Study On The Combustion System Of An Opposed-piston Two-stroke Gasoline Direct Injection Engine

With the advantages such as compact structure,high power density and excellent balance,opposed-piston two-stroke(OP2S)engine has attracted wide attention at home and abroad.OP2 S gasoline engine uses the electronically controlled gasoline direct injection(GDI)combustion system which is the effective solution to solve the emission and fuel consumption problem.At the same time,OP2 S gasoline engine has these potential advantages.The double crank shaft type OP2 S gasoline engine combustion system was investigated with a combined method of numerical simulation and experiment.OP2S gasoline engine was equipped with “port-to-port” uniflow scavenging system and GDI system,and its injector and spark plug were placed on the cylinder liner.OP2 S gasoline engine combustion system can be divided into the following three subsystems: 1)gas exchange system to complete gas exchange and flow organization;2)fuel injection system to realize fuel spray and mixture formation;3)combustion chamber system to organize ignition and combustion.In order to study OP2 S gasoline engine combustion system,the influence law between gas exchange,flow organization,mixture formation and combustion process were investigated.On this basis,the multi-system coupling simulation model was established including gas exchange,fuel spray and combustion.The matching design among subsystems and subsystem parameters were analyzed.In-cylinder flow was a coupling point among subsystems of combustion system which was interaction influences on gas exchange,mixture formation and combustion process.On the basis of scavenging system parameters optimization,organization of in-cylinder tumble and swirl for OP2 S gasoline engine were investigated.Non-uniform intake chamber can be used to organize tumble and intake port radial angle can be used to organize swirl.Pit top surface of piston with non-uniform scavenging chamber was favorable to organize tumble and maintain the in-cylinder turbulence intensity.It helped to form the combustible mixture around spark at ignition time.Compared with flat piston non-uniform scavenging chamber,turbulent kinetic energy was increased by 1.5 times.Combined with the characteristic that the injector was placed on the cylinder liner and organization of oblique axis tumble in-cylinder,asymmetric spray on exhaust and intake side mode was proposed.This spray mode not only realized homogeneous mixture in heavy load operation,but also helped the engine adopt two-stage injection strategy to stratified mixture in low and medium load condition.Increasing the injection angle on the exhaust side will decrease the fuel evaporation rate but the mixture uniformity was improved by increasing the injection angle on the intake side.The reasonable matching of injection angle on both side of exhaust and intake could reduce the probability of spray collision and falling flat.This reasonable arrangement can improve the macroscopic mixture of air and fuel in spray process.By analyzing the effect mechanism of injection timing,pressure and strategy on mixture formation,the parameters optimization of fuel injection system were completed in the OP2 S gasoline engine.OP2S gasoline engine combustion chamber was consisted of the opposed-piston top surface and cylinder liner.Because spark plug was placed on the cylinder liner,flat piston matching dual spark plug scheme and pit piston matching single spark plug scheme were designed for OP2 S gasoline engine.Flat piston and dual spark plug scheme can shorten the combustion duration and its maximum in-cylinder pressure phase was 10°CA earlier than single spark plug.Pit piston and single spark plug scheme can reduce surface-volume ratio of combustion chamber and increase combustion rate.In low and medium load condition,stratified mixture distribution accelerated combustion process by two-stage injection strategy,shorten 30% time of flame developing period and shorten 15% time of rapid burning period compared with homogeneous mixture.In this paper,some achievements have enriched the matching method in studying OP2 S gasoline engine combustion system to some extent,and made some useful results in exploring the mechanism of working process.It provided supports to optimize the performance of OP2 S gasoline engine and guided the optimized design in the double crank shaft type OP2 S gasoline engine.