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Interesting EGT.....

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
I've been doing some work on teh Valkyrie, and had a funny signature a few minutes after take-off the other day. The airplane runs dual P-Mags and auto plugs - very smooth, easy to get LOP, even though it is carbed. I still have what I consider to be the best display of EGT's on any of the EFISes - GRT's plots of EGT versus time (I used to love reading strip chart recorders too .... dating myself!).

So shortly after take-off, I leaned to peak, and as I did this, all four EGT's came together nicely, as they always do. LOP, all was smooth and CHT's started coming down. Then all of a sudden, a few misses - a touch of roughness. I looked down, and EGT #2 had jumped up and out of the pack by almost100 degrees! I had to riches things up to smooth it out, and that meant I was now ROP, and had to go a long ways rich to bring the CHT's down. Teh errant EGT rejoined the pack, but not completely.


I had removesd intake tubes as part of my maintenance work, so immediately suspected a leak - but why it would start inflight was a mystery. The mystery cleared when I took the cowl off and looked at cylinder #2 - the lower plug wire had popped off! The P-Mag automotive plug wires have to be "burped" to make sure you get the air out, and that the metal cap has clicked on to the plug.

Easy to fix - and the EFIS made it easy to know where to look!
 
Ironflight said:
I've been doing some work on teh Valkyrie, and had a funny signature a few minutes after take-off the other day. The airplane runs dual P-Mags and auto plugs - very smooth, easy to get LOP, even though it is carbed. I still have what I consider to be the best display of EGT's on any of the EFISes - GRT's plots of EGT versus time (I used to love reading strip chart recorders too .... dating myself!).

So shortly after take-off, I leaned to peak, and as I did this, all four EGT's came together nicely, as they always do. LOP, all was smooth and CHT's started coming down. Then all of a sudden, a few misses - a touch of roughness. I looked down, and EGT #2 had jumped up and out of the pack by almost100 degrees! I had to riches things up to smooth it out, and that meant I was now ROP, and had to go a long ways rich to bring the CHT's down. Teh errant EGT rejoined the pack, but not completely.


I had removesd intake tubes as part of my maintenance work, so immediately suspected a leak - but why it would start inflight was a mystery. The mystery cleared when I took the cowl off and looked at cylinder #2 - the lower plug wire had popped off! The P-Mag automotive plug wires have to be "burped" to make sure you get the air out, and that the metal cap has clicked on to the plug.

Easy to fix - and the EFIS made it easy to know where to look!
Click to expand...

Paul, I know you've talked about burping the plug caps before. I think you should have a serious talk with your mechanic :).
 
As I read your story, the very first thing that came to mind was that you lost a plug.

If you really want an eye-opener, do a mag check at altitude while 50 LOP. Even with all the spark advance of the EI, one plug just can't get everything burned.

Unlikely an induction leak would go to higher EGT if you are LOP - it would go leaner still and get cold. That would explain a miss though. So the EGT trend on the strip chart is the key discriminator.

I'd like to see you change a paper roll in flight, though, or what you do with all that chart paper piling up in the floor of the -8. I suppose you could have a take-up roll on the outfeed. ;)
 
scard said:
Paul, I know you've talked about burping the plug caps before. I think you should have a serious talk with your mechanic :).
Click to expand...

Yup - spent a little quality time after this with a set of long needle-nose pliers persuading the contacts to grip the plug tightly.
 
I wonder if there might be a spring clamp just the right size to fit over the boot?

Like those spring clamps you see on modern cars for hose clamps. It would need to have just the right size/clamp pressure to provide more positive retainment, but not chafe/cut the boot. And the clamp ears would have to be short enough not to get too close to the cylinder head fins, etc.
 
I got a good suggestion off--line from a long-time experimenter who uses just a touch of Dow #4 on the inside of the boot - slides right on, and makes for a nice click without any doubt. Great suggestion. I've ben using the P-mags and their plug wires for six years now - never had one come off in flight - so not a big deal. Just annoying!

