Thrust-specific fuel consumption: Difference between revisions

→‎Significance of SFC: ref supersonic engine
(→‎Significance of SFC: ref supersonic engine)
SFC is dependent on engine design, but differences in the SFC between different engines using the same underlying technology tend to be quite small. Increasing [[overall pressure ratio]] on jet engines tends to decrease SFC.
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 (physics)|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.
SFC varies with throttle setting, altitude and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force times distance, mechanical power is force times speed. Thus, although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed to get a way to compare engines that fly at different speeds.
For example, [[Concorde]] cruised at [[Mach number|Mach]] 2.05 with its engines giving an SFC of 1.195&nbsp;lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51&nbsp;lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines; it was the world's most efficient jet engine.<ref>[ Supersonic Dream]</ref><ref>"[ The turbofan engine]", page 5. ''[[SRM University]], Department of aerospace engineering''</ref> However, Concorde ultimately has a heavier airframe, and due to being supersonic is less aerodynamically efficient, i.e., the [[lift to drag ratio]] is far lower. In general the total fuel burn of a complete aircraft is of far more importance to the customer.