Curious CHT's

[tedrichey]

I have a 1976 Cessna 182P, has 150 hours on rebuild engine with new Millenium cylinder assemblies. I recently installed at 6 probe digital CHT gauge which seems to work perfectly. The interesting thing I have only just noticed is that when in cruise with cowl flaps closed or nearly closed, the CHT's can drop as much as 40 degrees when the cabin heater is cracked open about an inch or so. Close the cabin heater and the temps go back up. The aircraft standard CHT does not seem to move but perhaps it is not sensitive enough. Does anyone have any ideas on this, or has anyone else noticed this? Is it because the cabin heat allows a bit more air circulation through the cowling? I would appreciate any input. Ted

 

[Dave Siciliano]

Ted: I don't know a lot about your plane, but a friend has a Sky Hawk and the POH instructs him to use the carb heat when leaning. It affects his CHTs. It will be interesting to see what kind of answers you get. If they don't help you, let me know and I'll send an e-mail or two and see if we can get you some good answers.

Dave

 

[gismo]

Ted: I don't know a lot about your plane, but a friend has a Sky Hawk and the POH instructs him to use the carb heat when leaning. It affects his CHTs. It will be interesting to see what kind of answers you get. If they don't help you, let me know and I'll send an e-mail or two and see if we can get you some good answers.

Dave

Dave, did you miss that Ted was talking about cabin heat, not carb heat? I don't see the connection unless the controls are swapped.

Ted, as to opening the cabin heat valve causing CHTs to drop, I'd first suspect that there is some kind of electrical rather than thermal issue here. Perhaps the cabin heat control cable is affecting a ground connection on the engine or something like that. The only thermal connection I can think of is that there is a slight airflow change through the cowling on many single engine cabin heat systems because the heated air from the cabin heat muff is dumped into the engine compartment when heat is de-selected. Normally that heated air simply exits the cowling and doesn't affect temps inside the cowl, but I could see how something might interfere with the extraction of the waste cabin heat air allowing it to raise the air temp under the cylinders and/or causing a higher pressure under the cylinders that would reduce the cooling airflow over them.

Is this temp change on all cylinders? There is also the possibility that the waste heat from the cabin heat control valve is impinging directly on one of your new CHT probes and causing an abnormal reading that's not really reflecting a cylinder head temp change. What I can't see is this happening on all cylinders.

 

[Dave Siciliano]

Dave, did you miss that Ted was talking about cabin heat, not carb heat? I don't see the connection unless the controls are swapped.

No Lance, I didn't miss it; just didn't have any idea how cabin heat could affect EGTs or CHTs; so, I pointed out some use carb heat to lean.


You seem much better versed on why cabin heat could be an issue. I thought he might have been describing the problem incorrectly and was subtly trying to fish that out

Dave

 

[tedrichey]

Lance and Dave, the change in CHT's is on all cylinders, not just the one. I can't see any physical connection between the cabin heat cable and the wiring for the instrument. For your info it used to be eight channels with 6 egt's and 2 cht's but I got sick of replacing egt probes and so converted it to 6 cht's.

 

[tonycondon]

well doesnt turning the cabin/carb heat on allow heat to be drawn away from the engine a little faster? I have heard of people getting their overheating cars home by driving with the heater on full blast and the windows rolled down. if more heat can escape the cylinder during each exhaust stroke, the CHTs could go down a little, i think. i dunno, just an idea

 

[jesse]

well doesnt turning the cabin/carb heat on allow heat to be drawn away from the engine a little faster? I have heard of people getting their overheating cars home by driving with the heater on full blast and the windows rolled down. if more heat can escape the cylinder during each exhaust stroke, the CHTs could go down a little, i think. i dunno, just an idea

It is a little different. On your car you have a heater core..Picture this being like a mini version of your radiatior. Coolant is pumped through this core. A fan then blows air thru it..which heats the air up whcih is blown into the vehicle. You are dropping the temperature of the coolant which is god with your liquid cooled car.

