Study On Combustion And Emission Characteristics Of A Combined Injection Hydrous Ethanol/Gasoline Dual-Fuel Engine

The application of alternative fuels can alleviate environmental pollution and energy crisis.Ethanol is a kind of alternative fuel with great development potential,it has the advantages of low toxicity,reproducibility and mature production and preparation technology.However,the fermentation broth of ethanol contains a lot of water,in order to get anhydrous ethanol,the fermentation broth must go through the distillation and purification process,which will consume a lot of energy and lead to the increase of ethanol production cost.Therefore,the use of hydrous ethanol in engines can improve the life-cycle competitiveness of ethanol as an alternative fuel.Hydrous ethanol/gasoline dual-fuel is preferred to pure hydrous ethanol fuel because of its excellent cold start performance at low temperature.This study unites the combined injection technology with hydrous ethanol/gasoline dual-fuel,and explores the combustion and emission characteristics of a hydrous ethanol/gasoline dual-fuel combined injection engine.The results show that the combination of the combined injection technology and hydrous ethanol/gasoline dual-fuel can improve the engine power performance,reduce CO emissions,NO_x emissions and PN（Particle number）emissions.The main research work and conclusions are as follows:（1）Fuel combustion process is the key factor affecting engine performance.Clarifying the combustion characteristics and chemical reaction kinetics of hydrous ethanol/gasoline dual-fuel is the theoretical basis for studying the combustion and emission characteristics of the combined injection hydrous ethanol/gasoline dual-fuel engine.Therefore,this research uses CHEMKIN-PRO software to explore the effects of boundary conditions and the water ratio in hydrous ethanol on laminar flame speed,ignition delay period and key combustion groups,and compares the differences of chemical reaction paths between hydrous ethanol/gasoline fuel and ethanol/gasoline fuel.The results show that the laminar flame speed increases with the increase of temperature,decreases with the increase of water ratio in hydrous ethanol,and the higher the initial temperature is,the stronger the inhibition effect of water on flame propagation is.When the water ratio in hydrous ethanol is less than 12%,the laminar flame speed increases as hydrous ethanol ratio rises.However,when the water ratio is greater than 12%,the laminar flame speed decreases with the increase of hydrous ethanol ratio.When the excess air ratio value is between 0.9 and 1,the laminar flame speed is the highest,and the inhibition effect of water on flame propagation is the most obvious.The ignition delay period is prolonged with the increase of water ratio in hydrous ethanol.The increase of initial temperature will shorten the ignition delay period,and the prolonging effect of water on the ignition delay period decreases as temperature increases.The higher the ratio of hydrous ethanol is,the longer the ignition delay period is.The decrease of the excess air ratio shortens the ignition delay period,and as the excess air ratio decreases,the effect of water on the extension of ignition delay period becomes weaker.The thermal and chemical effects of water reduce the peak concentration of key combustion groups and delay the peak time.However,higher initial temperature diminishes the influence of water on the peak concentration and peak time,but larger hydrous ethanol ratio makes the influence of water on the groups more obvious.With the decrease of the excess air ratio,the weakening effect of water on the peak concentration is strengthened,but the delaying effect on the peak time is weakened.In addition,the chemical path of hydrous ethanol/gasoline dual-fuel is different from that of ethanol/gasoline dual-fuel,but only the decomposition process of some groups is quite different.（2）The engine bench test is difficult to investigate the working process of engine in cylinder deeply.To conduct an in-depth and detailed study of the engine work process,simulation analysis is often used.Therefore,this research uses the CONVERGE software to observe and analyze the evolution process of in-cylinder mixture,in-cylinder temperature field,flame front surface and the combustion intermediate group distribution field,clarifying how hydrous ethanol injection strategy and water ratio influence the engine combustion process,which provides a scientific basis for studying the combustion and emission characteristics of the combined injection hydrous ethanol/gasoline dual-fuel engine.The results show that the delay of direct injection timing breaks the uniformity of ethanol distribution in the cylinder.Under early injection mode,the delay of direct injection timing accelerates axial flame propagation but decelerates radial flame propagation.However,the effects of the delay of direct injection timing under late injection mode on the flame propagation are opposite to that of the early injection mode,and the after-combustion phenomenon is aggravated.The CO emissions under early injection mode are mainly from CO₂ dissociation at high temperature.The CO emissions