| Automobile is a convenient tool for human’s travel,it has become a inseparable part of our life,at present,the fuel of automotive is almost traditional oil-based energy,with the decline of the oil reserves,the automotive industry is facing a severe challenge,in order to alleviate the energy dependence on oil and reduce vehicle exhaust pollution to the environment,using efficient combustion methods and searching for clean renewable energy is imperative.Hydrogen as a green renewable clean energy,based on the characteristics of its high self-ignition temperature and low ignition energy,it’s more suitable for spark ignition engine.Hydrogen has the advantages of fast diffusion speed and rapid flame propagation speed,mixing hydrogen in the gasoline engine can promote the complete combustion of fuel and shorten the combustion duration to achieve rapid combustion and improve the thermal efficiency.In addition,hydrogen direct injection in the cylinder forming stratified mixture can solve the problem of low efficiency of traditional gasoline engine,with the help of low ignition energy,rapid flame propagation speed and wide fire boundary of hydrogen,the mixture can be ignited in a wide range of concentration gradient,a lean burn gasoline engine with hydrogen direct injection can achieve stratified combustion easier,at the same time it can improve the fuel economy of the engine and reduce engine emissions.Based on the project from National Natural Science Foundation “Study on Energy-saving Mechanism of HEV Equipped with Lean-burn Gasoline Engine with Hydrogen Direct Injection System and with Local Hydrogenous Mixture”,aiming at improving the characteristics of combustion,increasing the dynamic and economical performance of the engine while reducing the engine emissions by using hydrogen direct injection in the cylinder.In this paper,the effects of hydrogen injection timing and hydrogen injection pressure on the formation and distribution of mixture were studied,and the influence of different hydrogen injection pressure with different conditions on the stratification of the mixture was studied,at the same time,the contrastive analysis of the combustion and emission performance between a lean burn hydrogenation engine with excess air ratio of 1.5 and a gasoline engine with the theoretical air-fuel ratio was studied.The main conclusions of this thesis are as follows:(1)Hydrogen injected into the cylinder during the compression stroke could form a stratified mixture in the cylinder at the ignition timing,and the rich mixtuue near by the spark plug,and gradually became thinner in the axial direction of the combustion chamber,it was advantage for the burn of the engine.The boundary of stratified distribution and uniform distribution of mixture was at 135 ° CA BTDC,when hydrogen injection timing was later than 135 ° CA BTDC(including 135 ° CA BTDC),the mixture would be effectively stratified,while the in-cylinder mixture was homogeneously distributed before 135 ° CA BTDC.(2)When hydrogen injection timing advanced from 75 ° CA BTDC to 135 ° CA BTDC,with the advance of hydrogen injection timing,the cylinder peak pressure increased first and then decreased,and the crank angle corresponding to the cylinder peak pressure advanced first and then delayed,the maximum temperature in the cylinder also increased first and then decreased,and the duration of the high temperature increased first and then decreased,when the hydrogen injection timing was 105 ° CA BTDC,and ignition timing was 20 ° CA BTDC,the stratified state of mixture was most effectively,and the combustion condition is the best.When the ignition timing was constant,with the advance of hydrogen injection timing,NOX emissions increased first and then decreased,and the HC emission decreased first and then increased,both of which reached extreme value when hydrogen injection timing was 105 ° CA BTDC,hydrogen injection timing had a little effect on CO emissions,CO emissions have been at a very low level.(3)With the increase of hydrogen injection pressure,the cylinder peak pressure increased first and then decreased,and the crank angle corresponding to the cylinder peak pressure advanced first and then delayed,the maximum temperature in the cylinder also increased first and then decreased,and the duration of the high temperature increased first and then decreased,when hydrogen injection pressure was 5MPa,the stratified state of mixture was most effectively,and the combustion condition is the best.When the other conditions under certain conditions,and hydrogen injection pressure was 5MPa,NOX emissions were maximum,HC emissions were minimum,hydrogen injection pressure had a little effect on CO emissions,CO emissions have been at a very low level.(4)The effect of hydrogen injection pressure on the performance of the engine was greater than that of hydrogen injection timing,the maximum effect of the hydrogen injection timing on the cylinder peak pressure was 3.56%,meanwhile the maximum effect of the hydrogen injection pressure was 18.54%.For the impact on NOX emissions,ignition timing was the most influential,and hydrogen injection pressure was the second,the impact of hydrogen injection timing was the least.While for the impact on HC emissions,hydrogen injection pressure was the most influential,hydrogen injection pressure was the second,the impact of ignition timing was the least.CO emissions had been at a very low level,it indicated that the oxygen concentration in the cylinder was the most important factors which affected CO emissions.(5)With the increase of the engine speed,the optimum hydrogen injection was increasing,when the engine speed is 1200r/min and 1500r/min,the best hydrogen injection pressure was 5 MPa,while when the engine speed increased to 1800r/min,the best hydrogen pressure increased to 6MPa.With the increase of the engine speed,the turbulent flow in the cylinder became stronger and the flow rate of the mixed gas became faster,higher hydrogen injection pressure was needed to increase penetration and shorten the hydrogen pulse width,so that it could form a stratified mixture in the cylinder,and the rich mixtuue near by the spark plug,and gradually became thinner in the axial direction of the combustion chamber;under the conditions of hydrogen blended,at the small load,the hydrogen injection pressure had greater impact on combustion and emission performance of the engine,with the increase of load,the maximum effect of hydrogen pressure on the average effective pressure was 10.47%,7.07% and 3.44%,while the maximum effect on the cylinder peak pressure was 29.39%,18.54% and 2.39%.(6)Comparison of hydrogen-blending engine and pure gasoline engine,it could be found that when there was a small amount of hydrogen injected in the cylinder,even in the excess air coefficient of 1.5 under the rare conditions,the cylinder peak pressure was higher than pure gasoline engine under the relative theoretical air-fuel ratio,and thermal efficiency could be increased by 16.92%.What’s more,NOX emissions of hydrogen-blending engine were only half of the pure gasoline engine,HC emissions remained at a similar level,and CO emissions was almost "zero emissions ",it indicated that a lean burn engine with hydrogen direct injection and mixture stratifed could maintain a certain power output and increased thermal efficiency and reduced engine emissions.(7)The improvement of the combustion performance of the engine was better under the medium and small load when hydrogen blending,and the advantage of the improvement of thermal efficiency was better under the medium and small load.Under the small and medium load,the NOX emissions were only half of the pure gasoline engine,in the high-speed load,NOX emissions of hydrogen-blending engine were higher than the pure gasoline engine;HC emissions of hydrogen-blending engine were lighter than pure gasoline engine under the theoretical air-fuel ratio,but the CO emissions from the hydrogen-blending engine were almost "zero-emission" compared to the pure gasoline engine,and the CO emissions were reduced to a greater extent as the load increased. |
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