[BillL]

The topic of "derating" an IO360 by dropping the "rated" RPM from 2700 down to 2400 came up in another thread.

Here are some comments about it:

David, are you sure we disagree?

The engine wear of a 2700RPPM engine operated at that RPM is no more than 2400 RPM provided the oil is OK.

The forces on the crank of a 360/2400 are greater than those at 2700 in a 320 at the same HP. Do the physics/maths.

The majority of its life it will be at a cruise RPM anyway so much the same number. If there was any wear difference, it would be negated by the above.

I think you actually agree.

I wouldn't worry about de-rating the 360. The lycoming family is already de-rated. Think about how much HP you got from a 350 CI Chevy with no emission constraints! Heck, I have a 383 CI Ford that puts out over 500HP. The aviation engine philosophy is to use large, de-rated engines to improve reliability. My BMW 5 series rarely has cruise RPM's above 2300 and it has 120K miles and doesn't burn a drop of oil.

The only issues is heat. You don't want to run it so low that that CHT's fall out of the normal range. I don't think that would be the case. Lots of the RV's out there with 360's are cruising at 2400 RPM with no issue. The fact that they take off with 2700 is doing nothing to enhance their longevity.

Larry

The fact that they take off with 2700 is doing nothing to enhance their longevity.

Larry[/Quote...

Can you explain why this is true? Can you show the data or provide the math and physics that prove this particular crankshaft's frequency is happier at 2100 rpm than it is at 2700 rpm with no mention of what propeller its swinging, its inertia , the compression ratio, it's timing and the resultant power pulses.

Lets make some base assumptions and then address what happens as the RPM drops on this engine.

Given:
1. IO360 - no compression ratio change, same cooling system as standard Vans, 100LL, no ignition timing change for the RPM change.

2. A prop with no speed operating limitations for this engine. ref:Hartzell Composite

That should do it.

Now, let's break down the factors what will change and affect the life.

1. inertial forces - including piston weight, conn rod, and lack of counterweights on the crank to balance piston/rod weights.
2. Heat flux - BTU per unit area per time
3. Friction - ring/bore, bearings, air pumping losses.
4. Cylinder pressure due to firing.

I will post some comments on each of these, give me a little time to properly construct them.

 

[N941WR]

Bill,

It is my understanding that our engines were designed to make TBO while turning 2700 RPMs at 75% power.

Is that not correct?

 

[Mike H]

I don't think anyone will argue that in steady cruise operation that turning less RPM will, in theory, result in less engine wear. Even if this is true I doubt it would be great enough reduction to quantify, or show tangible results at time of engine tear down.

I think the real issue comes when people operate their engine at reduced RPM during takeoff and climb phases of flight. The former owners of the local flight school in my area used to teach students to pull the power back on the flight school's 172s shortly after lift off and never run the engines over 2300-2400 RPM so as to "save fuel". You could watch them take off and at around 100 feet you could hear them pull power then continue a normal climb. Operating at reduced RPM during highly loaded conditions will cause additional stresses on connecting rods, bearings, crankshaft and Pistons.

So I think the point that was trying to be made in the last thread was if you limit RPM to say 2400 maximum in an attempt to derate engine output, then what affect would that have on engine wear during highly loaded phases of flight?

 

[BillL]

1. Inertial forces

What inertial forces are important and how do they change with RPM?

The piston (and rings and pin) and small end of the rod translate from BDC to TDC repeatedly. As this happens they must accelerate from zero to max velocity and accelerate in the opposite direction back to zero. F=MA applies and these forces are reacted at the rod bearing. In addition, the big end of the rod bearing is spinning around with it's mass being "accelerated" by the circular motion.

The maximum force is at TDC where the rod is fully extended, and is fully felt 360 degrees from TDC at firing. The inertial stresses vary with the square of the engine rpm. With a flying web crankshaft, the forces on the crank tend to distort the crank and misalign at the main bearings.

