Two Stroke Marine Diesel Engine

TWO – STROKE CYCLE

This operation cycle is so called because it takes two strokes of the piston, or one revolution of the crankshaft, to complete the process needed to produce power. In this cycle, each event is accomplished in a very short time. Moreover, the engine requires some special arrangements. First, the fresh air is forced in under pressure. The incoming air is used to clean out, or scavenge, the exhaust gases and then fill the space with fresh air charge. Instead of valves, there are special holes, called ports, which are opened and closed by piston sides as it moves up and down.

So the piston is at the top of its stroke after fuel injection and combustion have taken place. The piston is then forced down on its working stroke with the valves in the cylinder head opening the exhaust port. The burnt gases then begin to be expelled and the piston continues down until it opens the inlet or scavenge port. Next pressurised air enters and drives out the remaining burnt gases. The piston closes these ports as it returns. The air is then compressed as the piston moves to the top of its stroke. This is the explanation for the name “two stroke”, with a downward power stroke and an upward compression stroke.

Adapted from: MARENG Software - Van Dokkum's Ship Knowledge