I think it is a mystery, and not heating
by the pump – for two reasons: 1) The heat load added by the pump has to
VERY small compared to the heat load coming from the engine, 2) The pressure
the pump is working against is basically the same as determined primarily by
the pressure regulator valve (as confirmed by his data), and the flow is
roughly the same because it is positive displacement pump.
Al
-----Original
Message-----
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Bill Schertz
Sent: Monday,
September 27, 2004 8:33 AM
To: Rotary motors in
aircraft
Subject: [FlyRotary] Re: Oil
viscosity
Some thoughts on Rusty's improved cooling with the
lower viscosity oil. If you recall, there was an earlier discussion about
drilling out the oil passage in the iron end plate and taking the oil from
there. I believe Ken Welter did this, and obtained about a (15 degree?)
improvement in oil temperature. The only way this effect could be possible is
that the pump is working less hard, and there is therefore less heat being
generated in the oil by the action of the pump.
This could relate to Rusty's experience, namely a
lower viscosity oil would require less power input to pump, and the heating due
to the pump would be less. Add to this the beneficial effect of lower viscosity
improving the heat rejection in the heat exchanger, and I can believe the
improvement is related to viscosity change.
There is an interesting NACA paper
"Cylinder-Temperature Correlation of a single cylinder liquid cooled
engine" published in 1946 where they studied the effects of flow rate and
coolant properties on cooling effectiveness. The correlation of lots of data
where they varied the ethylene glycol concentration from 0 to 97% allowed them
to determine the relative effects of heat capacity, thermal conductivity, and
viscosity of the cooling medium. They found that high specific heat, high
thermal conductivity, and low viscosity are desirable characteristics of a good
cooling fluid. In order of importance,
Thermal conducivity (k) > heat capacity
(Cp) > viscosity (v).
Rusty is not changing k or Cp with the lighter weight
oil, and the viscosity change only affects the heat transfer effect slightly,
so I am left with the conclusion that it is the decrease in absorbed work that
makes the biggest difference.
Bill Schertz
KIS Cruiser # 4045
----- Original Message -----
From:
Russell Duffy [mailto:13brv3@bellsouth.net]
Sent:
9/26/2004 4:39:16 PM
To:
flyrotary@lancaironline.net
Subject:
[FlyRotary] Re: Oil viscosity
5W30 seems a bit low.
>
>
I was thinking the same thing. Leon, will an
extremely light oil be able to keep the seperation
between the bearings and shaft? On the other hand,
will it 'wet' the walls of the cooler better and
thereby transfer heat more efficiently?
Since when it 30 weight
oil is "extremely light"? You might call it extremely standard, or
extremely normal :-) As I understand it, the first number is the cold rating,
which is just for cold starting, so it doesn't have any effect on normal
operation.
BTW, I made about 3 trips
around the airport (on the ground of course), and just made it to 140 degrees.
I'm convinced it's cooling much better with the lighter (though not extremely
light ) oil. Can't wait to fly it, but next Sunday is the earliest possible
opportunity, and probably more like the next weekend.
Rusty (Airbike project
for sale cheap on eBay now, gotta make room in the garage)