LOP Questions

[jlunseth]

15 hours ago, PT20J said:

When you run LOP, the burn rate decreases and this results in the pressure curve having a lower and delayed peak. This is really just showing the power loss from the timing no longer being optimum. If you could advance the timing you could get back to MBT and the peak would be back where it should be. The available power would still be less LOP than ROP because all the air is not being utilized. But, as long as you can get the power you want, LOP is more efficient.

Skip

I followed and agree with everything you said, but the last paragraph or two left me grimacing a little. I think the last paragraph is mainly accurate, except it reflects a purely theoretical viewpoint. In other words, if one wants the most perfect combustion cycle that makes the best use of available fuel and produces the most power, one would want to operate in the area slightly rich of stoichiometric. I will call that “best power.” The practical side, though, is the “red box” concept - that also happens to be a method of operation that produces the highest ICP and is hardest on the engine, and getting away from that, whether LOP or ROP or by reducing power is the important part. “Best” or “optimum,” are relative terms, best for what? Certainly, if you want the perfect combustion event, you can lower the power setting below 65%HP, which lowers the ICP. However, the aircraft will also slow down. The important message is that operating at the cruise settings that are in many of our POHs, which use best power and something around 75%, is right in that “red box” spot. The whole object of LOP operation, or healthy ROP operation for that matter, is to get out of that. While theoretically the “best” combustion event, looking at charts, it is not the “best” for engine health.

I must be missing something in your statement that “available power would still be less LOP than ROP because all the air is not being utilized.” I think you must have a specific ROP setting in mind, viz. a “best power” setting, that you are comparing LOP operation to, because as a general proposition, available power can be less both LOP and ROP. A rich setting, one richer than best power, reduces available power because an increasing amount of fuel is not being utilized. Both LOP and ROP inhibit flame front spread.

We can dream about changing timing, but we cannot change timing in our engines. Therefore, we lean or enrich the mixture to get away from what has, in the past, been called a “best power” setting. It was only theoretically “best” or “optimum” power. It would result in the best power output, yes, but it also resulted in the most heat and stress on the engine. This could be deemed acceptable only if, as in the POH for my TSIO360, the upper temp limits for CHT, TIT and the like, were stated at unrealistic levels, like the 460 CHT redline in mine.

[Shadrach]

4 hours ago, jlunseth said:

I followed and agree with everything you said, but the last paragraph or two left me grimacing a little. I think the last paragraph is mainly accurate, except it reflects a purely theoretical viewpoint. In other words, if one wants the most perfect combustion cycle that makes the best use of available fuel and produces the most power, one would want to operate in the area slightly rich of stoichiometric. I will call that “best power.” The practical side, though, is the “red box” concept - that also happens to be a method of operation that produces the highest ICP and is hardest on the engine, and getting away from that, whether LOP or ROP or by reducing power is the important part. “Best” or “optimum,” are relative terms, best for what? Certainly, if you want the perfect combustion event, you can lower the power setting below 65%HP, which lowers the ICP. However, the aircraft will also slow down. The important message is that operating at the cruise settings that are in many of our POHs, which use best power and something around 75%, is right in that “red box” spot. The whole object of LOP operation, or healthy ROP operation for that matter, is to get out of that. While theoretically the “best” combustion event, looking at charts, it is not the “best” for engine health.

I must be missing something in your statement that “available power would still be less LOP than ROP because all the air is not being utilized.” I think you must have a specific ROP setting in mind, viz. a “best power” setting, that you are comparing LOP operation to, because as a general proposition, available power can be less both LOP and ROP. A rich setting, one richer than best power, reduces available power because an increasing amount of fuel is not being utilized. Both LOP and ROP inhibit flame front spread.

We can dream about changing timing, but we cannot change timing in our engines. Therefore, we lean or enrich the mixture to get away from what has, in the past, been called a “best power” setting. It was only theoretically “best” or “optimum” power. It would result in the best power output, yes, but it also resulted in the most heat and stress on the engine. This could be deemed acceptable only if, as in the POH for my TSIO360, the upper temp limits for CHT, TIT and the like, were stated at unrealistic levels, like the 460 CHT redline in mine.

