Science of Aircraft Piston Engines

[Keef]

So, I found myself on Cessna's website checking out some planes, and I noticed that 162's engine makes 100 hp. Sounds decent, considering air isn't all that thick. But at 2,800 rpm? Hmm...

So I went looking for some info on aircraft piston engine science. All I found was this article, which (I guess) is debunking and/or agreeing with myths concerning engine operation.

I read through that a bit, and all the stuff about wear and friction being higher the more rpm you run makes obvious sense. That's the way it works, and it's something us car guys have to deal with. Plus, I'd assume, and hope, that reliability would be an airplane engine designer's number 1 concern, unlike the automotive world that seems to strive for efficiency and power. But then I kept reading a bit more, and the author was talking extensively about engines stress and this and that. I'm thinking, I don't even consider engine stress until I'm cranking out 300 hp, so what is this guys deal? Just punch it and go! But he kept on about wear and stress and rich and lean and inches of mercury and intake manifold pressure and crazy abbreviations and all sorts of this stuff that I don't hear car guys talking about. What is the deal with these airplane people and their engines?

I've come to the conclusion that aircraft piston engine science is just...different than car engine science. Down on the ground we slap a turbo on there and run it a tad rich to keep the EGTs down, and put an intercooler on there to cool the intake charge. Manifold pressure? Yes please, until it blows a gasket. Now I can understand that you might not want to push any limits when you're 5,000 feet up, but what is this guy's deal with reliability? Are airplane piston engines made of paper or something?

I'd appreciate it if someone could explain the concept of reliability and philosophy in general in airplane terms, because it just doesn't seem like that big a deal to me.

 

[Sharky.]

You wouldn't want your engine exploding at 10,000 feet, would you?

 

[nk4e]

I am going to lurk some more because this is interesting.

 

[Sniffs]

well, one thing I DO believe is that all that talk about pressures, etc has to do with the rather startling lack of oxygen for burning the fuel at those heights, and the miniscule air pressure. remember, they're literally several MILES in the air at points, straight up, and a car starts to gag pretty low.

from the specs I read for WW2 aircraft engines, fighters, bombers, etc...we're talking engines that make those in rigs/Lorries look tiny. the ones you were looking at are personal craft engines, and are the only Piston Engines in regular production atm.

 

[wfooshee]

A car engine doesn't have to literally re-jet itself every time it runs. A driver adjustment for mixture? Ha!

Most private piston-engine planes have a service ceiling under 10,000 feet, and spend most of their time about 5 or 6 thousand feet above ground level. That puts them in an atmospheric altitude of 5,000 to 10,000 feet. Higher and you have to carry breathing oxygen.

The mixture changes with altitude. Well, the ideal mixture doesn't change that much, but the carb setting to achieve the mixture change.

Aircraft engines also do not coast, accelerate, rev, idle, shift gears, or any of those automotive tasks. They hit maximum power for takeoff, then throttle back to cruising RPM, around 2500 to 3000, and they live there, for hours and hours. What we look for in car engines, things like throttle response, ability to accelerate (the engine's acceleration, not the car's - we want it to rev freely) and smooth idle are almost irrelevant in an aircraft engine. Throttle adjustments are small and slow, RPM is steady, not fluctuating, and they hardly ever actually run at idle.

The other thing about an aircraft engine is that the aircraft has to be able to carry it. Water cooling is heavy, so you run an air-cooled engine. Plenty of air up there, huh? But air cooling brings you into a limit for specific output, which brings up the stresses your writer was discussing.

If you want to fly really high, like for a fighter, bomber, or an airliner, you need to get above 20,000 feet. Now you're talking supercharging just to get enough air into the cylinder for combustion. The P-38 had a pair of really big honking turbos on the backs of those tail booms. And a pressurized cockpit. All of that is too expensive for a private plane on the order of your average Cessna.

 

[Keef]

The other thing about an aircraft engine is that the aircraft has to be able to carry it. Water cooling is heavy, so you run an air-cooled engine. Plenty of air up there, huh? But air cooling brings you into a limit for specific output, which brings up the stresses your writer was discussing.

Ah yeah, I think the whole stress thing was the part most bothering me. I didn't consider the fact that they'd be air cooled, even though that's obvious. I was thinking the air would be shoved through a radiator as the plane is flying along. Being air cooled not only raises the temps but by design the engine block and heads aren't quite a structurally sound as car engine. I suppose they're also much lighter, eh? I noticed in a few diagrams that they all seem to have pushrod valvetrains, no doubt for an excellent combination of simplicity, reliability, small size, and lighter weight.

I wonder if these engines have ECUs like a car. Do they have computer controlled variable this and that? Seems like adjusting the mixture per altitude would be something a computer could do pretty easily. Or once again is it a simplicity and reliability issue that makes those controls manual?

