High Engine Oil Temps.

[WesRagle]

I've never cut into one of these but I assume (I'll do a lot of that in this post) that there is a thermistor connected between the terminal on top of the sensor and the sensor body.

Notice that there is no thermal barrier between the "nut and threads" portion of the sensor and the portion of the sensor that is immersed in oil.

While it's hard to find cut-aways of such a simple device as a temperature sensor, from the things I've found the outside only "looks" like it's a solid mass, but in fact there are separations between materials. These links may not be exactly what we have in our engines, but they are at least similar. And it looks like the engineers were smart enough to have thought to separate the actual probe element from the rest of the body:
https://www.youtube.com/watch?v=TC7iTRWZ0bM
https://www.slideshare.net/HardikRathod ... re-sensors
http://premierautotrade.com.au/news/exh ... rs-EGT.php (see EGT cut-away near bottom of page)

Hi Mike,

If you take a look at the video you referenced and freeze it at the 38 second point, that is what I envisioned the sensor would look like. If I had a sensor to burn, I would cut it open. I would bet that the thermistor is embedded in thermally conductive potting compound which is in turn enclosed in the sensor housing . For the sensor to do it's job it has to be thermally conductive from the case of the sensor to the sensing element. And the case is "solid" metal which implies thermal conductivity to the engine case as well. An easy way to reduce unwanted thermal effects is to *not* fill the cavity all the way with potting compound. I'm sure the engineers did everything they could to make an acceptable device withing the design constraint of a $10 retail price. I'm equally sure they weren't able to achieve perfection.

Looking at slide seven of the the second link shows similar construction.

I don't think the third link is as relevant. Measuring up to 1832 Deg. F. requires a totally different animal.

Also, I think Mike Farley's post goes a long way toward proving the point that the sensor is sensing more than just the oil temperature.

I'm not trying to be an antagonist, and I know it's inconvenient to consider all the implications of admitting that the Oil Temp sensor might be subject to "installation errors". But, with the evidence you guys have presented, it's hard not to draw that conclusion.

I'm simply making some observations.

Wes

[mike.smith]

Not mad :-) But I disagree with your conclusions based on what I see of the construction of these devices. We'll just agree to disagree.

[WesRagle]

We'll just agree to disagree.

Agreed :-)

Wes

[MichaelFarley56]

Also, I think Mike Farley's post goes a long way toward proving the point that the sensor is sensing more than just the oil temperature.

I have no real axe to grind in this discussion; even when my engine was indicating higher oil temps I never hit 240 degrees (regarded by many as being a standard for maximum oil temp) and my oil pressure was always solidly in the green.

Just to clear up any confusion on my post, I do want to make it clear that I did not install any sort of blast tube or direct air source onto my sensor; rather, all I did was notch a 1.5" slot in my pilot side baffling with the hope that some air flow over the front of the entire engine would help reduce the errors I was seeing in my indicated temperature. For whatever reason, it worked just fine on my setup and I'm happy with the results. As far as I'm concerned, I was reading inaccurately high readings which could be the result of radiant heat coming off the nearby exhaust pipe, and allowing some air movement solved the issue. I didn't have this issue with the vertical split cowling and I'm glad it's resolved with my new horizontal split cowling as well.

Just my two cents of course!!

[WesRagle]

Also, I think Mike Farley's post goes a long way toward proving the point that the sensor is sensing more than just the oil temperature.

I have no real axe to grind in this discussion; even when my engine was indicating higher oil temps I never hit 240 degrees (regarded by many as being a standard for maximum oil temp) and my oil pressure was always solidly in the green.

Just to clear up any confusion on my post, I do want to make it clear that I did not install any sort of blast tube or direct air source onto my sensor; rather, all I did was notch a 1.5" slot in my pilot side baffling with the hope that some air flow over the front of the entire engine would help reduce the errors I was seeing in my indicated temperature. For whatever reason, it worked just fine on my setup and I'm happy with the results. As far as I'm concerned, I was reading inaccurately high readings which could be the result of radiant heat coming off the nearby exhaust pipe, and allowing some air movement solved the issue. I didn't have this issue with the vertical split cowling and I'm glad it's resolved with my new horizontal split cowling as well.

Just my two cents of course!!

Hi Mike,

Since you gave me an opening, I'm going to take another stab at this.

I spent over 30 years of my career directly involved in the design and programming of data acquisition systems supporting gas turbine engine operation and maintenance. That included developing equipment to test the sensors. Given that background, I am naturally interested in topics such as this. As a project engineer I was frequently on the hook to provide solution to customer reported problems. I was never on site when these events occurred so my only recourse was to sort through emails to try to get to the root of the problem. Anyway, life experience leaves me sensitive to implied meanings when I read messages on a list like this.

So, if you guys will indulge me, imagine us sitting at the pub having a spirited discussion over a pint of your favorite beverage.

