Experimental Study On Combustion And Emission Characteristics Of Methane-gasoline Dual Fuel SI Engine

Dowansizing and enhancement is an important development direction of thermal efficiency improvement for modern gasoline engine,but gasoline engine brings knocking problem under large load conditions by adopting high intake pressure.Methane has higher octane number and better antiknock performance,which is helpful to solve the problem of knock on engine.At the same time,the H/C of methane is relatively high,and there is almost no emission of particulate matter.Therefore,methane-gasoline dual-fuel engines are considered to have good combustion and emission characteristics.However,the current research on the influence of key engine parameters on the combustion and emission characteristics of methane-gasoline dual fuel is not very clear.Therefore,the combustion and emission characteristics of methane-gasoline dual-fuel engine under different operating conditions were studied based on single-cylinder and four-cylinder SI engine.First of all,in order to study the basic influence of engine key parameters on the combustion and emission characteristics of methane-gasoline dual fuel engine,the influence of parameters such as intake temperature,intake pressure and compression ratios were studied based on a single-cylinder engine with full throttle condition,fueled with gasoline,methane and methane-gasoline dual fuels respectively.The results show that:Under low compression ratio,lean burn and fuel stratification conditions,the methane-gasoline dual fuel ratio is shortened compared with that of gasoline under low methane blending ratio,and the cylinder pressure phase is advanced.The torque of the common gasoline engine is reduced due to the knock limit by raising the compression ratio.This study shows that adding methane at high compression ratio can increase the torque by inhibiting knocking and effective thermal efficiency;increase the intake pressure allows methane-gasoline dual fuel engine gets more torque;increasing the intake air temperature has less effect on the knock tendency but decreases the maximum torque.In terms of emissions,the conventional gas emissions from the methane-gasoline dual fuel engine both decrease as the mass fraction of methane increases.In addition,in order to study the practical application of methane-gasoline dual fuel,the combustion and emission of gasoline and different blending ratio of methane–gasoline for different load and excess air coefficient were studied based on a turbocharged gasoline direct injection four-cylinder gasoline engine.The results show that,in the aspects of combustion:With the increase of methane content,the maximum in-cylinder pressure and heat release rate increase under the condition of medium and small load at fixed torque,and the ignition delay period is shortened and the combustion phase is advanced.Under small load and lean burn condition,the cyclic fluctuation decreases sharply and the combustion duration shortens.However,at medium load,the combustion duration decreases with the increase of excess air coefficient and the effective thermal efficiency shows an upward trend when the methane content and the excess air ratio are relatively high.In heavy load conditions,methane-gasoline dual fuel can effectively suppress knocking and broaden the knocking limit,and can increase the effective thermal efficiency by about 2%compared with gasoline fuel.Emissions:For medium load and fixed output torque,for example,the emission of CO,CO₂,THC and NOx at the stoichiometric ratio is reduced with the increase of methane content.In summary,the use of methane-gasoline dual fuel can reduce the cyclic fluctuation under small load to achieve stable combustion,suppress the gasoline engine knocking under large load,and increase the effective thermal efficiency of the engine.For medium-load emissions,methane-gasoline can reduce emissions compared to gasoline.