Turbo - Static MAP vs. in-flight MAP?

[daleandee]

I don't think a hot spot on the cylinder wall can cause scoring, since overheating the cylinder will only make it expand more.

What David refers to happened to Michael on the ground while tuning the Aerocarb (follow the video links):

https://www.youtube.com/watch?v=22z4HvFkfbc

https://www.youtube.com/watch?v=9CkSQR2HTek

https://www.youtube.com/watch?v=-ZC0wtjme9Y


Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
173.4 hours / Status - Flying
Member # 109 - Florida Sonex Association
Latest video: https://www.youtube.com/watch?v=1VP7UYEqQ-g

[DCASonex]

Spots on cylinder walls can get hot enough to break down the oil, and that results in rings scuffing the walls. Early CAMit engines, like mine, had steel alloy rings and scoring resulted very quickly in real world cooling airflow conditions (uniformity of cooling air flow was likely much better in test setup) . Further testing and examination or affected rings and cylinders by tech guys at the company that made the rings pointed to oil breakdown, Those rings and cylinders were quickly replaced. However, both Jab and CAMIt use steel oil scraper rings, and if oil breaks down it is steel on steel. Scoring that happened on that cylinder wall while in flight looked to be far enough down the barrel to have been caused by the oil wiper rings. In any case, it is well to remember that thin steel cylinder walls can heat very quickly, much faster than the thick heat conducting heads where we take temperature readings.

David A.

[lutorm]

PIston rings are always steel and my impression is that at least the top ring basically lives without oil anyway. If the hypothesis is that the rings cause the scoring of the cylinder, why would the piston also be scuffed? Since the piston is scuffed, I would have thought it would be the piston overheating and pinching. Now, I think the piston is mostly cooled by the cylinder walls, so if the barrel overheats, the piston probably will, too, and then it'll pinch. And it's probably worse for an engine that's not broken in since it''ll have a lot of extra friction between the rings and the cylinder.

[NWade]

Returning to the main topic:

I tried a few more things today, and I have DATA! (...Always superior to anecdotes)

I added a bunch of tracerline dye to my fuel tank last night. I also plumbed in a mechanical boost gauge (leaving the RDAC out of the loop), using strong/stiff tubing. After warming the oil, I performed a couple of brief-but-smooth applications of WOT (followed by a minute of idle). Here's what I observed:

• RPMs climbed to 3050 and refused to go higher, per my usual results.
• The mechanical boost gauge refused to budge above 3 psi. This works out to approximately 6" of MAP. With an ambient pressure of 29.68 this put me right into the same 35.5" to 35.8" range I have seen on the MGL.
• I wheeled the airplane inside and used the special flashlight and glasses from the Tracerline kit. While it clearly showed oil spots from my previous oil-cooler blowout, as well as the minor oil weeping I'm getting out of my oil pump cover and left-side head bolts/case-savers, I saw zero spots of glowing dye around the intake elbows, tubes, silicone sleeves, turbo body, or intake down-tube.

From this I concluded that my RDAC/EFIS is reporting pressures correctly and I'm not dealing with an instrumentation error. I also am not dealing with an intake leak that opens up under positive pressure. I plumbed the RDAC MAP sensor back into the turbo intake, took a deep breath, and did something slightly scary: I disconnected the wastegate actuator and safety-wired the wastegate closed. I let the engine sit and cool until the CHTs were reading around 100 again, then I wheeled her out for another engine run.

I was very concerned about over-boosting the engine, so I didn't even take her to WOT. All I did was idle the engine for 30 seconds (since the oil was still at 90+ degrees from the previous run), smoothly throttle up to 2400 RPM and let the readings settle for 3-5 seconds, and then I began creeping up on the throttle smoothly. I gently walked it up over 2800 RPM (where the MAP starts to go higher than BARO). From there I solely focused on watching the MAP reading. I have no idea what RPMs I got to, but as I crept forward with the throttle lever I saw the MAP climb up smoothly through 35, then 36, and creep up towards 37. Knowing that I was above the highest reading I'd ever seen, I swiftly and smoothly throttled back down to idle. The MAP reading peaked at 37.2 and then fell back as the RPMs dropped quickly.

