<P> The four - stroke diesel engine has been used in the majority of heavy - duty applications for many decades . It uses a heavy fuel containing more energy and requiring less refinement to produce . The most efficient Otto - cycle engines run near 30% thermal efficiency . </P> <P> 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 are utilized . The resulting cycle, which closely resembles the actual operating conditions, is the Otto cycle . </P> <P> 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% . </P> <P> The problem with compressed charge engines is that the temperature rise of the compressed charge can cause pre-ignition . If this occurs at the wrong time and is too energetic, it can damage the engine . Different fractions of petroleum have widely varying flash points (the temperatures at which the fuel may self - ignite). This must be taken into account in engine and fuel design . </P>

The cycle involved in the operation of an internal combustion engine