 

[nk4e]

According to my older brother, there isn't alot of computers in a Aircraft Engine. When the engine hits a certain RPM, a certain switch is activate

 

[Sniffs]

the germans DID flirt with water cooled aircraft engines, but that was back in WW1, nearly a century ago.

 

[Bleached]

The concern about wear comes from the fact that the engines are constantly run at 70-100% throttle. If you ran your car at those throttle levels at near red line for hours at a time, wouldn't you be more concerned with wear than you are now?

 

[Keef]

Yeah, when the redline is 8 grand. But 2,800 rpm? My car can drive on one tank the same distance as a 162 can fly, and it does that at 4,000 rpm the whole time. One-hundred-fifty thousand miles later, it's had nothing more than regular maintenance and still works just fine.

Of course once an airplane is off the ground an engine failure would be catastrophic. I understand the need for reliability, but it just seems a little overboard. Maybe I'm not understanding something about air-cooled engines. I don't know. The Harley boys seem to get along just fine. But again, my argument comes back to this: are airplane piston engines made of paper?

 

[Venom800tt]

Most everything in aircraft engines is massively redundant, both piston and jet powered. Things are also overbuilt to help reduce the chance of failure. Aircraft piston engines almost always have two of everything, all in the name of safety and reliability. As others have said already, a failure while flying can be catastrophic, especially during landing and takeoff.

As far as engine rpm goes, it has to be kept low. Engines and propellers are matched to each other from the factory, and are designed to keep the engine from exceeding a certain speed with that propeller attached. If rpm is too high, the prop can exceed the speed of sound, which reduces efficiency and thrust. The only possible way to run higher rpm would be through the use of a reduction gearbox, such as on a turboprop jet engine. But, that adds more mechanical complexity and weight to an already complex engine (turbines are much lighter and far less complex than piston engines in operation).

Also, automotive engines have been tried in kit aircraft, along with some mass produced aircraft. One was a massive failure, the Porsche powered Cessna 172 from the late 1980's, which suffered from poor reliability compared to traditional aircraft engines.

 

[Bleached]

Yeah, when the redline is 8 grand. But 2,800 rpm? My car can drive on one tank the same distance as a 162 can fly, and it does that at 4,000 rpm the whole time. One-hundred-fifty thousand miles later, it's had nothing more than regular maintenance and still works just fine.

Of course once an airplane is off the ground an engine failure would be catastrophic. I understand the need for reliability, but it just seems a little overboard. Maybe I'm not understanding something about air-cooled engines. I don't know. The Harley boys seem to get along just fine. But again, my argument comes back to this: are airplane piston engines made of paper?

Right, but 2,750 is the redline for a reason. The engine is producing 180hp at 2,700. Comparably, my car that makes 200hp at 7,800 rpm makes around 50 hp at 2,700 rpm. That's less than one third of what the IO-360 makes at that rpm. 3 times the power will create much faster wear. Also, note that this engine has a displacement of 360cu in (5.9L). That would make for a large V-8 in cars, but this has all that displacement in 4 cylinders. So there is a lot of air/fuel being combusted per cylinder compared to car engines.

 

[Greycap]

the germans DID flirt with water cooled aircraft engines, but that was back in WW1, nearly a century ago.

I must be missing something quite crucial here but if we replace "water cooled" with "liquid cooled", ie. using glycol as the coolant we'll have such engine names as Allison V-1710 (P-38, P-40), Mercedes-Benz DB60x (Bf 109) and Rolls Royce Merlin (Hurricane, P-51, Spitfire) that surely must mean something to the history of aviation? And they were definitely more than a flirt with liquid cooling. But it's more expensive, heavier and most importantly less reliable than air cooling so it's not as widely used in civilian applications.

Disclaimer: the text above contains simplifications to get the point across easier.

 

[wfooshee]

It's an interesting comparison to make with different WWII fighters, which ones were liquid-cooled, which ones were air-cooled.

Many, if not most of the land based fighters were liquid-cooled. Compact V engines, more powerful than similarly-sized air-cooled engines, especially if that power came from supercharging. Bombers used giant multi-layer radial air-cooled engines, as did naval fighters. The Corsair had 4(!) rings of 7 cylinders, for example.

As for WWI, here's a Fokker D-VII on display at the Naval Aviation Museum at Pensacola Naval Air Station, with the radiator grille clearly visible at the nose:


But the Germans weren't the only ones (click the pic for a bigger one, maybe you can read the headline on the sign):


That engine was used on the large plane seen in this pic, the NC-4, first successful Atlantic crossing by air. Took 19 days to reach Lisbon from New York in 1919.


An obvious issue with liquid-cooled engines is a cooling system failure, such as a bad water pump or a bullet through the radiator, which would put the aircraft on the ground in short order. Air cooling had none of those worries.