A critical factor in improving performance of crankcase-scavenged two-stroke gasoline engines is to reduce the short-circuiting of the fresh charge to the exhaust in the scavenging process. To achieve this, the authors developed a reciprocating exhaust control valve mechanism and direct air-fuel injection system. This paper investigates the effects of exhaust control valve and direct air-fuel injection in the all aspect of engine performance and exhaust emissions over a wide range of loads and engine speeds. The experimental results indicate that the exhaust control valve and direct air-fuel injection system can improve specific fuel consumption, and that HC emissions can be significantly reduced by the reduction in fresh charge losses. The pressure variation also decreased by the improved combustion process.
CRANKCASE SCAVENGED two-stroke gasoline engines suffer from fresh charge losses leading to poor fuel economy and it is a reason for large increases of HC in the exhaust. In recent years, two-stroke engines have been reevaluated in terms of improvements to the drawbacks, involving the in-cylinder fuel injection, exhaust catalysts, and control of the optimum port area(1, 2 and 3).
The research reported here aimed to improve the performance and exhaust characteristics by reducing short-circuiting of fresh charge with equipment to control the exhaust port and in-cylinder air-fuel injection. The results found that the piston type exhaust control valve and air-fuel injection both were effective to reduce short-circuiting of fresh charge, and that both equally helped improving the trapping efficiency. However as the loss due to pumping friction was large with the exhaust control valve, the improvements in the specific fuel consumption was better with the in-cylinder injection. With the piston type exhaust control valve short-circuiting of charge was blocked leading to a large increase in the indicated output, and the net output was largest in the order with exhaust control valve, base engine, and in-cylinder injection.
In this way the investigation showed that the piston type exhaust control valve and in-cylinder air-fuel injection improved scavenging over a wide range of engine speeds and that there was a substantial improvement in CO and HC concentrations. At the same time there was some increase in NOx, but with air-fuel in-cylinder injection there was some reduction in NOx due to the lowering of the charge temperature by injected air.