Too cool Cyl Temps?

[garytex]

Actually I was hoping someone would chime in regarding minimum cruse CHTs.  

Cylinders are ground with several thousandths choke.  At ambient, they are tighter at the top of the cylinder, the theory being they as they heat up more near the top of the cyl. that they will expand a little there & the cyl. walls become become parallel.  This next part may be entirely OMT (Old Mechanics Tale) is that it is important to have parallel cyl. walls so the rings do not have to expand and contract as the piston reciprocates to avoid embrittlement of the rings and to get better oil control.  I don't know how much of that I buy, but it is intuitively attractive logic.  Ring gaps are (should be) measured in the choked area at room temp.  You can actually see the ring gap close as you bump the ring down the cylinder walls.    

So I wonder, how warm does the cyl. have to be for the choke not to be squeezing the oil off the piston?  We know that wear happens in this area, as cyls. with the choke worn out of them and worn piston skirts are not unusual.  Admittedly that type of wear is more common on big bore Continentals than Lycomings, but it still goes on.  

I would guess that Lycoming has twiddled their slide rule (I am assuming that this was done years ago) and come up with a minimum temp. to make all the clearances work.

Knowledge? Opinions?

Thanks, Gary

[Skybrd]

Good question. Is running 300 degrees CHT too low? Also how can a high value EGT show cool CHT? Is it helpful to reduce RPM's on decent's to keep the engine loaded and keep the CHT's up?

[Cruiser]

without objective evidence to support or reject the theory it is impossible to give an answer. Best guidance in this case would be the engine manufacturers recommendations or the POH. All others are opinions.

[Bob - S50]

First, my POH ('78 J) says the normal range for CHT it 300 to 450 but I will not allow it to go over 400.

 

Second, Mike Busch has a very good webinar on cylinders.  He does not mention a minimum temperature or anything about a 'choke' in the cylinder.  He does show that at room temperature there is a pretty big gap between the piston and cylinder.  He also states that since the cylinder is made of steel and the piston is made of aluminum, the piston expands twice as much (coefficient of expansion that is) as the cylinder as it comes up to temperature.  At operating temperature he says the gap is very small.

 

In another webinar about engine condition he talks about compression checks and mentions that several factors influence the reading including position of the ring gap and direction of travel of the piston.  Since the fit is loose at room temperature, the piston will rock back and forth in the cylinder a little as it moves up and down because of the angle between the wrist pin and the connecting rod.  Depending on the gap location and whether the piston is going up or down, the gap will change size.  That could be the reason you see the gap change.

 

Bob

[NotarPilot]

I routinely fly short hops of about 15 minutes and because I'm doing such short hops I tend to do them at low power settings of 55%-65%.  I often see one or two cylinders at about 285-295F CHT.  Any thoughts?

[Bob - S50]

Good question. Is running 300 degrees CHT too low? Also how can a high value EGT show cool CHT? Is it helpful to reduce RPM's on decent's to keep the engine loaded and keep the CHT's up?

I'm only writing based on what I've watched/read on the internet, mostly Mike Busch and John Deakins.

 

EGT is a measure of how much energy is left in the gases as they exit the cylinder.  CHT is more a measure of the peak pressure being generated inside the cylinder.  If you start at peak EGT you can go either LOP or ROP.  Going LOP 50 degrees for example will change the mixture to make it burn slower and will reduce the peak pressure.  This will reduce CHT.  Going 50 ROP will theoretically create the ideal mixture for the quickest burn and the peak pressure (all other things being equal).  That will increase the CHT.  In both cases you have the same EGT but two different CHT's.

 

As for RPM during descent, I've read you want low RPM in the descent to reduce ring float (not sure that is the correct term).  At high RPM and low MP, the pressure in the cylinder may not be high enough and will allow the rings to float in the grooves.  The prop is driving the engine rather than the other way around.  This supposedly can lead to ring and or landing failures.  Using low RPM and higher MP instead reduces the likelihood of that.  During descents I try to use long shallow descents at relatively high power settings and periodically try to check the mixture and try to keep it near peak EGT.

 

Just my opinion.

 

Bob

[Seth]

I had a fellow Mooney pilot let me know that for LOP ops during decent he likes to keep his CHTs above 250 degrees.  In cruise, mine are in the low to mid 300s with one or two just under.  In decents, I try to lean and add power (thought hard to do sometimes due to speed and decent rate) to keep the CHTs above 250.

 

I have not read the scientific backing but followed his reccomendations in the blind as he has had great success running his 231 LOP.  This is a very fascinating subject.

 

-Seth

[jlunseth]

The minimum CHT temperature in my 231 POH is 250.  I believe, though, that Mooney was more concerned about the related Oil Temp. that cylinder wall dimensions.  The manual says that allowing Oil Temp to drop to low (below 100) and then applying full power could result in a stoppage of oil flow.

[rbridges]

I read an article by shell (?) that mentioned keeping cht at a certain temp to minimize lead fouling. Something about a lead scavenging agent that works at a certain temp. For some reason, I was thinking it was around 300 or so.

[Shadrach]

Lead scavenging is the real issue. The lead scavengers need a certain amount of heat to work.  In the winter, I have seen CHTs readings as low as 275 in cruise with no evidence of lead fouling.

[Buster1]

Do you guys read John Deakin's stuff over here in Mooney-land?  It may not entirely answer the low CHT question, but most of the Bonanza owners treat his word as gospel and manage our engines as such.  Some great articles below!

 

http://www.avweb.com/news/pelican/pelicans_perch_84_mixture_cht_194816-1.html

 

http://www.avweb.com/news/pelican/182176-1.html?redirected=1

[Earl]

My CHTs in cruise typically have a difference of around 40-50 degF and the coolest is usually just under 300 degF.  No idea if this is a problem but I can't imagine it would be since I have good compressions and oil usage of about 1 quart every 8-10 hours of flight.

[Shadrach]

My CHTs in cruise typically have a difference of around 40-50 degF and the coolest is usually just under 300 degF.  No idea if this is a problem but I can't imagine it would be since I have good compressions and oil usage of about 1 quart every 8-10 hours of flight.

Compression and oil usage don't seem to be the issue. lead scavenging is the theoretically the issue with low temps. Fortunately, it really seems to be a non issue with our engines.  My theory is that the delta between Cyl wall/plug temps and CHTs is much higher in cold weather and therefore even at low CHTs, the plug and cyl walls are still plenty hot for effective scavenging...

[Piloto]

One significant cause for cold CHT is the cowl flap adjustment. In cruise a 1/4" gap can decrease the temperature by 25F. Check the cowl flaps gap in the closed position. If the cowl flap trailing edged is not in line with the cowling it will cause the lower CHT. There is nothing wrong about having the added cowl flap gap. In fact it helps on preventing overheating when you forget to open them on take off.

 

José 

[garytex]

Thanks to all.  I am considering inviting the hangar fairies to block the front of #1 with a small piece of aluminum, it always runs cooler than the rest by about 30 df.

Gary

Also thanks Piloto, I'll check the flap trailing edge.  I have only seen little dif in CHT's between open and closed cowl flaps. Maybe it is running slightly open.