The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston; therefore, the complete cycle requires two revolutions of the crankshaft to complete.
The diesel engine is similar to the four stroke petrol engine, but uses a different method to ignite the fuel.
During the intake stroke, the piston moves downward, The intake valve opens, and fresh air (containing no fuel), is drawn into the cylinder.At the same time the exhaust valve is closed position.
As the piston rises, the air is compressed, causing its temperature to rise. At the end of the compression stroke, the air is hot enough to ignite fuel.
Near the top of the compression stroke, the fuel injector drives fuel into the cylinder. The fuel immediately ignites upon contact with the hot compressed air.
As the fuel burns, the gas in the cylinder heats and expands, driving the piston.
At the bottom of the power stroke, the exhaust valve is opened and the exhaust is driven out of the cylinder. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
The valves are operated by a variety of mechanisms on diesel and four stroke engines. The engine illustrated here features dual overhead camshafts, sometimes abbreviated DOHC. These are usually driven by a chain or cog belt as shown here.
With his first practical engine in 1897, "Diesel proved to the world that his was the most efficient engine ever built."8 Diesels are still among the most energy efficient engines available. They are widely used in large trucks, boats, earthmoving machinery, etc.