High Performance engine wear.

Aztec Driver

Aztec Driver

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Bryon
So as I go along studying for the next written, I come along a bit of information that seems contradictory to everything I was ever taught on engine wear.
I was always taught to avoid low rpms with a high MAP, and the reason has always been the resulting high pressures developed in the cylinders. Reasonable, straightforward, and understandable.

Then I come across the following statement and actual question on the written test:

Under normal operating conditions, the most severe wear, fatigue, and damage to high performance reciprocating engines occurs at high r.p.m. and low manifold pressure.

Unfortunately, there is nowhere that I can find an explanation for it. It is something that we all do on approach to landing every time we fly. Why would a low power condition result in more wear?
 
Aztec Driver said:
So as I go along studying for the next written, I come along a bit of information that seems contradictory to everything I was ever taught on engine wear.
I was always taught to avoid low rpms with a high MAP, and the reason has always been the resulting high pressures developed in the cylinders. Reasonable, straightforward, and understandable.

Then I come across the following statement and actual question on the written test:

Under normal operating conditions, the most severe wear, fatigue, and damage to high performance reciprocating engines occurs at high r.p.m. and low manifold pressure.

Unfortunately, there is nowhere that I can find an explanation for it. It is something that we all do on approach to landing every time we fly. Why would a low power condition result in more wear?
Click to expand...

AC20-103 - Crankshaft Failure
 
Clip4 said:
AC20-103 - Crankshaft Failure
Click to expand...

Reading that circular says much the same thing, although it points out that it damages the crankshaft counterweights. It doesn't explain how that happens, however, or why. Or what one can do to prevent it, especially when you are commited to doing it on every landing.
 
What section of this document supports the OP's statement?
 
Aztec Driver said:
So as I go along studying for the next written, I come along a bit of information that seems contradictory to everything I was ever taught on engine wear.
I was always taught to avoid low rpms with a high MAP, and the reason has always been the resulting high pressures developed in the cylinders. Reasonable, straightforward, and understandable.

Then I come across the following statement and actual question on the written test:

Under normal operating conditions, the most severe wear, fatigue, and damage to high performance reciprocating engines occurs at high r.p.m. and low manifold pressure.

Unfortunately, there is nowhere that I can find an explanation for it. It is something that we all do on approach to landing every time we fly. Why would a low power condition result in more wear?
Click to expand...

Check with Savvy Aviators archived webinars or call them. Much more reliable advice than an internet forum where opinions are like **tt- h**es, everyone has one.
 
Those crankshaft failures have more to do with the wind driving the props on radial engines.

You don't have to worry about damaging the crankshaft on a flat engine in most GA aircraft.
 
dans2992 said:
What section of this document supports the OP's statement?
Click to expand...

No specific section. I just think the extensive discussion helps clarify the relationship so it makes more sense.

What I don't know about flying complex aircraft would fill volumes. But it seems pretty straightforward to me that high RPM / low MAP would imply throttle near idle and prop full forward, so the relative wind is powering the engine, which causes the pressure against bearings and bushings to fluctuate because the engine is producing low, but consistent power versus the inconsistent force of the wind spinning the prop. That can't help but increase wear on bearings, bushings, counterweights, pushrods, and probably other moving parts.

-Rich
 
Aztec Driver said:
So as I go along studying for the next written, I come along a bit of information that seems contradictory to everything I was ever taught on engine wear.
I was always taught to avoid low rpms with a high MAP, and the reason has always been the resulting high pressures developed in the cylinders. Reasonable, straightforward, and understandable.

Then I come across the following statement and actual question on the written test:

Under normal operating conditions, the most severe wear, fatigue, and damage to high performance reciprocating engines occurs at high r.p.m. and low manifold pressure.

Unfortunately, there is nowhere that I can find an explanation for it. It is something that we all do on approach to landing every time we fly. Why would a low power condition result in more wear?
Click to expand...

They have it right, as taught by Lindbergh and proven in WWII.
 
RJM62 said:
No specific section. I just think the extensive discussion helps clarify the relationship so it makes more sense.

What I don't know about flying complex aircraft would fill volumes. But it seems pretty straightforward to me that high RPM / low MAP would imply throttle near idle and prop full forward, so the relative wind is powering the engine, which causes the pressure against bearings and bushings to fluctuate because the engine is producing low, but consistent power versus the inconsistent force of the wind spinning the prop. That can't help but increase wear on bearings, bushings, counterweights, pushrods, and probably other moving parts.

