Why are aircraft engines sooo thirsty?
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Why are aircraft engines sooo thirsty?
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?
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?
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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.
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.
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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!
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?
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!
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?
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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
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.
So:
Why do they produce so little power from massive displacements?
Take a Lycoming O-360 - Wikipedia, the free encyclopedia
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.
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.
Last edited by Mariner9; 8th Aug 2013 at 13:11.
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.
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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.
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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.
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.
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. 
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So my car, 134 hp with 314 lb torque, acheives 7.5 litres on constant velocity highway driving at 3,000 rpm.
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?
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.
Last edited by BackPacker; 8th Aug 2013 at 14:24.
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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.
Last edited by Echo Romeo; 8th Aug 2013 at 14:17.
. Bet the Lightning pilots were glad they didn't pay the fuel bills 
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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.
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.
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.
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.
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.
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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.
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.