Airplane..is not liquid cooled. You can't do it that way..SO
you essentially take ram air and blow it over the muffler which is then blown into the cabin. (If there is a leak in the exhaust..carbon monoxide posioning will result)...I don't think this process is going to drop your CHT temperature at a measureable level.

 

[tonycondon]

yea understandable. i was thinking along the lines of giving the heat off the exhaust another path of exit, which allows a little more heat to be taken out the pipe, out of the cylinder. eh i dunno. probably should just go to bed

 

[Dave Siciliano]

Different aircraft use difference systems. On my A-36 there is a pickup off the exhaust manifold, but I don't know how that would affect engine performance; air is just allowed in or not and heated when flowing over the exhaust. In the Baron, I have a Janitrol heater in the nose compartment that actually burns avgas to produce heat.

I just don't have any idea how his plane is set up and how changing the heat seating would affect the readings he mentions. Perhaps Tom or one of the more mechanically inclined guys have an idea.


Dave

 

[Joe B]

Lance and Dave, the change in CHT's is on all cylinders, not just the one. I can't see any physical connection between the cabin heat cable and the wiring for the instrument. For your info it used to be eight channels with 6 egt's and 2 cht's but I got sick of replacing egt probes and so converted it to 6 cht's.

Any connection between the cabin heat cable and the cowl flaps cable? If they're bound together somewhere and one is binding pulling on it could make the other move. Just speculating...

Regards,
Joe

 

[tedrichey]

Thank you all for your responses. I have checked today and there is no connection between the cabin heat and the cowl flaps, they are well separated and movement of one does not effect the other. I can see no physical connection between the cabin heat cable and anything to do with the wiring for the CHT gauge. It is possible there is an earth that I can't find but the gauge movement is so regular and so consistent, it takes about 2 to 3 minutes for the temperature to change which it does in a linear and repeatable manner. It is quite uncanny and I am runnig out of ideas, I still have a feeling that opening the cabin heat knob lets more air flow through a relatively tightly cowled engine and thus lowers the temperatures by transferring heat to the cabin, but I wonder how much heat a 230 hp engine at 60% power, ie 140 hp can dissipate into the cabin of the aircraft. I need some more ideas...Ted

 

[Dave Siciliano]

Geesh Ted. I would sure like to know what you find out. I just don't know your plane and systems well enough to contribute much. If you were talking mixture, prop., throttle, I would be able to pitch in more.

Dave

 

[Steve]

Any time you put in instrumentation with higher resolution display you'll notice smaller deviations that you haven't noticed before (digital tach vs analog for example).

It would not be suprising to see a change of this magnitude with small variations in airflow over the cylinders. We're only talking in the neighborhood of a 10% change. To get some quantifiable data you'd need to measure the volumetric flow of the cabin heat duct, the OAT, the heater muff inlet and outlet temperatures on the exhaust side and the cabin heat side and then do an energy balance analysis. I suspect there are routines available online to do a simple heat exchanger performance calc such as this. You could then determine if the temperature variations observed on the cylinder heads were driven by the cabin heat system configuration.

Personally I'd rather have the EGT probes.

 

[jesse]

Perhaps you could do the following:

1.) Heat off
2.) Barely open the cowl flaps. Enough to see a 40 degree drop on your new digital CHT gauge. See if that drop is noticeable on the old analog CHT gauge.

You'll now know if the analog gauge is capable of indicating that 40 degree drop, which will let you know if the drop caused by the heat is actually a drop in CHT temperature or some other weird electronic error.

I don't really know if this is possible. I have very little experience with airplanes that have cowl flaps.

 

[Joe B]

... the gauge movement is so regular and so consistent, it takes about 2 to 3 minutes for the temperature to change which it does in a linear and repeatable manner...

That is exactly how a real CHT drop should behave. It sounds to me like you're observing a real phenomenon. I can think of two things: (1) if you're getting a real blast of air out of the cabin heat system, the drop could be simply due to the increased airflow through the cowling, or (2) turning on the cabin heat changes the airflow patterns inside the cowling just enough to make the normal cooling flow that much more efficient. The airflow in the cowling is anything but streamlined - lots of backflows and eddies, so who knows. Only way to tell would be to tuft everything inside your cowling and put a video camera (and light) in there.

