Low Power Cruise settings

[A64Pilot]

16 minutes ago, Bolter said:

This shows some relationships of power-mixture-BSFC (from Lycoming)

Unfortunately Lycoming chose to not put numbers on that chart, rendering it pretty much useless

[PT20J]

Regarding the formula for power LOP, I think it is good to understand its origin:

Looking at the curve posted by @Bolter you will notice that the BSFC curve is pretty flat in the best economy range LOP. The Advanced Pilot Seminars folks observed that most normally aspirated aircraft engines have a compression ratio of about 8.5:1 and a LOP BSFC of about 0.39 lbs/hr/hp. Similarly, they observed that turbocharged (NOT turbonormalized) engines have compression ratios of about 7.5:1 and BSFC of around 0.425 lbs/hr/hp. Taking the weight of a gallon of avgas to be 5.85 lbs., simple math gives:

5.85 / 0.39 = 15.0 hp/gph for normally aspirated engines

5.85 / 0.425 = 13.7 hp/gph for turbocharged engines.

Keep in mind that this is an approximation.

Skip

EDIT: Corrected typo to indicate that the weight of a gallon of avgas was taken to be 5.85 lbs.

[gsxrpilot]

19 minutes ago, A64Pilot said:

Unfortunately Lycoming chose to not put numbers on that chart, rendering it pretty much useless

What numbers do you want? All the relevant numbers are there.

[PT20J]

Here's a chart for a Continental IO-550G. 

[Andy95W]

17 minutes ago, A64Pilot said:

Unfortunately Lycoming chose to not put numbers on that chart, rendering it pretty much useless

No, it’s not, because it’s different for every airplane.

EGT is dependent upon where the probe is placed, so each installation will differ.  CHT depends upon cooling efficiency, so that’s dependent upon the airplane type and how good the baffling is.  Fuel consumption differs by engine type.

What remains consistent is the relative relationship of each of the numbers, which the chart shows pretty well.

[PT20J]

Just now, gsxrpilot said:

What numbers do you want? All the relevant numbers are there.

BSFC is missing 

[Hank]

24 minutes ago, PT20J said:

Wear vs. rpm and the effect of rpm on engine output:

A Lycoming IO-360 has a stroke of 4.375". Assuming it runs for 2000 hours at 2600 rpm vs 2500 rpm, the piston rings will travel within each cylinder an additional:

4.375 in./stroke x 2 strokes/rev x 100 rev/min x 60 min./hour x 2000 hr. / 12 in./ft. / 5280 ft./mi. = 1,657 miles.

The extra piston travel distance is 4% comparing 2600 vs 2500 RPM. Actually, it will be less than that unless you taxi, takeoff, climb, cruise, descend, fly IFR approaches and VFR patterns and land at 2500. I cruise and descend there, but takeoff and climb at 2700; pretty much everything else is at lower RPM. Is 3% piston travel over the life of the engine going to cause significant wear? If so, then the 2000 hour TBO is far too generous. But how many people run their engines 10-30% past TBO? Tells me there's a good safety factor built into engine design, as I would expect from a good engineer [or group thereof].

[PT20J]

Just now, Hank said:

The extra piston travel distance is 4% comparing 2600 vs 2500 RPM. Actually, it will be less than that unless you taxi, takeoff, climb, cruise, descend, fly IFR approaches and VFR patterns and land at 2500. I cruise and descend there, but takeoff and climb at 2700; pretty much everything else is at lower RPM. Is 3% piston travel over the life of the engine going to cause significant wear? If so, then the 2000 hour TBO is far too generous. But how many people run their engines 10-30% past TBO? Tells me there's a good safety factor built into engine design, as I would expect from a good engineer [or group thereof].

Your point is well taken. However, my point was simply that there is more friction at higher rpm and this shows up as wear (though I don't think it's significant) and heat and wasted fuel. So, if you have the option, you are better off to run lower rpm and WOT. Of course, with a little IO-360, you don't have a lot or latitude unless you run at low power or cruise at low altitude.

Skip

[A64Pilot]

3 minutes ago, PT20J said:

BSFC is missing 

Fuel flow is missing as well as actual EGT, now I know EGT will change based on location but the delta won’t. as well as fuel flow differences, they ought to be the same.

