Four Stroke Diesel Combustion Engine
A four-stroke diesel engine (also known as a four cycle diesel engine) is an internal combustion engine in which the piston completes four separate strokes while turning a crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:
As the piston moves down the suction valve opens, air is drawn into the cylinder liner. At bottom dead centre (BDC) the suction valve closes trapping the air within the cylinder.
The piston moves up compressing the air. As the air is compressed the volume decreases whilst the temperature increases. The compression ratio on the average diesel engine is in the range of 8:1 to 10:1.
Diesel is injected into the cylinder just before the piston reaches top dead center (TDC); the high temperature within the liner causes the diesel to ignite. The 'explosion' pushes the piston down turning the crankshaft in the process. This part of the cycle is often referred to as the 'power' stroke. The firing order determines when each cylinder fire's, with cylinders firing at different times power can be transferred more evenly.
When the crankshaft completes two revolutions (4 strokes) each piston will have completed one power stroke. Most engines have a air to fuel ratio of between 12.5:1 and 16:1 (lots of air, not so much fuel).
With the piston at BDC, the exhaust valve opens to allow the burned exhaust gas to be expelled to the exhaust system. The piston moves up the cylinder expelling the exhaust gas. With the piston at TDC the exhaust valve closes and the process begins again.
Note: The above cycle is for a compression ignition engine (diesel engine) not a spark ignition engine (petrol engine).
Introduction text: https://en.wikipedia.org/wiki/Four-stroke_engine
2D Animation source: Source: By Zephyris (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
Four-stroke cycle used in gasoline/petrol engines. Petrol engines have spark plugs, diesel engines do not, but the four stroke process is common to both types of engines.
Stroke 1 = Intake (suck)
Stroke 2 = Compression (squeeze)
Stroke 3 = Power (bang!)
Stroke 4 = Exhaust (blow)