LOP Too Cold

[jlunseth]

Here's a new one for the LOP/ROP argument.

I flew up to International Falls last night for dinner from Minneapolis (KFCM->KINL).  I have been flying my aircraft LOP quite a bit for the last year, but I could not do it last night.  The cylinders just ran too cold for my comfort.  #6 is the coldest cylinder in my aircraft because it is at the front and has an unobstructed airflow from the cowl opening.  The conditions were otherwise perfect for LOP, I was at 5,000, so not too high.  The problem was that #6 ran so cold I had to make a change.  The graph line on my JPI 930 for CHT starts at 224.  When the CHT on #6 stopped graphing I got a little worried, and when it slowly drifted down to 219 (I get a number displayed even if there is no graph line) I figured I needed to do something, so I ran ROP and with some extra power and everything was fine.

Part of the problem was that the cowl flaps were stuck in the half open position, and I could not fully close them.  But the bigger problem was that it was -22C at low altitude where the air is dense, and the cooling combined with LOP ops was just too good.  I don't know, probably that was just fine, I don't think there is a minimum CHT in my manual.  But I was not going to tempt fate.  Besides, it was getting really cold in the cabin with all the cylinders in the 200-somethings.  That is at 75% power (11.5 GPH and about 33 MAP @ 2450).

LOP ain't perfect in my aircraft. I can't usually fly LOP above 12,000 in the summer, the temps are just too hot.  It sure works well most of the time, below 12.  Just was too cold this time, which is a new one on me.

Edited February 17, 2016 by jlunseth

[carusoam]

Somebody published a number for that...?

I would see 280°F CHTs in winter and 320°F in the warm while cruising...

The O has an oil cooler blocking device and an OAT that is advised to use it.

Does your POH have anything related to CHT limitations or placarding?

Best regards,

-a-

[jrwilson]

46 minutes ago, jlunseth said:

Here's a new one for the LOP/ROP argument.

I flew up to International Falls last night for dinner from Minneapolis (KFCM->KINL).  I have been flying my aircraft LOP quite a bit for the last year, but I could not do it last night.  The cylinders just ran too cold for my comfort.  #6 is the coldest cylinder in my aircraft because it is at the front and has an unobstructed airflow from the cowl opening.  The conditions were otherwise perfect for LOP, I was at 5,000, so not too high.  The problem was that #6 ran so cold I had to make a change.  The graph line on my JPI 930 for CHT starts at 224.  When the CHT on #6 stopped graphing I got a little worried, and when it slowly drifted down to 219 (I get a number displayed even if there is no graph line) I figured I needed to do something, so I ran ROP and with some extra power and everything was fine.

Part of the problem was that the cowl flaps were stuck in the half open position, and I could not fully close them.  But the bigger problem was that it was -22C at low altitude where the air is dense, and the cooling combined with LOP ops was just too good.  I don't know, probably that was just fine, I don't think there is a minimum CHT in my manual.  But I was not going to tempt fate.  Besides, it was getting really cold in the cabin with all the cylinders in the 200-somethings.  That is at 75% power (11.5 GPH and about 33 MAP @ 2450).

LOP ain't perfect in my aircraft. I can't usually fly LOP above 12,000 in the summer, the temps are just too hot.  It sure works well most of the time, below 12.  Just was too cold this time, which is a new one on me.

I think your minimum CHT is 250, but not positive.  I have to check my manual.

[jlunseth]

I looked it up.  There is a "Green Arc - Normal Operating Temperature" range of 250-460, but no stated minimum.  If you think about it, the cylinder operates at all temperatures from ambient (which could be -20F, up to the max.  I often see temps in the low 200's on that cylinder while taxiing. So one could start a takeoff at, say, 212 from idle, and the cylinder would quickly heat up when full power is applied and that would be ok.  I don't know, I just thought that was too cold.  OT and all the other temps except CHT's were good.

