Engine Operation — What’s best for longevity?

[RyanB]

Okay, I know this a very loaded question and one that doesn’t have a one size fits all answer, as all engines are different and something that I’m sure we’ve hashed out several dozen times over the years.

For this subject, let’s assume a Lycoming O360, an engine that some may consider ‘bulletproof’. I’m curious what’s generally regarded as being “better” in terms of operational conditions.

Is it better to run up at altitude at a constant power setting for hours at a time, or is the ‘flight training’ regime better suited with lots of varying power changes and such? Of course I’m sure the latter isn’t bad per say, since most flight school aircraft seem to make TBO and beyond just fine, but the thought of making lots of rapid power changes and such seems like it would be worse than just climbing to altitude and letting ‘er rip for hours at a time at a continuous power setting.

What say’eth you?

 

[DesertNomad]

Flight school airplanes have the advantage of flying every day regardless of how they are operated. I think if the flying times were equal, the constant power is better, just like highway vs city miles on a car.

 

[Stewartb]

Simple. Start it and run it non-stop. It’ll blow through TBO easily. Thermal cycles and corrosion are the enemies.

 

[Jim K]

Corrosion is the biggest enemy. Running an engine regularly is the best thing for it. I'm not sure what that number is; I try to fly mine at least weekly. I suspect that's enough in all but maybe coastal Florida.

Number two is starting. Starting in cold weather without preheating is even worse. Engines in irrigation applications that run continuously, only shutting down for oil changes, go for 10's of thousands of hours between overhauls.

Number 3 is probably lead. Mike busch recently did an article on an o-360 a client took to 5000 hours in a club plane that flew a lot. It eventually succumbed to lead fouling clogging up the rings. The bottom end wasn't perfect, but it was still servicable. Lead also tends to deposit on lycoming exhaust valves which can cause them to stick. Leaning the engine appropriately I think would make a big difference.

Number 4 on my list would be heat. Running cht's too high is hard on everything. Of course running too cool exacerbates the lead deposit issue.

The last thing on the list would be power changes. If you could run an engine at constant power, properly leaned, not too hot, not too cool, I bet an o-360 could go 8k. The lower rpms the better. Every time you leave that goldilocks setting you shorten that life, just a bit.

 

[RyanB]

Corrosion is the biggest enemy. Running an engine regularly is the best thing for it. I'm not sure what that number is; I try to fly mine at least weekly. I suspect that's enough in all but maybe coastal Florida.

Number two is starting. Starting in cold weather without preheating is even worse. Engines in irrigation applications that run continuously, only shutting down for oil changes, go for 10's of thousands of hours between overhauls.

Number 3 is probably lead. Mike busch recently did an article on an o-360 a client took to 5000 hours in a club plane that flew a lot. It eventually succumbed to lead fouling clogging up the rings. The bottom end wasn't perfect, but it was still servicable. Lead also tends to deposit on lycoming exhaust valves which can cause them to stick. Leaning the engine appropriately I think would make a big difference.

Number 4 on my list would be heat. Running cht's too high is hard on everything. Of course running too cool exacerbates the lead deposit issue.

The last thing on the list would be power changes. If you could run an engine at constant power, properly leaned, not too hot, not too cool, I bet an o-360 could go 8k. The lower rpms the better. Every time you leave that goldilocks setting you shorten that life, just a bit.

All excellent points! In my situation, I have no CHT information and really no EGT info, except for a single probe indicator on #4 which is generally how I lean. Without knowing what my CHT’s and EGT’s are, I can’t ever be sure that I’m operating in the right range. I usually run at 65% and operate a bit rich of peak EGT. Oil temp and pressure are both fine, so I can only assume everything is within the proper range. Since annual in April, I’ve put about 60hrs on the Tach and two oil changes.

 

[pburger]

All excellent points! In my situation, I have no CHT information and really no EGT info, except for a single probe indicator on #4 which is generally how I lean. Without knowing what my CHT’s and EGT’s are, I can’t ever be sure that I’m operating in the right range. I usually run at 65% and operate a bit rich of peak EGT. Oil temp and pressure are both fine, so I can only assume everything is within the proper range. Since annual in April, I’ve put about 60hrs on the Tach and two oil changes.

