Timing vs CHTs with PMAGs

[Toobuilder]

Originally Posted by DanH

Just to play devil's advocate, I'll suggest that best longevity is found well away from any limits, rather than simply not exceeding them.

Remember, the original LOP mantra was running cooler at less peak pressure; with fixed timing, LOP delayed thetaP.

Being speed demons, we've twisted that into "How fast can we go on less gas?"

Sure, but we know that the vast majority of the Lycomings we fly will live long, happy lives at 100% (sea level) power. That's our baseline. Any increase in altitude reduces power, and by the time you are at cruise altitude and LOP, even the most perfectly timed ignition event is not going to boost power to anywhere near the sea level baseline. Therefore, a well timed ignition will allow you to recover power lost to fixed magneto, but you are still well inside the limits of the engine.

[Randy]

Nigel, thanks for posting your Savvy Analysis chart from your test flight. Although your flight profile was very different than mine it is still useful for some comparison. Your chart has some interesting data up at the higher altitudes as I assume you were experimenting with running very lean.

I did as you suggested, plotting FF along with EGTs on one chart and Palt along with CHTs on the other. This ability to put operating parameters on a chart like this is a great tool.

One thing I take away from it is that I have been running much less fuel through my engine than you do. I suspect that if I richen my mixture to what you were running, my CHTs will come down considerably. My practice has generally been to keep it rich enough to achieve CHTs under the 400F mark, but it seems like alot of fuel to run through the engine.

Since you climbed up to about 17,000' there is only a brief period where we were at the same altitude but some comparison can be grabbed as you went through the altitude I leveled off at.

With your Palt at 4500' FF was about 15.5 GPH with CHTs 320 to 347
With my Palt at 4500' FF 13 GPH with CHTs 366 to 390
With my Palt at 4500' and LOP, FF was 8.5 GPH with CHTs 345 to 356

I have generally used LOP operations as kind of a band aid approach to keep my CHTs in check. I want to get to the point where i can actually use all the power available from my engine.

Yesterday I carefully checked that my Pmags were set up properly at Top Dead Center and they were spot on. BTW this is pretty easy to check with the spark plugs out. With the Pmags in setup mode I got cylinder 1 on the compression stroke and gently nugged the prop with my foot while watching both of the pmag led lights. They go from green to red at TDC. I was careful to get all the slack out of the gears etc during the check. I also double checked that I was using the TC mark and not the 20 or 25 deg. marks. All good. With such inexpensive spark plugs to use I went ahead and replaced them, why not and about $2.50 ea.? I had Iridium NGKs in, went with regular BR8ES with solid tops this time.

I have always thought my injection system dumps too much fuel at full rich on take offs. Especially up at my home airport elevation of 4500'. As I push the mixture to full rich I can hear the engine kind of groan and RPMs drop somewhat. I generally see 17 GPH at full rich and maybe this is ok for sea level power, not too sure about that. I need to develop a good technique for leaning before take off at higher elevations like where I live. I know it is simple to do but pushing that lever to the wall is an old habit.

OK so back to my original question or idea. Based on the idea that running a richer mixture has nearly the same effect as retarding the timing, why not go ahead and retard the timing and burn less fuel?

Is there another CHT cooling mechanism at work when going full rich other than slowing down the fuel burn which is same as moving peak pressures later in the cycle, ie retarding timing?

Now that I know for sure that my timing is what the EI Commander thinks it is, I can get on with some more testing. I will head up to 4500' and try 15 plus GPH and I would bet, based on my experience, that my CHTs will be similar to yours.

Then perhaps I will see if I can accomplish the same CHTs at a lower fuel burn with less timing advance.. I welcome comments and would love to see others' Savvy Analysis charts under similar conditions. Funny how it seems like posting Savvy Anaylsis data is like sharing some kind of intimate secret. I say go ahead and share them and we can all learn more.

[nigelspeedy]

Hearing that some low compression engines struggle with high CHT with PMag and then that some high compression engines do just fine made me wonder if all PMag are equal.