-Rich
Click to expand...
This is what I was thinking, but there is no discussion on this point anywhere for reference.
 
Aztec Driver said:
But what is the reason for this phenomenon?
Click to expand...

The weakest link in an engine are the rod cap bolts, the higher the RPM the greater the reciprocating mass's kinetic energy to turn at the top of the stroke. With low MP you have more ring flutter force, real low and your intake stroke is starting to pull serious vacuum. The most kind and efficient operations will occur right before the point of detonation.
 
Aztec Driver said:
This is what I was thinking, but there is no discussion on this point anywhere for reference.
Click to expand...
Any basic mechanical engineering text will do. The faster the engine spins, the more frictional, centrifugal and rotational forces, and the more force, the more wear.

The limit on the other end is the potential for pre-ignition ("detonation") too early in the compression upstroke due to excessive MP for a given RPM. If our aircraft engines had computer control of ignition like our cars do these days, the ignition timing would be automatically retarded at lower RPM's to take care of this, but we're not there yet. The only time ignition timing is retarded to prevent early ignition is at the very low RPM's of engine start, which is done by the impulse coupler on one or both magnetos, but that is mechanically negated above about 500 RPM or something like that.
 
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Ron Levy said:
Any basic mechanical engineering text will do. The faster the engine spins, the more frictional, centrifugal and rotational forces, and the more force, the more wear.

The limit on the other end is the potential for pre-ignition ("detonation") too early in the compression upstroke due to excessive MP for a given RPM. If our aircraft engines had computer control of ignition like our cars do these days, the ignition timing would be automatically retarded at lower RPM's to take care of this, but we're not there yet. The only time ignition timing is retarded to prevent early ignition is at the very low RPM's of engine start, which is done by the impulse coupler on one or both magnetos, but that is mechanically negated above about 500 RPM or something like that.
Click to expand...

So you don't think that very low MP at a given high RPM, in itself, makes a difference?

I'm not an expert on these sorts of things, but it seems to me that the fluctuating forces of wind driving an engine that isn't generating much power of its own would add vibration and wear because it would alternately increase and decrease the load on the engine, therefore changing the direction of forces acting on the bearings, bushings, connecting rods, and so forth.

To use a manual-transmission car as an analogy, if you hold the transmission in a low gear and the engine at a fixed, high RPM while on a hilly road, there's a reversal of forces acting on the engine's moving parts when switching from going downhill (when the engine braking retards forward movement) and going uphill (when the engine is providing power for forward movement). That increases fatigue and wear on the engine's moving parts.

On an aircraft engine, I'd think there would also be fluctuations in the forces exerted on the crankshaft bearings along the engine's longitudinal axis as variations in the wind cause the prop to alternate between "pulling" the airplane and "braking" the airplane.

That's my hunch, anyway. But of course, I defer to your knowledge and that of others who, unlike me, actually know what they're talking about.

-Rich
 
Driving the engine with the prop adds a whole new level of factors.
 
Ron Levy said:
Any basic mechanical engineering text will do. The faster the engine spins, the more frictional, centrifugal and rotational forces, and the more force, the more wear.
Click to expand...

That is easily understood. Higher rotational forces and higher speed will generally equate to more wear and tear. What is not mentioned in that is the lower forces of the lower manifold pressure, creating less force and pressure. Why is that combination harder on the engine than the high MAP and high RPM?


The limit on the other end is the potential for pre-ignition ("detonation") too early in the compression upstroke due to excessive MP for a given RPM. If our aircraft engines had computer control of ignition like our cars do these days, the ignition timing would be automatically retarded at lower RPM's to take care of this, but we're not there yet. The only time ignition timing is retarded to prevent early ignition is at the very low RPM's of engine start, which is done by the impulse coupler on one or both magnetos, but that is mechanically negated above about 500 RPM or something like that.
Click to expand...
 
Aztec Driver said:
That is easily understood. Higher rotational forces and higher speed will generally equate to more wear and tear. What is not mentioned in that is the lower forces of the lower manifold pressure, creating less force and pressure. Why is that combination harder on the engine than the high MAP and high RPM?
Click to expand...

Lower manifold pressure is not lower force on the weak link, it's increased force. Reduce the fuel to reduce the force, not the manifold pressure. Power reduction is first RPM, then fuel flow, then manifold pressure.

