Oil temp during cruise

[Birdseyeview]

I've got 7 hours on my 912 ULS and all the temps and pressures are good during takeoff. However, after 10 minutes at cruise (5000rpm and 70F ambient) the oil temp creeps up to the top of the yellow band (240F). It doesn't seem to go any higher but I have backed off the rpm so it hasn't really had a change to get any higher. When I back off on rpm it comes back down into the middle of the green band and stays there. I'm nervous about what will happen during really hot day ambient conditions. I have an oil cooler and an oil thermostat, both purchased from Kitfox. This cooler wouldn't fit well directly behind the lower cowl inlet hole so I put it in front of the radiator. All the oil temp posts on the forum that I've read so far seem to show that most builders have a problem not getting the oil temp up high enough to boil off any water in the oil and they have put on all kinds of different mechanisms to close off air to the cooler to force it to run hotter. My oil cooler is about 3" in front of the radiator so there is no direct ram air into the oil cooler and I don't have a big NACA duct like on the series 7 lower cowls (not much ram effect to the oil cooler or the radiator).

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One previous post I read that seemed to make some sense to me theorized that the three openings in the front of the cowl let in a lot of air and the radiator restricts that total air flow out of the engine compartment. My sheet metal around the radiator is close to the lower cowl and so it doesn't let much air bypass the radiator. As a result of this restriction across the radiator, a slight air pressure may be built up in the engine compartment that produces a given air pressure drop across the radiator. This pressure drop allows a given airflow through the radiator and this airflow enables the heat transfer. The oil cooler also needs an air pressure drop across it to enable air flow through it. However, the slight positive pressure in the engine compartment just doesn't allow much if any airflow through it. The low placement of my oil cooler doesn't allow much ram air effect either and the gap between it and the radiator doesn't help the pressure drop across it that is needed to get air flowing through it. It seems like I might have a few options to improve air flow through my oil cooler, as follows:

1) Do nothing and just monitor it and adjust rpm and air speed accordingly to keep the oil temp within the green band. Maybe the oil temp will lower itself as the engine breaks in?
2) I could remove part of the sheet aluminum shroud at the bottom of the radiator and add a shroud to the back of the oil cooler to direct all its exit air out of the engine compartment via the open hole under the radiator
3 I could put some sort of a shroud between the oil cooler exit and the radiator to force more air through the oil cooler. I can't relocate the air cooler as close to the radiator as I would like because there is a radiator support tube in the way.
4) I could attach a shroud to the front of the oil cooler between it and the lower cowl inlet to force more ram air directly into the oil cooler.
5) Do both 3 and 4 above.
6) Close off the existing lower cowl inlet and install a small NACA duct pointing straight into the oil cooler air inlet side to increase the ram air effect.
7) Remove the sheet metal shroud at the bottom of the radiator that conforms closely to the shape of the inside cowl surface. This might let more air flow through the engine compartment in general but it seems the air will flow out this open hole as the path of least resistance and thereby it might negatively impact the radiator's effectiveness.
8) Increase the overall air flow through the engine compartment by creating a lower air pressure zone on the aft side of the radiator and hope that some of that air flow increase goes through the oil cooler. I could accomplish this by adding a downward slanted "spoiler" on the aft edge of the lower cowl to create a lower pressure on the exit side of the radiator. I've seen some builders do this on pictures of their lower cowls. This would be fairly easy to do but I don't need more air flow through the radiator because my CHT's (and equivalent coolant temps) are good as is. The picture below shows a little slope up of the lower cowl exit. This upward slop is going in the wrong direction to create the negative pressure zone I'm seeking in this option. I assume this upward slope in the existing cowl shape is to help stiffen the cowl and isn't designed in to control and restrict the airflow (via inlet to outlet area ratio).

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Any opinions on what I might try first to help bring my oil temps at cruise down to a more reasonable level? What other options did I miss?

[rv9ralph]

From what I can infer from the information provided, an analogy would be that the oil cooler is sitting in the middle of the room and the radiator is sitting in front of the open window... not breeze in the middle of the room, plenty at the window.

Suggestion. Use the bottom air inlet (below the prop) for a scat tube to a plenum in front of the oil cooler, forcing air to go through the oil cooler.

Ralph

[Birdseyeview]

From what I can infer from the information provided, an analogy would be that the oil cooler is sitting in the middle of the room and the radiator is sitting in front of the open window... not breeze in the middle of the room, plenty at the window.

Suggestion. Use the bottom air inlet (below the prop) for a scat tube to a plenum in front of the oil cooler, forcing air to go through the oil cooler.

Ralph

Good analogy. It appears to me like this is a vote for my option number 4. Thanks

[Jcard]

Had oil cooling issues on my old model V until it got a 13 row setrab oil cooler.
Did additional ducting with minimal improvement, the larger capacity cooler did the trick.

