Experiments on two-stroke engine exhaust ports under steady and unsteady flow conditions 1

A theoretical and experimental study of the discharge from an engine cylinder to atmosphere was made. The objective of the work was to establish suitable data for the design of exhaust ports. Two theoretical methods were used, in the first method the discharge was analysed by the method of characteristics and it was shown that the discharge was by wave action. The second method neglected the particle velocity in the cylinder. Comparison of the latter method with the solution by characteristics showed good agreement over the main part of the discharge and it was possible by that method to develop a simple design formula for the estimation of the port area.

The experimental investigation was carried out on a special machine which simulated in all respects the exhaust-port arrangement of a two-stroke cycle engine. The machine was designed to investigate the effect of port configuration, port timing, cylinder length, engine speed, release. pressure, and temperature. By suitably arranging the variables in non-dimensional groups a wide range of engines could be simulated. The flow characteristics of the exhaust ports were examined under both steady and unsteady flow conditions. The following conclusions were drawn:

1. the coefficient of discharge for the exhaust ports increased with cylinder pressure and decreased with increased port area;

2. the coefficient of discharge was lower under dynamic conditions then under steady flow conditions;

3. the dynamic coefficient of discharge decreased with reduction in cylinder length at release;

4. the effect of early port opening was to reduce the blowdown time;

5. the effect of piston movement during discharge was to reduce the effective port area required.