Specific fuel consumption
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Specific fuel consumption, often shortened to SFC, is an engineering term that is used to describe the fuel efficiency of an engine design. It measures the amount of fuel needed to provide a given power for a given period.
SFC is dependent on engine design, but differences in the SFC between different engines using the same underlying technology tend to be quite small. For instance, typical gasoline engines will have a SFC of about 0.5 lb/(hp·h) (0.3 kg/(kW·h) = 83 g/MJ), regardless of the design of any particular engine. Generally, SFC within a particular class of engine will increase with the compression ratio. Diesel engines have better SFCs than gasoline largely because they have much higher compression ratios, the way they burn their fuel is actually less efficient.
In practical applications, other factors are usually highly significant in determining the fuel efficiency of a particular engine design in that particular application. For instance, in aircraft, turbine (jet and turboprop) engines are typically much smaller and lighter than equivalently powerful piston engine designs, both properties reducing the levels of drag on the plane and reducing the amount of power needed to move the aircraft. Therefore, turbines are more efficient for aircraft propulsion than might be indicated by a simplistic look at the table below.
| Engine type | SFC |
|---|---|
| Ramjet | 1.0 lb/(hp·h) (0.61 kg/(kW·h)) |
| Turbo-prop | 0.8 lb/(hp·h) (0.49 kg/(kW·h)) |
| Otto cycle | 0.5 lb/(hp·h) (0.3 kg/(kW·h)) |
| Diesel cycle | 0.4 lb/(hp·h) (0.24 kg/(kW·h)) |
| Otto-Compound engine | 0.38 lb/(hp·h) (0.23 kg/(kW·h)) |
| Turbocharged Diesel | 0.38 lb/(hp·h) (0.23 kg/(kW·h)) |
| Turbocharged & Intercooled Diesel | 0.36 lb/(hp·h) (0.22 kg/(kW·h)) |
| Diesel-Compound engine | 0.34 lb/(hp·h) (0.21 kg/(kW·h)) |