Based on this result, I'm 99% sure I have a wastegate actuator problem. I'm not sure if I am failing to put enough tension on the actuator arm (despite the "no preload" warning in the instructions), or if I have an issue with the diaphragm or spring inside my actuator. But whatever it is, taking that unit out of the loop definitely allowed the engine to hit boost levels its never shown before.

My next plan of attack is to remove the safety wire, connect the actuator back up to the turbo while shortening the arm by 1/2 to 1 turn, and then perform 1 more engine run. If I am able to get close to 40" of MAP with that configuration, I'll assume I've solved the issue. If I still don't get more than 35.5" of MAP, then I'm returning the actuator and asking for a fresh one! :-P

Take care,

--Noel

[kevinh]

great diagnosis! I look forward to your next update!

[NWade]

Quick update:

While reattaching the wastegate actuator arm onto the turbo, I iddly twisted the arm itself and suddenly it "snugged up". Having read online about the arm turning if you don't use the locknut, I assumed that the wastegate actuator arm was on some kind of free-spinning pivot joint inside the wastegate assembly. However, this "snugging up" of the arm makes me believe that it is indeed threaded into a fastener at that end, and the entire time I've had my wastegate, it wasn't actually properly assembled/tightened inside the housing!

Lesson (re-)Learned: Don't assume the parts you receive have been QC'ed; or haven't gotten loose or otherwise misconfigured during the whole logistics flow.

Its continuing to be a miserable wet spring here in Seattle so I can't run the engine today; but I hope to do another test in the next day or two and see if that arm being tightened makes a difference.

--Noel

[NWade]

One more quick update:

I got a break in the weather this evening and squeezed in a quick engine run. As before, the engine refused to go above ~35.5" of MAP. This time I advanced the throttle smoothly and quickly enough I actually saw the MAP "hit its head off the ceiling" - which I believe is a further sign of the wastegate opening.

I caught this "smoking gun" graph on the MGL recording tool. The MAP is the white line and you see the way it goes from a sharp rise to basically a flat line (a decrease of less than 0.01 bar) - and note that all happens while I continued advancing the throttle the last ~1/8 inch of travel and then held it at WOT for 3 more seconds.

I think its time to investigate a different wastegate.

--Noel

[NWade]

...Aaand just to be triply-sure, I wired the wastegate back closed (only change to the engine) and did one more test run today.

The engine easily hit 40" of MAP with the wastegate wired shut! In fact, even with the throttle being advanced slowly the engine continued up past 38" so easily that I briefly pulled the throttle back a smidge out of caution! I watched it drop back a bit, and then advanced it again until the engine crept up past 40" MAP.

Here is the graph of today's engine run with the wastegate wired shut, and notice how easily it climbs above the 1.22 Bar (~35.5") MAP level from my previous post. The prop also hit 3100 RPM and there was still throttle travel I could have used; but I didn't for fear of overboosting the engine. I believe this definitely proves the problem is in the wastegate actuator, as the engine & prop obviously have no trouble producing the power specified in the manual!

--Noel

[Sonerai13]

The engine easily hit 40" of MAP with the wastegate wired shut! In fact, even with the throttle being advanced slowly the engine continued up past 38" so easily that I briefly pulled the throttle back a smidge out of caution!

When I worked on the turbo engines at Sonex we always found that the waste gate opened at around 44 inches MAP, give or take an inch. That was considered "normal". We didn't expect them to open at 40 inches. The operating procedure was to control the boost with the throttle, not to let the waste gate have anything to do with it. The waste gate was only there to protect against a pilot who didn't pay attention to the MAP and just pushed the throttle all the way forward.

[NWade]

When I worked on the turbo engines at Sonex we always found that the waste gate opened at around 44 inches MAP, give or take an inch. That was considered "normal".

Thanks, Joe! I appreciate the info & context.

That reinforces my thoughts on the wastegate actuator being the source of my trouble reaching full power. I've got an email in to the factory and will see what they say. Otherwise the engine is running great! Just the canopy and a few fairings left to go, and I should be ready for first flight...

--Noel
Sonex #1339