Editing Otto cycle (section)

==Processes==
The cycle has four parts: a mass containing a mixture of fuel and oxygen is drawn into the cylinder by the descending piston, it is compressed by the piston rising, the mass is ignited by a spark releasing energy in the form of heat, the resulting gas is allowed to expand as it pushes the piston down, and finally the mass is exhausted as the piston rises a second time. As the piston is capable of moving along the cylinder, the volume of the gas changes with its position in the cylinder. The compression and expansion processes induced on the gas by the movement of the piston are idealized as reversible, i.e., no useful work is lost through turbulence or friction and no heat is transferred to or from the gas during those two processes. After the expansion is completed in the cylinder, the remaining heat is extracted and finally the gas is exhausted to the environment. Mechanical work is produced during the expansion process and some of that used to compress the air mass of the next cycle. The mechanical work produced minus that used for the compression process is the net work gained and that can be used for propulsion or for driving other machines. Alternatively the net work gained is the difference between the heat produced and the heat removed.
[[File:OTTO_CYCLE.png|right|600px|Otto Cycle]]

===Process 0–1 intake stroke (blue shade)===
A mass of air (working fluid) is drawn into the cylinder, from 0 to 1, at atmospheric pressure (constant pressure) through the open intake valve, while the exhaust valve is closed during this process. The intake valve closes at point 1.

===Process 1–2 compression stroke (''B'' on diagrams)===
Piston moves from crank end (BDC, bottom dead centre and maximum volume) to [[cylinder head]] end (''TDC'', top dead centre and minimum volume) as the working gas with initial state 1 is compressed isentropically to state point 2, through [[compression ratio]] {{math|(''V''<sub>1</sub>/''V''<sub>2</sub>)}}. Mechanically this is the isentropic compression of the air/fuel mixture in the cylinder, also known as the compression stroke. This isentropic process assumes that no mechanical energy is lost due to friction and no heat is transferred to or from the gas, hence the process is reversible. The compression process requires that mechanical work be added to the working gas. Generally the compression ratio is around 9–10:1 {{math|(''V''<sub>1</sub>:''V''<sub>2</sub>)}} for a typical engine.<ref name="Electropaedia">{{cite web|url=http://www.mpoweruk.com/heat_engines.htm |title=Heat Cycles - Electropeaedia |publisher=Woodbank Communications Ltd |access-date=2011-04-11}}</ref>

===Process 2–3 ignition phase (''C'' on diagrams)===
The piston is momentarily at rest at ''TDC''. During this instant, which is known as the ignition phase, the air/fuel mixture remains in a small volume at the top of the compression stroke. Heat is added to the working fluid by the combustion of the injected fuel, with the volume essentially being held constant. The pressure rises and the ratio <math>(P_3/P_2)</math> is called the "explosion ratio".

===Process 3–4 expansion stroke (''D'' on diagrams)===
The increased high pressure exerts a force on the piston and pushes it towards the ''BDC''. Expansion of working fluid takes place isentropically and work is done by the system on the piston. The volume ratio <math>V_4/V_3</math> is called the "isentropic expansion ratio". (For the Otto cycle is the same as the compression ratio <math>V_1/V_2</math>). Mechanically this is the expansion of the hot gaseous mixture in the cylinder known as expansion (power) stroke.

===Process 4–1 idealized heat rejection (''A'' on diagrams)===
The piston is momentarily at rest at ''BDC''. The working gas pressure drops instantaneously  from point 4 to point 1 during a constant volume process as heat is removed to an idealized external sink that is brought into contact with the cylinder head. In modern internal combustion engines, the heat-sink may be surrounding air (for low powered engines), or a circulating fluid, such as coolant. The gas has returned to state 1.

===Process 1–0 exhaust stroke===
The exhaust valve opens at point 1. As the piston moves from "BDC" (point 1) to "TDC" (point 0) with the exhaust valve opened, the gaseous mixture is vented to the atmosphere and the process starts anew.