RPM effect on engine life.

[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:

Originally Posted by RV10inOz

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.

Originally Posted by lr172

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

Originally Posted by jj_jetmech

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]

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 - 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]

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]

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.