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Oil Cooler design related Q.

Bcone1381

Well Known Member
I have an Aero Classics 7 row oil cooler I will use to cool the oil in my Lycoming O-360 engine. I'm curious what airflow volume is needed to pass though it to cool the oil properly. A fan could substitute for a typical oil cooler installation, but I wonder how big it would need to be.

To know the CFM (Cubic Feet per Minute) demand would be handy, but I think the answer needs to be a Mass Airflow value like Pounds/minute.

Also, do we know the temperature change that takes place as hot oil passes through it or the volume that a Lycoming will pump through the engines oil system?
 
Air flow

Cowling differential pressure is normaly between 4-8" H2O.
(This depends on the air speed).
If you look at the bottom of the diagram in Kyles post, you will find DP.
Note that air flow is given in lb/min and that density decreases with altitude.
Yes, it gets colder as we climb and this will help, but air density is 75% at
FL 90, standard temp.
Temperature difference will depend on the oil cooling load and air density.
Flying high require a bigger cooler.
Specific heat for air.
https://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html

Good luck
 
Lots of different variables determine oil cooler requirements. How tight are your baffles? Do you have high compression pistons - more horsepower = more heat. Do you have piston squiters? Roller lifters or flat hydraulic lifters? Then there is the efficiency of your setup. Oil cooler on the #4 aft baffle, or somewhere else?

Just for a relative comparison, I have an O-360 on my newly finished RV6. It is a standard compression (8.5) carbed engine, with one EI and one Slick mag. I’ve mounted my Vans supplied 7 row cooler on the firewall, fed with a 3” scat tube on the #4 aft baffle. My biggest problem relative to oil cooling is getting the oil warm enough. This time of year (Cincinnati), I have the 3” scat flange on the baffle completely blocked off. If it isn’t too cold outside, I can get the oil temp up to 170*.

If you’re worried about getting enough cooling, you can do a couple things to help with this. First, make sure your baffles are sealed completely. I know this has a bigger effect on CHT, but it also helps with cooling your oil. Secondly, if you are running a cruise oil pressure that is on the high end (85-90 psi), you can adjust your oil pressure down to 65-75 psi. That is more than adequate to lubricate your engine, and the oil will pass through the cooler slower for more cooling.
BTW, my 3” scat tube will feed more airflow through it normally than my 7-row cooler is able to handle.
 
Kyle and Avanza;

Thanks so much for the help. This will get me going in the right direction I think.

Scott;

I've got your engine. Also a small fan (Actually two) that is rated 105 CFM, made in Germany, and rated for a service life of 40,000 hours. It draws 480 milliamps. I'm building a Bearhawk Patrol and kind of wondering if it (or they) would cool the oil.

Regarding varibles, there is one way oil is cooled in Lycomings. Energy gets accepted and energy is rejected by the oil when air flows through a cooler. Maybe I can use a fan to move air through it. Maybe we can engineer something and remove all the variables that make our status quo so mysterious.
 
OK……. I obviously wasn’t answering your question, but sounds like the other two did. Sounds like an interesting system you are designing with the fans. Will the fans be thermostatically controlled?
 
I'm not sure yet how the fans will be controlled. My first goal is to understand if its feasible to cool the oil with a fan. I've fabricated nothing. Right now my mind sees a 4 inch fan blowing air into a housing connected to the cooler.

I've thought maybe fabricating a housing to discover the water pressure build up on the intake side of the cooler. That would give me one of the known variables on the chart Kyle provided. If I can get 6-8 inches of water pressure it will be promising.

If we knew how many BTU's of heat rejection an O-360 needs, then we would know the pressure drop we would need and estaimate our need at higher altutudes. More research is needed.

I've considered thermostat control. I like simple, I don't like the idea of having the fans run all the time and a switch seems simple, but then oil temp requires babysitting. I'm at home so I build when I'm home. Researching this puts a stop to my build. So this will be a good project to research when I'm on the road.
 
SNIP

Secondly, if you are running a cruise oil pressure that is on the high end (85-90 psi), you can adjust your oil pressure down to 65-75 psi. That is more than adequate to lubricate your engine, and the oil will pass through the cooler slower for more cooling.
BTW, my 3” scat tube will feed more airflow through it normally than my 7-row cooler is able to handle.

While this is true with regard to the oil exit temperature, it is not true with regard to total heat rejected by the oil per unit time. It will produce a lower indicated temperature, since it is at the return where the temperature is measured, I believe. This might be a fun discussion: which is preferred, low exit oil temp or max rejection?

I've often wondered if the oil temperature going into the cooler might be an interesting measurement.

Re scat size: There are many variables, but I ran a 3" scat to my firewall mounted 7 row Niagara cooler (O-360) for about 20 years. It always nagged at me that I'd see temperatures over 200 on hot days/climbs (this really is not a problem except in my head...). A bit over a year ago I built a new plenum and supplied it with 4". Prolonged slow flight performance was improved somewhat, but otherwise not much change.
 
. Also a small fan (Actually two) that is rated 105 CFM, .

a cubic foot of air weighs .08 pounds and that fan moves about 8 pounds of air per minute. Most of the charts are showing the need for about 50 pounds per minute. It would seem that fan is well short of what is required. If you look at the chart posted, most coolers don't reject ANY heat below 10-15 pounds.

Larry
 
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a cubic foot of air weighs .08 pounds and that fan moves about 8 pounds of air per minute. Most of the charts are showing the need for about 50 pounds per hour. It would seem that fan is well short of what is required. If you look at the chart posted, most coolers don't reject ANY heat below 10-15 pounds.

Larry

Must be a typo above - math doesn't agree with conclusion.

Also, fan ratings must be de-rated for pressure heads against which they are pumping. Fans generally not too good at much head pressure. My gut says a half amp motor at 12v is simply way underpowered.
 
475 Btu per minute

Provision is made for aircraft manufacturer-supplied full flow oil cooler. Oil
flow through the cooler system will be approximately 7 gallons per minute
(26.5 liters minute) and heat rejection will not exceed 475 Btu per minute for
IO-360-P1A1A engines. The oil cooler must withstand continuous pressure
of 150 psi (1034 kPa) and have a minimum proof pressure of 400 psi (2758
kPa). A thermostatic bypass valve and pressure relief valve are supplied as
standard equipment. The pressure relief valve limits the pressure drop
between cooler connections to 35 psi (241 kPa). The valve closes at 185ºF
(85ºC) routing all engine oil flow through the cooler. If pressure drop across
the oil cooler system is more than +75 psi (517 kPa) ±15 psi (103 kPa), the
pressure relief valve opens to bypass the cooler.

https://www.lycoming.com/sites/default/files/IOM-IO-360-P1A.pdf

Good luck
 
Very Good! Data, Science and Math tell me to move on. The fan is insufficient. Thats probabl why no one else is using it! Haha.
 
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