What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

EGT during Lean of Peak operations

mchargmg

Well Known Member
Hi all,

I fly an RV-6A with an IO-360 (180 HP) and CS prop out of KLFY Colorado. I routinely run Lean of Peak (LOP) when just cruising around. Typical cruise numbers up at our altitude are:
WOT
MAP: 21"
RPM: 2350
Fuel flow ROP: 9.5 gal/hr
Fuel flow LOP: ~7 gal/hr
Cylinder head temps:
ROP: 370
LOP: 330
Exhaust Gas Temps:
Peak: 1420-1450
LOP: Peak -30
The engine has about 580 hours on it, all compressions in mid-high 70s.

So my issue is this. Over the last couple of flights, I have noticed that when I lean back for LOP operations, my EGT will come down as normal, and then suddenly number 2 cylinder EGT will start fluctuating and then wind up higher than normal by 50 degrees or so. The cylinder head temps are all normal, and the engine "sounds fine." No noticeable decrease in performance or roughness.

I would chalk this up to a flakey sensor, but it only happens when LOP. When I am ROP all the EGTs are close to each other (typically within maybe 20 degrees or so). This is very repeatable. Last flight I think I went from ROP to LOP 3 times, with the same result each time.

So my question is: what is my engine telling me? The only thing I can think of that we have changed were the spark plugs during the last conditional. When I think of a higher EGT I think of running lean. Could this be a sign that that the injector on that cylinder is either partially blocked, or somehow otherwise compromised? Any suggestions are appreciated.

Blue skies

Geoff
 
Assuming your EGT indication is correct, then from what you said:
- This does not indicate a reduced fuel flow as you are already LOP. So I would not be looking at a clogged injector.
- Could more fuel be going to that cylinder? Perhaps but not likely. As the cylinder does not do this ROP the extra fuel could be masked by EGTs at that point not being a sensitive to fuel flow as LOP - but still not likely.
- Air leak? Not likely as an air leak would cause EGT to go down at LOP, up at ROP.

So what does that leave? Ignition.

Just like EGTs go up when you do a mag check, I would assume a plug misfire at LOP will do the same. I’d start by pulling the plugs on that cylinder, cleaning and checking internal resistance to make sure they are good. Perhaps one of the new plugs you put in has a high internal resistance. Such things happen.

Carl
 
Carl's logic is spot-on. A weak spark or plug will cause EGT to go higher.

Savvy has its customers do a LOP mag check at 65% power and 50° LOP. Run on each mag for at least a minute, longer if your sample rate is more than a 1 second interval. Then upload the flight, zoom in on the mag check area and click the M button on the right.
 
This is not a clogged injector when LOP, because you would also see higher CHT on that cylinder if ROP.

The likely fault is one of the spark plugs on that cylinder is poor. Under the lean mixture condition, any weakness in the ignition system will show up, and a plug that fires better some times than others with a lean mixture exhibits this symptom.

When running rich mixtures, a weak plug may still be able to get the combustion event going because rich mixtures are easier to light off.

There is a slight chance you have a faulty probe that only plays up when a higher temp is reached. But I would go for the plugs first. Do an inflight mag check and you will know exactly which one.

If you are using massive electrode plugs and they have not been gapped in a while (do them every 50hrs) this may be all your drama is.
 
Thanks for the help

I will go check that spark plug this weekend. Not sure I have ever done an inflight mag check.

Thanks again!

Geoff
 
Roughly speaking with 2 spark plugs, each burns half the chamber.

With only 1 spark plug firing in a cylinder it takes longer for the mixture to completely burn in that cylinder. (It burns it's half of the chamber, then it has to burn the other half)

This would be the same as having less ignition advance.

With less ignition advance, the mixture can still be burning as it's exiting the exhaust valve giving you a higher EGT.
 
Roughly speaking with 2 spark plugs, each burns half the chamber.

With only 1 spark plug firing in a cylinder it takes longer for the mixture to completely burn in that cylinder. (It burns it's half of the chamber, then it has to burn the other half)

This would be the same as having less ignition advance.

With less ignition advance, the mixture can still be burning as it's exiting the exhaust valve giving you a higher EGT.

Not at all true.

The burn is a swirling mass, with two plugs firing it is kicking off a more effective flame propagation across a large diameter cylinder. Removing one spark event does mean it takes a bit longer to complete the burn.

The slower burn means the volume of the cylinder is larger (piston if further on the way down) when the peak pressure is obtained. This means the peak pressure is of a lower magnitude.

When the valve opens (long after the combustion is complete), the gases expand to atmosphere, and when they expand from a lower pressure to atmosphere they cool less. Thus a higher EGT.

Boyles Law...... PV=nRT
 
The slower burn means the volume of the cylinder is larger (piston if further on the way down) when the peak pressure is obtained. This means the peak pressure is of a lower magnitude.

When the valve opens (long after the combustion is complete), the gases expand to atmosphere, and when they expand from a lower pressure to atmosphere they cool less. Thus a higher EGT.