, and

"...a bit rich of peak EGT" is not where you want to be. If you look at the Lycoming graph, you'll see that CHTs actually increase as you move to the rich side of peak EGT, peaking at around 75deg ROP, then they drop off again. 100 to 125 deg ROP is considered the MAX power setting, and keeps your CHTs lower. That's 4 to 5 divisions on your EGT gauge from peak (if it says 25 deg per division). I'm no expert, but I would say either run at peak, or go to 100 deg ROP, but don't stay in the middle.

 

[RyanB]

, and

"...a bit rich of peak EGT" is not where you want to be. If you look at the Lycoming graph, you'll see that CHTs actually increase as you move to the rich side of peak EGT, peaking at around 75deg ROP, then they drop off again. 100 to 125 deg ROP is considered the MAX power setting, and keeps your CHTs lower. That's 4 to 5 divisions on your EGT gauge from peak (if it says 25 deg per division). I'm no expert, but I would say either run at peak, or go to 100 deg ROP, but don't stay in the middle.

View attachment 108734

Interesting. That chart is a bit confusing though. I don’t see 65% power and who runs at 80-100%?

 

[Dan Thomas]

Flying an engine regularly is the best thing for it.

FTFY.

The lower rpms the better.

No, especially with a constant-speed prop. That loads the bearings more. And low RPM often means too cool, aggravating carbon or lead fouling of everything. The ignition advance settings are designed for book RPMs. Unusually low RPM can place more stress on the pistons and cylinders due to the combustion pressure peaking too early, especially in CS prop applications. Follow the POH, but I sure wouldn't cruise around at 2300 RPM all the time.

These engines already run really slow compared to engines in any other application. What other vehicle of piece of equipment with an engine of similar HP runs at 2700 RPM?

 

[EdFred]

, and

"...a bit rich of peak EGT" is not where you want to be. If you look at the Lycoming graph, you'll see that CHTs actually increase as you move to the rich side of peak EGT, peaking at around 75deg ROP, then they drop off again. 100 to 125 deg ROP is considered the MAX power setting, and keeps your CHTs lower. That's 4 to 5 divisions on your EGT gauge from peak (if it says 25 deg per division). I'm no expert, but I would say either run at peak, or go to 100 deg ROP, but don't stay in the middle.

View attachment 108734

If your peak CHT's temps aren't in the danger zone temp wise, why can't you run it between best power and best economy? That's pretty much where I am all the time with CHTs under 350 at their hottest.

 

[GaryM]

I don’t see 65% power and who runs at 80-100%?

Renters?

 

[RyanB]

Renters?

Maybe, but I don’t know of any O360 making 80-100% power at altitude.

 

[EdFred]

Maybe, but I don’t know of any O360 making 80-100% power at altitude.

Fly at a lower altitude.

 

[RyanB]

Fly at a lower altitude.

Heh, guess if you like to tool around at 2500ft and WOT, otherwise that chart doesn’t include applicable numbers for most every operation.

 

[Jim K]

FTFY.



No, especially with a constant-speed prop. That loads the bearings more. And low RPM often means too cool, aggravating carbon or lead fouling of everything. The ignition advance settings are designed for book RPMs. Unusually low RPM can place more stress on the pistons and cylinders due to the combustion pressure peaking too early, especially in CS prop applications. Follow the POH, but I sure wouldn't cruise around at 2300 RPM all the time.

These engines already run really slow compared to engines in any other application. What other vehicle of piece of equipment with an engine of similar HP runs at 2700 RPM?

I own four 9L 300hp 6 cylinder engines that run 2200-2700rpm. One in the airplane and 3 in tractors. Actually doing the math, the 350 Chevy in my pickup is making about 160 hp at 2700rpm. Torque is largely a function of displacement, and horsepower is a function of torque and rpm.

I agree that running lower rpm's will result in higher cylinder pressure. The cylinders have more time to "breathe" so volumetric efficiency will be higher, and the flame front will have more time to propagate, both of which make the engine more efficient and raise peak cylinder pressure. That should raise cht as well, except that there's fewer combustion events in the same time, so they kind of balance out. Higher cylinder pressure would load the bearings more, but you really have to get into the pressure spikes of detonation to overcome the oil film. My belief is that whatever additional stress is added is more than offset by the increase in efficiency and the reduced number of cycles.

