Couple aircraft engine questions

[crashxpert]

By trade I’m in the auto industry and I’m trying to figure a couple things out. Say on the Comanche when I have the RPMs set at 2000 that is the prop speed correct? Is the prop connected to a reduction gear? Have a hard time believing that is also the engine speed. If they are different how do aircraft engines deal with red lines or with the engine load from the prop do they just not rev high enough to get there?
Also what kind of air fuel ratios do these engine typically run? Auto industry aims for stoichiometric which is around 14.7 to 1. Give or take. Newer cars try to get leaner but combustion chamber temps get too high and then you get more NOX emissions. Which then makes it harder to meet emissions. Also if chamber temps get to high you can melt or crack pistons. Forged parts help. When you lean an aircraft engine what kind of air fuel ratio are you getting? Do they run really rich and when you lean do you lean it into that 14.7 to 1 range or is it even leaner than that? If it is how do these engines not burn holes in piston?
Im not a real world pilot but I’m a gear head and was curious to learn more on how these engines work.

[alan CXA651]

Hi.
Prop lever controls RPM , throttle controls manifold , mixture fuel/air mix varies according to outside air temps and altitude , to get correct mixture for a given alt / temp lean untill manifiold needle osscelates then enrich till needle just goes steady , and props to engine go through reduction gearing , also carb icing can play its part others on here can explain it better than i can , but its to do with airflow into the engine and can leed to a loss of power .
And yes if engine is too hot or too cold , you can cause damage to the cyclinders and pistons .
regards alan.

[Tomas Linnet]

Not all engines have a reduction gear. I’m pretty sure the C172, C182, 180 and the 250 has direct drive engines

[Oracle427]

Most of the Continentals and Lycoming engines are direct drive.

These engines are designed to run at sustained relatively high power settings that produce a lot of torque that would quickly destroy an automotive engine. This is why must auto engine adaptations in experimental aircraft have reduction gearing.

These old engine designs do not have the ability to change the laws of physics, so chemical properties still apply and ideal ratio of fuel and air still applies for best power output. However, the cylinders in these engines are huge and they air cooled, so a lot of efficiency is sacrificed for cooling.

At lower cruise power settings (75% range), leaning very aggressively is okay because the temperatures and pressures in the cylinders are generally not high enough to cause damage.

There are no engine management computers, timed injectors or electronic ignition on these old engines. Everything is mechanically filled with fixed ignition advance timing. Simple, certified, and reliable enough for their price tag. There are definitely more efficient designs using modern engine management components, but their high costs have limited adoption.

I can generally get at least 3 factory new of these legacy engines for the price of one modern design.

[crashxpert]

Most of the Continentals and Lycoming engines are direct drive.

These engines are designed to run at sustained relatively high power settings that produce a lot of torque that would quickly destroy an automotive engine. This is why must auto engine adaptations in experimental aircraft have reduction gearing.

These old engine designs do not have the ability to change the laws of physics, so chemical properties still apply and ideal ratio of fuel and air still applies for best power output. However, the cylinders in these engines are huge and they air cooled, so a lot of efficiency is sacrificed for cooling.

At lower cruise power settings (75% range), leaning very aggressively is okay because the temperatures and pressures in the cylinders are generally not high enough to cause damage.

There are no engine management computers, timed injectors or electronic ignition on these old engines. Everything is mechanically filled with fixed ignition advance timing. Simple, certified, and reliable enough for their price tag. There are definitely more efficient designs using modern engine management components, but their high costs have limited adoption.

I can generally get at least 3 factory new of these legacy engines for the price of one modern design.

Thanks for the info. Good stuff to know. Funny how Internal combustion engines are all the same but the theory can be so different between the two different applications.

[Paughco]

One of the main things to remember in a controllable pitch aircraft is to push in the pitch before you push in the throttle. This is analogous to downshifting before you stomp on the gas pedal in your Mustang GT. There are some pretty cool videos about engine management theory and practice: https://www.youtube.com/watch?v=_VfiPuheeGw. That's one on leaning, which might become important when you are about 750 miles into one of those 800 mile hops across the Amazon Basin. Lots of other good stuff out there.

One of the A2A videos that came out around the time of release of the Comanche discussed how rpm is sometimes adjusted to find a sweet spot with minimal vibration. Later I read that vibration is reduced when you have the composite prop and when you have either no GPS or the in-dash GPS. I guess the vibrations are more pronounced with the metal prop and the Garmin 295 on the RAM mount on the right side of the panel.

Sounds like a fun flight. Happy landings!

Seeya
ATB

[Killratio]

Re "red line".

With a CSU controlling RPM, the red line is MOSTLY taken care of.. it is still possible to overspeed the prop though under certain circumstances.