Boyles Law...... PV=nRT

Sorry, no. Lower peak pressure isn't relevant. The only pressure of interest here is the pressure present at valve opening. Given later peak pressure, pressure at valve opening is higher, not lower.

Pressure and temperature are proportional when temperature is expressed in degrees Kelvin.

Illustration here:

https://vansairforce.net/community/showpost.php?p=1422901&postcount=5
 
Sorry, no. Lower peak pressure isn't relevant. The only pressure of interest here is the pressure present at valve opening. Given later peak pressure, pressure at valve opening is higher, not lower.

Pressure and temperature are proportional when temperature is expressed in degrees Kelvin.

Illustration here:

https://vansairforce.net/community/showpost.php?p=1422901&postcount=5

Dan,

Sorry, no. Peak pressure is very relevant.

You are missing a very valuable point. Let me explain below, and hopefully this is not detracting from my earlier simplified post which explains perfectly for the majority of readers.

Your red herring about degrees Kelvin (while true for calculation purposes) does nothing in terms of helping people understand a concept. That is not relevant to the concept.


THINK EXPANSION RATIO ! ! !

Definition: EVO = crank angle at exhaust valve opening.


Consider PP1 = 65 BAR vs PP2 = 50 BAR.

Assume that the pressure in the combustion chamber 1 degree before EVO is 10 BAR (roughly that by measurement on the dyno in Ada.).

Assuming the same starting temperature for the bulk gases - -

Then the Temperature of a defined number of moles of gas at theta(EVO) is going to depend (inversely) upon the ratio of P1/P(EVO) or P2/P(EVO).

If that expansion ratio is 65:10 then the bulk gas temperature will be lower than if the expansion ratio is 50:10.

Therefore, the statement: “Lower peak pressure isn't relevant” is not accurate.
(candidly this is an over simplification. One can argue that the pressure at EVO will be lower for a lower PPressure, which is true. But it illustrates the point that Peak Pressure does matter)

I think what you are “missing” is the critical concept of Expansion Ratio. It is NOT the compression ratio that is important. It is the EXPANSION RATIO from:

tPP => theta(EVO).

What you are not focusing on is the change in VOLUME of the combustion chamber from the crank angle at different thetaPPs to the constant crank angle at EVO.

That change in VOLUME gives you the EXPANSION RATIO. That is the dominant consideration in evaluating the temperature of the bulk combustion gases at EVO

HIGHER EXPANSION RATIO => LOWER TEMPERTURE.

I have never seen this explained in a text book. Not sure why.

Your graph clearly demonstrates the concept of expansion ratio.
 
I'm not sure that I understand what you are trying to say... but one thing that is missing is the work done turning the prop.

When you burn the fuel you get a certain amount of energy. That can end up as:
- work turning the prop
- heat in the cylinder head/barrel/piston
- heat in the exhaust gas

It takes energy to turn the propeller. Extracting that energy MUST cool the exhaust gas (particularly since it also results in additional heat lost into the head/barrel/piston).

The work done turning the prop is defined as force (pressure) x distance (stroke). The higher the pressure at the start of the power stroke, the more energy is transferred to the propeller, and as a result the exhaust gas MUST be cooler.

Boyles law requires that energy is not lost from the gas. An engine is designed to extract energy from the gas.
 
My M1B acts this way and with no particular cylinder.

Yes - misfire on one plug.

Yes - misfire on one plug effectively retards timing, longer burn

Yes - retarded timing yields longer burn and lower volumetric expansion ratio from firing (angle) to exhaust valve opening (angle). The exhaust valve always opens at the same angle. Therefore: Expansion (work) performed by the combustion charge is less=> less work in expansion, higher temperature at exhaust valve opening. Basic thermodynamics.

OK - All true but if all jugs were working the same, (A/F, etc) then the misfire would be random and most EGTs would go back up together. You may still likely have one cylinder that is different for fuel and/or spark (plug). Either could initiate the one plug/one cylinder misfire scenario.

Added: To check to see of this is possibly fuel related, slowly note the EGT peak vs fuel flow and see if #2 peaks first, that could mean a tiny bit off to consistently initiate lean misfire in this cylinder first. Running at peak EGT and doing a mag test (assuming you have two mags and they were overhauled at 500 hr) may tell you if there is a single plug (system) firing issue.

I spent many hours of flight and data analysis to support the conclusions. For my all stock M1B, the LOP range improves as RPM drops.
 
Last edited:
Yes David, expansion explains why, given later peak pressure, temperature is higher at exhaust valve opening.

Pressure and temperature are proportional. Given the same cylinder volume, higher temperature means higher pressure.

But that's not what you wrote.

When the valve opens (long after the combustion is complete), the gases expand to atmosphere, and when they expand from a lower pressure to atmosphere they cool less. Thus a higher EGT.

and

One can argue that the pressure at EVO will be lower for a lower PPressure, which is true.

Here's the linked PV diagram, from a respected textbook. Note higher pressure at exhaust valve opening with delayed peak pressure. Indicated EGT can be viewed as proportional to either cylinder pressure or in-cylinder temperature at exhaust valve opening.

Spark%20Advance%20PV%20Diagram.jpg
 
Back
Top