I've never blown an engine standing into the throttle, I have spun rod bearings several times when I lifted my foot in the traps.
 
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RJM62 said:
http://www.advancedpilot.com/downloads/prep.pdf

-Rich
Click to expand...

I've read Deakins' articles several times. I have question about this passage:
It is a good habit to note the MP gauge reading before engine start, and do a quick calculation to see how close it is. Set your altimeter to the field elevation, note the altimeter setting in the Kollsman window, subtract one inch per thousand feet above sea level, and your MP gauge should show very close to that value with the engine not running. At a 6,000-foot elevation airport, for example, set 6,000 on the altimeter, read (say) 29.5 in the Kollsman window, subtract six, and check that your MP gauge shows approximately 23.5 before start.
Anything else is an error in the instrument.
Click to expand...

Shouldn't the MP gauge and the Kollsman match?
 
RJM62 said:
So you don't think that very low MP at a given high RPM, in itself, makes a difference?

I'm not an expert on these sorts of things, but it seems to me that the fluctuating forces of wind driving an engine that isn't generating much power of its own would add vibration and wear because it would alternately increase and decrease the load on the engine, therefore changing the direction of forces acting on the bearings, bushings, connecting rods, and so forth.

To use a manual-transmission car as an analogy, if you hold the transmission in a low gear and the engine at a fixed, high RPM while on a hilly road, there's a reversal of forces acting on the engine's moving parts when switching from going downhill (when the engine braking retards forward movement) and going uphill (when the engine is providing power for forward movement). That increases fatigue and wear on the engine's moving parts.

On an aircraft engine, I'd think there would also be fluctuations in the forces exerted on the crankshaft bearings along the engine's longitudinal axis as variations in the wind cause the prop to alternate between "pulling" the airplane and "braking" the airplane.

That's my hunch, anyway. But of course, I defer to your knowledge and that of others who, unlike me, actually know what they're talking about.

-Rich
Click to expand...
That might be an oversimplification. For example, motorcycle engine designers tweak the internal cavities and valving to get the right amount of engine braking when the engine is aggressively downshifted. The engine is at high RPM and the rider desires to slow for corner entry and is on the front brake, the bike's inertia is being absorbed through the front brake/ tire and the engine/ transmission/ rear tire.
 
Henning said:
Lower manifold pressure is not lower force on the weak link, it's increased force. Reduce the fuel to reduce the force, not the manifold pressure. Power reduction is first RPM, then fuel flow, then manifold pressure.
Click to expand...

Lower MAP reduces fuel flow anyway. People do not generally reduce the mixture to control fuel flow when landing. At least I do not. I did read Deakin's articles suggesting that very thing, but it has never been suggested or taught to me in either primary training, or advanced training for part 135 ops.


I've never blown an engine standing into the throttle, I have spun rod bearings several times when I lifted my foot in the traps.
Click to expand...

That would be more a product of sudden deceleration than running at a low power high rpm setting. Suddenly removing power from the engine,
I can understand.

I just cannot understand why a power setting that barely keeps the engine at the higher rpm (ie below governor range)(brought down slowly) can be MORE detrimental to the engine than a high power setting, which brings with it much higher cylinder pressures and rotational and frictional forces. There has to be some other force at work here that I am not seeing/understanding.

At 15" MAP I can sustain full rated RPM on my engine. Why is that setting worse for my engine when coming in to the pattern to mix it up with the Skyhawks, than when I am cruising along at 27" and 2700 RPM?
 
There's more to wear than the number of turns. Sometimes a higher spin rate increases cooling/lubrication which defrays the effects of the alleged additional wear.

Pretty much once you keep detonation under control (which usually is only a problem at fairly low altitudes where sufficiently high power settings can be obtained), the KILLER of engines has pretty much been shown to be excess heat. Once you have decent cooling and you're in the max continuous power range, you can make any combination of MP and RPM and any setting of the mixture control without much concern over what's happening to your engine.

Deterioration (corrosion, failure of rubber components) from disuse is another killer.

A lot of 172's used in primary training blast way past TBO even with the alleged abuse of student and renter pilots, where someone's pampered hangar queen isn't likely to make it halfway to TBO before needing top end work.