In the end the difference between cruising 2200 and 2500 would be negligible over the life of the engine regardless of which one of us is right. I will say that I've never seen anyone claiming running higher rpm's is easier on the engine, and my observation over years of reading about and working on engines has been that in general the slower an engine turns the longer it lasts. I'm sure you have more experience with aircraft engine than I do though.

I cruise at WOT and as low an rpm I can get and still make 65% power. The poh lists power settings down to 2100 rpm on my 2700 rated engine. Of course @RyanB doesn't have to worry about this with his fixed pitch prop. When I fly a fixed pitch plane.... it's a rental so I run it balls to the walls lol. Seriously though I do exactly what the poh says: pick my power setting, lean it until it stumbles, then richen until smooth. If I'm running 75% maybe a little extra gas to get a little bit ROP.

Heh, guess if you like to tool around at 2500ft and WOT, otherwise that chart doesn’t include applicable numbers for most every operation.

What the chart is trying to show is what % of the power available at that MP/RPM is being produced at that mixture setting. At 65% power setting on your poh chart, there's a "best power" mixture. Leaner or richer you will make less power, even if your rpm/mp stay the same. Of course with a fixed pitch prop rpm drops as power drops. Best power will be just a bit rich of peak, and where your rpm maxes out. Generally it's the just a little rich of the point the engine smooths back out again. Best economy will be pretty close to where you are right when the engine gets smooth again.

You can't really hurt anything at 65% power. Running peak egt/cht/rpm is perfectly fine down there and the most efficient way to fly. Above 65% power and you need to start worrying a bit about staying a bit richer or leaner than peak.

 

[RyanB]

I own four 9L 300hp 6 cylinder engines that run 2200-2700rpm. One in the airplane and 3 in tractors. Actually doing the math, the 350 Chevy in my pickup is making about 160 hp at 2700rpm. Torque is largely a function of displacement, and horsepower is a function of torque and rpm.

I agree that running lower rpm's will result in higher cylinder pressure. The cylinders have more time to "breathe" so volumetric efficiency will be higher, and the flame front will have more time to propagate, both of which make the engine more efficient and raise peak cylinder pressure. That should raise cht as well, except that there's fewer combustion events in the same time, so they kind of balance out. Higher cylinder pressure would load the bearings more, but you really have to get into the pressure spikes of detonation to overcome the oil film. My belief is that whatever additional stress is added is more than offset by the increase in efficiency and the reduced number of cycles.

In the end the difference between cruising 2200 and 2500 would be negligible over the life of the engine regardless of which one of us is right. I will say that I've never seen anyone claiming running higher rpm's is easier on the engine, and my observation over years of reading about and working on engines has been that in general the slower an engine turns the longer it lasts. I'm sure you have more experience with aircraft engine than I do though.

I cruise at WOT and as low an rpm I can get and still make 65% power. The poh lists power settings down to 2100 rpm on my 2700 rated engine. Of course @RyanB doesn't have to worry about this with his fixed pitch prop. When I fly a fixed pitch plane.... it's a rental so I run it balls to the walls lol. Seriously though I do exactly what the poh says: pick my power setting, lean it until it stumbles, then richen until smooth. If I'm running 75% maybe a little extra gas to get a little bit ROP.

What the chart is trying to show is what % of the power available at that MP/RPM is being produced at that mixture setting. At 65% power setting on your poh chart, there's a "best power" mixture. Leaner or richer you will make less power, even if your rpm/mp stay the same. Of course with a fixed pitch prop rpm drops as power drops. Best power will be just a bit rich of peak, and where your rpm maxes out. Generally it's the just a little rich of the point the engine smooths back out again. Best economy will be pretty close to where you are right when the engine gets smooth again.

You can't really hurt anything at 65% power. Running peak egt/cht/rpm is perfectly fine down there and the most efficient way to fly. Above 65% power and you need to start worrying a bit about staying a bit richer or leaner than peak.