You can change the timing on your engine if you fly behind a four-cylinder Lycoming. Per the TCDS 25° is optional (it was standard when my aircraft was manufactured). I have heard of people with 20° engines re-stamping data plate for 25°. But I’ve never been involved in the process.  The additional advance definitely helps maintain power at efficiency ratios leaner than stoic. Your point about best power is a good one but probably of less practical importance to you given that you fly a turbo.  For someone in a normally aspirated bird like mine there is little reason to lean to 100ROP in cruise above 10k. Max power will be ~40ROP. The difference in speed between 100ROP and 40ROP will hardly be noticeable but the difference in fuel burn, while small, is noticeable.  
 

These threads often go down the combustion science rabbit hole. Becoming more and more familiar with combustion science over the last 25 years has actually simplified my engine management. I chose setting closest to 40° ROP that maintains healthy temps from the lean side of the mixture spectrum. I cruise at altitudes ranging from 2500MSL up to 12,500MSL. At low cruise altitude I run around 35 LOP. As I get higher I get closer to peak. Above 10,000 I’m absolutely trying to get as much power out of the thing as I possibly can. The above is a general rule, I monitor oil and Cyl temps and adjust as necessary for outside conditions. I am much more conservative with CHT at low altitudes in the winter given the F model's propensity to run cool. In cold weather it can be a challenge to keep #1 and #2 above 300°.

Edited June 7, 2021 by Shadrach

[jlunseth]

Thanks Anthony! Ah, the Ideal Gas Law. 

[PT20J]

3 hours ago, jlunseth said:

I followed and agree with everything you said, but the last paragraph or two left me grimacing a little. I think the last paragraph is mainly accurate, except it reflects a purely theoretical viewpoint. In other words, if one wants the most perfect combustion cycle that makes the best use of available fuel and produces the most power, one would want to operate in the area slightly rich of stoichiometric. I will call that “best power.” The practical side, though, is the “red box” concept - that also happens to be a method of operation that produces the highest ICP and is hardest on the engine, and getting away from that, whether LOP or ROP or by reducing power is the important part. “Best” or “optimum,” are relative terms, best for what? Certainly, if you want the perfect combustion event, you can lower the power setting below 65%HP, which lowers the ICP. However, the aircraft will also slow down. The important message is that operating at the cruise settings that are in many of our POHs, which use best power and something around 75%, is right in that “red box” spot. The whole object of LOP operation, or healthy ROP operation for that matter, is to get out of that. While theoretically the “best” combustion event, looking at charts, it is not the “best” for engine health.

I must be missing something in your statement that “available power would still be less LOP than ROP because all the air is not being utilized.” I think you must have a specific ROP setting in mind, viz. a “best power” setting, that you are comparing LOP operation to, because as a general proposition, available power can be less both LOP and ROP. A rich setting, one richer than best power, reduces available power because an increasing amount of fuel is not being utilized. Both LOP and ROP inhibit flame front spread.

We can dream about changing timing, but we cannot change timing in our engines. Therefore, we lean or enrich the mixture to get away from what has, in the past, been called a “best power” setting. It was only theoretically “best” or “optimum” power. It would result in the best power output, yes, but it also resulted in the most heat and stress on the engine. This could be deemed acceptable only if, as in the POH for my TSIO360, the upper temp limits for CHT, TIT and the like, were stated at unrealistic levels, like the 460 CHT redline in mine.

I took the APS course live in Ada in 2007. I wanted to go deeper so I've spent a lot of time studying textbooks and technical papers which caused me to dredge up a lot of disused thermodynamics and chemistry (I'm an electrical engineer, not a mechanical engineer). This effort has given me an appreciation for how the George, Walter and John took a lot of complex information and distilled it down to a few simple concepts that we can get our minds around: ICP, Red Boxes, the relation between power and fuel flow when LOP, CHT limits, etc.

ICP is a useful concept for understanding the combustion event, but there's not any practical operational value in it because you cannot measure it with the instrumentation available. As far as the red box, I think in terms of what can damage an engine. The engine is designed to structurally handle any peak pressure generated during normal operation so the pressure isn't the issue. The issue is heat. Air cooled aircraft engines are capable of rejecting more heat to the cylinder heads than the cooling systems can carry away. At high powers (above 75% in Lycomings and 65% in Continentals) the engines need very rich mixtures for cooling. Get inside the red box and the heads get too hot. The valves and spark plugs run the hottest, so there is a possibility of damage and possibly preignition. The high combustion temperatures can also set off detonation. So, if you do the big mixture pull, when generating high power, you will transit the red box and it is best to know where it is. If you are operating below 75%/65% you can forget about the red box as long as you are mindful of CHTs.

