Why do piston engines in aircraft burn fuel at a higher rate than a comparable car engine?

I've never understood why piston powered plane engines burn such a large amount of fuel per time period than car engines.

Can anyone elaborate on this?

I know that a C172M will burn ~5-9+ gph at 2200-2550 RPM between 2k-12k feet based on the POH. This gives a decent idea of the fuel burn in a very narrow RPM range at various altitudes. According to the POH, she came with a Lycoming 0-320-E2D. Wikipedia states that the 0-320 provides 150-160HP using a 5.23l engine.

My car on the other hand has a 2l 4 cylinder engine. At 70mph in top gear @ ~2300rpm she gets about 30mpg for a calculated 2.33gph. To me highway speeds at constant RPM provides comparable engine conditions to that of cruise phase of flight.

5.23l / 2l = 2.61

2.61* 2.33mpg = ~6gph

Does that basic analysis hold? Is the larger displacement the main factor in fuel consumption difference between the two? I consider this figure of ~6gph close enough to the 5-10gph listed in the POH for such a back of the napkin calculation if the analysis is more or less correct.

This leads to a follow up question: Is there a reason for the large displacement and low HP? For comparison a 5.7l truck engine from the 70s output 250HP for a gain of 100HP for .47l additional displacement when compared to the Lycoming. I'm in no way very knowledgeable about engine's but am guessing the answer lies in engineering trade offs for making the engine lighter or out of a different material for aviation use or for higher compression ratios for the fuel. Any detailed input would be appreciated.

I know that Volkswagen and Subaru engines have been used for aircraft. Although my research has been very light, it still looks like the fuel consumption is in the same range.

Going back to my car vs the C172M, or the EcoTec vs Lycoming, 173hp@ 5500rpm for the EcoTec and assuming a (most probably false) linear fuel consumption based on RPM,

5500rpm /2300rpm = 2.39

2.39 * 2.33gph (@2300rpm) = 5.56gph

for comparable HP.

If my analysis provided here is close to the true answer, then I think I've cleared up my misunderstanding by writing out this question. However, I would like someone more knoowledgeable about such things (possibly an A&P who is also a gearhead) to pipe in and verify or enlighten my analysis.

Thanks!

• Comments are not for extended discussion; this conversation has been moved to chat. – Federico Mar 29 '18 at 18:38

As others have noted, airplane engines and car engines have very different duty cycles. An airplane engine will typically run at full power for a few minutes during takeoff & climb, then at a large fraction of max during level flight, which may last for hours, and be throttled back only during descent and landing.

An auto engine, by contrast, only operates at full power for a few seconds, as you for instance accelerate up that freeway on-ramp, then it will be throttled back to produce the small fraction of full power that's needed to maintain freeway cruise speeds¹. Since the power output is proportional to fuel consumed², your car engine uses much less fuel. Even though it might be rated to produce the same max HP as the airplane engine, in normal operation it produces much lower HP for the vast majority of its operating time. You can see this in operation if you drive a car with a real-time fuel consumption gauge. For instance, my Insight hybrid shows ~25 mpg when accelerating or climbing mountains (using near full power), but 75 mpg or better cruising on a level interstate.

Also note that the actual fuel economy of an airplane isn't that much different from say your average SUV. I don't have the exact figures any more, but I once calculated that my Cherokee 180 got about 18 mpg. When you figure that it's going twice as fast, and on longer trips can shorten the actual distance travelled by going in a straight line instead of having to follow roads, the comparison isn't that bad.

¹This, incidentially, is why hybrid cars can use electric assist to get much better mpg - the gas engine can be just big enough for efficient cruise, while acceleration is provided by the electric motor.

²To a first approximation. It's a lot more complicated in the real world. Look up BSFC maps if you're curious.

• "... then it will be throttled back to produce the small fraction of full power that's needed to maintain freeway cruise speeds" - I take it you've never been to Germany? ;) – falstro Jan 4 '17 at 10:14
• @falstro you seem to have never been to Germany either. Although the German Autobahn is famous for having no speed limit, traffic prevents more than short sprints almost all of the time. ;) – André Stannek Jan 4 '17 at 11:55
• @AndréStannek Right, that's why I drive at night... But either way, maintaining 90-100mph is usually not impossible, which is a substantial strain on car engines in comparison to the more lenient 60-70. – falstro Jan 4 '17 at 11:56
• @falstro Is maintaining 90 mph really that big of a strain on a car engine? I'm pretty sure I can maintain that speed in my car with less than 50% engine power and mine just has an average engine (around 140 hp, IIRC,) not anything fancy. I don't drive that fast very often, but I've needed to at times and haven't noticed especially high rpms. I do drive 75-80 mph fairly often and that uses maybe 1/3 of available engine power. – reirab Jan 4 '17 at 15:52
• @reirab obviously that would depend on the engine. I've driven several cars which were unable to maintain that speed. And yes it's not scientific but if I compare the gas mileage driving through Germany, with that of other countries, an Audi Q3 gains 150 miles in range once I cross the border. Oh and it sheds quite a bit less oil too. So yeah, the added drag is certainly making the engine work... ;) And yes I'm a lay person, I don't know how this affects the longevity of the engine. It was intended more as a humorous remark. – falstro Jan 4 '17 at 16:22

You are comparing apples to oranges.