‘For that matter, where do the numbers in red come from? Lycoming? or did someone else put them there? Not arguing their accuracy, just thinking that they ought to be in the chart for the chart to meaningful. Of course that’s why they were added wasn’t it?

‘This chart to me is meant to be representative, meant to illustrate something. What I’d like is a performance planning chart, one with numbers.

If correct it does seem to indicate that the efficiency peak is at -20F LOP and that by running even lower LOP is less efficient doesn’t it? Assuming the numbers in Red are real and correct of course, but either way it shows a efficiency peak, which is not what you usually hear from LOP converts, they often say the leaner the better

[A64Pilot]

Higher RPM is not just more distance travelled, it’s mostly the kinetic energy that has to be dealt with as reciprocating mass comes to a complete stop and rapidly accelerates in the opposite direction., those forces are transmitted through the bearings of course.

The rotating mass, crankshaft etc has essentially no max RPM, or said another way its actual limit is well in excess of the reciprocating parts. The upper RPM limit of a race motor is set by piston speed, the piston is the weak limit.

From a TBO perspective, an engine is designed to be run at max allowable continuous power until TBO, it’s if you wanting to go beyond TBO, or if at TBO your not wanting to buy a bunch of parts is where part power comes in.

My 540 at overhaul all the hard parts met new limits so I didn’t turn the crank or have to buy one, largely because it was a 235 and had a redline RPM of 2400.

Lycoming has stated in print that motors run at or below 65% power will last longer

[A64Pilot]

4 minutes ago, bluehighwayflyer said:

LOP converts who know what they are talking about don’t say that.  When LOP they might say “the leaner the cooler”,  though.  

What they don’t say is that LOP is simply another way of reducing power, you see identical results by decreasing manifold pressure once LOP.

‘What they also don’t say also is that there is an efficiency peak, they seem to indicate that the leaner, the more efficient.

‘I don’t want this to turn into a LOP discussion, if you want to operate deep LOP that’s fine.

My experience with it is at the altitudes I travel at that it just doesn't make enough power

[AH-1 Cobra Pilot]

12 minutes ago, bluehighwayflyer said:

OK.  There are exactly three engine controls in my airplane that will allow me to reduce power.  Yours is probably the same.  And I never said I wanted to operate deep LOP.  

Well, I can think of five...six if I had an O vs. an IO.

[gsxrpilot]

36 minutes ago, PT20J said:

BSFC is missing 

 

25 minutes ago, A64Pilot said:

Fuel flow is missing as well as actual EGT, now I know EGT will change based on location but the delta won’t. as well as fuel flow differences, they ought to be the same.

‘For that matter, where do the numbers in red come from? Lycoming? or did someone else put them there? Not arguing their accuracy, just thinking that they ought to be in the chart for the chart to meaningful. Of course that’s why they were added wasn’t it?

‘This chart to me is meant to be representative, meant to illustrate something. What I’d like is a performance planning chart, one with numbers.

If correct it does seem to indicate that the efficiency peak is at -20F LOP and that by running even lower LOP is less efficient doesn’t it? Assuming the numbers in Red are real and correct of course, but either way it shows a efficiency peak, which is not what you usually hear from LOP converts, they often say the leaner the better

The equation used to get BSFC is different depending on whether the engine is running LOP or ROP. On one side Fuel Flow matters and is part of the equation, on the other side it doesn't. Therefore it doesn't make sense to add either numbers to the chart. The chart can be read completely and appropriately using the numbers provided.

But as @bluehighwayflyer said, I know how it works and how to use it. I cruise my turbo 252 in the flight levels somewhere between 20° and 30° LOP and enjoy a 35% fuel savings for only a 10% speed penalty. It also keeps my cylinders cool and my turbo happy. 

And on long flights, nothing says speed like skipping a fuel stop. So when all the ROP guys have to stop for gas, I just keep going 

[AH-1 Cobra Pilot]

1 hour ago, PT20J said:

Taking the weight of a pound of avgas to be...

Is that like asking, "How many 3¢ stamps are there in a dozen?"

Edited March 2, 2021 by AH-1 Cobra Pilot

[PT20J]

1 minute ago, AH-1 Cobra Pilot said:

Is that like asking, "How many 3¢

 

stamps are there in a dozen?"