LOP and ROP are really two different operating regimes. ROP, the power curve is fairly flat and the BSFC curve is steep as a function of mixture. LOP, the power curve is steep and the BSFC curve is fairly flat. This means that when ROP, you can vary the fuel flow over a wide range without much affecting power, but when LOP the power is a strong function of fuel flow. This is because the engine is essentially an air pump. When operating at a fixed MAP and rpm, it can only pump so much air. Maximum power comes at a slightly rich air/fuel ratio. Adding more fuel causes a small power drop off, but the important point is that there is enough fuel to use all the available air. Going LOP causes a nearly linear decrease in power because the engine is pumping all the air it can but there is not enough fuel to use it. So, the power LOP will be less than the power ROP for the same MAP and rpm. This was what I meant by there being less available power LOP.

Internal combustion engines are fascinating and there are still a lot of things I don't know, so I enjoy reading everyone's posts because that helps me learn more.

Skip

[PT20J]

33 minutes ago, Shadrach said:

Becoming more and more familiar with combustion science over the last 25 years has actually simplified my engine management. 

I've noticed that also. Early on, it was easy: I'd runabout 65-70% power and lean until it got rough and richen until smooth, or if I was lucky enough to have a EGT I'd set it for peak. Then I started reading John Deakin's articles and it got complicated. Then I attended APS and worried about ICP and red boxes. Then I learned some more and it got simple again. Mike Busch told me at MooneyMax that people are amazed at how little fiddling he does with power and mixture. He knows what he wants and sets MAP, RPM and fuel flow accordingly cross checking CHT. He doesn't pay a lot of attention to the engine monitor for operating -- that's a tool for diagnostics. 

Skip

[GeeBee]

2 hours ago, PT20J said:

I've noticed that also. Early on, it was easy: I'd runabout 65-70% power and lean until it got rough and richen until smooth, or if I was lucky enough to have a EGT I'd set it for peak. Then I started reading John Deakin's articles and it got complicated. Then I attended APS and worried about ICP and red boxes. Then I learned some more and it got simple again. Mike Busch told me at MooneyMax that people are amazed at how little fiddling he does with power and mixture. He knows what he wants and sets MAP, RPM and fuel flow accordingly cross checking CHT. He doesn't pay a lot of attention to the engine monitor for operating -- that's a tool for diagnostics. 

Skip

I do the same thing. Now that I have a GAMI spread of 0 to 0.2 gph, I set the engine at 65% power and pull back to 12.3. Done. I glance at the EGTs and CHTs to see if they are right where they normally are and that is it.  If I am below 65% say at 12,000 DA or something, I might work the EGT but for the most part, I am in the 9 or 10K range or below so it is all quick and simple. The IO-550 loves a wide open throttle plate and LOP. It just purrs. 

I have had to run ROP a couple of times, not for the engine but for me. Recently I was out of KABR in the middle winter at 11,000 LOP and could not generate enough heat to keep the cabin warm. Went to ROP operation and all was toasty.

 

[carusoam]

+1 for a tight gami spread.

+1 for a tightly sealed cabin.

 

Just reminded me of how hard it is to get heat at 12.5k’ in the winter with an O360... and a leaky door....   brrrrrrrrr.....

Best regards,

-a-

[jlunseth]

Same here. I have a lean cruise power setting that I know and like. If I want to depower from that I can just pull the MP back and the interlink brings the fuel flow back with it, so the engine actually leans the mix out a little further at, say, approach speed. I use the monitor just to make sure that the TIT and CHTs stay in line, but other than that, I keep it simple. I ignore the EGTs and the %HP on the 930. It is nevertheless good to know the theory of operation and that your engine is not going to blow up on the lean side. What I have noticed is the long term effect, the lower pressures and CHTs are healthier for the engine over hundreds of hours. 

[PT20J]

If you want longevity, there is a lot to be said for keeping the power down. The airlines learned this. For instance the P&W powered DC-3s used R-1830 engines and the most common version has a rated power of 1200 hp. Most airlines (according to my collection of several DC-3 manuals) set cruise power at 550 hp. Bob Buck said in his autobiography that the only way TWA could get reliability out of the R-3350TC-powered Connies was to eschew using high blower.