As with much of the FAA written test nonsense, it is either a historical relic or just complete bull poop. Memorize the party line for the test and forget about it. Don't even get me started about the whole "balance of forces" nonsense about coordinated flight or the utter educational DRIVEL that's on the FOI (the FAA specifically exempts teachers from having to take that so avoid all the puking in the old GADO building by people trying to force that stuff down).
 
flyingron said:
As with much of the FAA written test nonsense, it is either a historical relic or just complete bull poop. Memorize the party line for the test and forget about it. Don't even get me started about the whole "balance of forces" nonsense about coordinated flight or the utter educational DRIVEL that's on the FOI (the FAA specifically exempts teachers from having to take that so avoid all the puking in the old GADO building by people trying to force that stuff down).
Click to expand...
I do have a problem with these tests. I try to learn how to do all of the tasks properly, so I can duplicate them any time in life in different circumstances, as I have always learned throughout my life. Rote memorization does not sit well with me if the answers are not completely correct. And yes, the FOI written test is complete swill.
 
Someone here said, years ago, that:

hi RPM = wear (friction & heat on moving surfaces)
hi MP = stress (higher icp, higher loads on bearings etc)

Both of which are bad.
I do not recall anyone providing a counterpoint to this statement.
 
Dekin does not share the view that high RPM is bad.
 
Let'sgoflying! said:
Someone here said, years ago, that:

hi RPM = wear (friction & heat on moving surfaces)
hi MP = stress (higher icp, higher loads on bearings etc)

Both of which are bad.
I do not recall anyone providing a counterpoint to this statement.
Click to expand...

The ATP test question is:

Under normal operating conditions, which combination of MAP and RPM produce the most severe wear, fatigue, and damage to high performance reciprocating engines?

High RPM and Low MAP
Low RPM and High MAP
High RPM and High MAP

The correct answer, according to the test, is HIGH RPM and Low MAP. I just wanted to determine why that was worse on the engine than the other two, when High MAP would certainly bring higher pressures and much more force against the various components than would a low MAP.
 
Let'sgoflying! said:
Someone here said, years ago, that:

hi RPM = wear (friction & heat on moving surfaces)
hi MP = stress (higher icp, higher loads on bearings etc)

Both of which are bad.
I do not recall anyone providing a counterpoint to this statement.
Click to expand...

The ICP, as long as kept to below detonation levels, is not particularly harmful, it's what the engine was designed around. Keep the CHT in check by reducing fuel flow with the red handle and ICP will not be a problem, the same handle also reduces ICP. The worst of both will be around 75°-125° ROP, high CHT + high ICP is a recipe for detonation. Let the engine breath freely by keeping the throttle plate open as far as you can, this also increases the efficiency of the engine. Raising RPM at high MP also helps stay out of detonation.

The long and short of it is that CHT(or TIT) is the limiting factor you will run into on all these engines. The kindest and most efficient way on an engine to reduce that is to reduce the fuel flow without reducing the airflow at any given RPM which will be the limiting factor on how much power you can make. Reducing RPM first increases ICP and retards the timing of when ICP occurs as well as extending its duration in the top of the stroke where it exerts the most force (creates the most torque) allowing for a more efficient use of that fuel, but increasing the risk of detonation and allowing a greater amount of heat to transfer to the cylinder head and piston rather than go out the exhaust, that's why we lean. Closing the throttle plates adds resistance to reciprocation which requires more fuel just to turn the engine, this means more waste heat. This is when operating under load. When unloaded such as in descent if you have the engine driving the prop, then a closed throttle really kills you because you have turned the engine into a vacuum pump, and it's not designed for that. In another thread it was said in order for a pilot to beat the jumpers down in a recip plane required stupid pilot tricks that tear up the airplane, but it's not true. If you pull the prop all the way back, lean the fuel to barely running (or ICO) and open the throttle WFO, you put very little stress on the engine and can drop out of the sky rapidly with the nose down (the other fun way is to just ride a falling leaf stall down to whatever altitude you feel comfortable recovering at).

As far as high rpm wear and tear go, as long as the engine is well balanced, I don't buy into it on these engines, even the geared ones that turn up at 3200-3400 RPM at the crank.
 
Let'sgoflying! said:
Someone here said, years ago, that:

hi RPM = wear (friction & heat on moving surfaces)
hi MP = stress (higher icp, higher loads on bearings etc)

Both of which are bad.
I do not recall anyone providing a counterpoint to this statement.
Click to expand...

In tight tollerance racing engines, high RPM and low load is when parts start banging into eachother. Engines are designed to be under load and when you zing them to high RPM's with no load, then the forces acting on the internal parts are reversed. That is to say they're being stressed in a direction they weren't designed to be stressed from. This problem is especially pronounced in tight tollerence engines but it occurs in all engines to some degree.
 