Oops. Funny typo. Thanks for catching it. I've corrected it.

Skip

[AH-1 Cobra Pilot]

40 minutes ago, A64Pilot said:

Higher RPM is not just more distance travelled, it’s mostly the kinetic energy that has to be dealt with as reciprocating mass comes to a complete stop and rapidly accelerates in the opposite direction., those forces are transmitted through the bearings of course.

The rotating mass, crankshaft etc has essentially no max RPM, or said another way its actual limit is well in excess of the reciprocating parts. The upper RPM limit of a race motor is set by piston speed, the piston is the weak limit.

From a TBO perspective, an engine is designed to be run at max allowable continuous power until TBO, it’s if you wanting to go beyond TBO, or if at TBO your not wanting to buy a bunch of parts is where part power comes in.

My 540 at overhaul all the hard parts met new limits so I didn’t turn the crank or have to buy one, largely because it was a 235 and had a redline RPM of 2400.

Lycoming has stated in print that motors run at or below 65% power will last longer

Quick and dirty, I would estimate the power loss as P=kmd²N³, (m is mass, d is distance traveled, N is frequency.)

Ergo, about 13% more loss between 2400 and 2500 rpm.

[Bolter]

On 3/1/2021 at 6:54 AM, A64Pilot said:

Primary efficiency gains from LOP is from slowing down, not so much from an increase in BSFC. However there are a few people who stand to gain financially from selling products that if they are to be believed are required for LOP operation so it’s not surprising there are converts.

 

53 minutes ago, A64Pilot said:

Fuel flow is missing as well as actual EGT, now I know EGT will change based on location but the delta won’t. as well as fuel flow differences, they ought to be the same.

‘For that matter, where do the numbers in red come from? Lycoming? or did someone else put them there? Not arguing their accuracy, just thinking that they ought to be in the chart for the chart to meaningful. Of course that’s why they were added wasn’t it?

‘This chart to me is meant to be representative, meant to illustrate something. What I’d like is a performance planning chart, one with numbers.

If correct it does seem to indicate that the efficiency peak is at -20F LOP and that by running even lower LOP is less efficient doesn’t it? Assuming the numbers in Red are real and correct of course, but either way it shows a efficiency peak, which is not what you usually hear from LOP converts, they often say the leaner the better

1 hour ago, A64Pilot said:

Unfortunately Lycoming chose to not put numbers on that chart, rendering it pretty much useless

There are numbers.  I added the notes in red by looking at the scales on the chart and drawing lines.  You are welcome.  

On this chart, Lycoming provides values for delta-T for EGT and CHT and then values for %-power.  BSFC is the only thing without units.  As noted this is for relative performance, but you can still begin drawing conclusions on where the highest thermal efficiency is, and what your power compromise would be.  You do not need fuel flow, delta-T is just as useful to find your place on this chart.  You can estimate FF from the 15xGPH estimate if you like.  Regardless, this shows there is a maximum engine efficiency point (BSFC) at 20° LOP. 

You state LOP'ers claim leaner is always more efficient, which I do not think is true, but is certainly not an engineering answer to your own question.  Your question was about efficient flight engine operation.  In your test experience, you found loss of drag to be the bigger element of LOP flying.  That may be true, but you asked about efficient flight and engine settings, and there is an answer, even in this chart.  

[PT20J]

These curves are really interesting. There's a lot of information in there.

Take the BSFC curve for instance. As I understand thermodynamic fuel-air cycle theory, it should decrease linearly with reducing mixture strength. There should be a break in the curve slope at stoichiometric (peak EGT) with the slope steeper ROP and shallower LOP. The actual curve approximates this until LOP and then levels out and eventually rises. Why? According to John Heywood, Internal Combustion Engine Fundamentals, Chapter 15, p 831, "The reasons for this are that cycle-to-cycle pressure fluctuations and the total duration of the burning process increase as the mixture becomes leaner: both these factors degrade engine efficiency." This is why the engines run a bit rougher LOP even with balanced injectors.