2700 vs 2400? = (24/27)^2 = .790 Conclusion - lower speed is better.

The same would apply to the valve train, where the acceleration of the parts are lower with speed and wear forces on cam, push rods, and rocker arms.

Side loads from the piston on the cylinder are also less.

But will it really have a big effect? From a stress standpoint, all the steel parts would already have near infinite life, but the aluminum parts would have extended life. Rod and main bearings should also already have adequate oil film thickness at minimums, so, that should not be a major factor for speed. The rod and main bearing misalignment could be a factor with more edge loading as these bearings have a healthy length.

Mains and rod bearings 2400 +
Stresses on steel parts 2400 0 (neutral)
stresses on al parts 2400 +
Wear on valve train - 2400 +

OK, what have I missed?

 

[Jesse]

While avoiding getting too technical or scientific, the O-470's used in 182's are 1,500 hour TBO engines unless you get the U model, which is the same horsepower, higher compression and 2,400rpm instead of 2,700rpm with a 2,000 hour TBO.

 

[BillL]

Heat Flux - Btu/area-time

Let's break this down in to friction and combustion.

Friction is dominated by piston and rings in the bore. Of course there are bearings (viscous), gears, and windage. All of this together add up to a linear effect with speed. While the reduction in piston speed is proportional, the total effects are less. Meaning that while piston speed reduction is .89, the friction reduction is lower, but less than a .89 reduction. Lower friction means less heat at the point of generation resulting in a lower temp, and heat transfer aka, flux.

Combustion - this comes down to slightly increased port flow characteristics yielding a larger fuel charge per combustion event. This also means higher compression pressures and slightly higher combustion pressures. So, even though the energy per event is higher, the number of events is proportionally lower. Thus lower total heat rejected, lower metal temperatures, and lower heat flux.

Friction - 2400 +, lower and better maybe 8% lower
Combustion heat loss - 2400 +, lower and better, maybe 10% lower

I think that covers that.

 

[BillL]

Friction.

Friction - That is pretty much included in the other posts for mechanical friction, but pumping losses basically increase greater than the proportional increase, at least from 2400 to 2700.

It is a design consideration for how much valve area can be used vs longevity of the head due to thin sections and what top speed the engine needs. An engine might have a rated speed where volumetric efficiency has dropped quite a bit (and torque), but is still increasing in power due to more combustion cycles. Normally aspirated of course, turbocharging changes a number of parameters.

Speed will drive up pumping losses more geometrically than proportionally as higher port velocities, create more pressure drop and piston work to get the air in and exhaust out. We could look at indicator diagrams and integrate the forces over time, but this is the result.

Moving on.

 

[sblack]

Here's another perspective. What kills airplane engines is not running them. Most majors are required because of corrosion issues, at least for our sport planes. We just don't fly enough. That and cold weather ops or excessively humid environments. If we ran them like a ground power unit they would run indefinitely. If there is oil in the bearings and they are at operating temps there should be no metal contact and hence no wear.

I would expect the way the engine is run and the environment it is run in (oregon vs arizona) will have a much greater impact on its life than whether it is a 320 or derated 360.

 

[BillL]

Cylinder pressure

This one has been touched on already. With slightly higher volumetric efficiency, the larger energy in the fuel charge at a fixed A/F, the compression pressure and firing pressures will be increased, thus adding load to this component of loads on the bearings. Temperatures of piston, rings, head, valves are likely still lower, due to fewer cycles, and a relatively fixed heat rejection paths.

This effect is totally the result of Volumetric efficiency or Vol-eff. If there is only a 1% change, then that limits the effect very small. I looked at my Lycoming performance chart for the parallel valve 180 hp and it shows almost exactly 160 hp at 2400 rpm, sea level. It might be because they like to draw straight lines on the graph since these were done by hand.

Detonation - with the slight increase in pressure, there would be a reduction in detonation margin due to pressures, but since the heat/temperatures of the head/piston would be lower, this might neutralized(at best). Regardless, if the engine is qualified for auto fuel, then margin would be much greater with 100LL.