RJM62 said:
So you don't think that very low MP at a given high RPM, in itself, makes a difference?
Click to expand...
The stresses at a given RPM may be lower at a lower MP than a higher MP, but based on what I learned in engineering school, I think the stresses related to rotational speed are the dominant effect. Note that this is generally accepted as a measure of engine wear in that higher RPM results in faster turning of the tach time, and lower operating clock hours before recommended TBO is reached.
 
Aztec Driver said:
That is easily understood. Higher rotational forces and higher speed will generally equate to more wear and tear. What is not mentioned in that is the lower forces of the lower manifold pressure, creating less force and pressure. Why is that combination harder on the engine than the high MAP and high RPM?
Click to expand...
Because detonation can damage the engine fast -- really fast, like minutes or even seconds. But it's the result of the detonation, not the more long-term/cyclic MP/RPM-related wear that's the problem when detonation happens.
 
Ron Levy said:
Because detonation can damage the engine fast -- really fast, like minutes or even seconds. But it's the result of the detonation, not the more long-term/cyclic MP/RPM-related wear that's the problem when detonation happens.
Click to expand...

Low RPM-High MP = Greater chance for detonation than High RPM-High MP. Increasing RPM is one of the ways to drive out of detonation because it moves the peak ICP event further ATDC. The best way to manage detonation risk is by reducing fuel flow and increasing RPM.
 
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Jaybird180 said:
Shouldn't the MP gauge and the Kollsman match?
Click to expand...
You set air pressure corrected to sea level in the Kollsman window, i.e., what you'd read if you drilled a hole in the ground down to SL and then measured air pressure at the bottom of the hole. That's where your altimeter gets a reference pressure against which to compare ambient static pressure to compute altitude above MSL. OTOH, before engine start, the MP gauge reads what the air pressure actually is right where you are. Since air pressure drops pretty close to 1 inch/1000 feet as you rise above sea level, the ambient pressure at 6000 (i.e., what you see on the MP gauge at that elevation with the engine shut down) should be about 6 inches less than the current altimeter setting.
 
dans2992 said:
Dekin does not share the view that high RPM is bad.
Click to expand...
It isn't "bad", but it wears many things out faster than low RPM. That's why the tach time rolls over faster at higher RPM. OTOH, the difference between low and high MP for any given RPM setting doesn't affect wear rates as much.
 
Aztec Driver said:
The ATP test question is:

Under normal operating conditions, which combination of MAP and RPM produce the most severe wear, fatigue, and damage to high performance reciprocating engines?

High RPM and Low MAP
Low RPM and High MAP
High RPM and High MAP

The correct answer, according to the test, is HIGH RPM and Low MAP. I just wanted to determine why that was worse on the engine than the other two, when High MAP would certainly bring higher pressures and much more force against the various components than would a low MAP.
Click to expand...
It's a bad question which oversimplifies the issue. There's no way one could argue that 21"/2700 RPM is harder on a typical light plane engine than 27"/2700 RPM. OTOH, 27"/2100 RPM may have significantly greater risk for such an engine than 21"/2700 RPM if the detonation margin is compromised at 27/2100. But on the third hand, as long as the detonation margin is not compromised and the overall power setting stays in the normal cruise range, the engine will probably last longer in clock hours or miles flown if you cruise with a lower RPM and higher MP (e.g., 24"/2200 RPM vs 22"/2400 RPM).
 
Henning said:
Low RPM-High MP = Greater chance for detonation than High RPM-High MP. Increasing RPM is one of the ways to drive out of detonation because it moves the peak ICP event further ATDC. The best way to manage detonation risk is by reducing fuel flow and increasing RPM.
Click to expand...
True if you're talking about MP/RPM combinations which compromise detonation margin, but not true if you stay out of that range. Like I said, the question is too simplistic.
 
So many wrong things said here, I don't even know where to start. So I won't.

But Bryon, I disagree with the answer. So just memorize the answer they're looking for, and go from there.
 
Henning said:
Low RPM-High MP = Greater chance for detonation than High RPM-High MP. Increasing RPM is one of the ways to drive out of detonation because it moves the peak ICP event further ATDC. The best way to manage detonation risk is by reducing fuel flow and increasing RPM.
Click to expand...

Nevermind. read it wrong.
 
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