Why are aircraft engines sooo thirsty?

[Storkeye]

I've been pondering...

Ignoring the distance per pence calculation, why are piston engines so thirsty? My (not so economical) car engine uses 6 litres per hour. But a typical PA28 can use some 40 litres per hour.

Even if the magnetos are sparking twice as much fuel(?) that should still only equate to 12 litres per hour.

Why are SEP engines so uneconomical?

[BackPacker]

What car engine do you have? I'd be surprised if there's any car engine that can do 65% of 160 HP all day long, on just 6 liters per hour, where an O-320 (PA28-161) uses about 30 liters to do so - assuming you leaned correctly.

Remember that at typical highway speeds, your car only needs 20% or less of its rated power. If you don't correct for that, the comparison will be meaningless.

All the calculations I have seen suggest that a properly leaned aircraft engine is pretty efficient, considering the power output.

[Storkeye]

Thanks backpacker, that's a very good point. I was generalizing on engine types so I agree we should factor in leaning and horse power etc.

My car has a 60 litre tank and will do highway driving at 80 mph (80% engine max) for 8 hours. I do a lot of motorway driving which is what sparked my thinking in the first place! :zzz:

So my car, 134 hp with 314 lb torque, acheives 7.5 litres on constant velocity highway driving at 3,000 rpm.

Conversely, I fly at around 2,300 rpm and use 40 odd litres per hour. While we need to factor in engine max and horse power there is still a huge difference in consumption. Just pondering why?

[GoProPilot]

I dont want to hi-jack your thread, but want to add something(question along the same topic)

So:

Why do they produce so little power from massive displacements?

Take a Lycoming O-360 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Lycoming_O-360)

Lycoming 0 306 - displacement is 5.9litres! it produces 180hp.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Car engine on the otherhand

some 2.0L engines are producing 180hp?

So why the massive displacement for A/C engines?

A 5.9L n/a car engine would probably produce 350-500hp

Cheers.

[Rod1]

Comparing my 2L MX5 and my Rotax powered MCR there is very little difference.

Rod1

[Mariner9]

Dunno what year your aircraft was built in, but if you took a car built in the same year the a/c engine was designed, and drive it at PA28 cruise speeds, I suspect the consumption wouldn't be much different.

[pulse1]

Could it be something to with the fact that your aircraft engine is constantly generating the energy to overcome drag and to generate lift. The car engine only produces lift when going up hill? The figures for car consumption above suggest that it is a diesel I.e. 50 MPG at 80! I understand that diesel a/c engines are more economical.

[Proteus9]

A car engine generally will be nowhere near full power output at normal road speeds. It probably isn't 6+ litres either. A car engine isn't having to keep itself off the ground either, just roll along. Reliability isn't quite so critical in a car engine as they're not working at high output for long periods and if they do break down people tend to just stop.

[dubbleyew eight]

my little aeroplane gets the same consumption as a vw beetle.

I used to think this poor until I spoke to the neighbour at the time. he had a racing car that could reach the same speed as my aeroplane.
I asked him the fuel consumption.
200 litres for 8 laps (of the wanneroo race track)

my aeroplane is saintly compared to that.

[LowNSlow]

My 3.5 litre petrol M-B 350 CLK does approx 15 lph at 80 knts compared to my Auster which did approx 18 lph at the same speed. The Auster went in straight lines but the Merc is a lot more comfortable. :)

[BackPacker]

So my car, 134 hp with 314 lb torque, acheives 7.5 litres on constant velocity highway driving at 3,000 rpm.

Both power and torque vs. rpm are not linear curves, and differ with the engine internal characteristics such as spark timing. A typical aero engine will deliver max power and torque at 2700 rpm or thereabouts, where a typical car engine will deliver max power and torque around 4-5000 rpm (diesel) or 5-6000 rpm (petrol). Motorcycle engines and F1 racers occasionally can go as high as 13K rpm, as I recall. So comparing the rpms will not work either.

What you need to look for is the SPC figure - Specific Fuel Consumption. It might be a bit hard to find or calculate, but is essentially the liter per hour per horsepower figure. And is possibly measured or given for a specific (most efficient) rpm only.

Could it be something to with the fact that your aircraft engine is constantly generating the energy to overcome drag and to generate lift. The car engine only produces lift when going up hill?

Indeed. An aircraft needs to produce lift, where a car doesn't. But furthermore wind resistance (parasite drag in aviation terms) increases with the square of your velocity. So if your car was the same basic shape as your aircraft (not counting induced drag from the wings here), then your car doing 50 knots would only have 1/4th of the wind resistance of the aircraft doing 100 knots. On the other hand a car also has roll resistance from the tires, but that will be nowhere near the induced drag of the aircraft.

