Explain piston aircraft engines to me

2Airtime2

2Airtime2

Pre-takeoff checklist
Joined
Nov 5, 2015
Messages
197
Location
Raleigh, NC
Display Name
Display name:
Cherokee 180c
I understand engines well. I've built 2 350 Chevy's over the yrs. A poorly built one will easily rev over 5000 and a balanced one can spin over 8000.

What is designed differently to make Lycombings or Continentals redline under 3000 rpm's?

Why do 360cu.in. normally aspirated aircraft engines only produce 180hp? When Chrysler makes that size engine it puts out close to 400hp.

Do they make them large displacement to get more tprque then limit the revs with that tiny one barrel carb?

I know props need to max out under 3000rpm's and it's best to not have a gear reduction but what keeps these engines under 3000rpm's without lugging?


You could bolt a propeller (with enough pitch) to a 350 Chevy to make it max out at 2800 rpm's. If you did that the engine would be severely lugging and wouldn't last a hundred hours.
 
Last edited:
Turn a prop that fast, tips go supersonic, excess noise and lost efficiency.

Hence, if you have engine revving that fast, you have to use reduction (gearbox or reduction drive with belts).

An aircraft engine has to deliver its rated power all the time; a bad-azz Chevy V8 spends most of its time doing 15% to 20% power.

Aircraft engines, thus, have to be over-engineered (beefier components, larger bearing surfaces, etc.).
 
Engines are purpose-built for the applications that the serve. In the case of aircraft engines, priorities are weight, reliability, and efficiency.

Airplane propellers typically can be more efficient at lower RPM. Much of this is due to not wanting the tips to go supersonic. Sure, you can shrink the propeller size, but that will lower the efficiency, especially since propellers are trying to move air around items like cowlings and fuselages.

To make a prop spin at a low RPM, you can either have an engine that spins at a low RPM by design, or by a gearbox. Gearboxes add weight, complexity, and failure modes. Additionally, engines are typically less efficient at higher RPM, so you end up finding that the direct drive engine is a good choice. That Chrysler 360 may make 400 HP, but it doesn't do it at 2700 RPM, it does it much higher. If you get it down to 2700 RPM, it might make more than 180 HP, but probably not much.

The 2000-2700 RPM range that you typically find is basically the best compromise. You can get a prop diameter that's big enough to get good efficiency, you can get enough power out of the engine to keep the weight down, and you're not spinning super fast which will hurt reliability. You don't need a gearbox which adds weight and hurts reliability. Really, our engines are very well designed for what they're intended to do. There's room for improvement, but most people who put in automotive engines find they don't work as well. Ben is the exception, because he designed his auto conversion specifically around aircraft requirements.
 
Market forces, multiplied by engineering limitations, multiplied by liability, equals the engines we see today. If there was money in building something better, we'd have it.

Maybe we do have it.

Attempts to build something better cost a fortune; when I was working on an SMA diesel in a 182 in 2011, SMA had already spent a billion dollars on the thing and there were 50 flying. And they had multiple issues. Lycoming can't seem to get OEMs interested in their iE2. Must be too expensive. And the governments still want big fees to bribe them to get out of the way.

If someone could build a lighter engine that gave more power and still had good reliability and lifespan and didn't cost any more than the current engines, we'd have that, too. Probably it costs too much. See above.

If any attempt to build something new didn't have the spectre of a massive lawsuit hanging over it as soon as some pilot flew a perfectly healthy airplane with a perfectly running engine into a rock wall hidden by clouds, we'd have a lot more new engines.

You can lug an aircraft engine. Too much manifold pressure and too little RPM, the same factors as in lugging an auto engine. They tend to come apart doing that. It's the reason for high-octane gasolines: to try to prevent it by increasing detonation margins.
 
Dan Thomas said:
You can lug an aircraft engine. Too much manifold pressure and too little RPM, the same factors as in lugging an auto engine. They tend to come apart doing that. It's the reason for high-octane gasolines: to try to prevent it by increasing detonation margins.
Click to expand...


Ok, it looks like Dan touched on it but since y'all didn't know:D I did some research on my own. I already knew about the need to keep weight down and why the rpm's need to stay low. I wanted to know how the engineers designed these low rpm engines. What design changes make them max out at under 3000.

Looks like it's a couple of factors. A 350 Chevy has a bore and stroke of 4" and just under 3.5" and turns over 5000 easily. A Lycombing 0-360 has a bore of 5.125" and a stroke of 4.375". I think the main thing that reduces rpm's is the long stroke. A long stroke also has the benefit of increasing torque. I'm sure a tiny one barrel carb limits the rpm's too.

Low rpm's : large bore, large stroke, small intake. I'm sure octane, timing, and the ability to control manifold pressure and mixture helps.


So I found the answer to my question that I didn't ask. Why are aircraft engines so expensive. Answer: because aircraft is in the name.

There's nothing complex about them at all. Basically lower volume and regulations take a rebuild/re-manufacture from $3000 (what it should cost for such a simple machine) to $25000.


HP = rpm x torque

if you reduce rpm and still want 180hp you have to increase the other side of the equation. Increase torque by using a larger bore and a larger stroke.

Simple
 
Last edited:
Piston airplanes are no different than a car in manifold pressure regulation, its just a different way of presenting power via instrumentation.