Skip

[N201MKTurbo]

50 minutes ago, PT20J said:

If you want longevity, there is a lot to be said for keeping the power down. The airlines learned this. For instance the P&W powered DC-3s used R-1830 engines and the most common version has a rated power of 1200 hp. Most airlines (according to my collection of several DC-3 manuals) set cruise power at 550 hp. Bob Buck said in his autobiography that the only way TWA could get reliability out of the R-3350TC-powered Connies was to eschew using high blower.

Skip

eschew obfuscation

[Shadrach]

On 6/7/2021 at 1:48 PM, PT20J said:

ICP is a useful concept for understanding the combustion event, but there's not any practical operational value in it because you cannot measure it with the instrumentation available. As far as the red box, I think in terms of what can damage an engine. The engine is designed to structurally handle any peak pressure generated during normal operation so the pressure isn't the issue. The issue is heat.

 

3 hours ago, PT20J said:

If you want longevity, there is a lot to be said for keeping the power down. The airlines learned this. For instance the P&W powered DC-3s used R-1830 engines and the most common version has a rated power of 1200 hp. Most airlines (according to my collection of several DC-3 manuals) set cruise power at 550 hp. Bob Buck said in his autobiography that the only way TWA could get reliability out of the R-3350TC-powered Connies was to eschew using high blower.

Skip

These two statements seem to be slightly at odds.   In the first you state that ICP doesn’t really matter as long as temps are kept in check.

In the second you state that running it reduced power levels leads to longer engine life. While higher temperatures typically track higher power settings, it isn’t always so.

As you said we have no I have way of knowing ICP. CHT is the best proxy that we have. Unfortunately the other factors affecting that number are very inconsistent from day today throughout the year.  I am sure that Lycoming and Continental both designed a great deal of margin into their cylinder design with regards to ICPs. I would point out that it’s easier for someone on the West Coast in a temperate zone to assume ICPs are not a concern if CHT’s are healthy. It’s likely the case in almost any situation. However, as an East coaster, I take a more conservative approach. I recall Walt Atkinson admonishing me against taking CHT probe temperature as a literal representation of the cylinder’s uniform temperature. Apparently the folks in Ada instrumented a number of points on an engines cylinder barrels and heads. IIRCC, they determined that temperature can vary considerably throughout the cylinder, yielding temperatures both hotter in cooler than at the probe location. I think it was during the same testing that they discovered that doing run-ups into the wind did not increase airflow/cooling over the cylinders and in fact did the opposite...they found more engine cooling could be had with the wind blowing into the face of a running prop...but I digress. Back to ICPs - someone living in Seattle probably does not encounter negative density altitude’s to the same degree that say someone in the Mid Atlantic might. It might also be fair to say that if you live on the East Coast more of your trips are into the prevailing winds (if I head east I’m over open ocean within an hour). On a high pressure winter day it’s not unusual for me to find my 700 foot airport DA to be -3000 or more with strong winds out of the west at altitude. Staying out of those winds sometimes means staying below 3000’ feet AGL. There are often days in the winter where I find myself cruising at a DA around or below sea level with OATs in the low teens.

My choices in this case are as follows:

1) run wide-open throttle full rich. 

2) throttle back and lean to an appropriate ROP setting below 75%

3) Lean for best of power and BSFC knowing that the surplus of sub freezing air through the cowling will keep the cylinders from overheating.

4) Run WOT LOP and set power for a conservative CHT number.

I almost always choose 4...

I say do so because under said circumstances I think it’s prudent to be mindful of the fact that power production is in the upper region of design limits. It would be very difficult under this scenario for me to do anything with a mixture that would raise CHT‘s anywhere near 400°.  However, it’s likely possible to exceed max rated HP at certain mixture settings.
It just seems prudent not to push the engine to such a degree. I shoot for CHT numbers just a bit higher than what is needed to ensure lead scavenging ..and even those conservative numbers are likely in excess of 75% power.

I realize that low altitude cruise at high power may seem foreign to many but its very useful in my neck of the woods. Especially in busy airspace where low altitude VFR corridors can be the most direct transition. It’s become my SOP when conditions dictate but it does present a unique situation in terms of engine management.

Edited June 8, 2021 by Shadrach

[Will.iam]

38 minutes ago, Shadrach said:

 

These two statements seem to be slightly at odds.   In the first you state that ICP doesn’t really matter as long as temps are kept in check.

In the second you state that running it reduced power levels leads to longer engine life. While higher temperatures typically track higher power settings, it isn’t always so.