The result is a neutral to slightly negative for 2700 to 2400 for this parameter.

 

[BillL]

Conclusion

Leaving out running or not running and limiting to this specific engine, not cars, lawn mowers or ship propulsion it seems pretty clear.

The 2400 rpm rated speed will be extend the life of the 2700 rpm operation. Having said that, in the scope of operations, this might not present much of the proportion of the operating time. We also know some RVers keep at 2700 from TO to landing. Certainly, the lower rating would not hurt the TBO.

As pointed out, some engines have a 1 minute (random number, read:timed) speed rating. That was not uncommon with Contis. In a test cell, it may take 30 min to warm up an engine and changing an operating test point can take 10 minutes to stabilize. Given that a TO event may be 30 seconds long, an engine that might exceed some given limit like piston top ring temperature. From start up to reaching the limit might take 10 min, and if there is a throttle back, then it is eliminated.

I really do not know this particular engine design and which of the many limits will occur and what order. Given its anecdotal tolerance for some abuse, probably means that all limits are not close by.

Will it improve the TBO - well in 100 engines average, yes for sure, but like all things there is some statistical distribution, so, maybe.

 

[BillL]

Bill,

It is my understanding that our engines were designed to make TBO while turning 2700 RPMs at 75% power.

Is that not correct?

I dont know the details about the specifics, Bill, sorry. I can say, typically, there is a "rated" condition that is considered to be operated intermittently. Sometimes an hour sometimes much less. Then, there is a continuous rating where it can be operated without time limitation. Typically, there is a reduction in power and speed for this continuous rating. Many engines have been found to have the capability to operate in the field just fine outside these typical design/rating parameters. Most engines, after years and 100's of thousands of hours of experience, are approved for operating conditions that would seem to violate many "rules" but the collected data give some confidence in the approvals. Because one engine design can do this does not mean others can. Naturally, each engine design has it's strengths and weaknesses.

 

[luddite42]

I dont know the details about the specifics, Bill, sorry. I can say, typically, there is a "rated" condition that is considered to be operated intermittently. Sometimes an hour sometimes much less. Then, there is a continuous rating where it can be operated without time limitation. Typically, there is a reduction in power and speed for this continuous rating. Many engines have been found to have the capability to operate in the field just fine outside these typical design/rating parameters. Most engines, after years and 100's of thousands of hours of experience, are approved for operating conditions that would seem to violate many "rules" but the collected data give some confidence in the approvals. Because one engine design can do this does not mean others can. Naturally, each engine design has it's strengths and weaknesses.

The limits on duration of full power with Lycomings that I'm aware of are associated with specific aircraft types and not blanket limitations on a particular engine model installed in any aircraft. Different aircraft types have different cowling arrangements, airspeeds, cooling properties, etc. Are there actual engine models with these limitations straight from Lycoming?

Those Lycomings in Robinson helos run continuous 3000 RPM and have a 2000 hr TBO. Slightly different cam profile, but nothing different about those engines that tolerate RPM better than our 2700 RPM "limit" Lycs.

 

[BillL]

The limits on duration of full power with Lycomings that I'm aware of are associated with specific aircraft types and not blanket limitations on a particular engine model installed in any aircraft. Different aircraft types have different cowling arrangements, airspeeds, cooling properties, etc. Are there actual engine models with these limitations straight from Lycoming?

Those Lycomings in Robinson helos run continuous 3000 RPM and have a 2000 hr TBO. Slightly different cam profile, but nothing different about those engines that tolerate RPM better than our 2700 RPM "limit" Lycs.

Interesting! It is good to know that the engine (internal parts TBD) can be robust to engine speed. So do they have the "standard" pistons, Cr, connecting rods, and wrist pins? From what I could find, this was stated for the O-360 in the R22:"this 145 hp engine is derated to 131 hp for five minutes at takeoff and 124 hp for continuous operation." Is that your experience? Rating an engine this way would aid in the design of a gearbox for lower torque and improve the drive ratio. read: lighter

Is it throttle plate (restrictor) limited or does it have an early inlet valve closing to limit the volumetric efficiency? And thus HP?

