| The rapid development of the automobile industry has brought a large amount of consumption of the petroleum resources,and at the same time,the problem of atmospheric pollution has also become increasingly severe.It is necessary to find the alternative fuels that are equipped with abundant reserves and clean combustion characteristics.Natural gas has the characteristics of high calorific value,large reserves,and easy to exploitation.It is a significant alternative energy source to reduce the petroleum demand and mitigate the environmental pollution.The composition of the natural gas is different in different regions.In order to avoid knocking combustion,the designed compression ratio of the nature gas engines is relatively low,which limits the increase of their compression ratio to realize the improvement of combustion and economic performance.Therefore,based on exploring the knocking evaluation index of the natural gas engine,this thesis researches the effects of methane concentration in nature gas and increasing compression ratio.On this basis of the research,the study adopted the way by adding hydrogen to natural gas.It studied the effects of compression ratio and hydrogen addition on the combustion and performance of natural gas engines,and ways to promote the economics of natural gas engines.The main research contents and conclusions are as follows:(1)The knocking evaluation index of the spark ignited natural gas engine was studied,and the maximum pressure rise rate and knock intensity were used as the evaluation index of knock intensity.It was found that the knock threshold of knock intensity in single cycle greater than or equal to 0.4MPa was more reasonable.(2)The influence rule of the compression ratio was studied.When the compression ratio was increased,the maximum explosion pressure,the maximum heat release rate and the rate of pressure rise were all increased,but the coefficient of cyclic variation was decreased.The combustion centre was moved forward and the combustion duration was shortened.The indicated thermal efficiency,brake thermal efficiency increased and effective expansion ratio were all increased,and maximum increase amplitude of brake thermal efficiency reached 4.2%.The maximum engine load was not affected by the hydrogen concentration.The engine brake specific fuel consumption was decreased,whose maximum increase amplitude reached 6.4%.NOx emission was increased.(3)The influence of methane content was studied.When methane content was increased,it had a little influence on mean effective pressure and maximum explosion pressure.The maximum pressure rise rate and knock intensity were decreased,while the combustion duration was extended and the brake specific fuel consumption was increased.Compared with the natural gas which has a methane concentration of 93%,when the natural gas with a methane concentration of 99%was adopted,the engine maximum pressure rise rate and knock intensity were decreased significantly,which indicated that the latter has better antiknock ability.(4)The influence of hydrogen concentration was studied.When hydrogen concentration was increased,the maximum explosion pressure,the maximum heat release rate and the rate of pressure rise were all increased,but the coefficient of cyclic variation was decreased.The ignition delay period and the combustion duration were shortened.When hydrogen concentration was continuously risen,the indicated thermal efficiency and brake thermal efficiency were increased first then decreased.The brake thermal efficiency was increased by 1.8%when under ideal hydrogen concentration conditions.Using a high compression ratio combined with the addition of hydrogen can achieve the maximum increase in effective thermal efficiency of 6%.And the NO_Xemission was increased.Appropriate measures could be taken to reduce NO_Xemissions,such as delaying ignition time or reducing hydrogen concentration. |
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