Study On Optimization Of Cold Start Performance Of Aviation Heavy Oil Direct Injection Engine

Aviation kerosene is becoming more important in the field of general aviation.However,aviation kerosene also has the characteristics of large viscosity and poor evaporation,which results in poor atomization in the state of cool engine and difficulty in the formation of the mixture,resulting in extremely low engine cold start performance.This is a fatal flaw for the aero engine with high safety and high stability requirements.There is great engineering significance to optimize the starting conditions of the heavy-oil engine.In this paper,a self-developed low-pressure air-assisted direct-injection singlecylinder testing machine was used as a test prototype.The one-dimensional and threedimensional models of the engine was built,and the simulation and experiment were carried out.CFD simulation was carried out to study the formation of mixed gas in cylinder under engine cooler condition by using AVL-Fire software.The basic influence of different variables on the distribution of mixed gas was analyzed,and the minimum ignition energy was calculated and analyzed.The starting process was decomposed into three rotational states of 300 rpm,500rpm and 700 rpm.The simulation results show that the influence of variables on the whole starting process was studied through the rule of the mixture state parameters.At the same time,the effects of cylinder preheating,cone angle of injection and secondary injection on cold start performance were simulated.The results show that:With the increase of rotational speed,the effect of fuel injection time on fuel evaporation is weakened.With intake stroke injection,the intake flow rate and intake air speed play a major role in fuel evaporation.When injection start in compression stroke,the best injection time advance with the speed.Cylinder preheating has a most significant impact on the cold start in the early of start process,as the speed increase preheating on cylinder can not be taken to ensure the continuity of combustion.The minimum ignition energy reaches the minimum value in the vicinity of the excess air coefficient 0.9,the ignition timing has a greater influence than injection timing on minimum ignition energy,and the minimum ignition energy gradually decreases with the delay of the ignition timing.Cylinder preheating can effectively reduce the minimum ignition energy,but at least heated to 323 K to ignite.Spray cone angle should be holden between 20 ° to 40 ° to the normal fire,spray cone angle 30 ° make a minimum minimum ignition energy.Secondary injection can form a mixture of stratified equivalence ratio,formation of leaner mixture at the position far away from the spark plug,making sure a complete combustion in the flame propagation process,and formation of a thick mixture in the vicinity of the spark plug to improve the ignition capability,but to sacrifice a certain degree of motivation.The influence of two schemes of preheating and secondary injection on the cold start performance of a direct injection engine test platform was studied.At the same time,the change of combustion process of the engine under different rotational speed was studied.Research shows:It is proved that the cylinder preheating scheme is beneficial to the establishment of the flame core and indirectly proves the rationality of the minimum ignition energy to analyze the cold start performance.If the fuel injection strategy is reasonable,the ignition delay can be shortened.However,because of the fuel injection timing deviates from the optimum fuel injection timing,the fuel evaporation was weakened,the combustible mixture was reduced,and the combustion was deteriorated.The ignition delay and the rapid burning period of the engine were shorter with the increase of the rotating speed,which indicates that the ignition condition of the engine is improved with the enhancement of the inlet turbulence and the increase of the charging efficiency.