Oh, and a third thing: You haven't mentioned it but you've been under the cowl enough that I assume you've checked the integrity of your heating system, ducts, and so forth.

Answering your question about heat (well, SWAG'ing it), your engine is about 25% efficient at converting avgas to locomotion. (Formulae here.) All of the extra avgas is converted to heat, but most of that goes out the exhaust stack. About 5% of the energy in the fuel is converted to waste heat that your engine has to get rid of. When the engine is producing 140HP, I figure it's producing 28HP of waste heat. Somewhere I saw that the oil cooler gets rid of about 20% of the waste heat, leaving 22 1/2 HP for the cylinder fins. That's about 17,000 Watts, or 17 hair dryers running full tilt.

That could make your cabin nice and toasty warm if you took that heat and dumped it into the cabin. But you don't. You take a bit of the air that's whistling through the cowling and toast it on the screaming hot exhaust muff before sending it into the cabin. So I'm guessing the change you're seeing is due to a more efficient airflow pattern than to an alternate way to get the heat out of the cowling.

Regards,
Joe

 

[tedrichey]

Thanks Joe B, I think that you are on the right track. I just wonder if the low temperature air, (ie OAT, perhaps 5 degrees) whistling over the engine, which is at 350 degrees, can contribute to the cooling of the cylinder fins by allowing a greater volume of air to pass by, that is greater than what can exit out the closed cowls flaps plus the extra volume of cold air which is allowed to pass over the muffler when the cabin heat knob is cracked. I hope this makes sense. By the way, I notice that when the new gauge changes temperature by about 30 degrees, the aircraft gauge does move but it is a tiny amount, barely discernable, probly less than one fifth of the needle thickness. I will keep testing it in the air but I am beginning to think the extra air coming into the cabin is cooling the engine somewhat before the air is heated by passing over the muffler.. Best wishes Ted

 

[gismo]

Thanks Joe B, I think that you are on the right track. I just wonder if the low temperature air, (ie OAT, perhaps 5 degrees) whistling over the engine, which is at 350 degrees, can contribute to the cooling of the cylinder fins by allowing a greater volume of air to pass by, that is greater than what can exit out the closed cowls flaps plus the extra volume of cold air which is allowed to pass over the muffler when the cabin heat knob is cracked. I hope this makes sense. By the way, I notice that when the new gauge changes temperature by about 30 degrees, the aircraft gauge does move but it is a tiny amount, barely discernable, probly less than one fifth of the needle thickness. I will keep testing it in the air but I am beginning to think the extra air coming into the cabin is cooling the engine somewhat before the air is heated by passing over the muffler.. Best wishes Ted

In virtually every engine installation on certified aircraft, the cold air feeding the cabin heat was sourced outside the engine compartment (don't want oil leaks creating smoke in the cabin do we?). With such a setup, I don't see how the cabin air flow would impact cylinder cooling at all. OTOH, most if not all exhaust muff based cabin heat systems divert the heated air into the lower cowling when the heat is "turned off" rather than blocking the flow of hot air through the exhaust muff (which would tend to make that muff get a lot hotter). That's why I posted earlier that the diverted hot air might be having an effect on either the cylinder head temp itself and/or the CHT probe(s).

 

[tedrichey]

Lance, thank you for your response. I will get the cowls off again early this week and have another look but I think, and another 182 owner could correct me here, that the cold air comes through a door on the starboard side of the aircraft and the intake for hot cabin air comes through a scat hose situated on the rear crosswise baffle on the top of the engine inside the cowling where it goes down to the muffler cowling and then to the mixer valve to mix with the cold air from outside. I am not really sure on this without looking at the aircraft again, (I should know, I have owned it for 20 years) but there are no external air intakes except the cold air door on the right hand side and the filtered air intake at the front underneath of the cowling. I will post again after getting the cowls off. Best wishes Ted

 

[gismo]

I think you are correct about the heated cabin air source, but taking air from the pressurized area above the cylinders would actually cause the cylinder temps if anything but in any case I doubt that the source of air to you cabin heating system is the cause of your problem.

