G1000 EGT Readout Variance

[Jeff_S]

Hi Folks. For those who have a G1000 system in their Mooney, a question about variance between EGT readings. I've been experimenting with different power settings, which is making me focus on the EGT/CHT gauges to see how my engine reacts.

 

Today I noticed an interesting thing: the EGT gauge in the standard Map mode (the small sliders on the left) which shows and denotes the hottest cylinder is reading out exactly 100° hotter than the same cylinder shows when you put it on the Engine page.  This is only true for the EGT reading...the CHT reading shows the same in both modes.

 

Anybody ever see this before?

[Txbyker]

Yep I saw that too and was revealed on MSpace that EGT summary on the Map page is actually a 7th probe mounted on one side where all 3 exhaust join a bit lower than the individual probes. Notice on the map page it does not show a number in the box revealing a single hottest EGT.

[Jeff_S]

Hmm...that's interesting. I wonder what the point of that is. And why would it show hotter than the individual probes higher up in the stack? Maybe the combination of all gases reaching that point somehow has an additive effect? But thanks for clarifying that one. I hadn't seen it in the manuals anywhere.

[carusoam]

The seventh EGT is the original ship's EGT.

You should be able to read about it in the POH.

It is about three lines in a 300 page document.

It just doesn't stand out in a memorable way. (Unless... You are reading this on a Sunday!)

As for being higher temperature...?

Individual EGT TCs are closer to the Exhaust valve and will typically see the hottest temperatures. Depending on conditions (ROP) the flame fronts may extend past the individual TCs and continue to burn and get hotter.

The ships EGT is in the stream of three cylinders. It will see three flames and three times the volume of the individual exhausts. But, It shouldn't get hotter because of the three into one geometry.

It will see a smoother averaged number. It only tells the story of half the engine...the older JPIs collect data every couple of seconds. The thermal mass of the TC is too large to allow for heating and cooling in step with the engine cycles. At 2700rpm there are 45 cycles per second, 27.5 are power strokes...

How often does the G1000 take samples? Does it intentionally average the data over time?

Overall, the ship's gauge should be seeing a colder stream as the flame is done and the remaining gasses are expanding (cooling).

In the older Os, the ship's gauge is analog, making a precise comparison to the JPI somewhat challenging (for me).

Apparently, what we are collectively seeing, is that the peak temperature occurs somewhere further down the pipe after the first EGT TC.

What to do to determine if that observation is real...

Collect data...

One data set in fire breathing dragon mode, maximum MP, deep ROP, like in the initial climb. If flames are going to be deep into the exhaust system, this is when it will happen.

Another set at cruise mode, 50dF LOP, much lower MP, minimum excess fuel burning in the exhaust system.

Once the individual TCs sense the peak, the flame is no longer going past them?, oxidation is essentially complete, expansion rapidly continues.

I would expect that under LOP conditions that the ship's EGT gauge should run slightly cooler than the individuals higher in the stack.

In the exhaust system, there are two opposing forces...

- fuel burning heating things up

- gasses expanding cooling things down

The odd part of this is, we are making observations about the engine's operation from the outside, and extrapolating it to what's going on on the inside.

Until we get T and P data from inside the cylinder, this is the best we've got...

Let us know what you see. As usual, I'm still only a PP, not a mechanic...

Best regards,

-a-