I once did the calculation in a different way. I found that your typical 4-seater (O-320, 160 HP, 30 l/hr for 100 knots) will do about 6 km for each liter of petrol (assuming no wind), where a typical petrol 4-seater car would do 13 km or so for each liter of petrol. Considering that the aircraft is twice as fast and needs to generate its own lift, I think that's not too bad at all.

[dont overfil]

Our C182T does about 14 miles per gallon. However that is at 150 miles per hour. Better than a Range Rover can manage.:hmm:

D.O.

[Echo Romeo]

My 3.5 litre petrol M-B 350 CLK does approx 15 lph at 80 knts compared to my Auster which did approx 18 lph at the same speed. The Auster went in straight lines but the Merc is a lot more comfortable.

Wish my Auster would do 18 lph mines more like 30 for which she delivers 85 kts :(

[Lightning Mate]

During a reheat take-off, the Lightning F3 used about 3 litres per second.

Thought you might like to know that.

[Mariner9]

Blimey Lightning, thats the same consumption as a steam-turbine powered VLCC at 14 kts :ok:. Bet the Lightning pilots were glad they didn't pay the fuel bills :mad:

[gasax]

Backpacker has it.

Aircraft engines - at least our piston engines are only marginally less economical than car engines. However they run at significantly higher power factors and so the fuel gets used to create the amount of power required.

An interesting comparison is a Toyota Prius (or otherwise the Pious). Not expecially economical as a car - put it on a teack and use the maximum power it can generate and you'll get lss than 10 mpg - and pretty poor handling.

The capcity of our piston engines allows the power to be generated at lower rpm - so allowing direct coupling to the propeller and no gearbox. It also gives large combustion chambers and generally aids combustion - hence the good values of specific fuel consumption that aero picton engines have.

The obvious economy of car engines is down to managing the power changes very cleverly and ensure no fuel is burnt on the overrun, when stationary, when warming up etc etc.

[Silvaire1]

Aircraft engines typically operate at slightly better efficiency than car engines, mainly because they are operating at a higher percentage of full power than a car engine, as others have me mentioned. All engines are more efficient when this is the case. in addition, low rpm engines with fewer cylinders are generally more fuel efficient due to lower internal friction.

Why don't aircraft get better miles per gallon, despite being much lighter than a car? Two reasons: one is that aircraft go fast, and the energy required to overcome parasite drag over a certain distance rises as the square of speed. The second is that induced drag (the drag that comes with holding the aircraft in the air) is high in relation to rolling friction, which is the equivalent energy loss for a car (or train).

Flying at high altitude helps - the engine efficiency decreases but that is overcome by much lower aerodynamic drag.

[Lightning Mate]

but that is overcome by much lower aerodynamic drag.

What? Not at the same IAS it isn't!

[Silvaire1]

At high altitude you higher get higher TAS while flying at an IAS that is closer to the (relatively low) best Lift/Drag speed. That provides lower induced drag for the same speed over the ground. Parasite drag is also lower due to lower IAS, dropping as the square.

[GoProPilot]

gasax



The capcity of our piston engines allows the power to be generated at lower rpm - so allowing direct coupling to the propeller and no gearbox. It also gives large combustion chambers and generally aids combustion - hence the good values of specific fuel consumption that aero picton engines have.

Thanks. That answered my question thrown in the mix.

[abgd]

So why the massive displacement for A/C engines?

You can either make a lot of power by filling and emptying a lot of small cylinders quickly (high RPM) or by having some larger cylinders that fill and empty slowly.

If you choose the former option on an aircraft, you end up needing a gearbox to reduce the RPM of the propeller, to stop the tips going supersonic. This adds weight, undoing some of the advantages of a smaller engine. So in the interests of simplicity, most aero engines simply go for large cylinders without a gearbox.

[GoProPilot]

Quote:
So why the massive displacement for A/C engines?
You can either make a lot of power by filling and emptying a lot of small cylinders quickly (high RPM) or by having some larger cylinders that fill and empty slowly.

If you choose the former option on an aircraft, you end up needing a gearbox to reduce the RPM of the propeller, to stop the tips going supersonic. This adds weight, undoing some of the advantages of a smaller engine. So in the interests of simplicity, most aero engines simply go for large cylinders without a gearbox.

Thanks, great answer :).fully understand now.

