The main unique feature of the head of a Lister variable compression engine is the auxiliary combustion chamber. Most non direct injection diesels use one, more or less spherical pre-combustion chamber per cylinder. The Lister uses a pre-combustion chamber as well as a smaller auxiliary combustion chamber - effectively, two combustion chambers per cylinder. The compression ratio changeover valve has sealing surfaces on the back edge and front point. The valve either blocks or opens the small entrance to the auxiliary chamber, thereby changing the compression ratio.
In the high compression position the front point of the valve seats against the entrance to the auxiliary combustion chamber, preventing the compressed air from entering the chamber, allowing the use of the main combustion chamber only.
In low compression the valve moves all the way back and seals against the back of the auxiliary combustion chamber thereby opening the chamber to the main combustion chamber, increasing the total volume in which combustion can occur.
The middle position on the compression valve control lever gives no compression. Along the axis of the compression control valve are four gooves. When the valve is not seated in either the high or low compression positions, the compressing air is allowed to escape to atmosphere via these grooves.
