Slight engine egt/cht weirdness when leaning

[Ragsf15e]

IO-360A1A:  I fly regularly and use both LOP and Rop depending on the circumstances.  I’m usually between 7-12000’, so 60-75% power.  I have gamis and the spread is very tight - like 0.1-0.3gph.  I also know the typical cht order when leaned for my engine (1,4 cool, 2,3 warmer with 3 warmest). Left mag is SF with advance, right is 300hr bendix.  Tempest fine wires all around. Power set to wide open, Ram air on, 2500rpm.

So flying home from KFHR yesterday at 9,500 (~11,000 DA), i lean to peak… except #2 won’t peak.  It peaks way late.  Like 0.5 gph late.  Noticeably later than usual.  I set the engine to just barely LOP, like #2 is pretty much at peak and everything settles except 2 has the same cht as 3 and it’s egt is much higher, like it’s at peak and 3 is more lop.  Cruised for a while, tried again, same result.  Of course, I was crossing the Cascades with my family on board.  Maybe 10 minutes later, with zero input from me, I see egt on 2 slowly decline to about where it should be and then cht slowly follows to be a normal amount less than #3.  Ff stays exactly the same.

 I didn’t do an inflight mag check but i will next time.

what can make a cylinder peak later than normal and the cht and egt read high?  It seems that the cylinder would have to be richer than normal?

[PT20J]

The servo meters fuel to the flow divider and the divider is supposed to send equal amounts to the cylinders. The restrictors in the nozzles are what control the flow division. Usually what happens is that one nozzle gets plugged and then all the fuel goes to the other four making them rich. But, if somehow the flow were slightly restricted to three nozzles, the fourth would run rich. I'd probably just clean the plugs and the nozzles and see if it ever repeats. 

[Ragsf15e]

26 minutes ago, PT20J said:

The servo meters fuel to the flow divider and the divider is supposed to send equal amounts to the cylinders. The restrictors in the nozzles are what control the flow division. Usually what happens is that one nozzle gets plugged and then all the fuel goes to the other four making them rich. But, if somehow the flow were slightly restricted to three nozzles, the fourth would run rich. I'd probably just clean the plugs and the nozzles and see if it ever repeats. 

Yeah, I think that seems reasonable.  I tried to think through how one could be richer than normal and nothing really makes sense?  

[A64Pilot]

14 hours ago, Ragsf15e said:

Yeah, I think that seems reasonable.  I tried to think through how one could be richer than normal and nothing really makes sense?  

Contrary to popular belief nozzles do get dirty, not always the jet fuel flows through though, our nozzles also have an air bleed under the screen you can just see on them

Our nozzles I believe are designed to mix air and fuel together and form a spray, if the air bleed gets a little dirty it can block airflow, that means the fuel won’t be as atomized and it can act as if it’s a richer mixture because combustion won’t be as efficient with a stream of fuel as it is if it’s nicely atomized, fuel burn is slower which drives EGT up and it will be richer as it gets more fuel flow from the divider. I won’t try to explain that I think the RSA manual does though and of course it’s very much more of a reliable source than I am.

This is an excellent description of our fuel system, and I’ll admit that even though I’ve read it several times I still don’t fully have the thing down pat.

P 17 discusses nozzles very well and what I’m talking about is discussed in the middle of the paragraph. A clogged air bleed screen is more noticeable at partial throttle due to the higher pressure differential between ambient and manifold pressure, but ac we aren’t studying EGT’s like we do when we lean got peak it’s not hard to miss it.

https://precisionairmotive.com/wp-content/uploads/2019/06/15-812_b.pdf

I’d cut-n-paste but the Ipad won’t do that with a .pdf?

Personally if I clean one, I clean them all. I use Hoppes and an ultrasonic. I have four of the smallest baby food jar that I put maybe an ounce of Hoppes in each jar and then put them in the cleaner

[PT20J]

51 minutes ago, A64Pilot said:

I’d cut-n-paste but the Ipad won’t do that with a .pdf?

I just take a screen shot.

[mhrivnak]

It would be interesting to try a mag check in-flight if you see that again. I can understand not wanting to try that with family onboard over the mountains.

With ram air on, is it possible something was ingested and partially obstructed the #2 intake?

Also consider that your unexpected measurement is past the exhaust valve, which is a classic source of cylinder issues. It may be worth taking a look at it with a borescope.

[Ragsf15e]

3 hours ago, A64Pilot said:

Contrary to popular belief nozzles do get dirty, not always the jet fuel flows through though, our nozzles also have an air bleed under the screen you can just see on them

Our nozzles I believe are designed to mix air and fuel together and form a spray, if the air bleed gets a little dirty it can block airflow, that means the fuel won’t be as atomized and it can act as if it’s a richer mixture because combustion won’t be as efficient with a stream of fuel as it is if it’s nicely atomized, fuel burn is slower which drives EGT up and it will be richer as it gets more fuel flow from the divider. I won’t try to explain that I think the RSA manual does though and of course it’s very much more of a reliable source than I am.

This is an excellent description of our fuel system, and I’ll admit that even though I’ve read it several times I still don’t fully have the thing down pat.

