Reciprocating engines are nothing more than energy conversion devices. They convert a fuel (usually gasoline or diesel fuel) into various other forms of energy such as heat, smoke, noise, vibration, and horsepower. What's really interesting is how well, that is how efficiently, a given reciprocating engine converts its fuel to useful work - horsepower. The relative energy conversion efficiency of any two engines can be determined by looking at their BSFC numbers. BSFC stands for Brake Specific Fuel Consumption. It's a measure of how many pounds of fuel an engine consumes per hour per useful horsepower that the engine produces.

To illustrate, let's compute the BSFC of a hypothetical engine: the Travis-300:

Dynamometer tests of the Travis-100 disclose that it is capable of a maximum of 300 horsepower. At a reasonable cruise power setting of 70% of maximum, or 210 HP, we find that the engine requires 17 gallons per hour of gasoline.

Gasoline weighs six pounds per gallon so our engine requires 102 pounds of fuel per hour. By dividing this number by our power output (210HP) we get a BSFC of 0.48 which is pretty middle of the road.

Diesel fuel weighs more than does gasoline per volume but its energy density is approximately the same. That makes it possible to compare the relative efficiencies of gasoline vs. diesel engines simply by comparing their BSFC numbers.

Why are BSFC numbers important to us? Simple economics. For example, over a 2000 hour TBO run, the Travis-300, with a BSFC of 0.48, will burn 34,000 gallons of fuel. If, somehow, we could either improve the Travis-300 or find an alternate engine with a BSFC of 0.38 (13.33 GPH) then we would reduce the fuel used over a TBO run to 26,660 gallons. At an AvGas price of $2.30 per gallon, that's a savings of over $17,000 for fuel. Also note that the same reduction in BSFC allows us to make a four-hour trip in the airplane using 15 gallons less fuel. That's equivalent to a full-fuel payload increase of 90 pounds.

Two caveats: Aircraft engine fuel consumption figures are generally specified in two ways. One is with a so-called "best power" mixture and the other is with a "best economy" mixture. In this article I have consistently used BSFCs obtained with the "best economy" mixture.

Also, be careful when comparing horsepowers between two different engines. Aircraft engines are rated in the old SAE "standard" conditions of 29.92 inches of atmosphere and an ambient temperature of 59 degrees F. Some auto engines (but not all) are rated with the SAE J1349 method which differs in that the standard atmosphere is 29.38 and the ambient temperature is 77 degrees F. The difference between horsepower obtained with the two tests is less than 5% or a BSFC difference of 0.02.

Typical BSFC ranges are as follows. As usual, there will be exceptions, but this should serve as a general guide:

0.26-0.34 Large industrial four-stroke diesel engines (very small hp/weight ratios)

0.28-0.36 Other four-stroke diesel engines

0.32-0.38 Two-stroke diesel engines

0.37-0.44 Fuel injected four-stroke gasoline aircraft engines

0.40-0.48 Fuel injected four-stroke gasoline automobile engines

0.43-0.48 Carburetted four-stroke gasoline aircraft engines

0.48-0.60 Carburetted four-stroke automobile engines

0.55+ Two stroke gasoline engines

0.55-0.70 Four-stroke aircraft engine takeoff fuel flows

Engines don't maintain a fixed BSFC over their entire range. Typically an engine's BSFCs when producing only a fraction of its rated power are quite high. This is due to thermodynamic factors which limit the engine's efficiency when it runs cold. BSFCs typically reach their lowest value for the engine in the 50-80% power range. Then then begin to trend upward again as friction begins to play a dominant role and/or the mixture must be enriched to provide for adequate engine cooling.