[phiggsbroadband]

Hi, if you look at it from a thermodynamic point of view, all 4 stroke petrol engines have approximately the same efficiency. It is related to the maximum gas temperature (during combustion,) compared to the minimum gas temperature (when the exhaust gas leaves the engine.)

This is why diesels are so good, they have a high compression ratio (say 20:1.) which can be seen as a 20:1 expansion ratio. This lowers the exhaust temperature adiabatically by much more than a 9:1 expansion ratio petrol engine. So the min to max gas temperatures are better in the diesel. i.e. it gets more power out of the fuel than a petrol engine.

Manufacturers can make slight improvements, by reducing friction losses etc. but the 9:1 ratio is pretty well fixed for petrol as any higher causes pre-ignition.

Absurdly an engine at zero rpm has the least amount of friction losses, so it is more efficient to have low engine rpms. Also exhaust valve timing, opening at bdc would utilise more of the gas energy in the expansion ratio. However a 27 litre engine that only revs to 50 rpm would have pistons the size of dustbins, and flywheels of several tonnes... maybe suitable for boats but not aircraft.

[Silvaire1]

if you look at it from a thermodynamic point of view, all 4 stroke petrol engines have approximately the same efficiency.Yes, and for that reason I think the bottom line is that from a technical perspective, airframe and operational considerations are more fertile ground to gain in overall aircraft efficiency than engine changes. Airframe weight reduction, increased wing aspect ratio, higher altitude cruise etc all tend to have a good payback.

Changes to engine design that increase efficiency tend to have associated drawbacks. Water cooled heads increase efficiency at climb power by allowing leaner mixtures, but the advantage tends to go away in cruise and you're left with a more complex design. Within the modern era of small piston aircraft engines, Continental did water cooled cylinders in the early 80s, but apparently didn't see fit to pursue the idea commercially. Rotax started with water cooled heads on higher rpm four strokes in 1987, and at full power the efficiency benefit of water cooling more than offsets the efficiency disadvantage of higher rpm. Everything looks comparatively good until you get to cruise, at which point you could lean out a simple, air cooled, lower speed engine and it would start looking OK again, without the extra stuff. You can actually achieve the same thing in climb on an air cooled engine using water injection but nobody has bothered for decades, instead using extra fuel as the evaporative coolant for full power climb.

Geared engines tend to be lighter for the same power, which helps reduce overall weight... a good thing. But as with the Rotax, cooling issues then become more critical, which may necessitate water cooling, and you may also need dual carbs or EFI - both changes adding complexity and some measure of weight. Propeller choice and gearbox resonances can also become an issue, which is one reason Rotaxes mostly use wooden props - they are both lightweight and like a big sponge for vibration, but they are also less aerodynamically efficient and less durable than metal props. The net result for the Rotax is a lightweight engine that isn't as 'industrially tough' as it might otherwise be. Good for sport planes.

Diesels have high efficiency for the reasons described by previous posts but are intrinsically heavy. To get them anywhere near competitive in weight for an aircraft they become complex (turbo'd, geared, water cooled, and non-rebuildable), maintenance intensive, and extremely expensive. Few private buyers benefit financially over the life cycle of the engine, even in areas where fuel tax is extreme. I think personally that their best application is a flight school that flies the aircraft a lot of hours/month and has a full time mechanic. The biggest current user of diesel aircraft engines, the US Army, shares those characteristics. Also benefiting would be those buyers who simply cannot buy aviation gasoline locally due to lack of infrastructure in their area - which is also true for US Army operations, and is the reason they went to diesel UAVs.

To me, the most promising change for privately owned and operated aircraft engines right now is simply to incorporate variable ignition timing, as per Lightspeed ignition which advances spark in the partial power, high altitude configuration and is thereby effective in the same regime where the airframe is most efficient, and where the plane operates for the longest period during a cross-country flight. The offsetting disadvantage, which is usually the necessity for an aircraft electrical system, is no longer a significant driver for modern aircraft. But again, the efficiency gains that can be realized with a lightweight, aerodynamically efficient airframe seem to exceed what can on balance be achieved with engine changes for the average user.

[Torque Tonight]

Why do they produce so little power from massive displacements?

Aircraft engines operate in a rarefied atmosphere. To burn a certain mass of fuel in one cycle of the engine, a certain mass of air is required. At altitude the air is less dense and so that mass of air occupies a greater volume. Therefore, all other things being equal an aero engine requires greater displacement to gnerate the same output as an equivalent car engine. Turbo and supercharging are ways of mitigating this.