Paul
 
If you pull off the rubber boot and look at the metal cap that makes the contact with the plug you can see a small circular clip that's job is to provide the tight fitting, snap-into-place connection. I have had two of those little clips break at different times, resulting in intermittent connection or no connection at all. Just another thing to be aware of and check. After the second one I replaced all the leads.

Erich
 
Ironflight said:
I got a good suggestion off--line from a long-time experimenter who uses just a touch of Dow #4 on the inside of the boot - slides right on, and makes for a nice click without any doubt. Great suggestion. I've ben using the P-mags and their plug wires for six years now - never had one come off in flight - so not a big deal. Just annoying!

Paul
Click to expand...

Paul, I might have been a bit hesitant to use an "insulating compound" such as Dow 4 on plug connectors but if you go to this technical blurb from Dow Corning you can see that it is in fact recommended for spark plug connectors.
http://www.dowcorning.com/content/news/molykote_news_4_Compound.asp
 
If you use the right amount of dielectric grease (which I think is what Dow #4 is) and you get the clicker clicked and you burp the cap, these things are almost hard to remove. I can't imagine they would come off in flight if all 3 are true.
 
Paul, did you try to lean more aggressively at all before going rich?

It obviously wouldn't have helped in this case, but the reason I ask is because I've had plugs foul in flight before, with the same rise in EGT and roughness. A couple of cycles of aggressive leaning has cleaned the plug and brought the errant EGT back in line.

I don't remember specifically what the CHT was doing, but the engines that it happened to me on run quite cool CHT's, so I was never concerned about them getting dangerously hot (both are Franklins, a 150 and a 220).

The EGT rise is due to the ignition timing effectively getting retarded by the one plug not firing, causing the mixture to still be burning as it's going out of the exhaust (imagine the flame front starting at each plug and meeting in the center of the cylinder for a properly timed combustion event, then imagine taking a plug out of the equation- the flame front now has to travel twice as far). I would expect the CHT's to lower a bit in the event of a fouled or not firing plug since the entire combustion event (and therefore the heat) is not taking place inside the cylinder....
 
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1:1 Scale said:
Paul, did you try to lean more aggressively at all before going rich?

It obviously wouldn't have helped in this case, but the reason I ask is because I've had plugs foul in flight before, with the same rise in EGT and roughness. A couple of cycles of aggressive leaning has cleaned the plug and brought the errant EGT back in line.

I don't remember specifically what the CHT was doing, but the engines that it happened to me on run quite cool CHT's, so I was never concerned about them getting dangerously hot (both are Franklins, a 150 and a 220).

The EGT rise is due to the ignition timing effectively getting retarded by the one plug not firing, causing the mixture to still be burning as it's going out of the exhaust (imagine the flame front starting at each plug and meeting in the center of the cylinder for a properly timed combustion event, then imagine taking a plug out of the equation- the flame front now has to travel twice as far). I would expect the CHT's to lower a bit in the event of a fouled or not firing plug since the entire combustion event (and therefore the heat) is not taking place inside the cylinder....
Click to expand...

I usually lean so aggressively that there isn't much more "lean" than I can go - but your idea might help someone in teh future.

I also figured that the reason that the EGT rises when one plug stops firing is the fire still goign on when the exhaust valve opens. Makes sense when you visualize it, but a serious combustion engineer woudl have to confirm it with science....;)
 
Ironflight said:
I usually lean so aggressively that there isn't much more "lean" than I can go - but your idea might help someone in teh future.

I also figured that the reason that the EGT rises when one plug stops firing is the fire still goign on when the exhaust valve opens. Makes sense when you visualize it, but a serious combustion engineer woudl have to confirm it with science....;)
Click to expand...