If the airplane is equipped with a variable pitch propeller, engine power is presented using two measures, manifold pressure & RPM. (since the load is variable RPM isn't an accurate measure of power)

If the airplane is equipped with a fixed pitch propeller engine power can be presented by using only RPM. (a fixed load takes so many HP to turn a given RPM)
 
Last edited:
If you look at most industrial or specialized engines where there is an expectation of or near 100% for long periods of time they tend to be low RPM engines. Tractors, small engine, diesels, etc. You are kinda on it with bore and stroke because those big pistons take longer to move which slow things down while that stroke allows a longer fuel burn to create power for longer durations. Adding in oil and air cooling also changes how heat is removed. They are closer to tractor engines designed in the 30's and 40's than modern engines of today (I know antique tractors). Simple yet proven design to keep pressures and temps low while producing the required torque to spin the prop at a required rpm. Individual head/cylinder allow both heat transfer along with simplicity or repair. The materials are similar but a bit more restrictive in airplane as tolerances because you blow that 350 you pull over with choice words. You blow the airplane and those words are a bit more severe and you look for the crash site.


Sent from my iPhone using Tapatalk
 
You have the stroke part backwards. The physics of props combined with the simplicity and weight savings of direct drive really dictate the optimal RPM for an aircraft engine. Once you have that figured out, a long stroke that works for that RPM will tend to give you more output.

However, the stroke itself on our engines isn't a limiting factor. For example. GTSIO-520s spin at up to 3400 RPM at the crank.
 
I cruise at about 4800 to 5600 rpm behind my aircraft engine depending on if I am just goofing and sightseeing or trying to get somewhere.
 
Capt. Geoffrey Thorpe said:
I cruise at about 4800 to 5600 rpm behind my aircraft engine depending on if I am just goofing and sightseeing or trying to get somewhere.
Click to expand...

Yes, but you have one of them new fangled lawn mower engines. :)
 
Originally Posted by denverpilot
Or Ben.




Clark1961 said:
too busy with the sheep right now, it's a cold night in Wyo
Click to expand...



I just sent the sheep home so I have a few minutes to answer...:D:D:D:redface:...

Ted is the man for honest answers on aircraft motors..

Personally, I am a V-8 kinda guy....:goofy:....;)

https://www.youtube.com/watch?v=

Feel free to ask any questions...
 
Yep, I flew for years behind the gopher35. 260HP by turning the engine at 3400 RPM but the prop at 2000. However, the savings you get from the efficiency of running then engine higher and the prop slower is pretty much wiped out by the weight (let alone the complexity and overhaul cost) of the gear box.
 
Horizontally opposed aircraft engines have to be designed to be comparatively narrow to fit modern cowls for two place aircraft. Hence the extra large bores to go with the short stroke.

They also have a low specific fuel consumption (gph per hp or whatever) which is helped by fewer large bores. The fuel consumption eliminated the Wankel despite it's lower weight and dimensional form factor.

The tractor engine (and add boat) internal application similarity is good.
 
The width is also why you see pushrod valvetrain and not overhead cams. You could make the engines narrower by shortening the cylinders and connecting rods, but again, design choices/compromises.
 
2Airtime2 said:
I understand engines well. I've built 2 350 Chevy's over the yrs. A poorly built one will easily rev over 5000 and a balanced one can spin over 8000.

What is designed differently to make Lycombings or Continentals redline under 3000 rpm's?

Why do 360cu.in. normally aspirated aircraft engines only produce 180hp? When Chrysler makes that size engine it puts out close to 400hp.

Do they make them large displacement to get more tprque then limit the revs with that tiny one barrel carb?

I know props need to max out under 3000rpm's and it's best to not have a gear reduction but what keeps these engines under 3000rpm's without lugging?


You could bolt a propeller (with enough pitch) to a 350 Chevy to make it max out at 2800 rpm's. If you did that the engine would be severely lugging and wouldn't last a hundred hours.
Click to expand...

Several factors at play, primarily the application, driving a propellor. A propellor is most efficient at low speed, and as the prop tips approach .92 Mach, the drag on the tips really goes up and all the excess horsepower is basically turned into noise. So, we have to design an engine around the speed of the prop, with a ~2700 RPM limitation as a compromise between efficiency and pressure in the cylinder to make the torque required, one needs to either use a very large piston to optimize the burn rate of the fuel against the speed of the piston through TDC, or you need a gear reduction. The next issue one faces in power plants is getting rid of the heat since we waste 75% of the BTUs in the fuel we consume. The majority comes out the exhaust, but a not insignificant portion goes out through the fins on the head and cylinders. Large diameter pistons again get the favored nod as they increase the surface area of the fins. That 360cuin Lycoming has 4 x 5.125" pistons at 4.375" stroke while that Chevy 350 has 8 x 4" pistons and a 3.48" stroke. To make the 180hp that the Lycoming makes at 2700rpm, the Chevy 350 is going to be turning about 4200 rpm to stay out of detonation. That means it needs a reduction drive to run a prop.

Next factor is load factor and heat shedding. You can definitely build a Chevy engine to tolerate these continuous loads, in fact I have no real issues especially with the LS series to take them to 1hp/cuin for continuous output, however you still have to deal with the heat, which means a fluid reservoir and radiator which are weight and drag, and you have to figure truck like capacity, not car.
 
You must log in or register to reply here.
Back
Top