Oil temp question.

[Lood]

I'd appreciate some expert opinion on a rather puzzling oil temp situation that I have now encountered, more than once, on my new engine. I'll just use the most recent flight as explanation.

We flew up to Johannesburg last Saturday morning. Conditions were fine, OAT 12 deg C and we cruised at FL075, WOT, 2500 rpm, 75 ROP. Before take off, the oil level was about 2mm above level 6 on the dipstick and I opted to add no oil in order to determine whether any decrease in oil level was due to the oil actually being consumed or blown out.

It is generally accepted that most IO360's blow out any surplus oil and 6 seems to be the level where it is happy.

Anyhow, on this flight, my engine ran perfectly, as it has been doing for the last 80 or so hours SMOH. The oil temp sat pegged at 180 deg for the 2.5 hour flight.

On Monday, during pre-flight before we returned, the oil level had dropped to just below the level 6 mark on the dipstick. Now was the perfect time to clean the belly, do the flight and on landing determine where the oil was going and subsequently, I would then know where the optimum oil level for my engine is and what the actual oil consumption is.

However, there was no time to clean the belly, we had only a small window in which to depart before the weather closed in and there was a possibility of isolated weather along our return path that might have called for diversions. So, I decided that worrying about the oil level with a fully loaded airplane while looking out for the weather at the same time was not on and I added one quart - to level 7.

The conditions were very much the same as the earlier flight, except for the fact that we were loaded to MAUW and the return flight was done at FL085. I found it rather strange when my oil temp was 20 deg higher on this return flight and this is not the first time that I've noticed a higher oil temp after adding oil.

My question now is, can this temperature phenomenon really be directly connected to the actual oil level? Another interesting observation on the return flight, was that my CHT's were also slightly higher than on the previous flight - about 10 - 15 degs. Can this have any relationship with the oil level and temp?

[201er]

Interesting question. If I may add another question to yours, how does altitude or density altitude affect cooling? To me it seems that above a certain altitude, even though OAT continues to drop, CHT/OT actually increase. I suspect that the thinner air provide poorer cooling? Can anyone support or disprove this? I am postulating that the change in altitude and temperature played a role.

BTW how did you set power/mixture for each flight? A difference in mixture can easily play that much of a role.

[PTK]

Going from 7500 to 18 or 20K would definitely play a role and CHT's would be "normally" higher. By virtue of thinner air. But I don't think 20 degree Oil T and 15 degree CHT difference can be explained by 1000 foot alt change.

I'd look for obvious things first like air flow through the engine and baffles. And try to run more ROP or LOP!

You'd be surprised what difference even a seemingly slight variation to airflow in the engine has on temps. Altering slightly the dynamics of the airflow (high pressure on top with low pressure out the bottom) can have a dramatic effect. I just had my airplane painted. I removed the RAM and closed the hole permanently. I noticed, much to my surprise, a marked decrease in temps! Cooler CHT's by about 5% from before and less so but cooler oil T. The RAM door apparently wasn't sealing well, disturbing airflow through the engine.

[Lood]

Apart from the different FL, everything was just about the same. Power settings are always WOT, 2500 rpm at 75 deg ROP.

[201er]

What about the fact that flying higher you are also going slower. Not only is the air thinner but your indicated airspeed goes down (even though the true airspeed may have gone up). Also if the temp/pressure change, because of density altitude it could be that it was 3000ft difference between the two flights and not 1000. Just some thoughts.

[Marauder]

This is all theoretical and i am not sure i buy into my own theory. If it were a function of cooling efficiency based on volume of oil to cool, I would think that 7 quarts would run cooler. With 7 quarts you would have 1 additional quart of oil to thermally adsorb the internal engine heat. And passing the oil through the cooler at the same rate whether it was 6 or 7 quarts, I would think less oil being in the engine would run hotter since you can only run oil through the cooler at a fixed rate. With less volume and the same heat, there is less thermal adsorbing mass.

My theory falls apart at the boiling water hypothesis. Think of it in terms of heating two containers of water on a stove, one has 8 ounces of water, the other 10. Heat the water to boiling. The 8 ounces reaches boiling faster, but the 10 will boil as well. Now throw 1 ounce of 40 degree water in each (this is the cooler), the now 11 ounce container will be hotter than the now 9 ounce container. More mass to cool.

I'm more of the belief that density altitude would play a bigger role.

Where the heck is a physicist when you need one?

[Hank]

I would think we have enough engineers on this board to not need a physicist.

My engineering perspective is that part of the higher temp is related to changes in DA, OAT, air pressure/altitude, etc.; some of it is related to the significant increase in aircraft weight [second flight was at Gross; loading on the first isn't mentioned]. The first things I would check on the engine is the baffling [is it in good shape, properly installed, etc.], and maybe the vernatherm on the oil cooler.

The oil cooler by default sends oil through the bypass; as oil temperature increases, the bypass slowly closes, forcing more oil through the cooler. Two things would make this not happen as intended--vernatherm not operating properly, or partially-clogged/improperly plumbed oil cooler.

[fantom]

My engineering perspective is that part of the higher temp is related to changes in DA, OAT, air pressure/altitude, etc.; some of it is related to the significant increase in aircraft weight [second flight was at Gross; loading on the first isn't mentioned].

Most probably that's what ALL of the small differences are caused by, and the head/tail wind may be a contributing factor, too.

[Lood]

Makes sense. I was very light on the first flight and, together with being loaded to max, we had a higher tailwind component on the second flight. The temp differences are not anything serious - it's just interesting that it differs. Baffles on my airplane are brand new and fits perfectly and the cowling were not tampered with between the two flights.

[Hank]

Sounds like it was the extra load.

[duke]

every 100 lbs = aprox 1knt less airspeed coupled with 1000 ft more alt (density alt ?) + possibly slight mixture setting differences coulld make a few degrees difference.

[1964-M20E]

When you take a system and run it close to it's maximum limits small changes in outside elements become noticeable to that system. Not an answer to the question but a reason for seeing the changes.

[maxfly]

I have a problem with the oil temp and CHT gauges. Oil temp sometimes works, sometimes doesn't. When I turn power on, both gauge needles dive to the left.  When I disconnect the sender for the oil temp, it does the same thing. When I ground the sending units lead (off of the sending unit), it deflects all the way to the right.

Cyl. head gauge does the same thing but when hooked up, it's dead all the time. Both the sending units show resistance.

Whats up and what to check next?

[N201MKTurbo]

I have a problem with the oil temp and CHT gauges. Oil temp sometimes works, sometimes doesn't. When I turn power on, both gauge needles dive to the left.  When I disconnect the sender for the oil temp, it does the same thing. When I ground the sending units lead (off of the sending unit), it deflects all the way to the right.

Cyl. head gauge does the same thing but when hooked up, it's dead all the time. Both the sending units show resistance.

Whats up and what to check next?

 

What plane do you have?

[DonMuncy]

I have a problem with the oil temp and CHT gauges. Oil temp sometimes works, sometimes doesn't. When I turn power on, both gauge needles dive to the left.  When I disconnect the sender for the oil temp, it does the same thing. When I ground the sending units lead (off of the sending unit), it deflects all the way to the right.

Cyl. head gauge does the same thing but when hooked up, it's dead all the time. Both the sending units show resistance.

Whats up and what to check next?

The cluster gauges are notorious for having intermittently bad grounds and power supply. If I had any problem with the gauges, the first thing I would do is remove and clean the posts to the affected gauge(s) and the ground connector to the entire cluster.