Run your car engine at full throttle and measure again. The aircraft engine will look very good in comparison.

Modern car engines optimise fuel consumption with electronic engine control, so the fuel/air mixture and the timing of the spark plugs and the valves can be adjusted to the actual condition. Aircraft engines are technically still stuck in the 1950s and much more simple, which helps to make them more reliable. This also means that at partial loads the car engine is very efficient. However, once you operate it at or near its maximum power, all efficiency is thrown overboard and the parameters are set for survival.

When you run your car engine at full throttle, it will use a richer mixture in order to use the evaporation energy from the fuel to cool the cylinder. The same is true for the aviation engine, where the mixture is controlled by setting a desired exhaust gas temperature (EGT). However, the aviation engine is designed for running continuously at or near full power, so its cooling system will be more adequate, while your car engine will need relatively more cooling from fuel evaporation because its cooling system is too small for continuous operation at full power.

• I'm curious about the "more reliable". Most auto engines these days get easily past the 5000-hour mark without any kind of 100-hour inspection or 1000-hour overhaul. That sounds pretty reliable to me. – Martin Argerami Jan 4 '17 at 14:05
• @MartinArgerami Aircraft piston engines can go past the 4000-6000 hr mark with little more than oil servicing and spark plug cleaning. It depends on how the engines are operated. If I see an aircraft engine that only makes it to 1000 hrs I know that it was not operated properly, was flown very little per annum, or suffered from manufacturing issues from the time it was assembled. – J Walters Jan 4 '17 at 14:33
• @MartinArgerami, the 100-hour inspection and 1000-hour overhauls are because you need much higher reliability from aircraft engines and that is because in a car if the engine quits, you pull over and call a tow truck, but in a plane, especially single-engine one, you are in a fix. – Jan Hudec Jan 5 '17 at 19:38
• @MartinArgerami would the same car engine reliably get past 5000 hours if it was, as discussed, run at full output most of the time? – rackandboneman Nov 24 '17 at 16:16

RPM and displacement are not sufficient to determine the power created or fuel consumed by an engine. You also need to consider the throttle position.

Put your Saab in neutral and try to hold 2300 RPM. How far do you have to push the accelerator? Not very far. Drive your Saab up a steep mountain and try to hold 2300 RPM. How far do you have to push the accelerator? Depending on how steep the hill is, you may be unable to maintain 2300 RPM at full throttle, and need to downshift!

The additional parameter you need to consider is the manifold absolute pressure of the engine.

At low power, the throttle plate is mostly closed. When the intake valve opens and the cylinder descends, it can only suck a little air through this restriction. This suction generates a strong partial vacuum. A properly mixed engine will introduce a small amount of fuel to burn correctly with this small amount of air, and burning this smaller amount of fuel with the smaller amount of air produces a small amount of exhaust gas, weakly pushing the cylinder down. But in neutral, this weak push is all that's required to maintain 2300 RPM.

At high power, the throttle plate is open. When the intake valve opens and the cylinder descends, it can suck lots of air in through this open throttle. It only generates a weak vacuum, because it's easy for the air to flow in. A properly mixed engine needs to introduce lots of fuel to burn with this large amount of air, and burning all this fuel and air generates lots of pressure and pushes down powerfully on the cylinder. In neutral, this strong push would cause the engine to race much faster than 2300 RPM, or with the resistance of a hill or a propeller, may maintain 2300 RPM.

Cruising on the highway requires less power than cruising in an aircraft. Your car has a low manifold pressure/strong vacuum/closed throttle on the highway, while the Lycoming has a high manifold pressure/weak vacuum/open throttle, as indicated by their different fuel consumption!

A few observations:

Aerodynamic (parasite) drag is proportional to speed squared. Power to overcome it is proportional to drag times speed - so is proportional to speed cubed. To go twice as fast you have to push 4 times as hard at twice the speed, requiring 8 times the power. And 8 times the fuel burn. But with twice the distance, yielding 1/4th the MPG, all else being equal. That's why the fastest legal sports cars, capable of 200 - 250 MPH, need nearly 1000HP (750kW) engines and get less than 5MPG when doing so.

Yes, aircraft engines are low tech compared to modern auto engines. They also generally drive their propellers directly rather than through a geared transmission. So they have to run at low RPMs and can't get high HP per unit volume. My 1982 Honda V45 Sabre had an 84 HP 45 in^3 engine but could run at 10000 RPM.