[A and C]

I think that Silvaire1 has the nub of the problem identified, the engine it's self is quite efficient however the management of the engine is poor, fixed ignition timing and poor fuel management devices are not helping.

All the devices the you need to increase the efficiency of your engine are available however the certification system is restricting the innovation due to over regulation.

I also agree that for the Private owner the airframe offers efficiency savings the only problem with this is that with savings in weight come lack of durability, the Sport Cruiser is the perfect example of this, in the hands of a careful private owner it is a good aircraft but the lack of durability would make it an economic disaster in a flight school, for that task you need a Rotax turbo engined Cessna152.

[abgd]

Interestingly, the LAA in the UK has recently been delaying approval for conversions from magneto to electronic ignition for aircraft with metal propellers only. The argument goes that with variable timing, previously benign combinations of engine and propeller may start to exhibit resonances...

Perhaps they're being generally overcautious, but it's surprising with machinery how even ostensibly minor changes can rise up and bite you.

[Zonkor]

There's airplanes that are comparably fuel-efficient (per mile) to cars, such as a number of motorgliders and light-sport aircraft. Additionally, they often can use the cheaper MoGas instead of AvGas.

Sadly, however, motorgliders seem to be less and less popular. Light-sport aircraft are booming at least in Europe.

[shortstripper]

An interesting comparison might be the humble VW. I used to have a 1600 VW camper van with a fuel consumption of around 25 mpg. Many LAA machines use the same engine. In the campervan you would only cover around 50 miles in an hour of average driving, so that's 2 gph. In a Falke two seat motorglider or single seat light aircraft such as a Jodel D9 you could expect around 2.5 gph. So the difference isn't huge really for a direct comparison on engine size and power.
In a car you rarely have your foot pressed 3/4 travel on the accelerator pedal. You use most fuel getting up to speed or climbing hills. The rest of the time you are coasting or holding a small amount of throttle to regulate speed. On the motorway doing 80 mph you are not using 80% of power unless perhaps going up hill or overtaking. In a light aircraft you climb a much higher hill then set the throttle for the cruise and that's where it stays until you descend. It is probably set at 75% most of the time and whilst that doesn't exactly equate to 75% "power", it's a pretty good indication. How often in a car do you have you foot 3/4 of the way down on the pedal for any amount of time? I'd say very rarely if you think about!

SS

[dubbleyew eight]

A and C cant fully agree with you there.

if I look at a typical 1 hour flight I spend 5 minutes taxying at 1,000 rpm where I dont even need that much thrust.
I then spend 5 or 6 minutes climbing at full throttle to an altitude.
then I spend 45 minutes or so at 2500rpm.
then I enter circuit and spend the next 5 minutes at about 1500rpm.

I need maximum power for about 6 minutes then I fly at a deliberately reduced power for fuel economy.

so provided the engine delivers adequate climb power and runs smoothly in cruise I dont need all the "modern automotive" spark adjusting doo dads.

what a lot of people never realise is that aero engines have power profiles more like stationary engines than car engines.

I also dont want delicate little doo dads in my engine because they inevitably become in flight failures. I also dont need the engine to run faster because the prop at 2500rpm is already running faster than the propeller optimum of 1700rpm.

Innovate all you like but until you get the thermal efficiency of the engine over 25% all you are doing is wasting your time on minor peripheral issues.
YMMV.

[A and C]

what a lot of people never realise is that aero engines have power profiles more like stationary engines than car engines.


Lets not forget the Lycoming started life as an agricultural ditch pump motor.

[Jonzarno]

When not in a hurry, or if there's a big tail wind, I often fly my SR22 for economy rather than speed. My flight yesterday at FL170 gave the following results:

9 GPH, 145 KTAS converts to 22.3 MPG (imperial, not US)

My car certainly won't do that. :)

[Zulu Alpha]

There are advantages to running an engine slowly.

Look at this beastie The World's Most Gargantuan Diesel Engine (http://gizmodo.com/5822447/the-worlds-most-gargantuan-diesel-engine)

109,000hp at 102 rpm

[abgd]

There's airplanes that are comparably fuel-efficient (per mile) to cars, such as a number of motorgliders and light-sport aircraft. Additionally, they often can use the cheaper MoGas instead of AvGas.


I've just bought the plans for Michel Colomban's luciole - 81 knots on 4.5l/hr or about 90 miles per gallon. The thing is, it's not really fair to compare this small single seater with no luggage capacity to e.g. a toyota prius. In automotive terms, it's equivalent would be a concept-car for commuters that might do 200-300 mpg.