Research Of N-butane Concentration On Knock Severity Of Natural Gas SI Engine

The automobile industry’s consumption of petroleum resources and environmental pollution cause the demand for alternative fuels is increasing day by day.As a clean alternative fuel,the advantages of natural gas conclude abundant reserves and low C / H ratio.The composition of natural gas is different in different regions,so the natural gas engine’s compression ratio is designed to be smaller to reduce the knock tendency.The low compression ratio inhibits the natural gas engine from further improving fuel economy.Therefore,this paper studied the effect of n-butane concentration on the combustion process and knock severity of natural gas engines at different speeds through experiments,The effect of n-butane’s concentration on the combustion characteristic and knock of a heavy-duty natural gas engine at different equivalence ratios was further studied through numerical simulation.the research content and primary conclusions are shown as follows:(1)The effect of n-butane concentration on the natural gas engine combustion process at different speeds and full load was studied.it found that as the concentration of n-butane increases,the maximum in-cylinder pressure and heat release rate increase continually,and the corresponding crank angle of them both advances,while the 50% combustion position advances and the combustion duration shortens.In addition,the increase in engine speed and the effect of n-butane on the pressure increase and heat release process are more and more pronounced.Under high-speed operating conditions,the CA50 and combustion duration are more sensitive to the increase in n-butane concentration.(2)the knock severity evaluation index of natural gas engine and the corresponding threshold are quantitatively analyzed.Through investigation of cyclic pressure oscillation,maximum pressure rise rate and maximum amplitude of pressure oscillation under different speeds and n-butane concentration.The threshold value of the maximum pressure rise rate and the maximum amplitude of pressure oscillation for light knock and heavy knock is proposed.Combined the probability of knock cycle,the severity of knocking under the operating conditions is quantitatively evaluated.During the experiment,it is found that heavy knock caused the erosion of the aluminum piston crown.(3)The effects of n-butane on the combustion and knocking of natural gas engines at different equivalence ratios were studied.The software was used to establish the three dimensional model for combustion system,combining the temperature field in the cylinder,the combustion process and the pressure rise rate to analyze.the study shows that with the increase of the equivalence ratio,the combustion rate and the auto-ignition area of the terminal mixture both increase first and then decrease.By analyzing the effect of n-butane concentration on the combustion and knocking of natural gas engines and optimizing the knocking evaluation index in this article.This research has certain theoretical and practical significance in achieving efficient combustion of gas engines.