Magneto can affect CHT?

txflyer

txflyer

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Ron re-built both my mags and he said the left mag had a brush that wasn't working or making contact due to a spring issue. The other mag was in good shape.

I never noticed anything on mag checks other than I remember complaining to Ron that the left mag would run rough and have to be cleared but that was very un-often. I thought it was me not leaning enough.

I take it up today after all the work and find that I can lean more and run up the CHT by quite a bit more than I could before. And by my seat of the pants sense and I swear I see another pound or two of MP, it just feels stronger. I guess I was running a sick horse for a while. :redface:
 
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Even a degree of timing can effect cht to large degree. So if your timing was a bit off and is now on that could account for the difference.

Bob
 
txflyer said:
Ron re-built both my mags and he said the left mag had a brush that wasn't working or making contact due to a spring issue. The other mag was in good shape.

I never noticed anything on mag checks other than I remember complaining to Ron that the left mag would run rough and have to be cleared but that was very un-often. I thought it was me not leaning enough.

I take it up today after all the work and find that I can lean more and run up the CHT by quite a bit more than I could before. And by my seat of the pants sense and I swear I see another pound or two of MP, it just feels stronger. I guess I was running a sick horse for a while. :redface:
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You bet! The timing is set with the assumption of two flame fronts, and when one mag isn't working right, it has the same effect as retarding the timing.
 
Yep. Timing and mag performance can impact power and economy significantly.

This, combined with variable timing, is why electronic ignition is a logical thing for planes to have. Just need to make sure you have redundant electrical sources.
 
Ted DuPuis said:
Yep. Timing and mag performance can impact power and economy significantly.

This, combined with variable timing, is why electronic ignition is a logical thing for planes to have. Just need to make sure you have redundant electrical sources.
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and practical and logical are two totally different thingys. :goofy::D
 
 
Henning said:
You bet! The timing is set with the assumption of two flame fronts, and when one mag isn't working right, it has the same effect as retarding the timing.
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hmm, when I go to one mag, my egt increases, and I think my cht eventually would.
retarding is supposed to result in lower cht.
 
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Let'sgoflying! said:
hmm, when I go to one mag, my egt increases, and I think my cht eventually would.
retarding is supposed to result in lower cht.
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Going to one mag increases the overall burn time. EGT goes up because much of the burn is happening later and more heat is going out the exhaust - the late burning stuff doesn't get the energy expanded out of it.

CHT goes down because there is less time for heat to transfer to the heads and peak temperatures go down.
 
I guess he could :dunno:

latest
 
Timing is only one factor. The strength of the spark itself is another. A weak spark might not even fire when the cylinder pressure comes up on compression or if the plugs are sooty or oily. I have had engines run stronger just by installing new points and anything else that was worn, and resetting the E-gap, in mags that had been running without complaint from pilots.

Dan
 
Capt. Geoffrey Thorpe said:
Going to one mag increases the overall burn time. EGT goes up because much of the burn is happening later and more heat is going out the exhaust - the late burning stuff doesn't get the energy expanded out of it.

CHT goes down because there is less time for heat to transfer to the heads and peak temperatures go down.
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It doesn't increase the overall burn time, it changes the combustion pressure profile. The difference in combustion duration isn't noticeable.

Late ignition increases EGT because just like you said, the peak combustion temperature happens later during the cycle, meaning there was less time to transfer that heat to the cylinder/piston. So CHT goes down, EGT goes up.
Then again, this only works for non-knocking combustion. Knock and other anomalies obviously makes all of this do a 180.

It's never a good idea to talk about engine performance based 100% on temperatures. You need to compare PCT and PCP, and especially the PCP profile over a combustion cycle.
 
mtuomi said:
It doesn't increase the overall burn time, it changes the combustion pressure profile. The difference in combustion duration isn't noticeable.
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Do you have data to support that?

I don't have data because all the combustion work I did was on single plug engines. But, I would think that the rate of heat release from one flame front has to be slower than from two which should have a significant impact on the 10-90 burn time.
 
Wish I had an analyzer but I'm still flying with a single EGT probe in the right header pipe.

Ron says "you don't need that chit!" With my kind of flying, I tend to agree. Lean to peak then back off 50-75Celsius has been working. The engine and plugs have been looking clean enough.

I switched to Phillips X/C from Elite, and just put new Tempest plugs and an entire new exhaust system and we rebuilt my carb heat box on the carburetor. So somewhere in the mix of all that and rebuilding the mags we found some more power.
 
txflyer said:
Wish I had an analyzer but I'm still flying with a single EGT probe in the right header pipe.

Ron says "you don't need that chit!" With my kind of flying, I tend to agree. Lean to peak then back off 50-75Celsius has been working. The engine and plugs have been looking clean enough.

