Acheving Proper Oil Temperature

[Tad Olmsted]

I posted this on another site and have been getting some feed back from some guys with older SW's with no oil temp bypass thermostats.

 

According to Rotax, the oil temperture must reach 191 degrees F to boil off any moisture in the oil. As we all know, moisture is a byproduct from combustion and ends up in our oil. This is why there is a white milky film on the bottom of our oil can cap. Tape on the oil cooler is not a good idea, and here is why:

 

I recently put an oil cooler bypass thermostat on a customers Tecnam Bravo IAW Tecnam letter of authorization. I ran the engine for 20 minutes and never saw the temp get above 170, but it did warm up very quickly. When my customer went home he reported that he never saw 191 degrees during the flight home. He achieved over 200 degrees with tape and wondered why the thermostat didn't work.

 

Here is the answer:

The oil temp is indicated at its coolest point (on oil pump when the cool oil comes out of the cooler) and reads low. So if the guage says 170 degrees at its coolest point, what is the oil temp at its hottest point??? There is no way of knowing that. If you use tape on the cooler be advised that your oil at its hottest point could be out of tolerance and do some serious damage to your engine.

 

The tell tale sign of proper oil temp is easy to see. Just take the oil can cap off and flip it over to see if there is any milky white film on it after a flight. If there is none , then your oil might be good to go, if there is, then consider an oil cooler thermostat. It is self regulating and I have installed several that work wonderful.

 

Now for all you guys who have an oil cooler bypass:

This is the reason your Dynon probably never gets into the green range on your EMS. I know we all want to take a quick glance at the panel and see all green. Unfortunately it will always read low, but rest easy, it is working properly.

 

Total cost is around $600.00 to retro fit an oil cooler bypass thermostat, material and labor. Hope this helps.

[Roger Lee]

Hi Tad,

 

A good question. I sent an email to Eric Tucker to ask about temp variance between the in and out, but I can do a temp differential test on my own and actually find out what the average temp differential really is.

 

Here is an oil flow diagram for those that don't know what the Rotax oil flow is.

 

 

I'm not sure it would be a large difference. For instance let's say my CHT reads 200F then my oil is about 215F (no tape). No tape will let the oil cooler dump the temp lower than the taped cooler, but neither should heat up over what the engine temp is already for either situation. The tape will only let the oil cool to a certain point and then the engine will only heat it up no hotter than what the CHT and other engine temps already are. The taped oil cooler just won't let it cool the oil as far down before re-entering the engine. If we are running the tape it is because the oil cooler is doing too good a job in the cold air so we don't want it to cool that far down anyway which would put us in about the same situation as the summer months.

 

 

Tad, what do you think?

If my reasoning is off here, I'm open for discussion rebuttals.

 

I just got a response back from Eric. Here it is:

 

Rodger,

It's very difficult to say, so much is dependent on the size and location of the cooler and the length of the lines. In general the temp has to be taken after the cooler and oil tank, the return line is full of air with the oil, so any reading there is unreliable. In some testing that I have seen the temp difference is at least 15 F between the outlet and the inlet, but as I say the outlet is not a reliable source for reading the temperatures.

Regards

 

 

 

p.s.

This is exactly why I love forums. It makes you think and learn.

Which way does my oil flow.pdf

[Tad Olmsted]

The oil cooler thermostat is like any other. It opens and closes by a spring that contracts and extends as fluid heats up and cools down traveling past it. It is self regulating and is only going to open so far. If the oil is too hot and the thermostat is wide open, the oil will never have a chance to cool down and may go out of limits.

 

The way I see it, the thermostat opens exchanging hot oil from the engine for cool oil from the cooler. Then the thermostat closes a bit giving the hot oil a chance to cool down once it reaches the oil cooler. Then the process is repeated, exchanging hot oil for cool oil. If the oil is hot, it will just be making a constant loop throughout the entire system getting hotter and hotter as it the engine runs due to the thermostat never closing.

 

BTW Roger, I love these forums as well and I am always striving to improve my knowledge. I like the way you think and I may be looking at this wrong. By all means, lets keep on talking thing out and improve each other. I do not, or will not always have the right answer and I rely on forums like this one to keep myself in check.

 

I hope others chime in as well.

 

 

CRAP...I just realized I can't spell "Achieving" on topic and I can't edit it. LOL

[Roger Lee]

Hi Tad,

 

I don't actually dislike the Perma-Cool oil thermostat except for it's in the way when I want to work in that area and it only will bring the temp up to the max of the thermostat which is usually about 180F. Once it is wide open then you should have a normal oil circuit flow. With the tape you can apply enough to bring the temp circuit up higher in cold weather. Usually a 2" wide strip across the top of the radiators to affect both the coolant and oil radiator oil flow. We don't have much of a drop between the in and out temps because we only deal with 3 qts of oil and the engine is flowing that 3 qts at 4 gal. per minute. If we had a larger oil system and larger cooler we could have a bigger differential oil temp between in and out, but with this small system the temp in and out differential isn't very wide.