Because of the low engine speeds, a two valve head allows plenty of air flow as there's time for it to happen (volumetric efficiency is as good or better than most 4 valve head auto engines). Similarly, the power used to push the pushrods, rockers and valves is returned by the valve springs - they have time to do it.

Why not run small, fast engines with gearing? Propellers and engines have terrific torsional vibration we don't see in cars due to the flywheels and vehicle inertia (and lack of propellers!). Those loads and potential resonances have destroyed many, many attempts to make such solutions work. Yes, designs can be made stiff and strong enough to be reliable and have. The problem is the added cost and weight that defeat the whole reason for going that route.

So, in short - most aircraft engines have a specific fuel consumption (amount of fuel per horsepower (or KW) per hour) similar to most auto engines. The latter are somewhat better due to modern fuel injection, variable valve timing, ignition control, water cooling allowing tight temperature control and mechanical tolerances and such. But for their older tech, the aircraft engines come remarkably close. That's how I've seen Rutan 2 place aircraft fly hundreds of miles with two aboard at about 140MPH+ while getting over 40 MPG (c.f. cafefoundation.org).

Want even better fuel consumption? Go heavy fuel (diesel). The engines in supertankers have BSFCs approaching .16 or so (versus air cooled aircraft 0.45 to 0.5; UAV two stroke engines 0.68 on up).

Another factor in this equation is drag. Drag increases exponentially with speed, requiring a non linear amount of increased power to hold a higher speed. Run your 70 mph test in your car again at 140 mph (a much more realistic cruise speed for the 172 used in your example) and see how much more fuel you burn per hour. It will be a lot more than double.

There are many variables involved: compression ratios, running a lean air\fuel mixture is good for mpg but makes an engine run hot, richer fuel\air mixtures make for cooler engine head temps, on an aircraft that's really important. Aircraft run at higher altitudes with thinner air (oxygen content). Aircraft engines are optimized for a much narrower RPM range than an automobile. Also, once a car gets to a specific speed it requires less horsepower to sustain that speed than an aircraft which is not just providing speed maintenance but also the lift necessary to overcome the weight of the airplane.

My airplane has a 360 cubic inch engine (just under 5.9L, 4 big aircooled cylinders), generates 180HP at sea level, and much less at 6-8-10 thousand feet. It is variable pitch prop, so I guess one could compare that to a transmission.

"Dry weight of the 0-360s, including carburettor, mags., ignition harness, engine baffles, spark plugs, tach. drive, starter and generator or alternator, ranges from 282lbs to 290 lbs."

Being AL on the outside (engine case halves and cylinder bodies (with steel sleeve liners) helps to keep it light. No radiator, water pump, sleeving for water to circulate thru.

It burns 10 gph and cruises at 145, so 14.5mpg. Not as good as my 180 hp 2L BWM turbodiesel, getting 42mpg or so on the highway, but only averaging 60mph over distance with toll stops & traffic & stiff.

I've made it from near Buffalo to near Boston in 2:15 (good tailwind at 10,500 ft), while the same drive has never been under 7:30. I'd rather fly!

As explained in an article entitled: 'Power struggle, or why car engines won't fly', that describes in detail the process of adapting a car V-8 for use in the Lancair Tigress twin engine airplane, street car engines rarely have a power output only not close to the 100% of its theoretical maximum, but are mostly used at 10%-20%, as much 33% of their nominal top power, and this, even if a larger throttle opening use, as in airplanes, equals a better volumetric efficiency, better MEP, and a better mechanical efficiency, is the reason why airplane engines use more fuel per hour, the standard aviation way of measuring fuel use, cruise airplane speed is done at some 50-60% of top power, than a car, expressed in mpg or lit/ 100 km; in a flat road, and under 64 mph, 100 km/ h, a car uses less than 10% of its engine available power. Regards, + Salut †

Not one person mentioned the air cooled engines piston/cyl clearance and the lower op temp which leads to expansion inefficiency and losses from blowby. Secondly ,maximum torque is where a reciprocating engine develops maximum efficiency and most automotive and aircraft engines are roughly equal in that regard. The person stating drag from air resistance is an exponential [squared] is correct. That relationship and the higher airspeed of an aircraft is largely why they burn more fuel is correct. As cars drive much over 100kph the fuel efficiency drops off sharply. Rick in Atlantic Canada

• This might be better posted as a comment since it does not offer a very detailed answer to the question, and it also addresses other posts. Once you have enough reputation you will be able to comment on other posts. – J Walters Jun 7 '17 at 3:15

Because many of them are freaking giant like 8 liters for some reason.

That article about why car engines don't fly is BS, things have changed a lot in 20 years, Cessna was building diesel car size engines into them.

They're horribly innefficient, 2 valves per cylinder, gear driven cam that robs power, gear driven magnetos rob power, flat tappet lifters that rob power, a valve clearance that can vary because it has no setting other than changine the pushrod tube length.

Inefficient combustion chamber design, heavy engine....