THE Humphreys engine is a conventional powerplant to which has been added a device for automatically varying the clearance volume of the combustion chamber. This allows the compression ratio to be varied so that:
At light loads the compression ratio is at a maximum, providing best thermal efficiency and therefore lowest fuel consumption.
At intermediate loads the compression ratio is somewhat lower but still high enough to provide good thermal efficiency and therefore low fuel consumption.
At full load the compression ratio is at its lowest point, so that the engine is able to operate without knock with fuels of quite low octane number.