[jrwilson]

9 minutes ago, jlunseth said:

I looked it up.  There is a "Green Arc - Normal Operating Temperature" range of 250-460, but no stated minimum.  If you think about it, the cylinder operates at all temperatures from ambient (which could be -20F, up to the max.  I often see temps in the low 200's on that cylinder while taxiing. So one could start a takeoff at, say, 212 from idle, and the cylinder would quickly heat up when full power is applied and that would be ok.  I don't know, I just thought that was too cold.  OT and all the other temps except CHT's were good.

I think that is too cold too...but yeah, based on what?  I'm not sure...  I wouldn't have liked it either and would have heated it up too...  My #5 is coldest also, and my oil temp gets around 260 often, so I will fully close the cowl flaps to get the temps up, then I have to trail them because a couple other cylinders start creeping up too warm...

[carusoam]

Balancing the temps and Pistons / rings with the cylinders is discussed along the way.  The cylinders have a change in dimension called the choke.  The choke is a constriction that expands with the increased CHT.

High power with low CHT may be a challenge.  Add the cold OAT and FT at below SL MP.  ICPs can get pretty high. The rings have the ability to absorb a lot of the fluctuation in dimensions.

This is where Ross usually comes in very handy!

Best regards,

-a-

[jlunseth]

Yes, that was what concerned me, running at cruise power and cold temps.  Engine sounded just fine and nothing else was out of the ordinary, just did not want to risk it.

Believe it or not, this was one time LOP was too good.

[CaptainAB]

A sticky exhaust valve?

[kortopates]

CHT limitation depend on the engine. The continental turbos have a min of 250 before you should apply full power just as Anthony suggest. You should be able to find it in your POH in multiple places with an explanation in the emergency procedures.

Sent from my iPhone using Tapatalk

[Shadrach]

Minimum CHTs are mostly about lead scavenging. Dibromoethane does not do its job below a certain temp.  That being said, if the temps were too cold, why not just run a little richer until you get more comfortable. There's no need to be drastic, just enrichen a bit see what happens. Then maybe a bit more until you see 250 or more. If you get to peak and it's still below 250, I'd consider winterizing the the plane by adjusting the cowl flaps.

[jlunseth]

I need to get the cowl flap fixed.  I was able to close it on the ground from the outside prior to my trip back to FCM, and just took off with the cowl flaps fully closed and climbed to altitude.  In that cold weather the cylinders were just fine during the takeoff and climb, in the mid-300's.  I was able to run LOP at 10500 on the way back without an issue, the CHT's were quite a bit warmer, but that does not surprise me.  I generally see higher temps LOP at higher altitude.

Typically, "winterizing" up here means putting in an oil cooler block (just a piece of foam stuffed in the cooler will do), and adjusting the cowl flaps to fully close.   In the summer we take the block out and sometimes set the flaps to trail slightly.

I didn't think about enriching back from LOP towards peak, but I attended the APS seminar in Ada a few years ago and they did warn that under conditions where cooling is too good, it would be possible to run in the "red box" and have very high ICP's without seeing excessive CHT's.  I just went to a ROP setting that I knew would work and got better CHT's.

[garytex]

  My understanding which comes from Alcor and another source that I cannot remember is that the break over temperature for the bromine lead scavenger to work is a function of gas temperature in the head, which is higher than the gas temperature at the EGT probe.  The number is 1200° but of course we have no way to measure it.  But engines at idle are lower than that temperature.

[Jeff_S]

I had a similar experience doing some winter cruising this year, although not at temps as cold as the OP's. But I was seeing my #6 (also the coldest, for the same reasons) drop below 240° into the twenties. I remember from a Continental webinar I saw that there is such as thing as CHTs that are too cold, so I just richened it up a bit until I was more comfortable. It meant burning more gas with no meaningful speed increase, but I guess that's the price you pay.

I have the oil cooler restrictor plate for the O, but in Atlanta it just doesn't make sense since the temps fluctuate so much and never really stay low that long. I'd be taking on/off the cowling all the time.

