Editing Reciprocating engine (section)

==Operations==

In steam engines and internal combustion engines, valves are required to allow the entry and exit of gases at the correct times in the piston's cycle. These are worked by cams, eccentrics or cranks driven by the shaft of the engine. Early designs used the [[D slide valve]] but this has been largely superseded by [[piston valve]] or [[poppet valve]] designs. In steam engines the point in the piston cycle at which the steam inlet valve closes is called the [[Cutoff (steam engine)|cutoff]] and this can often be controlled to adjust the [[torque]] supplied by the engine and improve efficiency. In some steam engines, the action of the valves can be replaced by an [[oscillating cylinder]].

Internal combustion engines operate through a sequence of strokes that admit and remove gases to and from the cylinder. These operations are repeated cyclically  and an engine is said to be [[Two-stroke cycle|2-stroke]], [[Four-stroke cycle|4-stroke]] or [[Six stroke engine|6-stroke]] depending on the number of strokes it takes to complete a cycle.

The most common type is 4-stroke, which has following cycles.

# '''Intake''': Also known as induction or suction. This stroke of the piston begins at top dead center (TDC) and ends at bottom dead center (BDC). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
# '''Compression''': This stroke begins at BDC, or just at the end of the suction stroke, and ends at TDC In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
# '''Combustion''': Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at TDC (the end of the compression stroke) the compressed air-fuel mixture is ignited by a [[spark plug]] (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to BDC This stroke produces mechanical work from the engine to turn the crankshaft.
# '''Exhaust''': Also known as outlet. During the ''exhaust'' stroke, the piston, once again, returns from BDC to TDC while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.