| Ethanol is considered as one of the most promising alternative fuels because of its high anti-knock characteristics and high combustion efficiency.However,to make full use of the advantages of ethanol under different working conditions,different mixing ratios of ethanol are required.In order to use ethanol fuel more efficiently and flexibly in SI engine,a dual-fuel dual-direct injection system was adopted.The thermodynamic and chemical advantages of the two fuels can be utilized more efficiently when using this system,and the stratified combustion of different octane fuel concentrations can be realized by changing the injection timing.It can be a good reference for further improving the thermal efficiency of engines in the future.Based on a single cylinder engine,effects of different injection modes on performance and combustion characteristics were systematically compared and analyzed.The injection strategies included gasoline direct injection plus ethanol direct injection,gasoline direct injection plus ethanol port injection and gasoline port injection plus ethanol direct injection.The results show that the engine load can be further expanded,and the power and thermal efficiency can be improved by the dual-fuel dual-direct injection strategy.The reason is that the fuel latent heat of vaporization is more efficiently used to reduce charge air temperature and increase volumetric efficiency when both gasoline and ethanol are injected directly into the cylinder.With the increase of ethanol ratio,the evaporative cooling effect is enhanced,the flame propagation speed is higher,and the octane number is increased,so the anti-knock performance and the effective thermal efficiency can be improved.With the increase of ethanol ratio,the power performance and brake specific fuel consumption of the engine can be increased under different injection modes for medium load.Moreover,with the dual-fuel dual-direct injection mode,the engine power performance and brake specific fuel consumption for medium load can be improved significantly.With the increase of ethanol ratio,the combustion phase are changed nonlinearly due to the comprehensive effect of the relative fast laminar flame speed and the cooling effect of ethanol.Then,based on the dual-fuel dual-direct injection system,the effects of different injection time on engine combustion and performance were studied,and the effects of different octane number fuel stratification on knock and combustion were explored.The results show that with the delay of injection time,the knock tendency and intensity increased initially and then decreased due to the influence of evaporation of ethanol fuel and the stratification of different fuel.With the delay of the fuel injection timing,the torque increased initially and then decreased,and the brake specific fuel consumption decreased initially and then increased at MBT spark timing.With comprehensive consideration,the best power and economy can be achieved when Gasoline injection was 300 °CA BTDC,and ethanol injection time was 240 °CA BTDC.Finally,in order to further improving the thermal efficiency for medium load,the effects of different injection timings on engine combustion and performance under lean combustion were studied.The results show that with the increase of excess air coefficient,the thermal efficiency was increased obviously and the brake specific fuel consumption was decreased under the condition of keeping fuel injection constant and increasing throttle opening.With the delay of injection time,the thermal efficiency increased first and then decreased.Based on these,when the excess air coefficient is 1.2,the gasoline injection timing is 180 °CA BTDC,and the ethanol injection timing is 300 °CA BTDC,the effective thermal efficiency can be reached the 40.5% of the maximum.The coefficient of variation under lean combustion conditions can be reduced with the addition of ethanol fuel,and the combustion coefficient of variation can be improved significantly with homogenization of ethanol fuel. |
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