There is lots of good info on how to do baffles and how to set the mixture at high power, so lets assume folks are all doing a reasonable job with these items. Could it be that some PMag introduce advance at a higher MAP than others? My thinking being that if more advance was introduced at some high MAP this could result in high CHT. If the advance on another unit was not introduced until a lower MAP and hence lower power CHT might be fine. I am not sure if there is a difference between units but though it might be interesting to investigate.

I did an experiment yesterday flying at 1,000'PA, which is as low as I can reasonably go in my local area. I set the prop at 2,500 RPM and the throttle wide open which gave about 29.1" MAP. I recorded the timing from the EICommander. In steps of about 1" MAP I reduced the throttle (leaving prop and mixture constant) and recorded the corresponding ignition timing/advance. I also noted the % Power calculated by the Dynon EMS.

Before starting I set the advance shift to 0 and the maximum advance to 36.4 degrees using the EICommander. Initially the advance was 28 degrees which I assume is the baseline plus any RPM advance. As the throttle is closed and the MAP reduces the advance is constant until 24.5" MAP where the advance increases up to the maximum of 36.4 degrees at 22.3" MAP. The ramp up of timing happens pretty quick so small throttle inputs were needed to capture each 1.4 degree step of ignition advance change.

While it is interesting to ponder just the ignition side of the equation I think I need to consider the power and likely mixture setting as well. Most literature I have read suggests that you don't want to lean until you are less than ~65% power, which is about 2500 RPM & 23" MAP (which you would get to with WOT at around 7,000'). At low alt and high MAP the mixture will likely be rich and you don't need any advance above ~25 degrees. So it seems to me that my PMag introduction of advance based on MAP is a little early, ~75% power. If the ramp up of advance based on MAP was delayed from 24.5" until 23" this would probably be less stressful on the engine (lower CHT with a bigger detonation margin) with little reduction in power when running ROP. 23" MAP would coincide with ~65% power where one would normally consider lean of peak operations and the increase in advance would be useful.

Anyone with dual PMag and an EICommander care to replicate the experiment and compare results?

[IMG][/IMG]

Cheers
Nige

[lr172]

Originally Posted by Randy

Is there another CHT cooling mechanism at work when going full rich other than slowing down the fuel burn which is same as moving peak pressures later in the cycle, ie retarding timing?

While ignition timing is often discussed as a means to move the EGT/CHT balance, as you elude to above, it is actually the peak pressure point that moves this balance and that is influenced by both the timing of spark and the speed of the fuel burn (air fuel ratio, MAP, etc.). Ross recently posted some good data on this.

In answer to your question, yes. Once you are richer than the stoichiometric (most noticeable after approximately max power), additional fuel (i.e. richer mixture) has a cooling effect on the burning charge (via latent heat absorption) and you will see that translate into lower CHTs, regardless of peak pressure.

Larry

[lr172]

Originally Posted by nigelspeedy

Hearing that some low compression engines struggle with high CHT with PMag and then that some high compression engines do just fine made me wonder if all PMag are equal.

There is lots of good info on how to do baffles and how to set the mixture at high power, so lets assume folks are all doing a reasonable job with these items. Could it be that some PMag introduce advance at a higher MAP than others? My thinking being that if more advance was introduced at some high MAP this could result in high CHT. If the advance on another unit was not introduced until a lower MAP and hence lower power CHT might be fine. I am not sure if there is a difference between units but though it might be interesting to investigate.

I did an experiment yesterday flying at 1,000'PA, which is as low as I can reasonably go in my local area. I set the prop at 2,500 RPM and the throttle wide open which gave about 29.1" MAP. I recorded the timing from the EICommander. In steps of about 1" MAP I reduced the throttle (leaving prop and mixture constant) and recorded the corresponding ignition timing/advance. I also noted the % Power calculated by the Dynon EMS.

Before starting I set the advance shift to 0 and the maximum advance to 36.4 degrees using the EICommander. Initially the advance was 28 degrees which I assume is the baseline plus any RPM advance. As the throttle is closed and the MAP reduces the advance is constant until 24.5" MAP where the advance increases up to the maximum of 36.4 degrees at 22.3" MAP. The ramp up of timing happens pretty quick so small throttle inputs were needed to capture each 1.4 degree step of ignition advance change.

