Dynamic effects on the gas exchange process in two stroke cycle engines

Recent investigations of fuel injection in spark ignition engines have aroused new interest in two stroke cycle engines. The performance of such engines is very much affected by the dynamic behaviour of the fresh air introduced and the exhaust leaving the cylinder through ports, receivers and silencers. The present investigation has as its aim the clarification and establishment of trends in the performance of two stroke engines with particular reference to crank case scavenged engines. The basic relations governing the flow of air and gas through the whole system are investigated one by one and their interdependence is established. The investigation covers: pulsations in the cylinder, accelerations in the ducts, effect of variable port area, effect of throttling in the ports. The basic differential equation for free oscillation of the exhaust gas in the system comprising receiver, exhaust pipe, and silencer is set up and a method is developed which allows calculation of an equivalent length for the system. With this equivalent length, the natural frequency of the system can be calculated. An experimental investigation is presented which was carried out on a single cylinder compression ignition engine with crank case scavenging. The volumetric efficiency of the engine was determined as a function of exhaust system configuration at various brake mean effective pressures and at constant engine speed. The pressure in the exhaust near the port was measured (by means of a quartz and crystal pickup) as a function of crank angle. The results are presented in the form of plots of volumetric efficiencies as function of brake mean effective pressure for various configurations and as photographic records of pressure as a function of crank angle. It is concluded that a simple frequency analysis is insufficient for the determination of the exhaust behaviour. Criteria for good exhaust systems are established. The trends predicted in the theoretical analysis of the report are, in general, well verified. The need for further investigation is indicated and an outline of research is presented