Also interesting on this engine is the EGT curve. Note that the peak is very broad. This is because it is the average EGT for all cylinders and this engine with stock injectors has significant mixture variation between cylinders - they all peak at different fuel flows and so the composite curve is very broad. A stock Lycoming IO-360 does much better. But even a single cylinder will not have the sharp peak predicted by theory. This is due to the fact that there is always cycle-to-cycle variation in the combustion event at any mixture. There is just more variation at leaner mixtures. So your engine is pretty smooth at takeoff power and gets a little bit rougher the more you lean it. 

Skip

 

[A64Pilot]

1 hour ago, gsxrpilot said:

 

The equation used to get BSFC is different depending on whether the engine is running LOP or ROP. On one side Fuel Flow matters and is part of the equation, on the other side it doesn't. Therefore it doesn't make sense to add either numbers to the chart. The chart can be read completely and appropriately using the numbers provided.

But as @bluehighwayflyer said, I know how it works and how to use it. I cruise my turbo 252 in the flight levels somewhere between 20° and 30° LOP and enjoy a 35% fuel savings for only a 10% speed penalty. It also keeps my cylinders cool and my turbo happy. 

And on long flights, nothing says speed like skipping a fuel stop. So when all the ROP guys have to stop for gas, I just keep going 

No, fuel flow does matter, it matters a whole lot.

What shoud be considered is that throttling by fuel flow is nothing new, Long time ago a guy named Rudolph Diesel developed an engine that was entirely throttled by fuel flow, it operates at all times without a throttle valve regulating air flow, it never has a vacuum in its intake manifold, it’s if you will the ultimate LOP engine.

It also demonstrates rather well that fuel flow does matter, as fuel flow is the way that power production is regulated.

One thing people lose sight of is that these engines are heat engines, that is the motive force is derived by the rapid expansion of air from heating, it’s not an explosion or anything else, it’s no different than a turbine in that respect, and just like a turbine the max power output is pretty much determined by how much heat the engine can handle.

‘So within limits increasing the amount of air that’s available to expand by heat will increase efficiency, but the heat has ti there to expand the air, and of course the heat comes from fuel flow..

A turbo is a pure heat driven turbine, and what makes a turbo “happy” is lower temps, and 25 LOP is hotter than 50 ROP, by 25 degrees of course. Replace those numbers with any others you want to, a turbo only know temps, it doesn’t care LOP or ROP.

‘Cylinder head temps follow power production, they are cooler at LOP because power is down, not because there is any “excess” air available to cool the heads. 

Edited March 2, 2021 by A64Pilot

[carusoam]

What we have learned...

Lycoming skips discussing LOP...

Continental prints it clearly, and handed it to Mooney to use in their POH...

APS defined and set up training for everyone to use defining 65% BHP as a way to avoid the red box...

Works the same for both engine manufacturer...

 

The goofy part that gets confused in POHs...

The 50°F, and why it is there...

It is a method of avoiding the red box... at high cruise power settings

50 isn’t enough for good cylinder wear when ROP... very high ICPs.

50 is too much when above 65% BHP LOP... very low power.

Both excess fuel or excess air can be used to cool what is happening inside the cylinders...

 

The next goofy part...

We aren’t ever going to be able to adequately measure engine wear... for several different reasons... and dozens of variables.

But practically speaking Mooney engines don’t wear out... until long past TBO.

The third goofy part... cylinders do wear out, and that is a discussion of CHT control. Mostly because he hardness of the cylinders changes with temperature...

 

If you agree to that...

Lets talk rpm vs. lubrication... look at each point as one plate sliding past the next...

We then use the lubrication approximation to describe how the pressure between the plates rises, as the plates near each other...

The closer and faster they move, the better the lubrication works... as the separating forces increase...

In an ideal world, this works well.

 

Last point...

We avoid wear by using our planes often...

Changing oil frequently...

Using a filter...

 

This helps keep foreign particles from disturbing the oil film that is required to allow the lubrication approximation... and broken oil molecules from losing their viscosity...

Even here, reality bites... engines sit... oil doesn’t get changed... temps don’t get controlled...

Wow, how was that?  Better than an LOP/ROP discussion?

Even our cylinders can be kept longer than TBO on TC’d Mooneys...

And some people enjoy cruising in flaming dragon mode, and change cylinders at the halfway point...

Up to the PIC to decide how to handle that...

