Figures I found on the web indicates 3% is a fairly
standard figure used for a planetary gearbox efficiency.
So if that figure is close and you are producing in
cruise - say 160 HP- then the gear box will produce heat with about 3%
of the input power. So 3% of 160 HP = 4.8 HP
which is mostly converted to heat.
I assume the oil is already cooled by the
oil cooler before it is fed to the PSRU. So the temp of the oil
going into the gear box might be around 180F. I am not certain how
many GPM of oil Tracy's PSRU flows but the old Ross flowed around 1 to 1 1/2
pint/minute, but that was apparently rather marginal. So assume
Tracy's flows 8 times that or 8 pints/minute
= 1 gal a min. Probably not that high but lower flow
just means the temperature calculation will be higher than in this
example.
The Cp of oil is around 0.4 and its mass is approx 7.3
lbs/gallon.
4.8Hp converts to 203 BTU/Min, so we need to find out
what temperature rise that might cause in the oil.
Delta T = Heat/(Mass flow *Cp) or if we start
out with 180F oil (Ti) then we need to find the final temperature (Tf)
. So Tf = Ti + (Heat/(mass flow *cp)
Tf = 180F + (203/((7.3 *.5) *0.4) = 180F
+ 69F = 249F
So using this example and assuming I haven't screwed
up, I would expect the oil temps coming out of the gear box to be
around 249F.
Less efficiency would mean higher temps. More
oil flow would lower the temp and less would increase it. Less power
being produced would also decrease the temperatures. There may be some
heat lost through air flow around the gearbox, but probably offset by heat
from the engine through the spacers and plate.
In my case, I would likely producing around 80
HP at cruise based on fuel burn numbers (8 gph). My oil temp at that
power is around 170F.
So Tf = 170F + (102/(7.3*0.4) = 170F + 35F = 205
F.
So don't know if that tracks or not with what you guys
are seeing, but that's my 0.02 on it.
Ed