# How do I estimate piston aircraft fuel efficiency?

Is there any way to figure out or calculate fuel consumption of piston engines for aircraft? I'm looking at different engines in a book, and while jet engines have useful values like thrust, bypass and pressure ratio, piston engines only have power output and weight.
Propeller efficiency plays a role here, I guess, but it's not at hand so I assume all engines use the same propeller in this case.
Would the easiest way to calculate such a number be to find the maximum fuel capacity and range and from there find out how much fuel each engine uses per hour?

EDIT: To clarify, I am looking at airliners powered by piston engines and wish to compare them.

• Do you mean in general for a model engine, or for a specific aircraft? Piston aircraft have fuel consumption charts in the POH that tell you how much fuel in gal/hr you will use at different power settings and different conditions. May 23, 2019 at 15:19
• To clarify, I am looking at airliners powered by piston engines and wish to compare them. Edited into original question. May 23, 2019 at 18:38

Yes there is a simple rule of thumb for what is known as Specific Fuel Consumption that you can apply to estimate the fuel burn of most piston engines:

An air cooled carbureted engine, leaned, will burn roughly 0.44 to 0.45 lb/hp/hr. With Fuel Injection, a bit less, maybe 0.41 or 0.42-ish.

Cars with electronic sequential fuel injection are in the high 0.3s. Diesels are in the low 0.3s

2 strokes are up in the mid 0.5s.

Turbo props are very thirsty and are in the .6 range.

Take a Cessna 150.

75% cruise is 75 hp. X 0.45 = 33-34 lb/hr. = 5.6 usg/hr.

Which is about what you will find in the POH, or close enough anyway.

Want to know the fuel burn of an R-985 powered airplane?

450 X .75 = 337 hp @ .44 lb/hp/hr = 148 lb/hr = 26 usg/hr, which sounds about right for something like a DeHavilland Beaver.

• Thanks for your reply. This means that by estimating the SFC and cruise speed %, I can get the (estimated) actual fuel usage per hour? Other than engine type, what might affect SFC? May 23, 2019 at 18:39
• If it's for a big radial say, whether the engine is in auto rich or auto lean. In auto rich SFC would be maybe .5-.6 because you are using fuel for cooling. Also, turbo charging helps reduce SFC since you are recovering lost energy that would be wasted.Also Power Recovery Turbine systems as used on the R3350s and other big radials, which recovered about 300 hp from the exhaust, reduced the R-3350's SFC into the high .3s. Was one of the most fuel efficient aircraft piston engines ever made. 75% power cruise of a Super Constellation, I'd use an SFC of around .38. Using 3500 TO hp, 665 usgal/hr. May 23, 2019 at 19:32
• With 80 ppl aboard going 340mph, the S Connie would be burning 8.3 gal per pax, getting about 41 per-passenger miles per us gallon. I don't think any jet can come close. May 23, 2019 at 19:46
• @MatiasLq: Speed will not affect fuel consumption per hour. But altitude will, and now you need to know how much the engine is super- or turbocharged. Generally, as density decreases with altitude, fuel flow will do so, too. May 23, 2019 at 19:53

You don't really need to calculate it, manufacturers take care of that for you (for every plane built in the past half century or more). These numbers are available in the POH for the given aircraft but you will need to chose your aircraft as engines may have different characteristics based on installation.

For example here is the POH for the Warrior I fly you can find the fuel burn info and performance data starting on page 5-12.

• Another example, for an O-360 (180HP) in a Cessna Cardinal. I lean to about 10 gallons/hour once leveled off in cruise. Maybe somewhat richer in combination with opening the cowl flaps in hotter weather to keep cylinder head temperatures to 380F or lower. May 23, 2019 at 16:35
• Thanks for the reply. I was looking more towards airliners rather than sports planes. Not many commercial piston aircraft have been manufactured the last 50 years, so it would seem difficult to look for/purchase POHs for many different aircraft. May 23, 2019 at 18:37
• @MatiasLq old POH's for planes of that size are not terribly hard to come by and have plenty of engine data the layout and organization is more or less the same as a small aircraft. Most GA planes that are commonly flow (172/Cherokee) were designed ~50 years ago anyway.
– Dave
May 23, 2019 at 19:47