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Fit_Lawfulness_3147 t1_jduslek wrote

Reply to comment by pavlik_enemy in Does living in an airplane flight path, near an airport, pose a health risk? What happens to the lead from the jets fuel? by [deleted]

As previously commented upon, lead is an anti knock additive. It increases the octane number. The higher the engine’s compression ratio, the higher the required octane number. The higher the compression ratio the more power the the engine can produce. SO… small aircraft need high octane fuel in order to keep the engine(s) small (light).

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troaway1 t1_jdvdkv9 wrote

Small, light AND SIMPLE. No liquid cooling, no knock sensor, no computer controlled fuel injection, no O2 monitoring or exhaust gas recirculation, no transmission. Often the pilot controls the air/fuel mixture manually watching rpm and exhaust temp. Very primitive, but not a lot can go wrong as long as you don't have detonation which tear up an engine pretty quickly.

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pavlik_enemy t1_jdv25sr wrote

What I meant by "advanced" was that modern aviation have rather low specific output compared to automotive engines where you could have a 200hp/liter engine in a Toyota so they don't have high compression ratio and/or boost pressure. And these highly efficient engines run on say 98 octane unleaded

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quietflyr t1_jdv7kl2 wrote

Airplane engines are designed for torque at low RPM vs horsepower at high RPM, hence the very large bore sizes in aircraft engines.

My Honda Civic has a 1.5L (91 cu in) turbo inline 4, and pulls 174 hp at 6000 rpm, and 162 lb-ft of torque.

A Lycoming O-360 aircraft engine is normally aspirated with a 5.89L displacement, and produces 180 hp at 2700 rpm, but 350 lb-ft of torque.

Pretty massive difference.

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Happycracker t1_jdvpj88 wrote

The other thing I frequently see called out about airplane engines is they run these power levels for hours on end. Typical car usage hits peak power for a very limited amount of time before dropping to something minimal.

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quietflyr t1_jdvxnop wrote

Yes this is a big one. An aircraft engine will run at 60% power or more for almost the entire flight. A car engine is typically running at far lower power levels than that.

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pavlik_enemy t1_jdv9x7j wrote

Interesting. In auto world bore being larger than stroke is usually a sign of high-revving sport-ish engine.

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DreamyTomato t1_jdvw9hj wrote

Why so much torque? Cars go up hills, but piston engines are pushing a propellor through air, which at LOW revs doesn't offer much resistance.

Or am I missing a point that cars have gearboxes and maybe planes have no gearboxes? So 2700rpm is the prop speed, same as the crankshaft speed?

(and I believe instead of changing rev speed, prop aircraft change the blade angle. So the engine stays at the same speed, but the load can vary greatly?)

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quietflyr t1_jdvx47w wrote

An O-360 is a direct drive engine, the propeller is attached directly to the crankshaft. And that's the case for most light aircraft piston engines. Some (especially more modern ones) have a single speed reduction gearbox.

The only thing an aircraft piston engine does is turn a propeller through the air, and that absorbs nearly 100% of the power produced by the engine. Swinging an 80 inch diameter propeller at 2700 RPM will use a lot of power and required a lot of torque.

You raise variable pitch propellers or constant speed propellers. These are used on some piston engine aircraft (the rest use a simple fixed-pitch propeller) and all turboprop aircraft. They do vary the pitch of the blades to find the most efficient angle both in climb and at higher speeds, due to the mechanics of how a propeller works. But this doesn't really affect the fact that a ton of torque is required to turn the propeller.

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motorslug t1_jdvxou6 wrote

That's correct. The bar majority of piston engine aircraft use a direct drive from engine to propellor. More complex aircraft use a constant speed propellor as a form of gearbox, allowing the blade pitch to be altered while maintaining a constant blade rpm.

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