The oil can't heat any farther than the surrounding coolant, air flow, fuel cooling and and surrounding metal temps allow. The oil can only adsorb the surrounding heat, but shouldn't be able to go any higher because nothing else is hotter or heating it. The only benefit I see with a oil thermostat is slightly faster rise in temp at start up and even then most of the guys don't seem to think its a very big difference.

[sandpiper]

Tad - using your example, but fast forward to summer days, wouldn't we be damaging our engine if we were operating at just below redline?

 

I use tape but would prefer a cockpit controlled shutter.

[Tad Olmsted]

Sandpiper,

Good question, the oil thermostat is designed to self regulate so no matter what the OAT is, it will keep the oil temp at a constant temp no matter what the altitude or time of year is. the oil thermostat actually keeps the oil at the low range, but works because there is never any milky white film on the bottom of your oil can cap.

 

My case is this: If tape is used, you can't pull over and quickly pull it off the front of your cooler at cruise if you get it wrong.

 

Roger and I had a great phone conversation on this one last night and realized we are forgetting we are 2 worlds apart. What may work for us here in the southeast, is different from our friends in the southwest. We came to the conclusion that we both are right, considering our geographical locations.

 

These forums are great for this kind of stuff and I now have a better understanding of operation limits in different parts of the country.

 

AS far as a shutter, I don't see FD buying off on that one. I know Remos had a manual damper that could be controlled from the cockpit. My opinion is it would be one more thing to do in an already busy situation. (talking about my landings, yeah Im learning and yeah I suck at landings LOL)

[sandpiper]

"it will keep the oil temp at a constant temp no matter what the altitude or time of year is."

 

Tad - therein is the problem, at least for me. The thermostat won't keep my temp constant "no matter the time of year." No doubt it would help in the winter and eliminate the need for tape. But, in the summer I'm still gonna see the same high temps I see now in prolonged climb. Maybe even higher due to extra restrictions of the thermostat plumbing? Since I am very often in the high yellow during a summer climb, I'm not in a hurry to put a restriction in the line. I know that supposedly, once the thermostat is open, the flow should be the same as if no thermostat were in place but I'm not sure that is the case.

[NC Bill]

What is a "taped cooler"?

[bitten192]

Sandpiper,

Good question, the oil thermostat is designed to self regulate so no matter what the OAT is, it will keep the oil temp at a constant temp no matter what the altitude or time of year is. the oil thermostat actually keeps the oil at the low range, but works because there is never any milky white film on the bottom of your oil can cap.

 

My case is this: If tape is used, you can't pull over and quickly pull it off the front of your cooler at cruise if you get it wrong.

 

Roger and I had a great phone conversation on this one last night and realized we are forgetting we are 2 worlds apart. What may work for us here in the southeast, is different from our friends in the southwest. We came to the conclusion that we both are right, considering our geographical locations.

 

These forums are great for this kind of stuff and I now have a better understanding of operation limits in different parts of the country.

 

AS far as a shutter, I don't see FD buying off on that one. I know Remos had a manual damper that could be controlled from the cockpit. My opinion is it would be one more thing to do in an already busy situation. (talking about my landings, yeah Im learning and yeah I suck at landings LOL)

[bitten192]

I have a REMOS G3. Oil Temp control is no problem. Just part of the checklist.

 

Ernie, CFI, LSRM -A

[sandpiper]

Bill,

 

A "taped" cooler is one that has some tape over it to block airflow thus allowing warmer oil temps in the winter. Flight Design actually has a LOA out on this. I use 1 to 3 pieces of metalic tape in parallel depending on the L+OAT.

 

Bitten 192,

 

I agree shutters are the way to go. Add it to the checklist. Those that have the D-120 can set a warning so there should be no problem.

[sandpiper]

Forget the L+OAT. It's just OAT.

[Philip Welsch]

I'm considering having the thermostats removed from my LS during the next inspection. Eric Tucker thinks (Feb. Service Class renewal)the coolant thermostat is pretty much useless and my experience with two previous 9XX engines w/o oil thermostats makes me think that unit is also not worth having - at least here in SW New Mexico. Removal would improve useful load by 4 - 6lbs. and make FWF inspections easier due to better access. It should be interesting to see if FD allows removal.

 

PRW

[Roger Lee]

Hi Philip,

 

You, Eric and I are on the same page. Our systems are so small in volume size and our engines so tightly cowled that a thermostat might save a minute in warm up, but does nothing for getting temps high enough.