Numerical And Experimental Study On A Small Two-Stroke S.I. Engine With Fuel Direct Injection Technology Of FAI System

Gasoline direct injection(GDI), which greatly reduces fuel short circuit out of exhaust port during scavenge process and decreases hydrocarbon emission and fuel consumption as a consequence, is a main research subject of two-stroke gasoline engine. Currently, most of small two-stroke GDI engine utilize air-assisted fuel injection technology, while single fluid fuel injection, such as FAI (Free Armature Injection) injection system, is seldom used. This paper explored the feasibility, advantage and problem of adopting FAI injection system on a small capacity, such as 50 cm3, two-stroke gasoline engine. Methods like variable transfer ports (VSP) and stratified combustion, stratified charging are also explored to reduce hydrocarbon emission and fuel consumption during middle to heavy load and low load respectively.Experiments certified the feasibility of FAI system. At full throttle, the minimum brake specific fuel consumption (BSFC) and brake specific hydrocarbon emission (BSHC) are 336 g/kW.h and 15.7g/kW.h, which are 47% and 88% lower than the base carbureted engine and comparable to those of four-stroke engines of the same capacity.In order to further reduce hydrocarbon emission and fuel consumption, mixture distribution in combustion chamber should be improved. Effects of scavenge ports on spray and mixture formation process in two-stroke GDI engine are studied for the first time. It is found that more rapid fuel evaporation and more homogenous mixture formation are achieved by shifting from three or five scavenge ports to dual scavenge port during middle to heavy load which results in obvious decrease of hydrocarbon emission and fuel consumption.Stratified combustion is achieved to certain degree by retarded injection. Hydrocarbon emission and fuel consumption are decreased to some extent, but the values are still high comparing to four-stroke engine. Injection timing, in-cylinder flow field and ignition timing are crucial factors that affect stratified combustion.The existence of overmuch residual exhaust gas together with its over mixing with fresh charge is one of the main causes of unstable combustion at low load. A new methods, EGR(exhaust gas recirculation) stratified charging, is proposed to form stratified distribution of exhaust gas and fresh charge in combustion. Simulation results show that this method gives clear stratification which is little affected by engine speed and combustion chamber shape, but obviously affected by piston shape and asymmetry of scavenge ports.