Four Stroke Engine

Animation of a four stroke cycle

A four stroke engine is the most common internal combustion engine of modern times. It uses a spark ignition system which is very similar to the compression ignition in a diesel engine. The 'four - stroke' is so called as it applies driving power and supplies force through four 'strokes' which, amongst other things, applies fuel, compresses it, lights it, provides a driving force as well as pushing out the exhaust gases. (To under stand the following you will have to understand the basic layout and terminology of a reciprocating engine if you don't know do a Google image search or try Wikipedia for a good diagram of a cylinder.)

THE FIRST STROKE: Fuel Induction


Image of the first: intake stroke

In the first stroke the piston moves downwards within the cylinder, this downward movement creates pressure which opens the inlet port and closes the exhaust port (at the top of the cylinder) The sucking effect of the piston moving down pulls a mixture of air and fuel into the cylinder, this is inducing the fuel. As the piston reaches its BDC (bottom dead centre) this is the end of the first stroke, the inlet and exhaust ports are closed and the mixture of fuel and air is sealed within the cylinder.

THE SECOND STROKE: Fuel Compression


Image of the second: Compression stroke

In the second stroke, the piston is moving upwards from its BDC to its TDC (Top dead centre) this is an upward motion providing an upward force which compresses the fuel, both ports stay closed at the top of the cylinder throughout the stroke, maintaining the pressure to compress the fuel.

THE THIRD STROKE: Ignition and Power


An image of the third (power) stroke

The third stroke or 'the power stroke' is what provides the kinetic energy which powers the engine. In this stroke the piston moves in the same way as it does in the induction stroke (stroke one), from the TDC to the BDC, This time however both ports remain closed. Due to the compression in the previous stroke, the fuel is very hot, compressed at the top of the cylinder. At this point, in a petrol engine, a spark is applied to the fuel which then burns very rapidly into a high pressure gas forcing the pison down the cylinder and providing the force which powers the engine and is what moves the piston throughout the other strokes in the cycle, the stroke ends with the piston at its BDC and both ports closed.

THE FORTH STROKE: Exhaust release


Image of the forth stroke of the cycle: Exhaust

In this stroke the piston moves from its BDC to TDC providing an upwards force and the exhaust port opens. With the piston moving up and the port open, the burnt gas mixture is forced out of the cylinder through the port. When the piston reaches its TDC and all the fuel has been released the exhaust port closes and the inlet opens ready for the fuel induction which follows straight away.

This system is for a four stroke petrol engine, in a diesel however it works slightly differently. During the first stroke in a diesel, the engine draws in a charge of air only which is then compressed and reaches high temperature, then during the third stroke high pressure diesel is injected into the air creating a combustible mixture which is ignited by the temperature of the air which burns. Other than those two differences, the diesel four stroke works exactly as a petrol four stroke engine.