This is effected by the provision of links between the pump piston and its corresponding crank which increase the motion of the piston towards the end of the charging stroke. The driving links also retard the motion of the pump piston, while the working piston is finishing its in-stroke
The pump is charged through a valve and discharges through a cam-actuated or automatic valve when the working piston reaches the end of its exhaust stroke. In this position the charge is ignited and, when this is effected by the heat of compression, a small quantity of combustion products is retained in the cylinder, while the two pistons move in opposite directions but maintain constant, until the closing of the discharge valve, the combustion space, formed between them.
To ensure the expulsion of the combustion products an exhaust-valve is provided in the cylinder head, and the clearance space is reduced to a minimum.