Research On Combustion And Emission Characteristics Of An N-butanol/Gasoline Combined Injection Engine Based On Hydrogen Direct Injection

In the context of the dual pressure of fossil energy consumption and air pollution on automotive industry,the application of clean and renewable alternative fuels based on existing efficient and low-pollution combustion technologies has gradually become another development trend to improve the traditional spark ignition engines.In this thesis,according to the analysis of the physico-chemical properties of n-butanol and hydrogen,a new combustion control strategy,namely n-butanol/gasoline(main working fuel)port injection combined with hydrogen(gas auxiliary fuel)direct injection method,has been put forward based on the experimental study of n-butanol/gasoline through combined injection technique.Furthermore,the combustion and emission characteristics of the proposed mode were extensively studied and multiply evaluated by means of experiments,so as to fully combine the technical route of combined injection with the respective advantages of different fuels'physico-chemical properties as well as explore a reasonable hydrogen direct injection strategy.In this way,making the n-butanol blending gasoline as alternative fuel instead of pure gasoline become an effective and implementable way to relieve the dependency on fossil fuels,obtain favorable combustion characteristics and reduce exhaust emissions.The main research work and conclusions in this thesis are as follows:Firstly,a test and control platform of n-butanol/gasoline combined injection engine was built to experimentally study the influence rules of n-butanol volume ratio and fuel distribution ratio on the in-cylinder combustion state and emission level of the engine.The results show that on the basis of the optimal n-butanol volume ratio,the combination of combined injection is an effective method to further the MEP,shorten the flame development duration and rapid combustion duration,increase the peak cylinder pressure and heat release rate,reduce the HC and CO emissions as well as the total particle number concentration under medium and small load condition.In the range of test conditions,the optimum combustion quality is obtained under25%n-butanol volume ratio with 20%fuel distribution ratio.Secondly,an independent hydrogen direct injection supply system was installed on theexisting engine platform to realize the combined injection mode of n-butanol/gasoline port injection with hydrogen direct injection.The influence of hydrogen injection strategy on the distribution of in-cylinder mixture and its improving effect on combustion and emission characteristics of the butanol/gasoline engine under different operating conditions of medium and low speed,medium and small load were quantitatively studied.The results show that a stratified distribution of hydrogen,which is concentrated around the spark plug and gradually dilute outward in the combustion chamber,can be formed through hydrogen direct injection during the compression stroke.Then plus the desirable properties of hydrogen like low ignition energy and high flame propagation speed can effectively ensure the stable ignition and concentrated combustion of n-butanol/gasoline mixture.Furthermore,the injection timing and injection pressure of hydrogen jointly determine the quality of the stratification of hydrogen.So under the condition of constant n-butanol volume ratio and total heat value per cycle,a reasonable hydrogen injection strategy can give full play to the ignition and combustion-supporting characteristics and significantly improve the engine power and reduce emissions.In addition,the distribution of hydrogen is also susceptible to the change of flow field in the cylinder.So different optimal hydrogen injection strategies need to be adjusted to match different speed and load conditions,and the improvement effect of hydrogen addition on combustion performance can be more prominent under low speed and small load conditions.Thirdly,the influence of hydrogen addition ratio on combustion and emission characteristics of the butanol/gasoline engine under different n-butanol volume ratio conditions were investigated to analyze the synergistic effect of hydrogen and n-butanol more comprehensively.The analysis of the separate effect of hydrogen addition shows that the improvement of combustion performance of n-butanol/gasoline engine is mainly reflected in whether hydrogen is added or not,and the sensitivity to the proportion of hydrogen is relatively low.The analysis of the synergistic effect of hydrogen and n-butanol shows that the promoting effect on combustion performance such as increasing the laminar flame propagation speed after adding a small amount of n-butanol can weaken the positive impact of hydrogen.However,under the higher n-butanol volume ratio conditions,the power performance and fuel economy of n-butanol/gasoline engine can be further improved through continuously increasing the hydrogen addition ratio to offset the serious negative effect of low saturation pressure and high latent heat of vaporization caused by n-butanol.This also indicates that the promoting effect of hydrogen addition is more obvious on n-butanol/gasoline engine with higher n-butanol volume ratio,which is one of the effective methods to expand the tolerance of engine to n-butanol.Finally,on the basis of the above-mentioned reasonable direct injection strategy and hydrogen injection strategy,the influence of n-butanol/gasoline combined injection mode and hydrogen direct injection combined with n-butanol/gasoline mode on lean burn characteristics of the engine under different excess air coefficients was investigated,so as to further explore the potential of improving the thermal efficiency and reducing emissions of n-butanol/gasoline engine.The results show that the combined injection technology or hydrogen auxiliary fuel can both effectively reduce the disturbance of the flame core caused by the change of the concentration of the surrounding mixture and the air flow in the cylinder,offsetting the problems of the instability and long formation time of the flame core and the uneven combustion process of the n-butanol/gasoline lean mixture,which can significantly improve the lean combustion stability of n-butanol/gasoline,as well as reduce the CoV_pmiand greatly extend the lean combustion limit.Furthermore,when the excess air coefficient is 1.2,the hydrogen direct injection combined with n-butanol/gasoline mode can observably increase the effective thermal efficiency and notably reduce the HC and CO emissions and the total particle number concentration under the premise that the MEP is basically unchanged.Hence,the combination of lean combustion and hydrogen direct injection can be used as an effective control strategy to further improve the thermal efficiency and reduce emissions of the n-butanol/gasoline engine under the premise of ensuring power performance.