 

So.... for low cruise setting... 

Select your objective...

max efficiency to build hours?

max use of each drop of fuel, while flying fast?  

max use of fuel, while preserving cylinder life, while flying fast (My favorite)?

 

PP thoughts only, not a tribologist...

Best regards,

-a-

[ArtVandelay]

‘Cylinder head temps follow power production, they are cooler at LOP because power is down, not because there is any “excess” air available to cool the heads. 

You can substitute ROP for LOP and fuel for air, and that statement will be correct as well.

[A64Pilot]

42 minutes ago, PT20J said:

These curves are really interesting. There's a lot of information in there.

Take the BSFC curve for instance. As I understand thermodynamic fuel-air cycle theory, it should decrease linearly with reducing mixture strength. There should be a break in the curve slope at stoichiometric (peak EGT) with the slope steeper ROP and shallower LOP. The actual curve approximates this until LOP and then levels out and eventually rises. Why? According to John Heywood, Internal Combustion Engine Fundamentals, Chapter 15, p 831, "The reasons for this are that cycle-to-cycle pressure fluctuations and the total duration of the burning process increase as the mixture becomes leaner: both these factors degrade engine efficiency." This is why the engines run a bit rougher LOP even with balanced injectors.

Also interesting on this engine is the EGT curve. Note that the peak is very broad. This is because it is the average EGT for all cylinders and this engine with stock injectors has significant mixture variation between cylinders - they all peak at different fuel flows and so the composite curve is very broad. A stock Lycoming IO-360 does much better. But even a single cylinder will not have the sharp peak predicted by theory. This is due to the fact that there is always cycle-to-cycle variation in the combustion event at any mixture. There is just more variation at leaner mixtures. So your engine is pretty smooth at takeoff power and gets a little bit rougher the more you lean it. 

Skip

 

They chart is useful, even though it only is at one operating condition, it does show HP outputs relation to mixture etc.

‘But another thing, you can’t really “balance” injectors as individual cylinders are not balanced, they have different sized combustion chambers resulting in different compression ratios and even the piston are not identical. rods different length etc.

‘A lean burn engine is nothing new either, Chrysler and I believe Honda did it in the 70’s but abandoned it due to emission problems it presents, the Current Suzuki outboards use lean burn at certain RPM’s when possible, it’s not at lower RPM as the engine woud run rough and at high power y9u can’t be lean or you’ll put holes in your pistons, but at mid power levels the Suzuki will go very lean and sip fuel.

Just now, ArtVandelay said:

You can substitute ROP for LOP and fuel for air, and that statement will be correct as well.

Absolutely

[A64Pilot]

Cylinder wear isn’t really so much a function temp as Heads are, cylinders don’t really get that hot, because combustion doesn’t occur there and mostly because hot gases are not expelled through the cylinder, but through the head.

That is why GM in the 90’s finally began to “reverse” coolant flow through their engines and put the coolest water into the heads first and then onto the cylinders.

previously the coolant flow was from the bottom up, because way back in the day engines didn’t have water pumps and were cooled by thermal siphon, which simply means heat rises.

‘But we don’t monitor cylinder temps because it’s simply not a issue, we monitor cylinder head temps because it is

Edited March 2, 2021 by A64Pilot

[carusoam]

The Continental IO550 can be balanced within 0.1gph GAMI spread...

Curvy pipes allow identical length intake runs... to balance air flow...

Gami injectors can be balanced to match...

Lean smoothly until the engine stops running...

twist the the mixture knob slowly for full enjoyment of the event...

turn it back, equally slowly, and the engine comes back to life...

Don’t wait too long raw fuel will still be delivered to the engine and collect in the muffler...

-a-

[carusoam]

I may have over-simplified my statements...

My cylinders become useless, when any part of them wears too much...

To avoid that part from wearing...

I have one tool,

a single CHT sensor near the exhaust port...

It isn’t ideal...

But, it does work pretty well...

Keeping CHTs below 380°F has been shown to work well for extending cylinder life...

Climbing to altitude, this may not be practical...

Climbing to altitude is most practically done using 2-300°F ROP, using the blue or white box, to avoid the red box...

IAS is your friend to control CHTs during the 2kfpm climb...

Go Excess power!

-a-