| The low thermal efficiency and high emissions could be obtained in gasoline engines under partial load and idle operations.Improving ignition and combustion characteristics has the great significance to realize the clean and high efficient combustion of gasoline engines under low load and speed conditions.Dimethyl ether has a high cetane number.Hence,adjusting mixing ratio of dimethyl ether and gasoline can achieve the control of the mixtures of cetane number and octane number.This could improve the combustion and emissions characteristics of gasoline engines by improving ignition and combustion characteristics of the mixtures according to working conditions.There were only a few research on the combustion process of gasoline and dimethyl ether mixtures on sprak ignition engines.In this paper,the mechanism of dimethyl ether/iso-octane blended fuel was firstly used to carry out the study of the chemical reaction kinetics simulation,which was proved that dimethyl ether addition could promote the combustion characteristics of gasoline engines.Then the dimethyl ether/gasoline blended mixtures were carried out in port fuel injection gasoline engines.The influence of the ignition and combustion characteristics was investigated by adjusting fuel octane number.In addition,a three-dimensional simulation model was established to investigate the influence of dimethyl ether on the mixture flow and combustion process of gasoline direct injection engines.Finally,the mixture formation in the chamber was optimized,which improved the performance of dimethyl ether blended gasoline engines through the coupling of ignition and injection strategies under different working conditions.Hence,the influence of premixed dimethyl ether on combustion and emissions characteristics of gasoline engines was studied through the combination of numerical and experimental studies.With the increase of the mole fraction of dimethyl ether,the premixed laminar flame characteristics of iso-octane and dimethyl ether blended fuels were analyzed based on the premixed laminar flame model of iso-octane and dimethyl ether blended fuels.The results showed that,with the increase of the mole fraction of dimethyl ether,the decomposition rates of macromolecules in flame and the concentration of H,O and OH radical were increased.Besides,the laminar flame velocity and adiabatic flame temperature of the mixtures were increased.Then,the combustion process of the dimethyl ether/gasoline dual fuel engine under different operating conditions was carried out on the port fuel injection engine.The results showed that the pure dimethyl ether engine had an obvious low-temperature oxidation and heat release process under low compression ratio,which enables the pure dimethyl ether engine to obtain the improvements of combustion temperature,thermal efficiency and heat release rate than the pure gasoline engine under lean combustion.However,the premixed dimethyl ether could improve fuel economy,enhance the combustion of mixed fuel in the cylinder,reduce cyclic variation,broaden the stable combustion range and control HC and NOx emissions,effectively.In addition,optimizing the ignition timing could shorten the combustion phase,increase the brake thermal efficiency and reduce emissions.The simulation model of the port/direct combined injection engine was established by using computational fluid dynamics software.Variations of the mechanism of fluid turbulent kinetic energy,velocity field,flame front and mean combustion temperature in cylinder with crank angle of the dimethyl ether blended gasoline engine were investigated by numerical simulation.The results showed that dimethyl ether addition could increase the turbulent kinetic energy and the velocity field of the cold fluid,which increased the turbulent flame propagation rate and the average combustion temperature in the cylinder.Based on the above research,the impacts of different injection and ignition strategies under the lean burn conditions of the dimethyl ether blended gasoline injection engine were studied on gasoline direct ignition bench test.The results showed that the early injection timing of original engine leaded to unstable ignition,high cyclic variation and slow heat release rate,producing low brake thermal efficiency.Reasonable secondary injection could organize high quality the distribution of the mixtures in the cylinder,shorten the combustion phase and reduce the cyclic variation,improving the HC emissions and brake thermal efficiency.The proper adjustment of injection ratio of gasoline made the high quality fuel concentration distribution around the spark plug,which ensured the stable and reliable combustion of the flame kernel,and made the sufficient combustion in the cylinder,improving the brake thermal efficiency and reducing emissions.When the injection strategies remained,optimizing the ignition timting could improve the combustion,reduce the cyclic variation,and thus reduce emissions.These were conducive to the dimethyl ether blended gasoline engine to obtain better brake thermal efficiency.Reasonable injection and ignition strategies were used to improve the mixture formation process of the gasoline direct injection engine under the lean burn conditions.However,dimethyl ether addition could improve the combustion characteristics by increasing the combustion oxidation rate.With the increase of dimethyl ether energy fraction,the brake thermal efficiency of the engine was increased,the combustion phase was reasonable,and HC and particulate matter emissions were decreased.In addition,through the comparison test results under different loads,it could be seen that under the low-load conditions,the combustion process was slow,the cyclic variation was high,and the emissions were high.With the increase of the engine load,the combustion duration was shortened,the cyclic variation was reduced,the brake thermal efficiency was increased,and the concentrations of CO and HC were reduced.The blending of dimethyl ether could further improve the engine performance and fuel economy,and reduce the emissions of the gasoline direct injection engine. |