[jlunseth]

Yeah, the EGT's were all fairly normal.  Cooler than in the summer, but in the high14's/low 15's and the TIT was mid-high 1500, like 1550.  The cylinder was burning stuff just fine.

We just get weird cold weather issues up here, stuff you would normally not think of.  I froze up the air/oil separator once.  I commonly don't open the cowl flaps on the ground in the winter, have to get some heat going in the engine.  This was a new one on me.

[Shadrach]

10 hours ago, jlunseth said:

I need to get the cowl flap fixed.  I was able to close it on the ground from the outside prior to my trip back to FCM, and just took off with the cowl flaps fully closed and climbed to altitude.  In that cold weather the cylinders were just fine during the takeoff and climb, in the mid-300's.  I was able to run LOP at 10500 on the way back without an issue, the CHT's were quite a bit warmer, but that does not surprise me.  I generally see higher temps LOP at higher altitude.

Typically, "winterizing" up here means putting in an oil cooler block (just a piece of foam stuffed in the cooler will do), and adjusting the cowl flaps to fully close.   In the summer we take the block out and sometimes set the flaps to trail slightly.

I didn't think about enriching back from LOP towards peak, but I attended the APS seminar in Ada a few years ago and they did warn that under conditions where cooling is too good, it would be possible to run in the "red box" and have very high ICP's without seeing excessive CHT's.  I just went to a ROP setting that I knew would work and got better CHT's.

I would love to have a discussion with them on this. While it is true that under "certain conditions", which means air in enough volume and low enough temperature that you could run at the most abusive power setting and never see anything close to alarming CHT wise, let's think about your situation. You were concerned about low CHTs so you went ROP to get those temps up. What that means is that you increased both peak ICP and mean ICP to get temps up.  In essence, you decided to run closer to the red box and did so ROP.  Not a bad choice, but you could have done the same from your LOP power setting just by enrichening the mixture. The added benefit would have been better fuel economy and higher mean ICPs without the high peak ICPs.  I find that peak EGT or a just lean of peak on the richest cylinder is a wonderful power setting in the winter time. It produces cooler, more even CHTs than 100ROP, the engine is nice and smooth.

[jlunseth]

My recollection from looking at the graphs they displayed at the APS, and that included some actual engine data derived on the test stand that day, is that the ICP slope is skewed, especially when ICP's start to approach detonation.  The slope is very steep to a narrow peak on the lean side, and then more gradual on the ROP side.  Stands to reason because the degrees from peak settings are much smaller on the lean side than on the rich side.  30 LOP is about the equivalent of 125 ROP for example.  Those are very rough numbers.  But my point is that if you are running at, say, 30 LOP and need to warm the engine up, going just a few degrees richer will rapidly bring you up the ICP slope.  That slope is nearly vertical.   And part of the issue with that, for me, is that no matter what you do, a 231 engine is never going to be very consistent cylinder to cylinder, and in fact, if you start at a particular LOP setting and then change it, the relationships between the cylinders can change.  For example, if Cyl 2 was your richest at 30 LOP, and Cyl 4 was at 45, and then you change the mixture, Cyl 4 might skip ahead of 2.  The intake manifold is very much unbalanced, and changing one thing, like mixture, changes the others, like MAP.  So if you get a good, safe-for-the-engine setting like 30 LOP, and then try to change it towards peak, it will go very rapidly towards peak and some cylinders may actually "skip ahead."  At least, that is my thinking on it.  

BTW I have no good way of measuring how many degrees from peak I am when on the lean side, despite having great instrumentation.  I just have a setting that has worked and keeps the TIT at or below 1600 during the summer months.  

That was why I defaulted over to the rich side.  I picked a ROP setting I have flown for a long time, and then powered it up a little because I knew I needed more heat in the engine.  I have more room to play without going to peak if I do it from the rich side, and the CHT's will always be warmer on the rich side than for an equivalent LOP setting.