JP Instruments EDM 700 / EGT Question

[piperboy84]

Trying to get my head around the EDM 700 by reading the manual and faffing around with it when flying. I notice the book says one of the diagnostic/troubleshooting features of this unit is if on take of you notice an exceptionally high EGT reading on one cylinder that would signify a gunked up plug. I would have thought that a fouled plug would have caused the EGT to be lower due to reduced or no spark and ignition and the unburned fuel having a cooling effect.

Anyone want to have a crack at explaining this?

[Mach Jump]

Having one of a pair of spark plugs inoperative will have the same effect as retarding the ignition timing.

This may cause the combustion process to continue after the exhaust valve has opened, heating up the EGT sensor.


MJ:ok:

[piperboy84]

Muchos gracias señor MJ.

[dirkdj]

If you do a magcheck you will see the EGTs rise on the EDM when running on one magneto. 'All Rise' is the standard callout when magchecking. With only a single plug firing, the flamefront travels slower and will be hotter by the time the exhaust valve opens.
EGT is almost instant, CHT takes much more time.

[piperboy84]

And another question on this if I may, when leaning for best power does approximately 100f lower than the "leanest" setting found by the EDM sound about right ? (Leaning for ROP on a naturally aspirated lycoming 4 banger)

[India Four Two]

I've been coming to terms with an EDM 700 in my gliding club's towplane, so I am more interested in avoiding shock cooling, rather than worrying about fuel economy.

However, although the instrument works well, I have to say that the manual is one of the worst I have ever come across and the design of the user interface is not much better.

[n5296s]

+1 for India 42. I like having the instrument but any kind of setting is a nightmare. Why they didn't put couple of extra buttons on it making up/down/enter/select or something is one of life's great mysteries. You can very easily get it into modes where the only sure way out is to pull the breaker. Oh well.

[dirkdj]

Piperboy,

I suppose your 4banger is carburetted? If so the fuel/air distribution between cylinders will not be perfect, each cylinder will not have the same power output.

Being on the rich side of peak EGT this will be less noticeable since the power curve is fairly flat there. At 75% power or higher, you want to be at least 100°F ROP on the first cylinder to peak on the EDM. If you are at 65% or lower, you can't hurt the engine with the mixture knob and I would lean to the onset of roughness. The EDM will tell you if the CHTs remain safe. The EGTs are for 'navigation' on the power curve only, the CHTs tell you how the engine is feeling. EGTs are nearly instantaneous, CHTs take more time to change.

With a carburetor, you may get slightly smoother LOP operation with a little bit of carbheat, or with the throttle just a tad closed from full open (more air turbulence in the intake channel).

As noted before, the John Deakin articles about engine management are the best you can find.

[piperboy84]

You can very easily get it into modes where the only sure way out is to pull the breaker. Oh well.

I cant count how many times I have had to reset the power to the EDM because I have strayed into some configuration page while trying to switch views or lean

[dirkdj]

The small buttons (left and right) may be rubbing against the instrument panel; you can take off the covers (about 3mm black plastic) and use the switches directly (about 1.5mm black plastic). if they are the 3mm covers, you can pull them straight off. There should be some air around the buttons so they don't bind.

[India Four Two]

I cant count how many times I have had to reset the power to the EDM So I'm not the only one! ;)

For the benefit of those who haven't had the pleasure (!) of using an EDM 700, one of the major user interface issues is setting the amount of fuel in the tanks. When in the appropriate mode (which is an issue in itself), pressing and holding the right hand button rapidly increases the value for fuel in the tanks. If you overshoot the correct amount (which you will), you have to repeatedly press the same button, to decrement the value, IN TENTHS OF US GALS!!!

If the fuel amount has been set to full tanks (74 Gals), which is easily done and I want to set it to half-tanks, I have to press the bl**dy button 370 times! :mad:

I've given up. I just write down the amount of fuel in the tanks and then compare it with the fuel used display, which can be easily reset to zero.

On the plus side, the fuel consumption calculation is very accurate. I carefully dipped the tanks at the beginning and end of a day's towing and added in the fuel that I added. The amount of fuel used, according to my calculation, matched the EDM 700 fuel used value to within half a gallon.

I monitor CHTs during the climb, enrichening the mixture and/or increasing speed as required to keep the CHT below 400° F and then during descent, I switch to the CLD display, which shows the highest rate of cooling of any of the cylinders in degrees per minute. With care in how I throttle back and manage the descent IAS, I can keep the initial rate of cooling below 20° per minute and reduce the rate to close to zero by the time I'm in the circuit. The CHT usually never drops below 350° F.

[N-Jacko]

I4T, there are a couple of short cuts to setting fuel on board:
1. Go into config settings, and set your Main tank capacity to whatever you usually fill to, and Aux tank capacity to 74 USG less whatever you set for Main.
2. If you hold the right button a short time, it will increase the fuel on board slowly, hold a bit longer and it will run away as you describe. The same should happen on the way down: tap a few times and it will decrease one tenth at a time, keep tapping fairly fast and it should switch from tortoise to hare mode.

This works on the EDM 800 and 830, and I guess the 700/730 should be much the same.

PB, if out to save fuel/cruise far in my MX-7-180 I set about 58 to 65% power (say somewhere around 24"/2250) and lean to minimum EGT difference. This usually happens at about max EGT and results in smooth (for an O-360) running and about 28-36 lph consumption.

[India Four Two]

set your Main tank capacity to whatever you usually fill to, and Aux tank capacity to 74 USG less whatever you set for Main.N-J,

Great idea. I never would have thought of that! I'll try that when I get home. I'm currently in Santa Monica.

Thanks.

[dirkdj]

I usually hit FULL when I fill the tanks, and after consuming say 100 liters, I add 35 at the pump, I do the ADD plus number of liters added. Even after several of these partial full cycles, on the next fillup at the pump I am rarely more than a few liters off.
After nearly 18 years of good service my EDM700 is now shipped back from JPI after installation of new display. The old one was fading.

[piperboy84]

This old chestnut again, I have another question. As stated before the engine is a bog standard naturally aspirated Lycoming 360, no Gammi injectors so I am only concerned with leaning for either best economy which is peak EGT or best power which is approximately 100f ROP of the hottest cylinder.

To date I have only ever leaned it for best power for two reasons. Firstly, I'm an impatient type of chap and like to get to where I am going. But secondly and more importantly, when I get to the cruise, set the MAP/RPM for desired power then commence using the EDM to search for peak EGT the time between the onset of engine roughness and reaching peak EGT as signified by the relevant cylinder stack flashing seems like an arse puckering eternity. I am never so relieved to start screwing that mixture back in for 100f under peak EGT/best power. I have no concern when at high altitudes but when over built up areas or water I couldn't imagine leaving it to run at best economy with the engine giving the roughness it does.


Am I missing something here?

Note: the aircraft has just had its annual so everything is in theory set and running correctly