 

[DanH]

Good stuff Bill!

Cylinder Pressure
This one has been touched on already. With slightly higher volumetric efficiency, the larger energy in the fuel charge at a fixed A/F, the compression pressure and firing pressures will be increased, thus adding load to this component of loads on the bearings.

The list should include peak pressure moving closer to TDC as RPM is reduced. The effect seems to significantly increase peak cylinder pressure.

I looked at my Lycoming performance chart for the parallel valve 180 hp and it shows almost exactly 160 hp at 2400 rpm, sea level.

The chart also shows 2400/WOT hard against the "Limiting Manifold Pressure For Continuous Operation" line. That's assuming max MP to be about 28.6" Hg, with best power mixture and otherwise Standard Day conditions. An effective ram inlet, or a hot day/hot engine puts it over the line.



This is probably why. Get everything hot, set the oversquare 28.6"/2400 combination, then pull mixture a little past best power:



Detonation frequency reaches about 12% at the worst. May not break it, but it is hard on the parts.

Note the above is with 20 degree timing.

If I were a regulating authority in our subject builder's home country, I'd probably require a MP limit to go with the 2400 RPM, and specify 100LL min with 25 BTDC max timing. That may indeed be overly conservative, but it's what the manufacturer's data suggests as prudent.

 

[Mike H]

Great information! Thanks for sharing this Bill and Dan.

The data seems to prove that there is more to than just limiting power output through prop pitch/RPM.

 

[BillL]

Very Interesting!

Dan,
I was looking at the M1B parallel valve performance chart for my comment. It has no limitation for manifold pressure on the chart like the one you posted. It also mentions minimum fuel grade as 91/96 . I ordered it in with a package of information from Lycoming. I will scan it and get it posted here too.

Very engine specific indeed. For one to address derating more appropriately, it would important to obtain the proper performance curves for the specific engine to be addressed.

Detonation curve, if available, would be vital data for decision making. Also your comments about timing are spot on. Timing is an important parameter for proper heat release and to manage potential issues.

 

[RV10inOz]

Folks, this is simple and not really warranting the over complication. but I guess it is a fun exercise for those who want to undertake it.

Dan Horton has correctly picked up on the subtle but measurable difference I had been referring to in the other thread.

Dan H;

The list should include peak pressure moving closer to TDC as RPM is reduced. The effect seems to significantly increase peak cylinder pressure.

This is the bottom line. The life of the engine in terms of RPM and thus revolutions made over its usable life has been shown many times to be negligable, and as many people love listening to Mike Busch rave on about his way past TBO experiment, I think you can take some comfort in his results. Your engine life will not be determined by running at 2400/2300/2563/2478 or 2723 RPM.

The forces on the engine if you think being kinder to it by lowering the peak pressure will help it through less stress would have you running higher RPM not less, and for any given power output that would be done LOP.

WOTLOPSOP did not become the expression of George Braly, John deakin and Walter Atkinson for nothing.

As for detonation margin, running a conforming engine on conforming fuel, even for this discussion it is irrelevant.

Great thought thread Bill!

 

[DanH]

Dan,
I was looking at the M1B parallel valve performance chart for my comment. It has no limitation for manifold pressure on the chart like the one you posted. It also mentions minimum fuel grade as 91/96 .

The original 390 chart didn't include a limiting manifold pressure line, but it has been one engine Lycoming has declined to certify for 91/96. Don't know if the most current charts draw the line, but lack of one may not be a good indicator.

Detonation curve, if available, would be vital data for decision making.

And naturally, it's the one chart they won't hand out. Baffles me. Why not?

