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Asked by: Jon Moore
Does anyone know why EGT rises when one magneto is turned off?
John D. Collins on Dec 28, 2012
When the fuel-air mixture is ignited, it burns at a predictable rate until all of the fuel-air that can be burned is consumed. The flame front starts at the location of the spark plug and radiates out in all directions. With two spark plugs, there are two flame fronts and the fuel load burns quicker and therefore completes burning earlier in the cycle. The hot gasses then have more time to cool down before they are pushed out past the exhaust valve and the EGT sensor. So with a single flame front, it can take as much as twice as long to burn the fuel load, and because the burning event lasts longer, there is less time available for the gases to cool before they are pushed past the exhaust valve and EGT sensor, thus a lower EGT temperature is sensed.
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on Dec 28, 2012
John already answered the question, I’ll just add something that should be obvious from his answer. A rise in EGT to the aware pilot is one cue that you may have a mag failure.
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on Jan 27, 2013
To add clarification to Johns explanation above, as the flame front is effectively travelling twice the distance per event, the peak pressure happens further after TDC, or at a greater Theta PP. (theta being the angle past TDC and PP= peak pressure.
Because the temperature of combustion is roughly the same, but the pressure change from peak pressure (which is now lower) to atmosphere is a smaller amount, or less expansion, the gas average temperature at the probe is higher.
So it is not so much the more time to cool, rather its the less expansion.
As an example, get a low compression engine, even a turbocharged lower compression like in a TC Bonanza, and compare the EGT and TIT to say a Turbo Normalised Bonanza or Cirrus. The EGT is lower on the TN with the higher compression ratio than the lower CR engine. Even at the same HP rating.
One of the reasons why a TAT TNIO550 is a better option and more flexible to use than say a standard TC engine.
Beats me why Cirrus have gone to the CMI TC engine lately??? But they did.
Hope that helps!
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on Jan 28, 2013
David, pardon me but I don’t follow your logic. P on the left side of PV=nRT is lower, so something on the right side of the equation has to be lower too–presumably T since n and R are fixed. On one hand you say T of combustion must be roughly the same, then you say it must be higher. But I think if you’re using PV=nRT, you are giving evidence for lower T. Can you elaborate?
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on Feb 09, 2013
The DELTA P is less thus the DELTA T is less too. From one event to another.
Think about preignition, a much greater Delta P, and a lower EGT, but boy the CHT’s go skyward. OUCH!!
on Feb 13, 2013
Maybe I am just new to this but why are there -1 votes to these two posts?
I thought I did a good job at explaining what goes on.
I am not bothered about being popular or scoring votes, but teaching things like this on a bulletin board is not easy.
Whoever is not happy, please ask. I won’t be offended too much
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