Fuel Burn vs Car

Fuel Burn vs Car

<HTML>How come our C172 burns 8 or 9 gallons per hour and my 100+ hp car does uses 2 or 3 gals per hour?</HTML>

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Re: Fuel Burn vs Car

<HTML>There are a lot of guys out there who can answer this better than me but here are a few thoughts. We generally run at 75% power, which would be the equivalent of towing a trailer up the steepest grade you find on an Interstate Highway for hours on end.

Eight gallons per hour is about what a tractor trailer loaded to 80,000 pounds going 60 miles per hour uses with it’s 425HP Caterpillar Diesel (which itself weighs 3,000 pounds).

So you have a puny little four cylinder aluminum engine weighing 280 pounds producing a similar amount of power to a cast iron diesel weighing 3000 pounds and we wonder why it only lasts 2000 hours. The diesel might be good for 10,000 hours, ten times the weight, five times the life. (For the technically minded there is a 20% difference in the weight and BTU’s per gallon, and some difference in pounds burned per horsepower but let’s not spoil the fun.)

Engine TBO on a gallons consumed basis 2000 hours X 8 GPH = 16, 000 gallons. That would take your car 480,000 miles. Lycoming engines don’t look so bad in that light.

When you compare a P210 engine burning over 15 GPH to a comparable diesel it’s a surprise they last at all.</HTML>

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<HTML>Great response, Jim!  Marine engines have the same relationship.  (Many folks think they can replace their aero engines and marine engines with automotive type engines successfully, only to find out that automotive engines can't withstand the abuse we place upon our aero engines.)  Remember that horsepower is a measure of work performed over a period of TIME,... So the relationship of miles per gallon consumed perhour in an aero engine also must be studied in the light of how much work is produced in that time-frame.  Although the typical light aircraft gets about 20mpg (compared to modern autos that get approx 35mpg), that aircraft does it at a speed that averages 3 times that of the automobile, i.e., does the same work in 1/3 the time.  Not bad for such a small fuel consumption per hour/per mile/per horsepower!</HTML>

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<HTML>I thought people might be more familiar with trucks. A medium sized marine diesel burns 100 gallons of diesel per hour while producing 1800 HP at 900 RPM, and weighs about 30,000 pounds.  Try to get that up to takeoff speed.

There was a small experimental helicopter in the 70's with a 100HP marine outboard motor derived engine. Thankfully not many of them flew.</HTML>

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<HTML>Actually, quite a number of them did, it was the "Scorpion", and it is still produced (but uses a different engine).</HTML>

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