 

[RV10inOz]

The EMS data ran several cylinders just like detonation does in terms of temperature rise. Under the state it was in it was not a conforming engine. We had concerns of fuel. It was very hot cylinders from long taxi and high OAT and IAT, it had all the signs for a non conforming engine to do that. But back then I did not know as much as I do today, and listened to some folk who peddled OWT's (mechanics).

 

[DanH]

As for detonation margin, running a conforming engine on conforming fuel, even for this discussion it is irrelevant.

Three detonation onset charts taken from DOT/FAA/AR-08/53, the Swift fuel/100LL comparison done at the FAA's Hughes Technical Center. It's one of the few public documents with both detonation and peak pressure point data for a Lycoming.

The engine in these charts is an IO-540K angle valve 6-cyl. I think the 540K uses the same cylinder and piston as the IO360 angle valve models (needs checked to be sure), and if so the data should be transferable. Conditions are worst case per the FARs; one CHT at 475F, all others within 50F, 245F oil inlet, 100F air inlet temps. It's fair to note most of us stay well away from 475 CHT and 245 oil, but manufacturer's maximum is the FAA standard for this test. It's also fair to note that the charts indicate detonation onset, which is not necessarily severe or doing damage. Still, it is not good.

I've removed the Swift fuel plots for clarity. What remains are runs made with 100LL sourced at local FBOs, as well as a special 100LL fuel formulated to the minimum standard ("100LLms").

First up, 2450 and 28". Running hot and oversquare, detonation onset is at peak power mixture.



Pull the manifold pressure back to 26", and detonation onset is thankfully leaner than best power. We can see peak EGT in this one; the risk zone is ROP, just as seen in GAMI and Lycoming documents.



2450 and 24": things settle into the familiar pattern. Detonation onset is now about 50~60 ROP, well away from best power mixture. One of the FBO fuel samples detonates near peak EGT.



The margins get very thin if a cylinder is allowed to run hot, lean, and oversquare. In fact, it detonates at max available power for that RPM, thus the "max manifold pressure" line in the (or some) power charts.

From a certification standpoint, an approval to limit RPM to 2400 without limiting other factors does not seem reasonable. Consider the hot day possibilities given a carbureted engine with poor fuel distribution, or an injected engine with a partially plugged nozzle.

 

[DanH]

It's also fair to note that the charts indicate detonation onset, which is not necessarily severe or doing damage.

On the other hand, it certainly has the potential to be severe. Another pair from the same document, these showing detonation intensity. When hot and oversquare, detonation intensity is off the top of the chart (or heading there) as mixture nears stoich (equivalence ratio near 1.0).

 

[lr172]

Here's another perspective. What kills airplane engines is not running them. Most majors are required because of corrosion issues, at least for our sport planes. We just don't fly enough. That and cold weather ops or excessively humid environments. If we ran them like a ground power unit they would run indefinitely. If there is oil in the bearings and they are at operating temps there should be no metal contact and hence no wear.

I would expect the way the engine is run and the environment it is run in (oregon vs arizona) will have a much greater impact on its life than whether it is a 320 or derated 360.

I agree that certain parts wear or fail due to corrossion. However, the BIG killer due to inactivity is accelerated wear of cylinder walls due to oil starvation. After about 30 days, the oil film leaves the majority of the cylinder wall due to gravitational forces. This leaves significant wear in the first few seconds of operation. This eventually leads to removal of the honing scratches due to the wear. The primary purpose of these scratches is to hold oil. Once their gone, cylinder wall wear increases dramatically, as each revolution is now creating slight wear as the oil layer to prevent it has been dramatically reduced (no valleys left to hold it).

Larry

 

[lr172]

The limits on duration of full power with Lycomings that I'm aware of are associated with specific aircraft types and not blanket limitations on a particular engine model installed in any aircraft. Different aircraft types have different cowling arrangements, airspeeds, cooling properties, etc. Are there actual engine models with these limitations straight from Lycoming?