As you said we have no I have way of knowing ICP. CHT is the best proxy that we have. Unfortunately the other factors affecting that number are very inconsistent from day today throughout the year.  I am sure that Lycoming and Continental both designed a great deal of margin into their cylinder design with regards to ICPs. I would point out that it’s easier for someone on the West Coast in a temperate zone to assume ICPs are not a concern if CHT’s are healthy. It’s likely the case in almost any situation. However, as an East coaster, I take a more conservative approach. I recall Walt Atkinson admonishing me against taking CHT probe temperature as a literal representation of the cylinder’s uniform temperature. Apparently the folks in Ada instrumented a number of points on an engines cylinder barrels and heads. IIRCC, they determined that temperature can vary considerably throughout the cylinder, yielding temperatures both hotter in cooler than at the probe location. I think it was during the same testing that they discovered that doing run-ups into the wind did not increase airflow/cooling over the cylinders and in fact did the opposite...they found more engine cooling could be had with the wind blowing into the face of a running prop...but I digress. Back to ICPs - someone living in Seattle probably does not encounter negative density altitude’s to the same degree that say someone in the Mid Atlantic might. It might also be fair to say that if you live on the East Coast more of your trips are into the prevailing winds (if I head east I’m over open ocean within an hour). On a high pressure winter day it’s not unusual for me to find my 700 foot airport DA to be -3000 or more with strong winds out of the west at altitude. Staying out of those winds sometimes means staying below 3000’ feet AGL. There are often days in the winter where I find myself cruising at a DA around or below sea level with OATs in the low teens.

My choices in this case are as follows:

1) run wide-open throttle full rich. 

2) throttle back and lean to an appropriate ROP setting below 75%

3) Lean for best of power and BSFC knowing that the surplus of sub freezing air through the cowling will keep the cylinders from overheating.

4) Run WOT LOP and set power for a conservative CHT number.

I almost always choose 4...

I say do so because under said circumstances I think it’s prudent to be mindful of the fact that power production is in the upper region of design limits. It would be very difficult under this scenario for me to do anything with a mixture that would raise CHT‘s anywhere near 400°.  However, it’s likely possible to exceed max rated HP at certain mixture settings.
It just seems prudent not to push the engine to such a degree. I shoot for CHT numbers just a bit higher than what is needed to ensure lead scavenging ..and even those conservative numbers are likely in excess of 75% power.

I realize that low altitude cruise at high power may seem foreign to many but its very useful in my neck of the woods. Especially in busy airspace where low altitude VFR corridors can be the most direct transition. It’s become my SOP when conditions dictate but it does present a unique situation in terms of engine management.

Sounds like you are describing what 231 pilots have to manage every flight. Over-boosting. Whether the condition is by turbo or mother nature, the answer is still the same. You can regulate the power via throttle or mixture or both. 

[Shadrach]

1 minute ago, Will.iam said:

Sounds like you are describing what 231 pilots have to manage every flight. Over-boosting. Whether the condition is by turbo or mother nature, the answer is still the same. You can regulate the power via throttle or mixture or both. 

Three sentences...Well now I feel like a blowhard... Indeed it's a similar scenario.  While I don't really trust my MP as accurate to the number, I have observed MP numbers well above a standard day while in low altitude cruise.

[Will.iam]

Just now, Shadrach said:

Three sentences...Well now I feel like a blowhard... Indeed it's a similar scenario.  While I don't really trust my MP as accurate to the number, I have observed MP numbers well above a standard day while in low altitude cruise.

Sorry didn’t mean to come off stand-offish. I bet death valley can have the same situation in the winter. 

[Shadrach]

32 minutes ago, Will.iam said:

Sorry didn’t mean to come off stand-offish. I bet death valley can have the same situation in the winter. 

You didn’t! Your point was well taken.

[PT20J]

4 hours ago, Shadrach said:

 

These two statements seem to be slightly at odds.   In the first you state that ICP doesn’t really matter as long as temps are kept in check.

In the second you state that running it reduced power levels leads to longer engine life. While higher temperatures typically track higher power settings, it isn’t always so.

