cool CHTs

[Shadrach]

I did a lot of short hops over the holiday. The weather finaly cooled to sub freezing temps on Christmas eve. I noticed that at between 30and 20 OAT that keeping all CHTs above 300 was a challenge. These were high pressure days (30.2X") so I was able to pull ~29" at 3500ft with the ram air open.   

At 15 LOP CHTs were:

#1 274

#2 309

#3 312

#4 299

At that power level, I would have preferred to be around 30 LOP, which is where I started, but there's something disconcerting about seeing everything under 300df with the lowest in the mid 250s and dropping.  In order to get everthing into low 300s I needed to run between 20 and 100 ROP, which is not something I want do at 29 inches.

I know that the CHT gauge bottom of the green arc is 250 or 275 ( I forget which), but it just seems oddly cool. I did a fair amount of baffle work during engine install last spring which may account for the cooler than typical temps. Cowl flaps make little difference in cruise, maybe 10df on CHTs. I verified they were closing to specs. 

Anyone else see this or consider it a concern. With the mid teen temps of Jan and Feb approaching I am concerned that this will be even more of an issue.

[Shadrach]

-Bump-

This issue continues... At 11'500ft yesterday, I was unable to to keep al CHTs above 300df at 100ROP and 21"... at peak #1 fell to 272df and at 5 LOP 262df while the rest fell somewhere between the 275 and 295df. I find this disconcerting... Is anyone else experiencing this? The OAT at altitude was only in the mid 20s (F)...

[johnggreen]

Ross,

My Bravo rarely breaks 310 degrees on any cylinder.  I think the only temp that should concern you as being too low is the oil temp which should be at least 160-170.  Your cylinders are well within operating range.

Be happy or as my aunt has told me for forty years, quit trying to buy trouble.

Jgreen

[m20j]

The J has an adjustment for the cowl flaps in the maintenance manual.  Perhaps your gap is too wide?

 

NOTE

For improved cooling during summer months or above normal temperature operations, cowl flaps may be rigged to have a 0.25 max inch gap at "B" when in the closed position.

 

 

http://i42.tinypic.com/io21hh.png

[Shadrach]

Me no have J...just a lowly F. I have checked the CFs and they close flush with the cowling.

[Immelman]

Your experience matches mine in running LOP on all but hot days.

[John Pleisse]

Quote: Shadrach

-Bump-

This issue continues... At 11'500ft yesterday, I was unable to to keep al CHTs above 300df at 100ROP and 21"... at peak #1 fell to 272df and at 5 LOP 262df while the rest fell somewhere between the 275 and 295df. I find this disconcerting... Is anyone else experiencing this? The OAT at altitude was only in the mid 20s (F)...

[Shadrach]

Quote: Immelman

Your experience matches mine in running LOP on all but hot days.

[Shadrach]

Quote: N4352H

In the quest to find a lead substitute for avgas.....famous experimentation was done by a fuel injector nozzle maker on a non Lycoming engine. Astounding, gorund breaking data was assembled where it was determined this frigid operating range would be refered to as..................... "The Blue Box"...........

[John Pleisse]

 C'mon...where's my hug?

[Shadrach]

I'm not that easy...

[Bolter]

Quote: Shadrach

I did a lot of short hops over the holiday. The weather finaly cooled to sub freezing temps on Christmas eve. I noticed that at between 30and 20 OAT that keeping all CHTs above 300 was a challenge. These were high pressure days (30.2X") so I was able to pull ~29" at 3500ft with the ram air open.   

At 15 LOP CHTs were:

#1 274

#2 309

#3 312

#4 299

At that power level, I would have preferred to be around 30 LOP, which is where I started, but there's something disconcerting about seeing everything under 300df with the lowest in the mid 250s and dropping.  In order to get everthing into low 300s I needed to run between 20 and 100 ROP, which is not something I want do at 29 inches.

I know that the CHT gauge bottom of the green arc is 250 or 275 ( I forget which), but it just seems oddly cool. I did a fair amount of baffle work during engine install last spring which may account for the cooler than typical temps. Cowl flaps make little difference in cruise, maybe 10df on CHTs. I verified they were closing to specs. 

Anyone else see this or consider it a concern. With the mid teen temps of Jan and Feb approaching I am concerned that this will be even more of an issue.

[Shadrach]

Dan,

In theory you absolutely correct. So, in practice one must make allowances for things like OAT. To get the disclaimer out of the way, I'm not recommending any specific mixture setting. Anyway, back to you question.  I handle it this way. In the hot summer months, I limit cruise CHTs to ~350df  and climb CHTs to ~380df depending on alt. Higher would not bother me, but I know that my engine usually runs <350df even with OATs of 90df unless I'm up high and running 100ROP above 10K and even then rarely gets to 370df.  In the dead of winter I limit cruise CHTs to ~340 in cruise and ~360df in climb. I have done a lot of experimentation comparing temp while running LOP at high power to ROP and full rich. The reality for my engine is that anything beyond about 35LOP is cooler than full rich from a CHT standpoint. As to Internal pressure (ICPs). ICP is how we make power. Mean effective pressure correlates to horsepower.   Power can be produced with a high peak pressure (80ROP is the highest) which causes high heat, or a more constant level of pressure (richer or leaner than 80 ROP) through out the power stroke which generates less heat.

In short, all other things being equal, it is highly unlikely that with a CHT of 300 or 310 or even 330df that my engine is witnessing anything close to abusive ICPs in cruise. Therefore, I'm happy to run there. Also, I've had the unfortunate benefit of seeing the inside of my cylinders at just under 90hrs. because of a case crack. They looked good and and had healthy compressions. Therefore, I manage my engine to CHT, but I change the parameters based on OATs.