With a fixed pitch prop, the pilot absolutely HAS to keep a firm eye on the rpm to prevent red line in anything other than normal flight.
If the prop approaches red line (say, on descent) the throttle can be retarded BUT depending on the dive, it is still possible to overspeed and, probably just as bad, if you don't, the prop is
still driving the engine if you are anything near red line. In that case you can only put the nose above the horizon and wait.

Aerobatics in a fixed pitch aircraft is a challenging and rewarding type of flying!

[shortspecialbus]

One of the main things to remember in a controllable pitch aircraft is to push in the pitch before you push in the throttle. This is analogous to downshifting before you stomp on the gas pedal in your Mustang GT.
Seeya
ATB

Technically, I blip my throttle in my WRX whenever I downshift (with the clutch in, obviously), whether it's to accelerate or slow down, to match up the RPM to save wear and tear on the gearbox and transmission

But it's a reasonable comparison anyways.

I learned a mnemonic of sorts for this sort of thing - Blue Sky, Black Earth, which is a reminder to push in the Blue (prop) knob first when climbing/increasing power, and to pull the Black (throttle) knob first when descending/reducing power.

-stefan

[Killratio]

Technically, I blip my throttle in my WRX whenever I downshift (with the clutch in, obviously), whether it's to accelerate or slow down, to match up the RPM to save wear and tear on the gearbox and transmission

Stefan,

I aim for a 3000 change just about every time, it makes the rev matching more instinctive! What year is she?


D

[shortspecialbus]

Technically, I blip my throttle in my WRX whenever I downshift (with the clutch in, obviously), whether it's to accelerate or slow down, to match up the RPM to save wear and tear on the gearbox and transmission

Stefan,

I aim for a 3000 change just about every time, it makes the rev matching more instinctive! What year is she?


D

2017 WRX Limited with a Cobb CAI and a Nameless Catted Turboback, then a Stage 2 Protune from a local shop. I don't worry too much about specific RPMs for the changes, but I've gotten pretty good at getting within 500rpm (usually closer to 0-100) with a quick blip just from practice and instinct, so to speak. Hopefully if nothing else I get a few thousand miles more out of the clutch from it

What do you have?

-stefan

[AKar]

I see everything is pretty much answered. I add just a couple of points of my own.

Say on the Comanche when I have the RPMs set at 2000 that is the prop speed correct? Is the prop connected to a reduction gear? Have a hard time believing that is also the engine speed.

As mentioned, typical GA engines made by Lycoming or Continental is direct drive engine. So, no reduction gearing. There are exceptions: both manufacture engines with reduction gearing as well, indicated by letter 'G' in the model designation prefix. You mainly find these in big, high performance piston twins, but there are other uses as well. Also some other engines, such as those manufactured by Rotax have a reduction gearbox.

If they are different how do aircraft engines deal with red lines or with the engine load from the prop do they just not rev high enough to get there?

In a static run, GA piston engines should not exceed redline due to propeller load. With a constant speed propeller (that has variable pitch on its blades), the minimum fine pitch setting should in general be such that the redline is not exceeded when standing still. This is to avoid overspeeding when the power is added for the takeoff. When forward speed increases, the governor adjusts the blade angles to keep the RPM at selected value with a constant speed prop. With fixed pitch propeller, you will eventually exceed the redline if speed is allowed to increase enough at full power (it may require you to dive, though).

Also what kind of air fuel ratios do these engine typically run? Auto industry aims for stoichiometric which is around 14.7 to 1. Give or take. Newer cars try to get leaner but combustion chamber temps get too high and then you get more NOX emissions. Which then makes it harder to meet emissions. Also if chamber temps get to high you can melt or crack pistons. Forged parts help. When you lean an aircraft engine what kind of air fuel ratio are you getting? Do they run really rich and when you lean do you lean it into that 14.7 to 1 range or is it even leaner than that? If it is how do these engines not burn holes in piston?

As noted, these airplanes employ manual mixture control, so the air-fuel ratio goes from very rich to infinitely lean. That is, lean enough to cut the fuel flow entirely. Engine is shut down with the mixture lever, so it is fairly obvious. Now, note that the lean mixture actually produces less heat than near-stoichiometric - if it is lean enough.

"Optimal" cruise mixtures in terms of fuel efficiency are leaner than stoichiometric. It is the excess air (or lack of fuel) that starts to cool things down when you lean further. The issue comes from the fact that many airplane engines have very poor fuel distribution in between the cylinders, and start to show some uneven running when leaned beyond 'peak EGT' as is typically the reference in the airplane world. During takeoff at or near sea level, airplanes use "full rich" which is adjusted by the mechanics. It is a very rich fuel setting, so leaning for cruise flight is all but mandatory.