As you said we have no I have way of knowing ICP. CHT is the best proxy that we have. Unfortunately the other factors affecting that number are very inconsistent from day today throughout the year.  I am sure that Lycoming and Continental both designed a great deal of margin into their cylinder design with regards to ICPs. I would point out that it’s easier for someone on the West Coast in a temperate zone to assume ICPs are not a concern if CHT’s are healthy. It’s likely the case in almost any situation. However, as an East coaster, I take a more conservative approach. I recall Walt Atkinson admonishing me against taking CHT probe temperature as a literal representation of the cylinder’s uniform temperature. Apparently the folks in Ada instrumented a number of points on an engines cylinder barrels and heads. IIRCC, they determined that temperature can vary considerably throughout the cylinder, yielding temperatures both hotter in cooler than at the probe location. I think it was during the same testing that they discovered that doing run-ups into the wind did not increase airflow/cooling over the cylinders and in fact did the opposite...they found more engine cooling could be had with the wind blowing into the face of a running prop...but I digress. Back to ICPs - someone living in Seattle probably does not encounter negative density altitude’s to the same degree that say someone in the Mid Atlantic might. It might also be fair to say that if you live on the East Coast more of your trips are into the prevailing winds (if I head east I’m over open ocean within an hour). On a high pressure winter day it’s not unusual for me to find my 700 foot airport DA to be -3000 or more with strong winds out of the west at altitude. Staying out of those winds sometimes means staying below 3000’ feet AGL. There are often days in the winter where I find myself cruising at a DA around or below sea level with OATs in the low teens.

My choices in this case are as follows:

1) run wide-open throttle full rich. 

2) throttle back and lean to an appropriate ROP setting below 75%

3) Lean for best of power and BSFC knowing that the surplus of sub freezing air through the cowling will keep the cylinders from overheating.

4) Run WOT LOP and set power for a conservative CHT number.

I almost always choose 4...

I say do so because under said circumstances I think it’s prudent to be mindful of the fact that power production is in the upper region of design limits. It would be very difficult under this scenario for me to do anything with a mixture that would raise CHT‘s anywhere near 400°.  However, it’s likely possible to exceed max rated HP at certain mixture settings.
It just seems prudent not to push the engine to such a degree. I shoot for CHT numbers just a bit higher than what is needed to ensure lead scavenging ..and even those conservative numbers are likely in excess of 75% power.

I realize that low altitude cruise at high power may seem foreign to many but its very useful in my neck of the woods. Especially in busy airspace where low altitude VFR corridors can be the most direct transition. It’s become my SOP when conditions dictate but it does present a unique situation in terms of engine management.

Ross, I appreciate your viewpoint because you think deeply about this subject. Allow me to clarify a couple of my observations.

The way I think about ICP and temperature is to consider what can do harm to an engine. Intuitively we understand that the ICP will be maximum when the engine is at maximum power. So the engine is designed for that with some margin. At lower powers the ICP will be lower. So, assuming normal operation, the ICP should not be a problem. While it may well be relatively higher or lower at some particular mixture, I really don't care if the power is in the normal cruise range.

It seems obvious, however, that heat can cause problems. It's well understood that the exhaust valves and spark plug electrodes are the hottest components of the cylinder. The spark plugs have relatively low thermal mass and are subjected to the heat of combustion for the longest time since the fire starts there. The maximum stress on the spark plugs will occur at maximum combustion temperature which is around 40-50 F ROP. The exhaust valve is bathed in the hot exhaust gasses, so the maximum stress on the exhaust valve occurs at peak EGT. The engine is designed to keep these temperatures at safe levels by fuel cooling at high powers and limiting CHTs at lower powers. It is well known that the temperature varies at different points on the cylinder head and barrel. There are old NACA reports from WWII and before that investigated this. I'm reasonably certain that Continental and Lycoming are aware of this and have allowed for it in their recommendations for CHT at the point where they measure it. But, again, it's not something that we can measure, so I don't worry about it any more that I worry about the stress on the wing attach bolts if I stay within the normal category g limits.

If the power is high enough and the mixture is not rich enough, the temperature and pressure can get high enough to risk detonation. As the flame front progresses, the temperature of the end gas (the fraction of the fuel-air mixture that has not yet burned) increases due to heat radiation from the burned gases and compression ahead of the flame front. At some point it will spontaneously ignite causing a abrupt pressure and temperature rise -- detonation. So, the red box is a consideration for high power settings. 

My point was that at conservative cruise power settings, there is no need to worry about ICP or the red box. Just keep the power setting at or below the recommended level for your engine and the CHTs at or below 380-400F.

