Can AFR's be optimized for a particular tune, but EGT's be, at the same time, extremely high? Or, just because my AFR's are perfectly fine, can I assume(w/o the benefit of a pyrometer), that my EGT's are at a safe level?

 

if you haven't done anything really far out there as far as timing goes, your EGT's should be fine if you have good AFR's. If you get your timing way off, all bet's are off.

 

During normal combustion, there is an intrinsic relationship between EGT and AFR. In fact, EGT was used to tune AFR for many years. And, in many small airplanes, AFR is still manually controlled using an EGT gauge today.

However, there are two distinct limitations to EGT. #1, The only AFR point EGT gauges can find with any real precision is stoichiometric (nominally 14.7:1). This is where EGT peaks. Any adjustments off this point are basically guesses. #2, For AFR purposes, EGT is a relative measurement. Again, the only point that can be found with any precision is stoich, but we don't find stoich at a particular temp, we find it where the exhaust temp peaks.

The absolute temperature of this peak will vary. Again, you can prove this easily in a small, normally aspirated plane.

So, when things are working normally, the absolute value of EGT is not as important as the relative changes in EGT. At 14.7:1, EGT should be at its highest, and get cooler at any other AFR (the dropoff in temperature is pretty steep on the lean side of stoich, and a bit more gradual slope on the rich side of stoich).

CHTs follow a similiar curve, but are shifted. Max CHT occurs just rich of stoich, not at stoich itself.

The time to get concerned is when EGT behavior appears to deviate from this (ex. you enrichen, and EGT goes up). Because this is often a sign that you are in areas of abnormal combustion (misses, detonation, etc.) or have conditions so out of norm that instrumentation errors are occuring. Although lambda (wideband) meters eliminate both the shortcoming of EGT measurement of AFR, they have their quirks, and the quirks vary based on the controller used.

So, if EGT appears to move in the wrong direction or soar at the wrong point. I wouldn't just dismiss it based on a wideband measurement. There are a number of cases were an EGT probe can be 'right' and a current based lambda meter 'wrong'.

On the other hand, if my EGT showed the same basic response curve as my buddy's car, just higher absolute temps, I generally wouldn't worry about it.

-jjf

 

Egt does not correcate with AFR directly. You can have different egt (higher and lower) with the same AFR, depending on timing,boost, etc.

egts are only usefull as a reference. i.e once you have a basline condition (tune,afr,timing etc) you record that egt. Then when you make changes you can watch which way the egt goes to verify.

Another thing egts are good for is telling what the temp before the turbo is. If its too high then turbine damage can occur.

 

For a given configuration, EGT will always follow the peak = stoich rule. However, absolute EGT (just how high EGT is in absolute terms) does depend a lot on things like boost and timing.

For example, more boost means more overall heat and pressure. More total heat means higher EGTs.

Similiarly, timing changes alter the overall distribution of heat. In simplest terms, combustion heat goes three places: Mechanical work, CHT, and EGT. When you advance and retard timing the opportunity for work and/or heating cyl heads changes. So what is left over - EGT, can change dramatically as well.

-jjf

 

I believe that peak adiabatic flame temp is at lambda = 1.05, but that's pretty close to stoich

 

Yes, calorific value for burning fuel is something like 1 - b / (1.05 * a). But peak flame temp does not equate to peak EGT. In fact, 1.05 ends up being in best economy (say 70 degrees LOP). Nor does peak flame temp equate to peak CHT, that ends up being just rich of peak EGT.

-jjf

 

Hmm.... always learning something new, good to know

 

The key is to look at the temps. The flame is waaaay hotter than what you typically see in absolute EGTs. EGT is just one of the places that heat/energy goes.

You were absolutely right about the flame, but EGT isn't a direct measurement of the flame, but what is left over after useful work and heating the cyl heads themselves. One of the biggest hurdles I had was to stop thinking of our engines as 'steam engines'. Energy wise, they are a lot more complicated.

If it is an area of interest to you, drop me a PM and I'll forward you some papers and articles.

-jjf

 

EGTs are more directly affected by timing.

 

ABSOLUTE EGTS reflect timing (see above), because timing directly change how much heat is used where.

RELATIVE EGTS (relative to peak for a given timing) follow AFR.

-jjf

 

very good info in here.

i think that egt is best used as a way to refenence eng mods and watch what the eng isdoing.

 

I have EGT and AFR in my Z and I have found the EGT gauges ( after a good tune ) are helpful to determine

1) sudden changes in your system ( manifold leak due to cracking ), downpipe leak due to cracking

2) state of ignition timing tune

3) boost limits ( push the turbo past its efficiency and everything starts to get " bubbly"

4) Also, tuning usually isn't done when the motor is heat soaked. So EGT's are helpful to see the state of the engine on Long WOT 4rth / 5th gear runs. It can be a basis on when to shut down

5) If you use water injection, you can see the effect on EGT's with and without water.