I switched to Phillips X/C from Elite, and just put new Tempest plugs and an entire new exhaust system and we rebuilt my carb heat box on the carburetor. So somewhere in the mix of all that and rebuilding the mags we found some more power.
Click to expand...

I wouldn't get one for the EGT probes, I would get one for the CHT probes. EGT is mostly irrelevant, CHT is were the $$$ is, either savings or waste. Mechanics are not the most objective people on the matter, they have kids and boat to support.:rofl:
 
Henning said:
I wouldn't get one for the EGT probes, I would get one for the CHT probes. EGT is mostly irrelevant, CHT is were the $$$ is, either savings or waste. Mechanics are not the most objective people on the matter, they have kids and boat to support.:rofl:
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The jugs have boss's for CHT probes. I could get just those, but then I'd want fuel flow and something else while I'm doing it, and there you go ...

She's flying great now like she has for fifty years. I know an analyzer is worth the money but I'm tired of working on her and spending money on the ***** and right now we've got the weather and I just want to fornicate her for a while and get my money's worth before she get's de-cowled again. :D
 
txflyer said:
The jugs have boss's for CHT probes. I could get just those, but then I'd want fuel flow and something else while I'm doing it, and there you go ...

She's flying great now like she has for fifty years. I know an analyzer is worth the money but I'm tired of working on her and spending money on the ***** and right now we've got the weather and I just want to fornicate her for a while and get my money's worth before she get's de-cowled again. :D
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Fuel totalizer alone is worth the cost to me. It came in my Travelair, and I put it in each since. I really like knowing what I'm flowing and what I have left. If you have a 430 or similar you get a heads up if the winds aloft change enough to get you in a bind for fuel way in advance so you can pick a cheap fuel stop. With the 310 at 100+ gallons a fill, saving $2-3 a gallon pays for it in a few fills.
 
Let'sgoflying! said:
during the Otto cycle power stroke, peak internal cylinder pressure and temperature optimally occur at 15° to 20° of crankshaft rotation after top dead center - a point denoted by θpp. Anything that causes θpp to occur earlier (i.e., closer to TDC) increases CHT, and anything that causes θpp to occur later (further from TDC) decreases CHT.

For example, advancing the ignition timing so that the spark plug fires earlier causes θpp to occur earlier and increases CHT. Retarding the ignition timing so that the spark plug fires later causes θpp to occur later and decreases CHT.
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That's sloppy talk... Theta PP is an angle. The angle doesn't occur earlier... The peak pressure (PP) occurs earlier.

Not good communication to muddle concepts, Savvy friends!

Paul
 
Capt. Geoffrey Thorpe said:
Do you have data to support that?

I don't have data because all the combustion work I did was on single plug engines. But, I would think that the rate of heat release from one flame front has to be slower than from two which should have a significant impact on the 10-90 burn time.
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We did some comparisons with twin spark/wasted spark/single spark systems. If the combustion chamber is optimized for dual-front flame, you will see slight benefits from it. But airplane engines really aren't. 10-90 isn't that important, CA50 is what matters when you have constant ignition angle aka. the profile of the pressure gradient, not the absolute burn speed. The whole duration of the process stays almost the same so your 10-90/0-10 are almost the same. (your heat release/mass fraction burnt graphs change when you modify your ignition system)

So, single spark changes the CA50, not 10-90. I would have to dig deep into my GT-Power logs to find relevant data so I can't really prove this, it just is what it is :) Don't care about the cumulative heat release, it stays roughly the same.

The initial flame propagation is slower with single spark, so you will reach CA50 later which means less power, loss of hp/rpm etc. But your 10-90 is roughly the same, combustion chamber design correctly spreading the charge in there. A HC measurement from the tailpipe would confirm this.
But, later CA50 means more complete combustion past-CA50, which compensates for this. That is why if your ignition angle is the same, you shouldn't see much variance in your CHT, but you will likely get higher EGTs.

The difficulty when comparing single/dual mag operation is also the PCP spread over the combustion chamber, which alters the results. That's why it's not that easy to draw = marks from mag operation to CHT.

All this obviously assuming zero wasted fuel = not running with the red knob through the firewall.
 
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Henning said:
I wouldn't get one for the EGT probes, I would get one for the CHT probes. EGT is mostly irrelevant, CHT is were the $$$ is, either savings or waste. Mechanics are not the most objective people on the matter, they have kids and boat to support.:rofl:
Click to expand...

TCM 470s and 520s use rubber induction connectors that are prone to leaks. 6 point EGT monitoring is the best indicator. CHTs are also nice to see. It allows you to control heat and to make adjustments to normalize the temperature differential between cylinders. Ideally the EGTs and CHTs will be relatively flat line and will remain that way. I'm less worried about seeing a deviation than appreciative of seeing stable, flat temp values when I'm wondering what that vibration or that hot smell might be.

Considering the reason for the thread? A monitor probably would have alerted you to the diminished ignition performance.
 
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