Erratic or Jumpy EGT and CHT

[jetdriven]

I have a "Masten Engine Analyzer" installed about 1991.  It had a digital display and has all 4 EGT, CHT,  and fuel flow. EGT and CHT are manually selectable, or "auto" mode which always displays the hottest  CHT or EGT to 1 degree, whicheve is selected. It also calculates an "average value" and triggers a flashing alert to this "out of limits" EGT or CHT. 

The problem is, whenever I transmit, a random CHT or EGT value jumps about 30-100 degrees and triggers this alert. Its getting annoying. Every time you transmit, engine monitor blinks 4 times then stays on that cylinder.  Reset.  Repeat.  I flew with a friend in his Bonanza and he has a JPI 700 engine monitor, and, when he transmitted, all the bars started blinking.  Different airplane, different unit, same problem.

Clearly its RF related.   I have replaced all the RG-58 coax on the COM1 and COM2 with RG-400, which is double shielded.  No change. Question I pose is, how to stop this interference?

[jetdriven]

They all peak within .2 GPH, so I put it in manual mode, EGT cylinder #1 (the hottest), and lean past peak on that one.

[N601RX]

If it is not radiated leakage ie coax then it may be coming back through the power lead.  Put a small filter on the power lead just before it goes into the engine monitor.

Double shielded coax will still leak to some degree, especially if it is bundled up close to he engine monitor wires.  Physical seperation (distance) is one of the best isolators. 

Thermocouples are very sensitive to noise because the output is high impdece and only produces a few millivolts.  The signal is then amplified and digitized, including any noise that gets into the system.

My JPI 830 also specifically said the common power connection should be made directly to the engine case and not the airframe to prevent what you are seeing.

[Piloto]

The problem is most likely to happen on those planes with composite cowlings like the M20J. Because there is no metal shielding between the COM antenna and the probes. The probe wires act like an antenna and convey the RF energy to the indicator. The problem can be solved by installing .01uF capacitors across the probe wires right at the indicator connector. Another option is to coil (10 turns) the probe wire around a pencil (remove pencil after coiling) as close as possible to the indicator.

José

 

[jetdriven]

Jose, would this filter like they are talking about, work?  http://www.vansairforce.com/community/showthread.php?t=63836&highlight=SHielding+transmit&page=3

[smccray]

Quote: N601RX

My JPI 830 also specifically said the common power connection should be made directly to the engine case and not the airframe to prevent what you are seeing.

[jetdriven]

Quote: JimR

Thanks, Byron.  The reason that I ask is that my Aerospace Logic EGT monitor works the same way as your engine monitor does.  Just to insure that I understand what you are saying, the key to your technique is that you know that all of your cylinders peak within .2 GPH, correct?  Don't we all agree that absolute EGT values are meaningless due to probe placement and accuracy issues?  That being the case, why do you select the hottest cylinder when leaning for LOP ops?  How do you know that it peaks last just because it is the hotest?

For what it's worth, almost out of necessity, I've been doing the same thing (using the hotest cylinder), but this technique doesn't pass academic scrutiny based upon my admittedly limited understanding of the science.  Can you provide any more insight or comments into this?

Thanks,

Jim

 

[N601RX]

http://www.specemc.com/dsub_700.asp  I've used these on several test chambers at work with good success.  They have more attunuation than the I-Com, but cost more also.

[Piloto]

Quote: jetdriven

Jose, would this filter like they are talking about, work?

[Piloto]

Quote: jetdriven

Jose, would this filter like they are talking about, work? http://www.vansairforce.com/community/showthread.php?t=63836&highlight=SHielding+transmit&page=3

[fantom]

Quote: jetdriven

Like most who drink the kool-aid, I think the absolute number is meaningless, unless talking "Target EGT" for climb, which is ~1350-1385.    Real peak EGT for cruise at low altitude falls somewhere around 1510 F.   All 4 probes read within 20-30 degrees of each other.

[jetdriven]

EGT itself is meaningless in climb.  But a 1350 EGT is what our particular engine does at sea level, 29.92, at 15C.  So, by maintaining 1350 EGT, it corresponds to a constant, sea level, safe from detonation (remember, >75% power) climb. I actuall y go leaner than that above 5K feet, and leaner still above 8k.

Before I had the 20 degrees timing changed to 25, the EGT would peak at 1580.  Now, 1510. Perhaps the probe is drilled closer to the head. We dont use EGT to measure exhaust heat.  We use it to measure where we are on thr Rich/lean curve.    Either way, you are below the temperature where you can damage engine components What does 321 stainless or Inconel become damaged?  EGT does not correlate to exhaust valve temp. It is closed 3/4 of the time.   231s have a TIT limit of, what, 1650?  The pipes are glowing cherry red.  Why is it hotter than EGT? Becaus the exhaust valve is closed more than open. Pull the RPM back from 2500 to 2300 and EGT lowers too. Because the valve spends less time open.  1650 TIT or 1500 EGT is just under the maximum that can be achieved at a high cruise setting anyway, The cylinders in turbo aircraft arent failing so long as CHT is kept under ~400.  a 1500 EGT is like a 360 CHT instead of a 330.   Neither one is of consequence.