under late injection mode mainly come from the incomplete combustion of fuels due to local oxygen-lack.Higher injection pressure accelerates the mixing process of hydrous ethanol and air,increasing the forward speed of flame front and expansion rate of high temperature region.The increase of injection pressure will strengthen the stretching effect of airflow on the flame and increase the deformation of the high-temperature region.Water has negative effects on the diffusion,accumulation and consumption of H₂O₂ group and OH group in the cylinder.With the increase of water ratio,the temperature and forward speed of the flame front decrease,and local high temperature areas decrease.When the water ratio is 0%,the diffusion rate of H₂O₂ groups accelerates with the increase of ethanol ratio,but the process of low temperature oxidation of fuels which can generate H₂O₂ groups is weakened.The concentration of OH groups in the cylinder increases as ethanol ratio increases.When the water ratio is 20%,the increase of ethanol ratio inhibits the diffusion process of H₂O₂ groups in the cylinder,and weakens the process of low temperature oxidation of fuels which generates H₂O₂ groups.The concentration of the OH group decreases as the ethanol ratio increases.When the water ratio is 0%,the increase of ethanol ratio accelerates the forward speed of flame front,and the high temperature area in the cylinder increases.When the water ratio is 20%,the increase of ethanol ratio has negative effects on the flame propagation process and the high temperature field in the cylinder.（3）Simulation analysis has some limitations,often cannot fully reflect the complex actual engine working process,while the engine bench test can directly observe the performance of the engine,and test the application of hydrous ethanol/gasoline dual-fuel in the actual engine.Therefore,the combustion and emission characteristics of the combined injection hydrous ethanol/gasoline dual-fuel engine were investigated by engine bench test.Injection strategy is important to the dual-fuel combined injection engine,this research studied the effects of direct injection strategy on the combustion and emission characteristics of a hydrous ethanol/gasoline dual-fuel combined injection engine through the engine bench test.When the hydrous ethanol ratio is 40%and the water ratio in hydrous ethanol is between 4%and 8%,the engine power performance and operation stability are good,and the harmful emissions are low.However,the delay of the direct injection timing increases the uneven distribution of the mixture and leads to the deterioration of the combustion state in the cylinder.The increase of injection pressure is beneficial to improve the engine’s power performance,operation stability and emission characteristics.In conclusion,the hydrous ethanol ratio is 40%and the water ratio in hydrous ethanol is between 4%and 8%,coupled with the direct injection time of300°CA BTDC and the injection pressure of 12MPa can ensure the comprehensive performance of the engine is great.（4）To investigate the performance of the hydrous ethanol/gasoline dual-fuel combined injection engine more comprehensively,avoid the limitation of the single operating condition,this research explores the combustion and emission characteristics of the engine under different operating parameters.Under lean-burn conditions,the power performance decreases as water ratio in hydrous ethanol increases.However,the engine performance is the best when the water ratio in hydrous ethanol is 8%under rich mixture or theoretical mixture conditions.Under different excess air ratios,HC emissions increase with the increase of water ratio,but CO emissions and NO_x emissions show downward trends.When the excess air ratios are 0.9 and 1,the total PN concentration decreases as water ratio increases.When the excess air ratio was 1.2,the increase of water ratio increases the concentration of total PN concentration.To ensure the engine power performance,the water ratio in hydrous ethanol should decrease with the increase of speed and the decrease of load.Although the optimal water ratio for power performance is different under different operating conditions,a water ratio between 4%and 8%in hydrous ethanol can obtain great power performance under different working conditions.The change of operating condition will not affect the trend of HC and NO_xemissions with water ratio.The increase of water ratio in hydrous ethanol leads to the increase of HC emissions and the decrease of NO_x emissions.The variation of CO emissions with water ratio is complicated and closely related to the engine operating condition.When the engine works under low speed and small load operating conditions,the total PN concentration will increase with the increase of the water ratio;however,the total PN concentration decreases with the increase of the water ratio,also decreases with the increase of the speed and load.Therefore,when the engine has a large demand for power performance,the water ratio in hydrous ethanol should be controlled within the range of 4%to 8%.The water ratio in hydrous ethanol should be controlled at 20%when the emissions of PN need to be controlled under heavy load condition.