Editing Four-stroke engine (section)

==Thermodynamic analysis==
[[Image:diagrama pv de ciclo 4tempos.png|right|300px|thumb|The idealized four-stroke Otto cycle [[Pressure volume diagram|p-V diagram]]: the
<span style="margin:1px; background-color: #10ff00;">&nbsp;intake (A)&nbsp;</span>
stroke is performed by an [[Isobaric process|isobaric]] expansion, followed by the
<span style="margin:1px; background-color: #ffae21;">&nbsp;compression (B)&nbsp;</span>
stroke, performed as an [[Adiabatic process|adiabatic]] compression. Through the combustion of fuel an [[isochoric process]] is produced, followed by an adiabatic expansion, characterizing the
<span style="margin:1px; background-color: #ff0001;">&nbsp;power (C)&nbsp;</span>
stroke. The cycle is closed by an isochoric process and an isobaric compression, characterizing the <span style="margin:1px; background-color: #639eff;">&nbsp;exhaust (D)&nbsp;</span>
stroke.
]]

The [[thermodynamic]] analysis of the actual four-stroke and two-stroke cycles is not a simple task. However, the analysis can be simplified significantly if air standard assumptions<ref name="airstandard">{{cite web |url=http://www.betp.net/2011/04/air-standard-assumptions/ |archive-url=https://web.archive.org/web/20110421085012/http://www.betp.net/2011/04/air-standard-assumptions/ |archive-date=2011-04-21 |title=Best Place for Engineering and Technology, Air Standard Assumptions |url-status=dead}}</ref> are utilized. The resulting cycle, which closely resembles the actual operating conditions, is the Otto cycle.

During normal operation of the engine, as the air/fuel mixture is being compressed, an electric spark is created to ignite the mixture. At low rpm this occurs close to TDC (Top Dead Centre). As engine rpm rises, the speed of the flame front does not change so the spark point is advanced earlier in the cycle to allow a greater proportion of the cycle for the charge to combust before the power stroke commences. This advantage is reflected in the various Otto engine designs; the atmospheric (non-compression) engine operates at 12% efficiency whereas the compressed-charge engine has an operating efficiency around 30%.