The specific fuel consumption, is the amount of fuel utilised relative to the mechanical power produced and is a measure for the quality of the energy conversion in the combustion engine. In the engine map, lines of constant specific fuel consumption form so-called "shell curves".
As the lines of constant power indicate, the combustion engine can achieve a certain power output at different operating points. Thus the layout of the transmission and axle ratio as well as the gear shifting point in stepped transmissions or the control strategy in the case of infinitely variable torque converters have a large influence on the fuel consumption.
If the road resistance curves dependent on speed are introduced into the engine map, while taking into consideration the transmission ratio and the losses in the drive train, the indicated demand curves result for constant velocity drives in plane.
The figure shows an engine map along with the demand curves for a stepped transmission and the control characteristic curve for a CVT tuned for minimum fuel consumption.