The pump scavenge two-stroke-cycle engine designed by Sir Dugald Clerk in 1879 was the first successful twos troke engine; thus the two stroke diesel engine is sometimes called the Clerk engine. Uniflow scavenging took place – fresh charge entering the combustion chamber above the piston while the exhaust outflow occurred through ports uncovered by the piston at its
outermost position.
Low- and medium-speed two stroke diesel engine still use this system, but high-speed two-stroke diesels reverse the scavenging flow by blowing fresh charge through the bottom inlet ports, sweeping up through the cylinder and out of the exhaust ports in the cylinder head
With the two-stroke-cycle engine, intake and exhaust phases take place during part of the compression and power stroke respectively, so that a cycle of operation is completed in one crankshaft revolution or two piston strokes. Since there are no separate intake and exhaust strokes, a blower is necessary to pump air into the cylinder for expelling the exhaust gases and to supply the cylinder with fresh air for combustion.
Scavenging (induction and exhaust) phase The piston moves away from the cylinder head and, when it is about half-way down its stroke, the exhaust valves open. This allows the burnt gases to escape into the atmosphere. Near the end of the power stroke,a horizontal row of inlet air ports is uncovered by the piston lands . These ports admit pressurised air from the blower into the cylinder. The space above the piston is immediately filled with air, which now blows up the cylinder towards the exhaust valves in the cylinder head. The last remaining exhaust gases will thus be forced out of the cylinder into the exhaust system.
This process of fresh air coming into the cylinder and pushing out
unwanted burnt gas is known as scavenging.
Compression phase Towards the end of the power stroke, the inlet ports will be uncovered. The piston then reaches its outermost position and reverses its direction of motion.