It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke. Most engines only use one plug. EGT is going up due to the retarded timing associated with firing only one plug vs two (your "effective timing" is probably retarding 10 - 15*). Timing is not really relevant here on the exhaust theory. The flame speed is SO much faster than the piston speed, it is not possible to ignite it late enough for it to go out the exhaust port. If your WAY late, and the piston goes down, you lose the compression and then a spark cannot ignite it. The fuel air mixture in an uncompressed state cannot be ignited by a spark plug.

Larry
 
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lr172 said:
It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke. Most engines only use one plug. EGT is going up due to the retarded timing associated with firing only one plug vs two (your "effective timing" is probably retarding 10 - 15*). Timing is not really relevant here on the exhaust theory. The flame speed is SO much faster than the piston speed, it is not possible to ignite it late enough for it to go out the exhaust port. If your WAY late, and the piston goes down, you lose the compression and then a spark cannot ignite it. The fuel air mixture in an uncompressed state cannot be ignited by a spark plug.

Larry
Click to expand...

Well, not "still burning" but lets say, less time since it stopped burning. Engines with only one spark plug usually have it closer to the middle of the cylinder head. Flame front is much slower for very lean mixture, which is why it takes so much spark advance to get it all burned efficiently. Loosing a spark means only one flame front, with a lot farther to travel to get it all burned.
 
scsmith said:
Well, not "still burning" but lets say, less time since it stopped burning. Engines with only one spark plug usually have it closer to the middle of the cylinder head. Flame front is much slower for very lean mixture, which is why it takes so much spark advance to get it all burned efficiently. Loosing a spark means only one flame front, with a lot farther to travel to get it all burned.
Click to expand...

Most engines light the mixture around 35* before TDC and we hit peak power around 10* after TDC. I expect that flame has extinguished by 25-30* after TDC, though the gas is still hot and expanding.

The lycoming, at 25* timing, is getting about the same peak pressure point with dual spark. With only one plug, that moves to around 20* and the flame would extinguish by 40* after TDC.

The exhaust valve doesn't open until around 190* afterd TDC.

Larry
 
lr172 said:
It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke.

Larry
Click to expand...

That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.
 
Noob question.

I now know what "Dow #4" is.

What does "burp the plug wire" mean?

Thanks.
 
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RV6_flyer said:
That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.
Click to expand...

Typical complete combustion (90 to 100% mass fraction burned) for a relatively non-turbulent four stroke (read "Lycoming") would be about 40 ATDC, well before exhaust valve opening. The delay due to single plug ignition is unlikely to be an additional 60 to 80 degrees.
 
DanH said:
Typical complete combustion (90 to 100% mass fraction burned) for a relatively non-turbulent four stroke (read "Lycoming") would be about 40 ATDC, well before exhaust valve opening. The delay due to single plug ignition is unlikely to be an additional 60 to 80 degrees.
Click to expand...

I agree. But the delay IS why the EGT is higher was my point.
 
RV6_flyer said:
That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.
Click to expand...

If this were the case, you would be dumping unburnt fuel in the exhaust and the next combustion cycle would ignite it, creating after-fires (i.e. pops in your exhaust). This doesn't happen when running on one mag, proving that the charge is fully combusting.

Larry
 
lr172 said:
If this were the case, you would be dumping unburnt fuel in the exhaust and the next combustion cycle would ignite it, creating after-fires (i.e. pops in your exhaust). This doesn't happen when running on one mag, proving that the charge is fully combusting.

Larry
Click to expand...

