B

Bavafa

Well Known Member
If border-line stupid questions are allowed, I have a few…

I thought I understood the concept of why EGT rises when we lean, but now I have doubts in my understanding after doing a mag check in their air. I had never paid attention to my EGT during mag check till recently when I read about it and its usefulness to do it in the air when leaned. With that, I would appreciate any inside as why

1 - We see a rise in EGT when leaning
2 - When we run only on one mag
and in a healthy running engine,
3 - how much should we expect to rise from our base line when switching to one mag.
4 - what is the proper mag check in the air with respect to EGT and all

As for a reference, I usually run my high-test EGT around 1400 and all my CHTs are in 345-350 range. When I did a mag check in the air, I really didn't notice much engine degradation and I wasn't expecting any RPM change since I have a CS. But my EGT raised quickly to around 1450 and higher at which point I did not want to see any higher and switched back to both mag. My both L & R behaved very similar.

As always, appreciate all the good tips and useful info.
 
Still burning

As I understand it, with only one plug of the two sparking, the fuel-air mixture is still 'burning' when passing by the egt probe as compared to with both plugs sparking. Therefore we see an increase in egt.

Just my anecdotal knowledge.

Jim
 
Jim's correct.

With only one mag running, there is leftover fuel that burns in the exhaust, raising the EGT's.

Why does EGT rise during leaning?

Well in the South, we have a lot of fish fries and if the burner is shooting a red flame, we know that it needs more air, so we open the damper until the flame turns blue:)...something very similar happens in your exhaust. If you could see the flame (like I could in my AT-6 at night), you'd see it progress from red to blue as you leaned, with a corresponding increase in EGT's.

Gotta go cook fish:D
 
That was my understanding but the part that has confused me is, by using that logic, I would have expected that a rich engine has excessive unburned fuel which should burn in the exhaust and therefore rise the EGT. But it does not work that way, does it?
 
Nope, it doesn't work that way.

If you've ever welded with oxy/acetylene, or watched it being done, you'd see the flame get bluer with the red disappearing as the welder turned up the oxygen, which is the same as leaning, in reality, and the flame getting hotter.

Best,
 
Well I was going to throw in my 2 cents, then I was thinking about heat and detonation. My understanding of this is.... under normal operation and leaning to 14.7 to 1, the protective layer of A/F is protecting the combustion chamber parts from actual flame front contact ( a few microns thick ) then once detonation occurs, it blows away this layer and the flames come in contact with the piston, and melting, So Then if we are rich there is molecular weight of the unburned fuel that can actually carry and absorb some of this heat with it out the exhaust for a cooler EGT. But I was asking my self if Detonation is the collision of two flame fronts colliding, is two opposing spark plugs causing some kind of controlled detonation? Or is detonation a rapid uncontrolled expansion of the flame front? OK Great, now I'm confused.
 
fehdxl said:
As I understand it, with only one plug of the two sparking, the fuel-air mixture is still 'burning' when passing by the egt probe as compared to with both plugs sparking. Therefore we see an increase in egt.

Just my anecdotal knowledge.

Jim
Click to expand...

It's not unburnt fuel, you're just burning the fuel less efficiently in the cylinder, which results in less energy being transferred to the crank, and more energy going out the exhaust. At the same time, you're actually reducing in cylinder temperature.
 
EGT is highest at the optimal fuel:air mixture. In a rich situation, there is excess unburned fuel, which can expand in the exhaust and, because expanding gases lose heat, it cools the exhaust stack. It DOES NOT burn in the exhaust, because there is no oxygen present to allow it to burn. It ultimately may burn once out the tail end of the exhaust.

In a lean situation, there is excess air in the system. All of the fuel burns (ideally), but because there is less fuel present, the temperature is not as high as it would be with the optimal mixture. The excess air simply passes through the system.

With regard to the single-plug operation, Jim has it right. Since the system is designed to utilize two plugs/two flame fronts, if only one plug is firing, then some of the fuel-air mixture will still be burning as it moves into the exhaust system. Thus one would see a rise in EGT in this situation, compared to both plugs operating.

greg
 
You must log in or register to reply here.