Thanks, Jim. I guess it’s just hard to wrap my head around the fact that CHT’s are actually lower running at peak, than they are a little ROP.

 

[pburger]

Interesting. That chart is a bit confusing though. I don’t see 65% power and who runs at 80-100%?

You are thinking about it wrong. It's a two-step process:

1. You set %power desired with throttle (and prop control if you have a constant-speed prop). You'll use your tables and adjust for temperature for those settings.
2. Now you lean for proper fuel/air ratio at your selected power setting. The chart I presented is for LEANING (setting fuel/air ratio). If you lean to BEST POWER (100 to 150 deg ROP), you will get 100% of whatever %power you set with the throttle and prop controls (i.e. - 100% of 75%power). If you lean to BEST ECONOMY (peak EGT), according to the chart, you'll actually get about 96% of whatever %power you set (i.e. - 96% of 75%, which would be 72% power), but you'll be WAY more fuel efficient -- note the specific fuel consumption curve at the bottom. This is the fuel used per horsepower-hour. At peak EGT, you get a slight drop in power but a big increase in efficiency.

Does that make sense?

 

[Checkout_my_Six]

Fly it at least once a month for 30 minutes or more.....get the oil temp up into the green (190F>)....keep the CHTs lower than 390F....change the oil every 40-50 hrs or once a year.

Oh, and add a pint of CamGuard in with your Philips 20w50

 

[RyanB]

You are thinking about it wrong. It's a two-step process:

1. You set %power desired with throttle (and prop control if you have a constant-speed prop). You'll use your tables and adjust for temperature for those settings.
2. Now you lean for proper fuel/air ratio at your selected power setting. The chart I presented is for LEANING (setting fuel/air ratio). If you lean to BEST POWER (100 to 150 deg ROP), you will get 100% of whatever %power you set with the throttle and prop controls (i.e. - 100% of 75%power). If you lean to BEST ECONOMY (peak EGT), according to the chart, you'll actually get about 96% of whatever %power you set (i.e. - 96% of 75%, which would be 72% power), but you'll be WAY more fuel efficient -- note the specific fuel consumption curve at the bottom. This is the fuel used per horsepower-hour. At peak EGT, you get a slight drop in power but a big increase in efficiency.

Does that make sense?

Helps a lot, thank you!

 

[Tarheelpilot]

Fly every day on long trips.

That’s the secret.

 

[SixPapaCharlie]

I thought it doesn't matter what you do with an engine as long as you're putting marvel mystery oil in it

 

[Daleandee]

I thought it doesn't matter what you do with an engine as long as you're putting marvel mystery oil in it

The new MMO is called "CamGuard"

 

[Let'sgoflying!]

So sad that the engineering of these engines is so lacking that we have to fuss so much about these issues.... and that so much engine damage is occurring from not doing one of these things 'just right'.

 

[Tarheelpilot]

So sad that the engineering of these engines is so lacking that we have to fuss so much about these issues.... and that so much engine damage is occurring from not doing one of these things 'just right'.

Most of it is just over active imaginations.

 

[Rgbeard]

If you stop flying the engine entirely it will last indefinitely. Reaching TBO won’t ever be an issue.

 

[Salty]

If you stop flying the engine entirely it will last indefinitely. Reaching TBO won’t ever be an issue.

That’s about the only answer. Running them wears them out, not running them wears them out.

 

[Kristin]

A local flying club has a C-172 that is about to get its engine overhauled at 5,000 hours. It flies about 100 hours a month.

 

[Pinecone]

No, especially with a constant-speed prop. That loads the bearings more. And low RPM often means too cool, aggravating carbon or lead fouling of everything. The ignition advance settings are designed for book RPMs. Unusually low RPM can place more stress on the pistons and cylinders due to the combustion pressure peaking too early, especially in CS prop applications. Follow the POH, but I sure wouldn't cruise around at 2300 RPM all the time.

Disagree. And go read Mike Busch, he also disagrees with you.

Lower RPM allows a longer power stroke time for the fuel to burn.
Lower RPM reduces wear in the engine
Propellers are more efficient at lower RPM.
Lower RPM will give lower EGT due to the fuel being burned more completely during the power stroke, but they are cold, and the CHTs will still be in the proper range.