[Shadowrider]

Had oil cooling issues on my old model V until it got a 13 row setrab oil cooler.
Did additional ducting with minimal improvement, the larger capacity cooler did the trick.

Yeah I think I would go larger oil cooler also. What is your water temps like?

[Birdseyeview]

Dustin,

My engine is of the older config where I don't have a separate water temp sensor port but I am sensing CHT and it is always in the middle of the green band at climb and cruise. Thats telling me that the radiator is sized properly and there's enough airflow through it to keep things cool during cruise.

The more I think about my install the more I think that there isn't enough dedicated air flow through my oil cooler. Even a small cooler has proven to be more than enough for a number of builders with too much oil cooling as evidenced by all the forum pictures of louver mechanisms and talk about taping off much of the fin surfaces. Take another look at the second picture of my install and note the 3" gap between my oil cooler and the radiator and its low placement relative to the radiator. It just doesn't seem that it is conducive to optimizing the air flow through it. Before I buy a bigger oil cooler I think I need to try getting more air flow through my existing one. Since I can't easily measure the air flows in my engine compartment I'm only going by what seems to make sense to me with the info I have so far.

My latest idea for improved air flow may be just to flip my existing oil cooler upside down so the oil ports are on the bottom. Then I can place the cooler higher in front of the radiator and it should give the air flow a more direct line through both. This idea wouldn't force me to change oil line lengths or add any ducting and might be just enough to see if my air flow idea is on the right track. You've played with air flow a lot on your install so you must have seen how air flow and cooler placement effects cooling. Am I all wet?

[Birdseyeview]

Follow up: My previous flight where I saw higher oil temps was at 70F ambient. My most recent flights were at 55F ambient and the oil temp was at the bottom of the green band at cruise (190F at 5000rpm). So it's obvious that like a lot of other builders have experienced, my 912 oil temp during operation is highly influenced by the ambient temp. I now suspect that the ambient temp is more of an influence than the exact position of my cooler in the engine compartment. Since fall weather is here now in the midwest I'm not too worried about the high temp I saw earlier (until next summer). So i've got time to figure out my final solution and experiment with a few ideas and options along the way. If I come up with any interesting findings or a "magic bullet" along the way I'll post it.

[109JB]

Is your engine new? If so oil temps tend to run hotter on new engines during the breakin period. That said, they still sound too high and a separate duct to the oil cooler will help.

As far as putting the oil cooler hose inlets facing down, this is something that Rotax mentions as a no-no. I believe the reasoning is that the ports down creates an air pocket inside the cooler where oil only flows in the lower part of the cooler. With the ports up, the cooler has to fill first before anything comes out of the outlet.

[Birdseyeview]

John,

Thanks for your input and the tip on the oil cooler installation position.

[Shadowrider]

Do you have any gaps around your radiator between your radiator and your naca? Just think of air like water and if you poured water with your engine vertical where where it would go? Any gaps you will lose a bunch of cooling air. Make sure between the bottom of radiator and cowl is sealed and sides of naca are sealed. I used baffle material for the sides and top and built a 1/4” fiberglass ramp in front of radiator to seal it and direct ram air up into the radiator.

[Birdseyeview]

Dustin, thanks for your input - I have a sheet metal shroud at the bottom of my radiator which extends down to the lower cowl surface. It follows the entire contour width of the lower cowl. However, there is a 1/4" gap between the lower edge of this shroud and the upper surface of the lower cowl, along the entire contour. I'm planning to install a rubber weather strip into this gap to seal it up. That should force a little more air through the radiator and oil cooler.

[ClickClickBoom]

Cooling air management is critical. I had a customer who had a Mooney 231 that in cruise had to have the cowl flaps 1/2 way open. After consulting with Loren Lemen at Continental, I discovered that the baffling was old and did not seal correctly. I replaced all the rubber with new silicone baffle seal and the plane after take off power was pulled back could climb with the cowl seals almost closed. Both Continental and Lycoming state that in a typical 6 cylinder application no more than 1 square inch of gaps between the high pressure side and low pressure sides of the cooling baffling. They even specify a delta difference in inches of water column. My Porsche 911 with a fender mounted cooler couldn’t cope with high ambient temps while the loud pedal was close to the floor. I built a baffle system to create a high pressure and low pressure area. Some aluminum and aircraft baffle seal solved the problem and kept the oil/engine temps in the middle of the normal operating range. If the high/low baffling isn’t enough perhaps a divergent/convergent duct would help. The essence, is air control. Optimize the existing radiator/cooler then decide if the sizing needs alteration. Most of the kit built aircraft I have seen have woefully lax airflow control relying on brute force, which often isn’t enough.