Specific Fuel Consumption and Efficiency

In engine test, the fuel consumption is measured as a mass flow rate per unit time second m f , a useful
parameter is the specific fuel consumption ( sfc ),as we can state is as the fuel flow rate per unit power output.

It measure how efficiently an engine is using the fuel supplied to produce work :

                      Sfc = ---------------------

when we express about sfc in units ( mg/J ),as the fuel mass flow express in (g/s),and power with ( kw )

A dimensionless parameter that relates the desired
engine output (work per cycle or power)to the necessary
input ( fuel flow ) would have more fundamental value.
The ratio of the work produced per cycle to the amount
of fuel energy supplied per cycle that can be released
in the combustion process is commonly used for this
purpose. It is a measure of the engine's efficiency.

The fuel energy supplied which can be released by
combustion is given by the mass of fuel supplied to
the engine per cycle times the heating value of the fuel.

The heating value of a fuel, Q_HV, defines its energy
content.It is determined in a standardized test procedure
in which a known mass of fuel is fully burned with air,
and the thermal energy released by the combustion
process is absorbed by a calorimeter as the combustion products cool down to their original temperature.
This measure of an engine's " efficiency." which will be called the fuel conversion efficiency nf.
Typical heating values for the commercial hydrocarbon fuels used in engines are in the range 42 to 44 MJ/kg (18,000 to 19.000 Btu/lbn) Thus; specific fuel consumption is inversely proportional to fuel conversion efficiency for normal hydrocarbon fuels.

Note that

* The fuel energy supplied to the engine per cycle is not fully released as thermal energy in the
    combustion process because   the actual combustion process in incomplete . When enough
    air is present in the cylinder to oxidize the fuel,completely,almost all (more than about 96 percent)
    of this fuel energy supplied is transferred as thermal energy to the working fluid.

* When insufficient air is present to oxidize the fuel completely, lack of oxygen prevents this fuel energy
    supplied from being fully released.