Yes, advance settings are set for book RPMs, but my book allows cruising at 2200 RPM, so that IS following the book.

Article by Mike, read the part about RPM - https://resources.savvyaviation.com...ticles_eaa/EAA_2012-10_flying-efficiently.pdf

 

[Salty]

No, especially with a constant-speed prop. That loads the bearings more. And low RPM often means too cool, aggravating carbon or lead fouling of everything. The ignition advance settings are designed for book RPMs. Unusually low RPM can place more stress on the pistons and cylinders due to the combustion pressure peaking too early, especially in CS prop applications. Follow the POH, but I sure wouldn't cruise around at 2300 RPM all the time.

These engines already run really slow compared to engines in any other application. What other vehicle of piece of equipment with an engine of similar HP runs at 2700 RPM?

What would you say if someone said not to use overdrive gear because "that loads the bearings more". It's also kinda funny you found a way to make a cooler engine a bad thing. I think that's the first time I've heard that one.

Cruising at 2300 and WOT may be a bad idea, but my POH recommends 2400 and < 75% for purt near all cruise situations.

 

[Dan Thomas]

What would you say if someone said not to use overdrive gear because "that loads the bearings more". It's also kinda funny you found a way to make a cooler engine a bad thing. I think that's the first time I've heard that one.

Cruising at 2300 and WOT may be a bad idea, but my POH recommends 2400 and < 75% for purt near all cruise situations.

Lots of pickups with overdrive have lockouts for towing. The extra load at low RPM is hard on stuff.

And cold aircraft engines will foul stuff sooner. It's why we have hot and cold versions of sparkplugs. The hot plugs are used in cooler engines to burn off deposits. The Lycoming O-235 is a notoriously cool-running engine, and it fouls its plugs real quick. On top of that, it has more problems with crankcase condensation and corrosion from blowby gases.

I'm not talking about super-low temperatures, just that babying the engine can result in other types of damage than that caused by heat.



Hot plugs for cooler engines stay cleaner longer.

 

[Dan Thomas]

Disagree. And go read Mike Busch, he also disagrees with you.

Lower RPM allows a longer power stroke time for the fuel to burn.
Lower RPM reduces wear in the engine
Propellers are more efficient at lower RPM.
Lower RPM will give lower EGT due to the fuel being burned more completely during the power stroke, but they are cold, and the CHTs will still be in the proper range.

Yes, advance settings are set for book RPMs, but my book allows cruising at 2200 RPM, so that IS following the book.

Article by Mike, read the part about RPM - https://resources.savvyaviation.com...ticles_eaa/EAA_2012-10_flying-efficiently.pdf

Lower RPM also allows more time for the complex hydrocarbon molecules to disintegrate under heat and become autoignitable, resulting in detonation. Detonation is a poorly understood phenomenon, and very few textbooks elaborate on it. Low RPM, high MP, and elevated induction air temperatures all figure into it, aside from octane ratings and compression ratios. It's why the POHs for constant-speed-prop airplanes have RPM/MP limits. Not much problem for fixed-pitch.

Propellers are indeed more efficient at lower RPM. The prop on your airplane is optimized for POH cruising speeds, and there's nothing you can do about it. Running extra low RPM does not increase its efficiency.

Lower RPM decreases wear. However, the low redlines on these engines already result in "low" operating RPM. They have far lower piston speeds than auto engines.

Use the POH, as you say. But VERY low RPM was the subject here, near max-endurance RPMs, and you might get low operating temps but your mileage will be lower since the airspeed will be low. IIRC, the 172's best endurance was found at around 1900 RPM and 80 MPH.

We ran the flight school Lycomings hard, all the time. Cruising 2500 most of the time. Those engines went to TBO with compressions in the high 70s and no metal in the filters, and no cylinder or valve repairs anywhere in their lifetimes. The only engine that gave us grief was the O-235, with its cold running and resultant crankcase condensation that cause cylinder bore corrosion, cylinder ridging from the rings, and shaving by those ridges of the aluminum piston pin plugs. I covered the oil cooler permanently to get the oil temps up high enough to minimize the problems there. I replaced its sparkplugs with UREM37BYs to stop the plug fouling; there were times we had the plugs out at 30 hours to clean them.

