71 Scavenging system layout of a 25 cc two-stroke engine intended for stratified scavenging 2002-32-1840

A sequentially stratified scavenged engine is characterised by the principle that the cylinder is first scavenged by pure air, followed by the air/fuel mixture. The air is introduced into the upper part of the scavenging ducts through a piston port or a reed valve. To take full advantage of the stratified scavenged principle, the scavenging ducts have to be designed in a way, so that they can accommodate all the air that is delivered into the scavenging ducts.

When converting a conventional two-stroke engine into a stratified scavenging engine, it is also important that the tuning and basic scavenging characteristics of the standard engine are not deteriorated.

In this paper it is shown how these two aspects can be combined. Together with a theoretical approach for dimensioning the volume and length of the scavenging ducts, it gives a guideline on how to design the basic engine layout, for a stratified scavenged two-stroke engine.

When performing this work it also becomes clear how the scavenging process can be delayed. Using longer than normal scavenging ducts, together with an increase in blow down of exhaust gases, when the scavenging ports open does this.

This effect can be advantageous on a standard two-stroke engine as well, and rightly done it will lower the emissions and increase the torque in the low speed range.

A 25 cc standard engine, and a 25 cc engine with modified scavenging ducts are used as examples. Test results from these engines will illustrate the conclusions. Theories and simulations will help to explain the upcoming improvements.