen blend ratio,hydrogen injection timing,hydrogen injection pressure and ignition timing on the combustion state in the cylinder under different engine operating conditions were explored.It is concluded that a small proportion of hydrogen can shorten the ignition delay,increase the maximum heat release rate and indicate mean effective pressure and make the engine combustion more stable.When the hydrogen blend ratio is greater than 10%,the improvement amplitude of engine power performance will be decreased.Due to the increase of flame propagation speed after hydrogen addition,the ignition advance angle needs to be postponed appropriately to prevent the increase of engine negative work.When the direct injection pressure is 5MPa,the hydrogen spray has adequate initial speed and penetration distance,which is not easy to be diverged by the influence of air flow and beneficial to the formation of the local hydrogen-rich stratified state.When the hydrogen injection time is 60°CA BTDC and 75°CA BTDC,it can not only ensure the hydrogen concentration near the spark plug,but also make the distribution of the remaining hydrogen in the cylinder more uniform.Under different working conditions,with the increase of engine speed and load,the power performance and economic benefits brought by the increase of hydrogen blend ratio decrease continuously,so as its impact on combustion becomes less and less obvious.3.The effects of hydrogen blend ratio,hydrogen injection timing,hydrogen injection pressure and ignition timing on engine emission characteristics under different operating conditions were investigated.With the increase of hydrogen blend ratio,the carbon monoxide and hydrocarbon emissions decrease continuously as the result of the increase of temperature in the cylinder and the shortening of wall quenching distance.5MPa hydrogen injection pressure with 60° CA BTDC hydrogen injection timing is conducive to the formation of stratified mixture in the cylinder,which can improve the combustion process and make the oxidation of emission products more complete.The carbon monoxide and hydrocarbon emissions can continuously be reduced,but the nitrogen oxides emission is increased.Furthermore,the addition of hydrogen can effectively reduce the production of large particulates.As the hydrogen mixing ratio increases,the amounts of particulate emissions are continuously reduced and the particle size shows a downward trend.4.Under lean burn conditions,hydrogen mixing can ensure the stability of flame core formation and achieve the effect of extending the lean burn limit.It is difficult for the original ethanol engine to ensure stable combustion when the excess air coefficient is 1.3.However,the combustion state in the cylinder can be more stable under the same excess air coefficient by adding hydrogen,as well as the occurrence of misfire can be significantly reduced.Hydrogen mixing can also increase the flame propagation speed and the constant volume level of lean combustion,thereby making up for the power loss.When the excess air coefficient is 1.3,the 10% hydrogen blend ratio can obtain a similar power performance as the original engine under equivalent air-fuel ratio.Meanwhile,the carbon monoxide and hydrocarbon emissions are at the lowest level,and the generation of nitrogen oxide emissions is inhibited due to the decrease of the temperature in the cylinder.In terms of unregulated emissions,the formaldehyde content of ethanol engine increases slightly with the increasing hydrogen ratio,while the acetaldehyde,ethanol and 1,3-butadiene content all decrease.Overall,hydrogen addition is an effective method to reduce the unregulated emissions of ethanol fuel engine.5.The effects of different hydrogen blend ratios and EGR rates on engine combustion and emissions characteristics were investigated.When the excess air coefficient is 1.0,the ethanol engine can obtain a better power performance while using a large EGR rate due to its high tolerance to the EGR rate.Moreover,the large EGR rate can also solve the issue of increasing nitrogen oxide emissions caused by the high proportion of hydrogen addition.In the case of excess air coefficient is 1.2,the tolerance of the ethanol engine to EGR can be improved by a small amount of hydrogen,but the improvement effect of continuously increasing the hydrogen blend ratio on the stability is not obvious.The condition of 5% hydrogen blend ratio combined with 15% EGR rate can be considered as an optimal control strategy,which can not only synergistically play the role of hydrogen to promote combustion and EGR to reduce pumping loss,but also reduce the surge of nitrogen oxides caused by hydrogen mixing through EGR. |
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