While it is interesting to ponder just the ignition side of the equation I think I need to consider the power and likely mixture setting as well. Most literature I have read suggests that you don't want to lean until you are less than ~65% power, which is about 2500 RPM & 23" MAP (which you would get to with WOT at around 7,000'). At low alt and high MAP the mixture will likely be rich and you don't need any advance above ~25 degrees. So it seems to me that my PMag introduction of advance based on MAP is a little early, ~75% power. If the ramp up of advance based on MAP was delayed from 24.5" until 23" this would probably be less stressful on the engine (lower CHT with a bigger detonation margin) with little reduction in power when running ROP. 23" MAP would coincide with ~65% power where one would normally consider lean of peak operations and the increase in advance would be useful.

Anyone with dual PMag and an EICommander care to replicate the experiment and compare results?

[IMG][/IMG]

Cheers
Nige

Wow, that is a pretty aggressive ramping of the advance. Some food for thought on your theory above. I have a D120 EMS and early in phase 1 I found the MAP bouncing around substantially. The dynamic range was about a full inch or more at times. I had seen something similar when I built an electronic injection system for my Porsche, so I installed a restrictor (~ .040") in the line from the #3 cylinder to the MAP sensor and this gave me very stable readings.

I am guessing the Pmag is doing some smoothing on it's MAP sensors input. However, if they ramp 10 degrees over 1" of MAP and your MAP is dynamically oscillating over that 1" MAP range, I could see how there will be variability across installations. Based upon your numbers above, they are advancing 1* per .1" of MAP. I think they would struggle to get that kind of resolution on the MAP sensor unless they built a restrictor into their sensor.

Just food for thought.

Larry

[McPilot]

Hello everyone..

I have been following this thread from the start.

There is a design flaw in the baffling/cylinder fin profile on many #2 cylinders.

The baffling fits closely to the cylinder and gas a "dam" in from of the fins. The fins are not completely open mid cylinder as well. The causes no airflwo to teh bottom of the #2 cylinder and also effects flow to the #4 cylinder.

I placed a washer in between my #2 cylinder and the baffling. This was after installing 2 PMAGS and noticing even higher CHTs than with my slick mags ~380 deg on #2 CHT in cruise.

Placing this washer has allowed my CHTS to run in the ~340-~360 deg range now.

Check your installation and see if yours is the same...

[FasGlas]

The aggressive advance on a pmag is a problem for some, not all. Moving the shift sometimes helps but not all. Adding the restrictor has made no difference on the high CHT temps, it does help to smooth the fast moving advance.


This is the curve of an ElectroAir, much smoother and less aggressive. When I ran an ElectroAir I did not see the higher CHT's. At higher altitude cruise the ElectroAir would advance as much as 40 degrees.

[AX-O]

Just to make sure, is the "0" on the Y axis equivalent to 25 deg timing?

[FasGlas]

Originally Posted by AX-O

Just to make sure, is the "0" on the Y axis equivalent to 25 deg timing?

This represents the MAP curve. The ElectroAir is normally set to 18 degrees advance at around 1000 RPM. So not taking into account for RPM advance this would be 18 degrees plus MAP advance, "0" being at least 18 degrees.

[nigelspeedy]

I too put a 0.040" restrictor in the MAP line prior to the EMS sensor and the PMags.

Does the reduced heat from running rich really come from the latent heat of vaporization? Lets say peak EGT is at 10gal/hr and I run rich at 13 gal/hr, so 3 gal/hr is just being evaporated.
Gasoline has about 19,000 BTU/lb being burnt and only 150 BTU/lb evaporating.

10 gal/hr * 6lb/gal * 19,000 BTU/lb = 1,140,000 combustion BTU
3 gal/hr * 6 lb/gal * 150 BTU/lb = 2700 evaporation BTU or only 0.2% of the energy of combustion. I cant imagine that mechanism changes the CHT very much.

I see that the PMag MAP advance is about 4 degrees/ inch of MAP which is about twice what the Electroair example is. Certainly small errors in measuring MAP would have a big effect on timing with that ramp up.

Cheers

Nige