IO-360 low rpm LOP cruise power?

[Guitarmaster]

I have huge interest in this subject.

Curious...

Mike Busch's article on plain bearings was very enlightening. What I took away from it was when the engine is running, there is essentially no friction on the mains due to oil pressure. The real wear comes from startup just before pressure rise. Hence the reason to not use synthetic oil.

It seems to me, as long as you have good oil pressure, the bearings should not be an issue... To a point of course. I think I would be more concerned with connecting rods and pistons. However, as long there is no detonation, I can't see low rpm being an issue. When I get a chance I am going to talk to the excellent engine shop we have here on the field and see what they have to say.

Admittedly, OS scares me a little. Probably my own carryover biases from years ago. I am always looking for new and possibly better ways to operate my airplane.

Cheers!

Matt

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[jetdriven]

The lycoming operators manual shows oversquare setting and so does the POH. In fact, the lycoming manual shows 1800 RPM 25" power settings, and that's ROP. All I hear is old wives tales on oversquare operations. Anyone got a limitation or an accident report to share?

[carusoam]

What happens as we slow the prop down?

I would think there is a bit of fuzziness in slowing the engine rpm with respect to ignition timing...

The time in relation to spark and pressure development has to match the placement of the piston with regards to dBTDC.

The physics measures time in fractions of seconds...

The engine, is a bit more simple, uses degrees to set timing...

The conundrum is...

At 2200rpm there is more time available (in seconds) to go from 25dBTDC to 0dBTDC than at 2700rpm.

So if a pilot is concerned about detonation at 2700rpm by turning back the timing from 20dBTDC to 25dBTDC.

Shouldn't he be more concerned by making this change and running the engine slower?

I would think that a calculation can be made to compare the effects of engine timing in degrees to actual time in seconds for a few cases.

2700rpm, 2500rpm, 2300rpm and 2200rpm

Keep in mind that rate of reaction could vary with the temperature and pressure inside the jug as well.

How much time in seconds is 5 degrees of timing at these different rpm settings?

What could we use to verify that we are not harming the engine?

What we know by setting the timing back 5 degrees... More power and higher CHTs...

What we surmise by setting the rpms back... increased time to get to TDC, more heat but balanced by fewer power pulses...?

The pilot proposing the question knows this situation better than most. If he says OWT.... I'm on board.

Then again, if the CHTs start to increase as the rpms decrease, would that be a sign to watch for?

Byron, how did I do?

Best regards,

-a-

[Guitarmaster]

I have lots of time to think at FL410 for hours on end. My thoughts for today.

I tune my truck myself. Under high load (high power, low RPM). The timing is retarded to near 0 dBTDC. Under light loads, the timing is more like 20 dBTDC. The reason is flame propagation and internal cylinder pressure. Quite simply, there is always negative torque in an engine, of course, advancing timing increases this. The trick is to get the perfect balance. Theoretically, this can be done with computer controlled timing.

Since our airplanes, as I understand, have a fixed timing (except for start), then low RPM with high power will increase ICP dramatically and at the same time, decrease EGT. without a doubt, this will increase power until you get to the negative torque region, then you have problems. The real question is, how much ICP will these engines take?

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[jetdriven]

I'm sorry I don't understand your statement. Negative torque, as I understand it, is where the prop is driving the engine. You can feel this start around 16-17" at 2400-2500 rpm at airspeeds over 100-110 knots in an M20J

[chrisk]

Does anyone have a concern for over square anymore?

Is OS officially dead?

 

 

What is your MP when you take off at sea level?  What is your RPM?  Are you not OS?  Also, turbo engines clearly operate OS for extended periods of time.    Unless the POH or engine manufacturer recommends against it, I suspect OS isn't an issue.

[N201MKTurbo]

Ok, here is the problem with letting the prop drive the engine: the oil holes on the crank rod journals are drilled to supply high pressure oil at the crank angle of highest pressure. With the prop driving the engine the pressure is 180 degrees from the oil holes. Increasing the risk of violating that oil film that Mike Busch talks about.

[Guitarmaster]

There are two kinds of negative torque. When the plane is driving the prop, but also in the engine itself.

The engine is nothing more than a big air pump. I my case, 360 in3 per stroke. On the intake we inject fuel and air. In the case of our airplanes, that fuel/air mixture is lit off at 20-25 dBTC. In other words, flame propagation starts there. Because of this fact there is a certain amount of gas expansion prior to TDC trying to push the engine backward. Negative torque.

To have no negative torque, you would have to start flame propagation at TDC, but this is inefficient as the flame chases the piston down and most of the energy goes out the exhaust. This produces high EGT. As the timing becomes more advanced, flame propagation and expansion begins sooner and EGT decreases. The more advanced the timing, the cooler the EGT. BUT... since energy cannot be created or destroyed, it has to go somewhere. In this case, ICP and CHT. Ideally you want the maximum expansion just past TDC. Easy to talk about, not so easy to achieve.

I can't say for sure, but I would guess Lycoming sets this at maximum RPM and MP at sea level standard day.

If you slow the engine down, and increase power, you are running at a high load. Like pulling a trailer up a hill in sixth gear. Since the timing is not variable, the flame propagation begins much earlier in the stroke. This will naturally show a decrease in EGT and you will have a increase in negative torque.

So, the real question is, how much can these engines take? The only real way to tell if you have passed the "equal torque point" where negative is greater than positive, is to dyno the engine at different load settings.

This is based on my years of experience tuning automobile engines. However, all internal combustion engines work basically the same.

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[jetdriven]

There are two kinds of negative torque. When the plane is driving the prop, but also in the engine itself.

