Why are aircraft engines sooo thirsty?

[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]

Motorgliders and light-sport aircraft

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

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.