Those Lycomings in Robinson helos run continuous 3000 RPM and have a 2000 hr TBO. Slightly different cam profile, but nothing different about those engines that tolerate RPM better than our 2700 RPM "limit" Lycs.

A key difference in longevity is the load placed on an engine at any given RPM. Dan touched on this by showing the MP to RPM relationship. Taking an auto analogy, cruising down the highway at 2400 RPM is not the same load on the engine as going up a hill at 2400 RPM. I see this in Marine engines. They do not get the same life out of the same 350 Chevy small block running in a car. That is because of the constant load not seen continuously in cars.

I feel that one must consider overall load when determining longevity and not just RPM. It's just too few variables in an environment with MANY variables in play. I still contend that RPM is not the driving factor in longevity. NOW, if the lower RPM is causing a higher load or Over square, as Dan referenced, then lower RPM could be worse. However, in this example, that condition represents such a small percentage of use that it makes the impact insignificant.

Larry

 

[RVDan]

This is all a very interesting discussion, but I have a question relative to the core issue of the thread- RPM effect on Engine life. The engine life is not time limited by the manufacturer, and parts can be replaced based on condition.
1. Define engine life- does this mean overhaul or replacing cylinders, or both or what?
2. What are the failure modes that limit engine life, or at least drive us to call the engine life ended (overhaul the engine?) Once this is understood we could apply some of the theory discussions to see how they effect those specific failures that limit engine life.

For example I could start the list with:
Loss of cylinder compression?
Failed crankshaft?
Valve failed/stuck?

And so on. Once we had a short list of , say the 10 most significant reasons we call an engine life ended, perhaps it could be seen how the theory actually impacts engine life.

 

[BillL]

This is all a very interesting discussion, but I have a question relative to the core issue of the thread- RPM effect on Engine life. The engine life is not time limited by the manufacturer, and parts can be replaced based on condition.
1. Define engine life- does this mean overhaul or replacing cylinders, or both or what?
2. What are the failure modes that limit engine life, or at least drive us to call the engine life ended (overhaul the engine?) Once this is understood we could apply some of the theory discussions to see how they effect those specific failures that limit engine life.

For example I could start the list with:
Loss of cylinder compression?
Failed crankshaft?
Valve failed/stuck?

And so on. Once we had a short list of , say the 10 most significant reasons we call an engine life ended, perhaps it could be seen how the theory actually impacts engine life.

Since I started this, I will answer, in general. There are temperature, stress and wear limits of many parts. Operation of an engine within parameters, even it short excursions above them occur, should result in a relatively balanced end of life wear out of rings, valve faces, seats, cams, etc. Some of those things will live much longer and are not likely reasons for "overhaul" If, however, say piston ring groove temperature, consistently exceeds the limit temperature then a collection of carbon, wear and possible ring sticking would occur prematurely and become the reason for shortened life. Allowing operation to result in outright failure of major components would not be acceptable, and is not considered a "life" issue, in the context of a 2000 hr expected life. Short life trade offs for racing or grenade ratings are another matter.

Back to our topic, One of the most likely catastrophic result would be caused by detonation. DanH has shown that the angle valve engine is very close in reduced RPM at high power (BMEP) measured as InManP. A bit off on leaning and it could reach a condition of failure, of accumulate a series of damaging events that reduce the life by premature piston failure.

While valve sticking is one failure mode, it seems to be a result of other out of bounds operating parameters. One exception seems to be the brand of oil in one case seen here on VAF. That one would deserve some investigation by an engine manufacturer as to the root cause.

Each engine design has different characteristics and could cross a limitation where others don't. This is in the context of Lycomings too. You can take a long list of things, pistons, pins, cranks, heads (angle vs parallel) valve clearances, compression rations, ignition timing, etc into consideration. Sneaking up on limits by taking engines from the field operated under known conditions and pushing up conditions can result in operation very close to limits, but often with little tolerance for "abuse". This is where the engine experts like Barrett come into play. For this reason one should be very careful in drawing conclusions about what can be done from other designs.