Analyses

The Efficiency of the engine in converting the heat of combustion of the fuel into useful work is of major interest. This quantity may be defined in a number of ways depending on the detail that is of interest. In a pragmatic sense, the heat provided by the fuel has to be purchased and the work output is what one gets for this expenditure. An efficiency in the form:
Efficiency = h = (What you Get) / (What you Pay For)

can be used to estimate the performance of the cycle under different operating conditions.
Efficiency can also be estimated in terms of thermodynamic quantities such as the work done on the piston by the "working fluid" and the heat addition to this fluid in the combustion reaction. In terms of the ideal Otto cycle (p,v) diagram:

Efficiency = h = (Work on Piston) / (Heat from Combustion) = (pdv)/ Q_3-4