Your point about very low density altitudes is well taken. Generally, the cruise power charts in the POH and the sea level and altitude performance charts in the engine manual will include a footnote with correction factors for low inlet air temperature. This is often overlooked in temperate climates but can be a factor in colder climates. John Deakin once wrote about the risks of setting mixture by CHT only: https://www.avweb.com/features/pelicans-perch-84-dont-set-mixture-with-cht/

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[Shadrach]

37 minutes ago, PT20J said:

Ross, I appreciate your viewpoint because you think deeply about this subject. Allow me to clarify a couple of my observations.

The way I think about ICP and temperature is to consider what can do harm to an engine. Intuitively we understand that the ICP will be maximum when the engine is at maximum power. So the engine is designed for that with some margin. At lower powers the ICP will be lower. So, assuming normal operation, the ICP should not be a problem. While it may well be relatively higher or lower at some particular mixture, I really don't care if the power is in the normal cruise range.

It seems obvious, however, that heat can cause problems. It's well understood that the exhaust valves and spark plug electrodes are the hottest components of the cylinder. The spark plugs have relatively low thermal mass and are subjected to the heat of combustion for the longest time since the fire starts there. The maximum stress on the spark plugs will occur at maximum combustion temperature which is around 40-50 F ROP. The exhaust valve is bathed in the hot exhaust gasses, so the maximum stress on the exhaust valve occurs at peak EGT. The engine is designed to keep these temperatures at safe levels by fuel cooling at high powers and limiting CHTs at lower powers. It is well known that the temperature varies at different points on the cylinder head and barrel. There are old NACA reports from WWII and before that investigated this. I'm reasonably certain that Continental and Lycoming are aware of this and have allowed for it in their recommendations for CHT at the point where they measure it. But, again, it's not something that we can measure, so I don't worry about it any more that I worry about the stress on the wing attach bolts if I stay within the normal category g limits.

If the power is high enough and the mixture is not rich enough, the temperature and pressure can get high enough to risk detonation. As the flame front progresses, the temperature of the end gas (the fraction of the fuel-air mixture that has not yet burned) increases due to heat radiation from the burned gases and compression ahead of the flame front. At some point it will spontaneously ignite causing a abrupt pressure and temperature rise -- detonation. So, the red box is a consideration for high power settings. 

My point was that at conservative cruise power settings, there is no need to worry about ICP or the red box. Just keep the power setting at or below the recommended level for your engine and the CHTs at or below 380-400F.

Your point about very low density altitudes is well taken. Generally, the cruise power charts in the POH and the sea level and altitude performance charts in the engine manual will include a footnote with correction factors for low inlet air temperature. This is often overlooked in temperate climates but can be a factor in colder climates. John Deakin once wrote about the risks of setting mixture by CHT only: https://www.avweb.com/features/pelicans-perch-84-I rememberdont-set-mixture-with-cht/

Skip

 

 

 

  Thanks for the thoughtful reply Skip! 
I remember when that column was first published. I knew in the first paragraph that John was trying to tactfully articulate his disagreement  with Mike Busch who was not named but I believe was the seminar speaker to whom he was referring.

[Will.iam]

I had an interesting observation about the importance of airflow.  I was flying at 2200 rpm 20"mp and 6gph ff.  My calflaps were closed and my CHT where in the 320dF range.  I have been coming in too hot for landings when I'm by myself as I was just using a general rule of 80kts on final 75kts over threshold and slowing to 70 by the flare.  Works good when heavy, not so much light.  So I was at 4000ft and pulled back on the yoke to get to a stall just to see when the stall horn goes off and when the plane would actually stall with just me in the plane and only 20 gallons of gas.  I found that the horn went off at 65kts and buffet felt at 61 with an actual stall at 60.  So I can revise my approach and landing speeds by subtracting 5kts to my speeds above.  I setup again and wanted to practice flying slow flight to get a little more feel of the controls at that speed so I slowed to 65 and was flying along when after about 30 seconds, I noticed my engine monitor flashing and was surprised to see my CHT on #3 was at 385 and climbing.  At my power setting referenced above that is about 40% power and I would have thought that 40% power not enough to heat stress the engine but yet here I am seeing it climbing past 388 so I lowered the nose and accelerated trading altitude for airspeed and the CHT touched 390 before coming back down to 350 and then slowly dropping from there.  All of this was without touching the power prop or mixture.  In the future if I want to practice slow flight I'll leave the calflaps open to help out.  I was just surprised that even at a low power setting the engine can start to overheat if you don't have good airflow even LOP.