P 17 discusses nozzles very well and what I’m talking about is discussed in the middle of the paragraph. A clogged air bleed screen is more noticeable at partial throttle due to the higher pressure differential between ambient and manifold pressure, but ac we aren’t studying EGT’s like we do when we lean got peak it’s not hard to miss it.

https://precisionairmotive.com/wp-content/uploads/2019/06/15-812_b.pdf

I’d cut-n-paste but the Ipad won’t do that with a .pdf?

Personally if I clean one, I clean them all. I use Hoppes and an ultrasonic. I have four of the smallest baby food jar that I put maybe an ounce of Hoppes in each jar and then put them in the cleaner

Thanks, that’s a good point about anything clogging the air/breather hole on the injector making it richer.

[Shadrach]

18 hours ago, Ragsf15e said:

Yeah, I think that seems reasonable.  I tried to think through how one could be richer than normal and nothing really makes sense?  

Do you recall if the raw EGT number was elevated on #2?  Your scenario very much looks like an intermittent ignition problem we had on #2.  My partner scrubbed a trip because he could not lean to peak on #2. Pre take off run up went fine. Leaning for cruise there was a noticeable power loss before #2 would peak. Our spread is tight with stock injectors but not as tight as yours.  I flew it in the pattern after a good run up. I noticed #2 EGT was fluctuating and elevated at high power settings. Mag check revealed that the bottom plug on #2 was intermittent. Ran it up to full power on the ground and watched a plug drop out around 2200 RPM.  I believe that in flight the process of leaning for cruise was speeding up the single ignition combustion event on #2 as the dual ignition combustion event on other cylinders leaned past peak and began to lose power.  The net effect was that the single ignition #2 (our richest) was making best power at a FF where the other 3 were very lean of best power.

[Ragsf15e]

1 hour ago, Shadrach said:

Do you recall if the raw EGT number was elevated on #2?  Your scenario very much looks like an intermittent ignition problem we had on #2.  My partner scrubbed a trip because he could not lean to peak on #2. Pre take off run up went fine. Leaning for cruise there was a noticeable power loss before #2 would peak. Our spread is tight with stock injectors but not as tight as yours.  I flew it in the pattern after a good run up. I noticed #2 EGT was fluctuating and elevated at high power settings. Mag check revealed that the bottom plug on #2 was intermittent. Ran it up to full power on the ground and watched a plug drop out around 2200 RPM.  I believe that in flight the process of leaning for cruise was speeding up the single ignition combustion event on #2 as the dual ignition combustion event on other cylinders leaned past peak and began to lose power.  The net effect was that the single ignition #2 (our richest) was making best power at a FF where the other 3 were very lean of best power.

Yes, #2 egt was a little higher than I expected but not much.  I’d have to pull the data off the jpi (duh, I should’ve downloaded it) and see how much though.  What you described sounds plausible, especially if it’s my “mag” plug on that cylinder.  The SF advance kind of masks problems with the other plugs because the engine is (almost) firing just on the SF at higher altitudes because of the advance.  There’s very little egt rise if I switch to left mag at cruise, lop.  The egts go way up and the engine gets rough if I switch to right mag without re-leaning.  The mag is working fine, it’s just the difference in timing.
The thing I didn’t understand was the slight increase in cht.  You guys have definitely given me a place to look.

Edited August 29, 2023 by Ragsf15e

[Shadrach]

4 minutes ago, Ragsf15e said:

Yes, #2 egt was a little higher than I expected but not much.  I’d have to pull the data off the jpi (duh, I should’ve downloaded it) and see how much though.  What you described sounds plausible, especially if it’s my “mag” plug on that cylinder.  The SF advance kind of masks problems with the other plugs because the engine is (almost) firing just on the SF at higher altitudes because of the advance.  There’s very little egt rise if I switch to left mag at cruise, lop.  The egts go way up and the engine gets rough if I switch to right mag.  The mag is working fine, it’s just the difference in timing.
The thing I didn’t understand was the slight increase in cht.  You guys have definitely given me a place to look.

I think an increase in CHT is plausible especially if you're running advanced timing. All the other cylinders are well past peak.  The combustion event on the single ignition cylinder is making peak pressure at a lower fuel flow than than the others because of the single point of ignition propagation. In term of ICP, it would be like having one extra rich cylinder. 

[Ragsf15e]

1 hour ago, Shadrach said:

Do you recall if the raw EGT number was elevated on #2?  Your scenario very much looks like an intermittent ignition problem we had on #2.  My partner scrubbed a trip because he could not lean to peak on #2. Pre take off run up went fine. Leaning for cruise there was a noticeable power loss before #2 would peak. Our spread is tight with stock injectors but not as tight as yours.  I flew it in the pattern after a good run up. I noticed #2 EGT was fluctuating and elevated at high power settings. Mag check revealed that the bottom plug on #2 was intermittent. Ran it up to full power on the ground and watched a plug drop out around 2200 RPM.  I believe that in flight the process of leaning for cruise was speeding up the single ignition combustion event on #2 as the dual ignition combustion event on other cylinders leaned past peak and began to lose power.  The net effect was that the single ignition #2 (our richest) was making best power at a FF where the other 3 were very lean of best power.

I felt pretty similar to what you described… I just couldn’t get #2 to peak before feeling loss of power which isn’t normal.