Engine Monitor Data Analysis

 
 

Most glass panels and digital engine monitors include the ability to save engine monitor data, and then export that data for later analysis. The Dynon does this extremely well, logging all the parameters that the instrument displays. The data can be imported into a number of different programs to display and analyze it, including spreadsheet programs like Excel and special-purpose programs. My favorite of these is SavvyAnalysis.com. I highly recommend you visit the site and create an account. It's an incredible resource with other great features as well.

Savvy Analysis is leading the charge on putting engine monitor data to good use in determining the health of your engine, and they are masterful at troubleshooting issues by combing through the data. Their success with Lycoming and Continental engines is inspiring, but the lessons and takeaways don't always transfer directly over to Jabiru engines (or other light aircraft engines for that matter). For example, Jabiru has far different limits on CHT's, different induction and carburation systems, and the detonation margins are largely unknown on the Jabiru prompting the very-low recommended max EGT's in the manual.

My goal here is to apply Savvy's techniques using their online platform to try and identify engine issues before they become serious problems, use the logged data to refine my engine operating procedures, thus improving efficiency and longevity, and possibly root out a few nuggets to better understand my engine.

Let's look at a recent flight I made in the fall of 2015. I uploaded the Dynon data into Savvy and displayed the flight below. I'll refer to several screen captures of the engine data, including a couple of zoomed-in sections. The data is arranged with all six EGT's color-coded lines on the top, with CHT's just below. I've also added an additional parameter overlaid on each set, with a blue line on top showing fuel flow, and a yellow line on bottom showing RPM (it gets cluttered, but each of those lines stand apart from the EGT or CHT clusters.)

Figure 1: 80 Minute Flight (Total Flight)

First, a few initial observations. The EGT's seem to track pretty closely between the six, rising and falling in unison. Second, there are a few spots where the EGT's seem to abruptly change values (e.g. 10 min, 26 min, 1:02, 1:16). These deserve closer inspection to see if there's a problem, or if there is an explanation for the change.

Zooming in to the first section of interest (below), we'll have a closer look at what's going on.

Figure 2: Take Off (Minutes 9-12)

Looking at the lower CHT set, specifically the yellow RPM line, we can see the takeoff event. RPM is low at the 00:09:00 mark (engine warm-up), increases slightly at 00:09:10 (run-up), then jumps up to max RPM (take-off power) at 00:09:30. We can see the EGT's jump up in unison with the application of takeoff power. They go above 1250 F, peak slightly, then fall back to about 1300 or so for the remainder of the climb-out (until about 00:10:40.

That initial peak of EGT 5 and 6 is interesting. I wondered why 5 and 6 peaked higher than 1-4 but then fell back in line after a few seconds. You can see from the RPM trace (bottom set, yellow line) that throttle remains pretty constant, and from the Fuel Flow trace (top set, blue line) that fuel flow takes just a few seconds to fully ramp up but then stays constant as well. I think what's happening here is that throttle goes full open, RPM increases to max, but fuel flow lags behind just a bit. For just a couple moments the engine is running lean, and that's what causes the short-lived high EGT peak. After fuel flow catches up, the mixture richens up and EGT's come back into line. There's something going on in the intake manifold here as well. The rear cylinders are acting different, probably due to the shape and flow inside the manifold.

Next, at 00:10:40 you can see a slight throttle reductions (200 rpm, or so). There's a corresponding drop in fuel flow, and EGT's, and CHT's peak around this time and start downward. I interpret this as the conclusion of the initial climb, and throttle back to cruise. In terms of percent power, the engine drops from 90% to something closer to 75% (2750 rpm, 2000 ft DA, 7 gph). Making less power allows the CHT's to cool off as well.

Lastly, at 00:13:30 we see evidence of a leaning event. Fuel Flow (blue line on top) falls, while all EGT's rise more or less in unison. RPM (yellow line below) increases very slightly. This is all consistent with the pilot slowly leaning the mixture. A few seconds later the RPM is reduced slightly, likely from a minor throttle reduction to restore cruise RPM after the RPM increased due to leaning. At the end of the event 00:14:00, EGT's are slightly higher than before, RPM is steady at cruise RPM, and fuel flow is lower - just what we want to accomplish with leaning. From the data, we can infer that the amount of leaning was conservative. CHT's remained steady, EGT's didn't run away or peak and then fall. We can conclude that the leaning shown here never put the engine into a dangerously lean condition, nor did it lean into the onset of perceptible roughness (which would have shown up as one or more EGT traces that deviated from the others).

The last section to examine more closely is the ending section of the flight. This corresponds to the final descent and landing phase.

Figure 3: Descent and Landing (1:16 - 1:25)

At 01:14 RPM comes down in a couple stages, with resulting reductions in CHT as the engine makes less power but airspeed remains high in the descent. Look at the increase in fuel flow at 01:16. Could this be due to pushing in the mixture knob somewhat as the final descent levels off at pattern altitude? Shortly after, throttle comes down to 2000 rpm, then 1800, then 1200. This must be the downwind, base and final legs. The whole while CHT's are cooling. There's a blip of power at 1:20, and then it looks like landing happens just after 1:22. A couple of short throttle blips and resulting EGT spikes probably happen while taxiing back to the hangar.

RPM drops to 0 at 1:24.5, indicating engine shutdown. Look at the fuel flow just then - it drops sharply but EGT's climb rapidly. This is consistent with pulling the mixture knob to full lean to kill then engine. The EGT spike just before it dies indicates it's idling pretty rich, something all AeroCarb users struggle with due to the design of the needles.

This whole time the red CHT line for cylinder 1 has been substantially lower than all the others. I've always attributed this to it getting a blast of cold incoming air right at the front of the cowling. It could be a bad CHT probe, but look how CHT1 climbs after shutdown to exactly match the other CHT's. No cooling air gets to the cylinder after shutdown, and at this point the CHT comes up to read just like all the others, as expected. I interpret this to mean the probe itself is likely OK, but maybe the blast of incoming air is causing some extra cooling of just the probe itself and the reading is lower than the actual cylinder temp. Tweaking the probe's placement is something I'll have to play with....

Next up, I'll run some leaning tests to see if I can detect how much leaning is OK, and what signs there might be within the data that the engine isn't happy.

Engine Monitor Data Analysis, Part 2 (coming soon)

 

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Updated: 05 Jan 16