First of all I did my best to explain to Mike S. why the construction of the oil temperature sensor made it susceptible to sensing errors to which Mike simply replied: "I disagree with your conclusions based on what I see of the construction of these devices." Since Mike and I have agreed to disagree, and since its obvious that I won't be able to sway him, and since your experience directly contradicts his assertion that the sensor only "sees" what is presented to the tip, and ... I'll proceed on the assumption that I'm right and he's wrong:-)

While it may be true that you have no axe to grind, you do want to believe that your current set up is reporting the proper oil temperature. That's obvious. Your first setup reported an oil temperature you were happy with. Your second setup reported an oil temperature your were "less happy" with. You immediately assumed that the second indication was in error and you set about finding a solution to the "problem".

Here is the assumption in action:

all I did was notch a 1.5" slot in my pilot side baffling with the hope that some air flow over the front of the entire engine would help reduce the errors I was seeing in my indicated temperature.

Here's another clip:

Obviously, getting a little bit of air flowing over the sensor made a big difference on my engine readings!

If you're struggling with high indicated oil temps and you don't have the lower cowling opening, you can consider either adding the cowling opening or modifying your baffling (or adding a blast tube) by the temp sensor and see if that helps.

The first thing I notice is the exclamation point. You were happy, I was startled, it works for both.

Anyway, where does that leave CPT. Tidy?

You offered two possible reasons that the sensor reading might be artificially high in the second setup.

I was still getting good oil pressure (around 48-50 psi) and suspected I was having an issue with the oil temp sensor getting heat soaked by the exhaust and hot, stagnant air in the lower cowling.

1) The sensor being heated by radiated energy from the exhaust.
That sounds reasonable to me.

2) Stagnant air in the lower cowling.
I doubt that there is static air anywhere under the cowl while flying around at 130 mph with big holes in the front and the bottom of the cowl.

The only way to determine if #1 is the culprit is to treat the disease rather than the symptom. That is to say shield the sensor from the radiated heat instead of dissipating the heat with cooling air. If the indication goes down, you were right, and knowing the problem we can then determine the best "fix".

If you were wrong on both counts it means you are now indicating lower than actual oil temperature. If you're OK with it that's fine. But it also means that if CPT Tidy wants lower oil temperature he will have to start down the road of cowl/plenum modifications, etc., etc.

A little sarcasm to drive home my point.

Hey CPT Tidy, here is an idea, take your oil cooler off, you don't need it. Just run a blast tube to the sensor and adjust the air flow in the blast tube until you get the reading you want.

I'm going to go work on my airplane now.

All in good fun:-)

Wes

[Onex107]

Remember folks, when comparing Sonex oil temps to Onex oil temps the Onex valve covers hang out in the wind where the Sonex engine is entirely contained in the cowling. There isn't a lot of oil flowing through the valve train but it all counts.

[Cpt.tidy]

Update on high oil temperatures.
I performed a test flight on Sunday. The following condition applied:
- Altitude 4000 ft
- Power setting 2800 RPM
- Temperature at altitude was 74 degrees
Results:
- Oil temp 236 degrees
- Here is where it gets interesting. Cylinder head temperatures:
#1-378 degrees #2–360 degrees #3-370 degrees #4-254 degrees
The number four cylinder is running 100 degrees or more cooler the other three cylinders. I believe this is where the problem is.
I did a differential compression check after landing. These were the results, (#1 80/75), (#2 80/78), (#3 80/80) (#4 80/75). The number four compression checked surprised me. Any ideals before I pull the heads?


Regards,
CPT.Tidy

[XenosN42]

Hello,

I wouldn't be too quick to pull the heads off. First I'd answer the question: are the temperature probes working correctly and reporting the correct values?

Two ways to check. 1) Switch the #4 CHT probe with the #3 probe. Is the #4 CHT still lower than the rest.
2) Dunk the #4 CHT probe in boiling water. It should read 212F.

If you can you should also check the oil temperature probe with boiling water.

The temperatures of the other CHTs look good for a new AeroVee engine.

What instrumentation are you using on your panel to view these temperatures? Did you get the probes from the instrument manufacturer?

[WesRagle]

Hi,

All of your indications and compressions sound perfectly reasonable except for the one outlying CHT reading. So I agree with Michael, verify the sensors first.

I'm not an engine expert so I have a question for you. What do you see in the data that would prompt you to remove the heads? All of the compressions look good.

It seems to me that if the odd CHT reading were actually real it would indicate that cylinder is not making proper power. It also seems that low power on a single cylinder would be accompanied by rough running and possibly a low EGT indication on that same cylinder.

Did you happen to notice how the EGTs were behaving during the flight?

Wes

[MichaelFarley56]

I also agree with Michael and Wes; your compression looks great, so unless you suspect some other damage or issue, don’t pull your head at this point. With one reading so far off, I’d start with swapping that CHT probe with another just as others have suggested. If the cold cylinder reading follows the probe, you’ll know it’s just a bad probe. I’ve had to replace several over the course of flying my Waiex the last 7 years.

In addition to the previous questions, I’d also like to ask how the engine is running in general. Smooth and powerful or is it running rough? Have you attempted setting up cruise and then flying with the secondary ignition turned off? If the secondary ignition is advanced, if you turn it off in flight you should see a drop in temperatures.

Keep at it! You’ll get there!