Specific Fuel Consumption
Note: This page refers to 4 stroke spark ignition gasoline reciprocating engines.
In engine tests, the fuel consumption is measured as a flow rate or mass flow (mf) per unit time. A more useful parameter for engine comparisons is the specific fuel consumption (sfc) which is the fuel flow rate per unit power (P) output.
sfc = mf / P
To calculate the mass flow, you first need to know the mass of gasoline fuel.
This is: 1 liter = ±740 g [1 US gal = 6.17 lb]
Typical average values of sfc for spark ignition piston engines are: 305 g/kW/h = 227 g/hp/h = 0.5 lb/hp/h
Sfc measures how efficiently an engine is converting fuel into work. The graph of an engine's sfc plotted against engine RPM will be "U" shaped. At high speeds the sfc increases due to increased friction, while at lower speeds the sfc also increases due to higher heat losses from the combusted gas to the cylinder and piston wall because of the increased time the hot gases remain in the cylinder. This is also the reason why air cooled engines are usually less fuel efficient than water cooled engines.
Typical best values of sfc for very efficient spark ignition engines are about 270g/kW/h = 201g/hp/h = 0.443 lb/hp/h. Normally fuel efficiency is less at high power levels (higher RPM). It is also possible that the manufacturer recommends (or programs the electronic motor management) to further sacrifice fuel efficiency by using a rich fuel/air mixture in order to cool down the engine to avoid detonation/knocking at high power output. Sfc figures of more than 400g/kW/h = 298g/hp/h = 0.658lb/hp/h are normal in this case.
A good fuel efficient carburettored 4 stroke spark ignition engine consumes: 200g/hp/h ÷ 740g/L = 0.270 L/hp/h = 0.071 USgal/hp/h. Electronic engine management, multiport fuel injection in combination with efficient intake manifold and porting designs can improve these figures.
If for example the engine had to produce 60 hp constantly (presuming that is the point where it is most efficient), then fuel consumption would be:
60 hp x 0.270 liter/hp/h = 16.2 liters/hour or
60 hp x 0.071 US gal/hp/h = 4.3 USgal/hour - 26.4 lb/hp/hr
Calculating power produced
If your aircraft has very low fuel consumption, then your engine is not having to work hard! This is good for your pocket and good for the wear on your engine.
If someone was claiming their engine only consumed 20 l/h [5.3 USgal/h] while constantly producing 85 hp then this engine would have a sfc of: 20 l/h x 740g = 14800g/h ÷ 85 hp = 174g/hp/h [0.06 USGal/h - 0.38 lb/hp/hr] !
Unless this engine has modern technology installed, and the figures are backed up by other users of the same engine/propeller/aircraft combination, he/she is probably lying because this is 15% less than the typical good figures for carburettored engines. The engine might have a maximum rated power of 85 hp (under ISA conditions at sea level) but with a fuel consumption of 20 l/h [5.3 US gal/h] the engine is probably only developing: 20 l x 740g ÷ 220g/hp/h [0.49 lb/hp/hr] (average good figure) = 67 hp !
The actual power produced could vary anywhere between 80 hp for a modern technology, very fuel efficient engine and 57 hp (or less) for one with poor fuel efficiency.