What is your maintenance experience?

 

[Daleandee]

Many years ago I read an excellent article on HALT (Highly Accelerated Life Testing) of IC engines and the conclusion (at least for the engines tested) was that more damage was found when engines were heavily loaded (lugged) as opposed to a lighter load at higher RPM. The wear from higher cylinder pressures for longer periods of duration causes accelerated wear on pistons, wrist pins, connecting rods, and bearings because of the increased pressures. Test results indicated that "letting it spin" gave better results than loading it down. Engines have a sweet spot they like to run in and that information will be provided by the engine manufacturer .

Understand that my experience is with experimental engines and not certified aircraft powerplants so YYMV ... but I doubt it.

 

[Checkout_my_Six]

Makes good sense....Did lots of HALT for DeWalt Power Tools back in the day.

Many years ago I read an excellent article on HALT (Highly Accelerated Life Testing) of IC engines and the conclusion (at least for the engines tested) was that more damage was found when engines were heavily loaded (lugged) as opposed to a lighter load at higher RPM. The wear from higher cylinder pressures for longer periods of duration causes accelerated wear on pistons, wrist pins, connecting rods, and bearings because of the increased pressures. Test results indicated that "letting it spin" gave better results than loading it down. Engines have a sweet spot they like to run in and that information will be provided by the engine manufacturer .

Understand that my experience is with experimental engines and not certified aircraft powerplants so YYMV ... but I doubt it.

 

[Jeff767]

Lugging a engine is bad but you are not viewing it in the right perspective. Running a engine at 2200 rpm that is designed for 2700 rpm is the same as running a 5400 RPM engine at 4400 RPM. Not exactly what most consider lugging. One thing the leaning chart posted did not show is internal cylinder pressure. It peaks in the 50 to 75 ROP range and is why it’s better to run at peak or at least 100 ROP if you don’t want to go LOP.

 

[JOhnH]

Unleaded avgas.

 

[Dan Thomas]

Lugging a engine is bad but you are not viewing it in the right perspective. Running a engine at 2200 rpm that is designed for 2700 rpm is the same as running a 5400 RPM engine at 4400 RPM. Not exactly what most consider lugging.

Nonsense. The flame front speeds are roughly the same at 2200 as at 4400, but at 4400 they don't have time to cause detonation. The pressures ahead of the flame front are rising, and rising pressures mean rising temperatures, and rising temperatures breaks down the complex hydrocarbon molecules that are resistant to autoignition into simpler, autoignitable molecules, and the possibilities of detonation go way up. Low RPM in large cylinders are ideal conditions for this. They give the time for the molecular breakdowns. The autoignitable stuff doesn't wait for the flame front to reach it; it all goes off at once, an explosion instead of a controlled burn. Flame front speeds of 5000 feet per second instead of 100 fps. Big pressure spikes and high CHTs. Broken rings, holed pistons, cracked heads, cylinders pulled off the case. Lots of opportunities to spend money.

As I said, in fixed-pitch prop aircraft this is not normally a problem. Raising the MP by opening the throttle results in higher RPM, getting the engine farther out of the detonation risk. But opening that throttle quickly can get brief detonation, which is why engine manufacturers tell you to take two or three seconds to go from idle to WOT. And brief detonation, done enough times, damages the engine.

 

[Shepherd]

I thought we tried to stay away from Religion and Politics?

 

[Jeff767]

Detonation is simply not a issue at cruise where one might use lower RPM’s. No one is talking about setting 2200 RPM for a full power takeoff on a 35C day.

 

[Tusayan]

No one is talking about setting 2200 RPM for a full power takeoff on a 35C day.

Depending on the engine, airframe and prop pitch that is exactly what you may be doing at takeoff with a fixed pitch prop, or 2300 rpm anyway. And it is not an issue either as long as you stay in the generous limits on continuous over square operation which apply to typical GA engines used with FP props. For e.g. a 150 HP Lycoming O-320 running on 100LL you are 20 octane points above the rating of the fuel on which the engine was certified to do that without detonation issues.

Also, if and when detonation does occur it is more common with high CHT resulting from continuous full power operation, not at takeoff.