Four-stroke principle
1. Intake (1st stroke): the piston slides from TDC to BDC. The intake valve opens, the fuel-air mixture flows into the cylinder.
2. Compression (2nd stroke): the piston slides from BDC to TDC and compresses the sucked-in fresh fuel-air mixture on its way. The intake and exhaust valves are closed.
3. Combustion (3rd stroke): before TDC, the compressed fuel-air mixture is ignited by the spark plug, the mixture burns like an explosion, due to the increase in pressure, the piston is pressed to the BDC position. The connecting rod transfers energy to the crankshaft and sets it in rotation.
4. Push (4th stroke): The inertial mass of the flywheel again moves the piston from BDC towards TDC. The exhaust valve is now open, so the burnt (spent) gases can exit into the exhaust system. In total, four strokes form the gas exchange in a four-stroke engine.
In principle, a diesel engine operates on the same principle. During the suction stroke, it sucks in clean air, compresses it much more strongly, so at the end of the compression stroke, the injected fuel (diesel) ignites independently from contact with hot air, without ignition from an external source (ignition spark). The rest of the gas exchange is then carried out completely identically to a gasoline engine.
The working volume 2 extends from the upper 1 to the lower 3 dead center. Between the TDC, which in the right cylinder is just limited by the base of the piston, and the convexity of the cylinder head 5 is the combustion chamber 4
Working volume: the space that the pistons travel through during their movement from BDC to TDC. The combustion chamber has no effect on the working volume. The working volume and combustion chamber together form the cylinder volume.
Compression ratio: The ratio at which the fresh gas sucked in is compressed in the combustion chamber. The size of the combustion chamber has a direct effect on the compression ratio. The compression ratio indicates how much fresh gas at 100% filling (wide open throttle) must be in the combustion chamber at the moment of ignition. Endura turbo diesel engines have a compression ratio of 19.4:1. Zetec-SE operate with a compression ratio of 11:1, and E models compress fresh gas at a ratio of 10:1.
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