 

[tedrichey]

Lance, my curiosity got the better of me and I went out and took the top cowling off. The air intake for the cabin heating is actually right at the right hand front side of the engine, underneath the oil cooler and not on top of the cylinders as I originally thought. It is in a pressure air area which explains why you can't get much heat in the cabin on climb out with the cowl flaps open, closing them pressurises the engine compartment and puts more hot air into the cabin. The only other thing I noticed was that the rubber baffle seal around the front of the cowling was inside out on the side near the cabin heat intake, I fixed it but heaven knows if that will make any difference. I am finding it hard to justify extra air circulation through the heater cooling the engine when the cabin heat intake is forward of the engine cylinders so I am back to square one..best wishes Ted

 

[tedrichey]

Found the answer. Nothing to do with the engine cowlings or anything like that. The instrument has became defective in some manner and changes, (reads lower) with an increase in cabin temperature. A heat gun under the panel very quickly caused a very large drop in indicated CHT's. Still waiting for a reply from the manufacturr, they initially said it was impossible. You just never know. Ted

 

[Skip Miller]

Found the answer. Nothing to do with the engine cowlings or anything like that. The instrument has became defective in some manner and changes, (reads lower) with an increase in cabin temperature. A heat gun under the panel very quickly caused a very large drop in indicated CHT's. Still waiting for a reply from the manufacturr, they initially said it was impossible. You just never know. Ted

If that is the case it sounds to me like a bad connection, which could be inside or outside the gauge assembly. Check the outside connections before replacing the gauge.

-Skip

 

[gismo]

If that is the case it sounds to me like a bad connection, which could be inside or outside the gauge assembly. Check the outside connections before replacing the gauge.

-Skip

A thermocoupe gauge actually reads the difference between two junctions, one at the point of measurement and the other typically at the gauge. If the reference junction was exposed to the direct blast of the heater air, that could easily drop the reading by a significant amount. That said, the majority of CHT gauges are either fluid expansion (older airplanes) or thermistor based (the probe's resistance varies with temperature and the gauge measures this resistance by passing a current through the probe). The three types are easily distinguished by the connection between the probe and the gauge. Fluid expansion units have a small diameter (typically metal) tube, thermistor types have a single fairly flexible wire, and thermocouple probes always have two (usually single strand and stiff) wires made from two different metals.

 

[tedrichey]

Lance, it has six thermocouple wires going to all six cylinders, all cylinder temps seem to be affected equally, there must be a master junction somewhere, possibly in the instrument.Ted

 

[gismo]

Lance, it has six thermocouple wires going to all six cylinders, all cylinder temps seem to be affected equally, there must be a master junction somewhere, possibly in the instrument.Ted

Oh, it sounds like an electronic instrument. They are thermocouple based, but also compensated for the cool junction temp by the software using a separate internal semiconductor junction temp sensor. I'd be talking to the tech folks at the company that made the instrument. Any chance that the thermocouple wiring wasn't done according to the installation instructions? Normally the special wire must go all the way to the connector on the instrument (or breakout box if it's separate). If someone used regular copper wire or even the wrong type of thermocouple wire (there are two common types) you would have an uncompensated cold junction wherever the wire type changes and that would be sensitive to heat.

 

[tedrichey]

Hello Lance, When it was changed we had to order four cht screw in probes and two or three thermocouple wires. They came from the Australian agents and had the right part numbers on them so assume they were the right ones, they certainly looked the same. I have contacted the manufacturer and they are a bit reticent with any bright suggstions, apart from send the instrument back to them, which is a bit tricky sending it from Australia. I thought that I would have had at least a few constructive suggestions about where to look, what to check,etc. Ted