The engine is nothing more than a big air pump. I my case, 360 in3 per stroke. On the intake we inject fuel and air. In the case of our airplanes, that fuel/air mixture is lit off at 20-25 dBTC. In other words, flame propagation starts there. Because of this fact there is a certain amount of gas expansion prior to TDC trying to push the engine backward. Negative torque.

To have no negative torque, you would have to start flame propagation at TDC, but this is inefficient as the flame chases the piston down and most of the energy goes out the exhaust. This produces high EGT. As the timing becomes more advanced, flame propagation and expansion begins sooner and EGT decreases. The more advanced the timing, the cooler the EGT. BUT... since energy cannot be created or destroyed, it has to go somewhere. In this case, ICP and CHT. Ideally you want the maximum expansion just past TDC. Easy to talk about, not so easy to achieve.

I can't say for sure, but I would guess Lycoming sets this at maximum RPM and MP at sea level standard day.

If you slow the engine down, and increase power, you are running at a high load. Like pulling a trailer up a hill in sixth gear. Since the timing is not variable, the flame propagation begins much earlier in the stroke. This will naturally show a decrease in EGT and you will have a increase in negative torque.

So, the real question is, how much can these engines take? The only real way to tell if you have passed the "equal torque point" where negative is greater than positive, is to dyno the engine at different load settings.

This is based on my years of experience tuning automobile engines. However, all internal combustion engines work basically the same.

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Its 360 Cu In evry two revolutions of the crankshaft.

 

If you light off the fuel at 25 BTDC the peak power pulse doesn't occur until somewhere around 10-17 degrees after TDC in an aircraft engine.  I am not sure if you can get pressure to build before TDC because of the time it takes for the flame fronts to spread. If it did, you would have massive loss of power at the crank, since the engine is trying to compress a burning and expanding mixture.  FWIW, the Electroair system uses only a MP reference for timing advance. Above 25" MP, its rated timing. However, below that, its up to 40 degrees of timing.  How can an engine produce power at 40 degrees of timing regardless of RPM, if 25 degrees somehow encroaches on the boundary of firing too soon?  Let me know.

[Guitarmaster]

I don't know where the boundary is. It might be well past 40 degrees. I just know the timing would be advanced and ICP increased with a low rpm/high MP cruise.

A question...

Are the internals for a turbo engine different/beefier than for the naturally aspirated?

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[carusoam]

CR is much different for the turbo engine.

Different enough to cause serious problems when intakes leak or turbos break.

Making the TN an interesting option for high altitude flight...

Best regards,

-a-

[pinerunner]

Does anyone have a concern for over square anymore?

Is OS officially dead?

At my preferred altitudes 10-12k' OS is not really an issue.

Immelmann, nice to see you pop in once in a while!

What do you fly at work?

Best regards,

-a-

I think in a day and age without good engine monitors the OS provided a good rule of thumb to protect your engine, especially when climbing out.  I think in this day and age its still a good rule of thumb but not a straight-jacket to stop you from trying things. I've read (Busch, Deakins, and the folks at GAMI) that the flame front propagates a bit slower when LOP so a lower RPM maybe makes sense there. I certainly watch the CHT's closer when dropping RPMs so OS is still making me increase vigilance. 

 

I still take the oversquare rule seriously when asking for high power at low speeds and high angles of attack (poorer cooling).  At lower power settings I drop the MP a bit and the RPMs a lot, avoiding the restricted RPM range I have in my old M20E. 

 

I don't think OS will ever be totally dead.

[carusoam]

Re-write...

Fear of over-square

Is the fear of over-square officially dead?

There doesn't seem to be a reason that stands out that would make it an unacceptable practice.

It is a discussion of going from high rpm to too low rpm with OS in the middle somewhere..

Still waiting for the ICP sensor to bring a light to the discussion. CHT is a bit too indirect...

Without a sensor, it is too challenging to determine when ICPs are increasing...

High MP mixed with low rpm sounds dangerous...

Low MP mixed with lower rpm sounds like LOP guidelines. Safe at lower % power...

Hope that clears up my thoughts...

Best regards,

-a-

[Linguinator]

Er . . . years later . . .

It's worth mentioning that Lucky Lindy pioneered the low-rpm, high manifold pressure economy technique.  It's equivalent to tall gearing in an auto:  low rpm, high throttle setting, high prop pitch, LOP mixture.

Lindy flew with the 475th Fighter Group in the South Pacific and taught them how to use this exact cruising-power protocol.  I believe he brought their cruising rpm from 2,200 down to 1,600.  By using his methods, the group almost doubled their P-38's range.  I've read that he was famous for leaning out the mixture to the point that his spark plug tips were just a little too white.  The man was an aviation savant.

This story is all over the internet.  Do a search for "Lindbergh 475th fighter group" and you'll get numerous hits.

 

FWIW, I would never worry about the bearings . . . unless it's detonating, of course.  Running oversquare doesn't scare me one bit.

 

[Andy95W]

Welcome to MooneySpace!

[Pinecone]

On 5/20/2014 at 11:27 PM, kortopates said:

The APS guys are always talking about faster RPM being more efficient due to delaying the spark a little so I am surprised and I also find at high power settings I can't get as smooth LOP ops at low RPM. But I like to go fast! 

Are you sure about that?

LOP means slower burn rate.  So timing needs to be advanced.  Lower RPM effectively advances the timing.

[kortopates]

Are you sure about that?
LOP means slower burn rate.  So timing needs to be advanced.  Lower RPM effectively advances the timing.

No you’re so right, must have been a big brain fart back then because i said that totally backwards. Slower RPM will permit the engine to run leaner smoothly than a higher RPM.

i’ll have to scroll up and find that old post and correct it.
thanks for letting me know!


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