Editing Otto cycle (section)

===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>