If you compare these thingies to automotive engines, the biggest fundamental difference is that the airplane engines use fixed ignition timing. So, lots of stuff that is tweaked by spark timing in a car needs to be done with selection of wise power-RPM-mixture combo. Also, cars use near-stoichiometric mixture for emission reasons, however, nowadays of course we see ultra-lean apparent mixtures in modern engines driven at partial load. I say "apparent", because the combustible mixture of a given air-fuel ratio fills only a part of the combustion chamber in those, called 'stratified charge'.

-Esa

[Killratio]

2017 WRX Limited with a Cobb CAI and a Nameless Catted Turboback, then a Stage 2 Protune from a local shop.

What do you have?

-stefan

Nice!

2015 WRX STi. 2.5l 305HP 1500kg in track trim. Dead stock standard apart from the Koya racing rims and Dunlop semi slicks that I fit just for track days.

Haven't been able to track much since wife got ill 18 months ago but hopefully in 12 months time......

She will do race car/race driver times on anything up to a 1600 metre track...falls a bit behind if the track is longer with a longer main straight.

[shortspecialbus]

2017 WRX Limited with a Cobb CAI and a Nameless Catted Turboback, then a Stage 2 Protune from a local shop.

What do you have?

-stefan

Nice!

2015 WRX STi. 2.5l 305HP 1500kg in track trim. Dead stock standard apart from the Koya racing rims and Dunlop semi slicks that I fit just for track days.

Haven't been able to track much since wife got ill 18 months ago but hopefully in 12 months time......

She will do race car/race driver times on anything up to a 1600 metre track...falls a bit behind if the track is longer with a longer main straight.

Nice car! I haven't taken mine on a track and don't really intend to, as it's my daily driver, but I did win a Charity TSD Rally with it:



I got mine to replace my Dieselgate Jetta TDI after VW gave me a huge pile of money for it. I thought about the STI but they still haven't changed out that old EJ25 for something newer. It's a good engine, but the FA20 is a bit more reliable, I think, and besides, for a daily driver with as long a commute (I live out in the country) as I have for work, I'd rather not have the STI suspension. I do want its diffs, though, haha.

I'm not sure how many HP mine's at right now. It was 265 stock, but with the work I've done, it's probably up over 300 somewhere. I don't really care to take it to a dyno to find out. Some day down the road I might start taking it to track days. It won't beat your STI due to the aforementioned suspension, diffs, and all the other stuff, but might hang close on a straight line

I hope your wife gets better :\ Long illnesses are rough on everyone.

-stefan

[Killratio]

Cheers Stefan,

And congrats on the win!!

Yes, the STi suspension is "quite firm" for a daily drive but really only affects passengers, the driver is too busy trying to control the stupid grin on his face from the engine sound

I bought mine specifically FOR the older engine. The diffs ARE magic. VERY little power gets wasted, as you really notice with the semi slicks on. They make about 1/2 a second difference for 0-100. (well, 1/2 on MY times, I am not prepared to abuse my gearbox by dumping the clutch at 5000, just to get the mythical 4.8 seconds. I go 5 flat WITH the semi slicks and slipping the clutch, 5.5 with standard tyres.

[shortspecialbus]

Cheers Stefan,

And congrats on the win!!

Yes, the STi suspension is "quite firm" for a daily drive but really only affects passengers, the driver is too busy trying to control the stupid grin on his face from the engine sound

I bought mine specifically FOR the older engine. The diffs ARE magic. VERY little power gets wasted, as you really notice with the semi slicks on. They make about 1/2 a second difference for 0-100. (well, 1/2 on MY times, I am not prepared to abuse my gearbox by dumping the clutch at 5000, just to get the mythical 4.8 seconds. I go 5 flat WITH the semi slicks and slipping the clutch, 5.5 with standard tyres.

Niceeee. I'm not sure what my 0-60 time is much less 0-100. My AccessPort has a thing to calculate it automatically, but I'm not about to do clutch drops either. I also don't have any semi-slicks - right now I've got on 245/40R18 Michelin Alpin PA4s, since it's winter and I'm in Wisconsin, and my summer tires are Michelin Pilot Super Sports, same size. I love them - a lot quieter than the stock tires were. I like engine/exhaust noise, not road noise.

The Nameless Turboback exhaust (5" Mufflers) sounds utterly fantastic. I imagine it'd be even better with the EJ25, but I'm really happy with the sound as it is. The stock STI sounds really good, although I'm not sure how I feel about that weird sound tube they have from the engine bay into the passenger compartment.

Anyways, this is probably quite the derail from the original question. I'll end my part with a quick picture:



Bonus backside (I've since changed out the tail lights):

-stefan