[KSMooniac]

Dan, you've got it correct and this is where I usually make my plug for the Advanced Pilot Seminar course so you can learn all of this stuff efficiently and correctly.

Ross knows it, and manages his engine accordingly and probably cruises high enough that he isn't making tons of power, so that helps in terms of giving a broader range of mixtures that are not harmful to the fatigue life of the engine.  

In your example, if you run 29" of MP in  J then you're pretty much screaming down the beach close enough to count freckles on the people, and your CHTs will vary based on OAT and your mixture setting.  It is theoretically possible to imagine a very cold day and a 10 dF LOP setting that still gives you a CHT reading of 380 or less.  This is NOT a good setting due to the high peak ICP that Ross mentioned, even though the cylinder is relatively cool.  High peak ICPs are not good, especially if they're combined with high CHTs (think bad, and more bad).  Running 29" is very close to full power (presumably RPMs are dialed back a bit) and you should be in the 150+ dF ROP or 70+ dF LOP range so keep those ICPs down to something reasonable.  As you reduce power (either by pulling MP back, or climbing higher) then you can set it less ROP or less LOP, and by the time you get to 65% power you can set it anywhere you wish and you won't be producing high enough pressures and temps to do any long term damage.  This describes the "red box" as coined by the APS crew and you want to stay out of the red box.

[Bolter]

Quote: KSMooniac

Dan, you've got it correct and this is where I usually make my plug for the Advanced Pilot Seminar course so you can learn all of this stuff efficiently and correctly.

Ross knows it, and manages his engine accordingly and probably cruises high enough that he isn't making tons of power, so that helps in terms of giving a broader range of mixtures that are not harmful to the fatigue life of the engine.  

In your example, if you run 29" of MP in  J then you're pretty much screaming down the beach close enough to count freckles on the people, and your CHTs will vary based on OAT and your mixture setting.  It is theoretically possible to imagine a very cold day and a 10 dF LOP setting that still gives you a CHT reading of 380 or less.  This is NOT a good setting due to the high peak ICP that Ross mentioned, even though the cylinder is relatively cool.  High peak ICPs are not good, especially if they're combined with high CHTs (think bad, and more bad).  Running 29" is very close to full power (presumably RPMs are dialed back a bit) and you should be in the 150+ dF ROP or 70+ dF LOP range so keep those ICPs down to something reasonable.  As you reduce power (either by pulling MP back, or climbing higher) then you can set it less ROP or less LOP, and by the time you get to 65% power you can set it anywhere you wish and you won't be producing high enough pressures and temps to do any long term damage.  This describes the "red box" as coined by the APS crew and you want to stay out of the red box.

[Shadrach]

 

 

Dan,

You've pretty much got the conceptual aspects of managing mixture down. I didn't mean for this to turn into a ROP/LOP discussion but to build on what you've mentioned:

While I did say we were in effect controlling timing with mixture to help people understand the concept, a propulsion engineer would probably get a bit "bent" over that comment if taken literally. What we do is use the mixture to control the speed of the combustion event which in turn affects peak and mean pressure. If we had variable timing ignitions, we would have an additional parameter to work with in controlling peak and mean pressure (by igniting the combustion charge sooner or later as measured from TDC), but we don't (for the most part), so the mixture is it.

As to the infamous "Red Box", it has been stated many times that it's a "fuzzy red box" (there's a joke in there, but I'll leave it alone). We can approximate it (as APS has done) to serve as a guideline for all NA and Turbo engines, but it is not the same for all engines and measuring it is not exact...as is stated in some Deakin quotes below.

"We suggest that you can safely set the mixture anywhere you please at 60% power, and 65% is probably "close enough." Remember, just because we teach the concept of the "red box" doesn't mean the engine will instantly explode if you get near it, and we cannot begin to state with assurance EXACTLY where the red box begins and ends. There's room for error here, and we're trying to be conservative in this."

Pelicans Perch #66

"As a general rule in normally aspirated engines, you won't go too far wrong by leaning until you have a MIXTURE setting of about 15 to 25F LOP (at lower manifold pressure settings) or 25 to 50F LOP at higher manifold pressure settings in order to obtain the most favorable (minimum) BSFC."

Pelicans Perch #65

This graph (taken from APS) shows the box beginning (100%HP) at around 230ROP and ending at around 45LOP. Remember it's fuzzy...

 

 

This graph is key to understanding why operating on the lean side is more efficient and how mixture affects the combustion event. Less peak pressure LOP but slightly more pressure during the later more effective stages of the power stroke.

 

 

 

You mentioned a yellow box. I don't think that I'm in any sort of yellow box. I am not hung up on any raw numbers other than CHT and OAT. If it's a hot day and all CHTs are under 340, then the likelihood of detrimental effects to my engine are nil. If it's a cool day then I adjust CHT limit down accordingly. 

The reality is that it is nearly impossible to get a 4cyl Lyc to detonate (maybe if I tried to by climbing steeply CF closed and at 29" and 80ROP).  These margins were built into the engine to meet certification standards. It's possible that you could abuse it daily and get TBO, but that doesn't mean it's a good idea.  

One other thing, my numbers could be skewed because I don't know how accurate my MP gauge is. It could be reading high. Your question has inspired me to check it out; I think it's in the ballpark because IAS tells me so. However, my engine seems to have a narrower "red box" than what APS suggests. My CHTs begin to fall drastically at anything leaner than peak.

Sorry for the novella...

Hope this helps.

 

 

 

 

 

[Bolter]

An excellent read! Thanks for the detail and effort. -dan

Quote: Shadrach

Sorry for the novella...

Hope this helps.