Larry, it kind of does, if you are at altitude, and way LOP, and you shut off a mag. It really pops and caughs and bangs. How much is mis-fires and how much is after-fires, I don't know.
 
scsmith said:
Larry, it kind of does, if you are at altitude, and way LOP, and you shut off a mag. It really pops and caughs and bangs. How much is mis-fires and how much is after-fires, I don't know.
Click to expand...

but it doesn't do that on the ground during a mag check. The pops and coughs can be lots of different things. I drop out a mag in the air all of the time. I have never heard an afterfire, disproving the cyl too big theory. If I drop my mag and leave the EI running, you can't even tell that I did it. This is because the timing on the EI is much more advanced and keeps the peak pressure point closer to optimum than with only the mag.

popping and coughing when very lean of peak is altogther a different animal and is not related to this theory that the cylinder is too big for one plug. The leaner you go, the harder it is to light the charge and ignition weakness become more apparent. Altitude also hurts a mags performance. THis is why they have pressurized mags for high flying planes.
Larry
 
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Guys, let me clear up WHY the EGT goes higher with one plug, or whole magneto is offline.

As DanH has already pointed out combustion is over way before the valve opens, and this is 100% true.

The answer lies in Boyles law, Pv=nRT which means that when you expand a gas from a small volume and high pressure to a lower pressure you get a temperature drop. Think a BBQ gas bottle being opened, it gets frosty!

So with one plug not firing, the peak pressure in the cylinder happens later, and the piston has travelled further down the cylinder, thus in a larger volume. Being in a larger volume the pressure is less.

Given the atmosphere where the exhaust is going remains the same, we have a lesser change in pressure and less expansion. If we balance up the equation the only other variable is T and with less expansion we have to have less temperature drop. Thus the EGT is higher!

So there ya go! Hope that helps. :)
 
RV10inOz said:
Guys, let me clear up WHY the EGT goes higher with one plug, or whole magneto is offline.

As DanH has already pointed out combustion is over way before the valve opens, and this is 100% true.

The answer lies in Boyles law, Pv=nRT which means that when you expand a gas from a small volume and high pressure to a lower pressure you get a temperature drop. Think a BBQ gas bottle being opened, it gets frosty!

So with one plug not firing, the peak pressure in the cylinder happens later, and the piston has travelled further down the cylinder, thus in a larger volume. Being in a larger volume the pressure is less.

Given the atmosphere where the exhaust is going remains the same, we have a lesser change in pressure and less expansion. If we balance up the equation the only other variable is T and with less expansion we have to have less temperature drop. Thus the EGT is higher!

So there ya go! Hope that helps. :)
Click to expand...

Certainly a function of basic gas law. But...

This is a p-V (pressure -volume) diagram for a research engine at 1200 RPM, borrowed from Taylor's Internal Combustion... The X-axis is cylinder volume, with TDC on the left. The Y-axis is cylinder pressure. I've added some notes; the only variable is ignition timing.

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




Note that when ignition is delayed, peak pressure is indeed reduced, as David notes. However, pressure later in the stroke, around exhaust valve opening, is higher. Cylinder volume is the same at exhaust valve opening for any ignition timing, so higher pressure simply means higher gas temperature.

Pressure and temperature are proportional when temperature is expressed in degrees Kelvin. Play with the numbers. In the first post, Paul wrote that his errant EGT jumped almost 100 degrees. Assume the jump was 1400F to 1500F, or from 1033K to 1083K. The required pressure increase at valve opening is only about 5%.
 
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Great post, last week I had the exact same problem Paul described only I did not read this thread before attempting to correct the problem. I spent yesterday cleaning all the injectors, seemed likely to me that the cylinder was getting too lean so there must being something in the injector. Today after the dinner and everyone down for a nap I went out back to the hangar and rolled the airplane out for an engine run. So I started the engine and after it decided to stop shaking from purging all the fuel system I let it warm up. This engine is a 100 SNEW IO360. CHTs have always been very good, EGTs always great with low spreads. After warm up at idle CHTs normal and all very tightly grouped. EGTs spread out but consistent with low idle warmup. Unfortunately after bringing the RPMs up to 1600 I was able to duplicate the problem again. EGTs normal with reasonable grouping but when I leaned it up number 1 took off again, adding mixture would bring it back down. CHTs remained good and tightly grouped at all times. After reading this thread starting to think I should change the plugs on that cylinder.
 
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