PDA

View Full Version : Oil coolers


Dan Luke
August 7th 03, 02:04 PM
Somehow, in 6 years of flying, I missed the fact that not all light piston
airplanes have oil coolers. Why is this so? What factor determines when an
O-320, for instance, must have an oil cooler? ISTR that the oil does a large
percentage of the cooling in aircraft piston engines. How does the engine
get rid of this heat without an oil cooler?
--
Dan
C172RG at BFM

Dan Luke
August 9th 03, 01:19 AM
"mikem" wrote:
> Oil doesn't do "coolling" per se;

> When an "oil cooler" is added to an engine, it simply increases the
> total surface area of the engine assembly that the cooling airflow can
> interact with.

These two statements don't quite jibe, do they?

The oil cooler removes heat from the engine and the medium that conveys heat
to the cooler is oil. I don't see any difference between that and how water
cools a water-cooled engine.
--
Dan
C172RG at BFM

mikem
August 11th 03, 03:57 PM
On Fri, 8 Aug 2003 19:19:30 -0500, "Dan Luke"
> wrote:

>"mikem" wrote:
>> Oil doesn't do "coolling" per se;

What I tried to convey is that the oil doesnt cool the engine; Oil is
used to reduce temperature gradients inside the engine. "Cooling"
takes place where the airflow comes in contact with metal...

>> When an "oil cooler" is added to an engine, it simply increases the
>> total surface area of the engine assembly that the cooling airflow can
>> interact with.

You started the thread by acting surprised that not all aircraft
engines have oil coolers. I tried to point out that aircraft engines
have such large external surface areas, that if appropriately cowled,
they can get rid of heat without an external oil cooler.

Adding an oil cooler has more to do with automatically regulating
engine oil temperature for water vapor boil off and constaining sizes
of cowling openings for streamlining (ala LoPriesti) than out of pure
heat-transfer necessity.

>The oil cooler removes heat from the engine and the medium that conveys heat
>to the cooler is oil. I don't see any difference between that and how water
>cools a water-cooled engine.

In those cases where a cooler has been designed in to acheive those
other criteria, niether do I, and I said so...

MikeM

Dennis O'Connor
August 11th 03, 08:04 PM
Don't underestimate the amount of heat the oil removes from the engine...
It is critical... Without it's help pistons would burn up within minutes,
valve springs would overheat, etc...High powered engines often have a an oil
spray directed to the bottom of the piston... The oil cooler is needed where
the oil pan, case, prop hub, and tappet covers, cannot radiate the heat load
adequately...
Denny

> >> Oil doesn't do "coolling" per se;
>

Wayne
August 11th 03, 10:43 PM
Ask a performance boater about a "Drive Shower". Oil cools well there too.
Doesn't make it necessary or even helpful in all cases though. Mikes post
makes sense too.
Wayne


"Dennis O'Connor" > wrote in message
...
> Don't underestimate the amount of heat the oil removes from the engine...
> It is critical... Without it's help pistons would burn up within minutes,
> valve springs would overheat, etc...High powered engines often have a an
oil
> spray directed to the bottom of the piston... The oil cooler is needed
where
> the oil pan, case, prop hub, and tappet covers, cannot radiate the heat
load
> adequately...
> Denny
>
> > >> Oil doesn't do "coolling" per se;
> >
>
>

Dan Luke
August 12th 03, 03:14 AM
"mikem" wrote:
> You started the thread by acting surprised that not all aircraft
> engines have oil coolers. I tried to point out that aircraft engines
> have such large external surface areas, that if appropriately
cowled,
> they can get rid of heat without an external oil cooler.

Where are you getting this from? I don't mean to sound insulting, but
this seems dubious. Appropriately cowled? All the light piston
aircraft I am familiar with are cowled the same: a forced air plenum
above the cylinders creates a differential pressure across the
cylinders and heat is conveyed out of the cowling by the resulting
airflow. In some installations, an oil cooler is installed to use this
same d. p. for additional cooling.

> Adding an oil cooler has more to do with automatically regulating
> engine oil temperature for water vapor boil off...

So in hot weather it's not needed?

> ...and constaining sizes
> of cowling openings for streamlining (ala LoPriesti) than out of
pure
> heat-transfer necessity.

On a Cessna 172?

> >The oil cooler removes heat from the engine and the medium that
conveys heat
> >to the cooler is oil. I don't see any difference between that and
how water
> >cools a water-cooled engine.
>
> In those cases where a cooler has been designed in to acheive those
> other criteria, niether do I, and I said so...

Well, you started out by saying that oil doesn't do cooling.
--
Dan
C172RG at BFM

mikem
August 12th 03, 05:50 PM
On Mon, 11 Aug 2003 15:04:58 -0400, "Dennis O'Connor"
> wrote:

>Don't underestimate the amount of heat the oil removes from the engine...
>It is critical... Without it's help pistons would burn up within minutes,
>valve springs would overheat, etc...High powered engines often have a an oil
>spray directed to the bottom of the piston...

Yes, but spraying oil on the underside of the pistons does nothing to
remove that heat from the engine! The oil just carries the heat from
the piston into the oil pan. The piston/cylinder is cooler than it
would be without the oil spray, but the oil/lower end of the engine is
now much hotter. This is why I said that oil just reduces temperature
gradients. Getting the heat out of the lower end is another matter
entirely.

How does the heat get carried away from the engine's lower end? If the
engine has no oil cooler, then the temperature of the lower
end/crankcase/oil pan will stabilize when the heat being carried away
by the airflow through the cowling just matches the heat being
produced inside the engine (convection). Heat loss from the engine due
to radiation does a very small part of the total heat flow.

>The oil cooler is needed where
>the oil pan, case, prop hub, and tappet covers, cannot radiate the heat load
>adequately...

I prefer to think that this is just a choice made by the designer.
Clearly in high performance engines/aircraft, it is advantageous to
"concentrate" some of the heat transfer from the engine to the cooling
airflow into one spot, which we call an "oil cooler". As I have
already pointed out, this makes possible automatic regulation of oil
temperature, and LoPresti-small opening cowling. However, there are
many aircraft flying without oil coolers, so a cooler is not
mandantory

The reason I got into this thread in the first place it to try to
dispell the OWT that you cited above: "Don't underestimate the amount
of heat the oil removes from the engine.". If an aircraft has no oil
cooler, then the amount of heat removed from the engine by the oil is
ZERO! (Unless you have a BIG leak).

MikeM

Dan Luke
August 12th 03, 10:24 PM
>
> Well, you're experience is somewhat limitted then.

Granted.

> The Navions (and
> some others for sure) come with a large intake on the bottom of the
> cowl and blow air UP on to the cylinders.

The principle is the same: a pressure differential is created across
the cylinders.

> Other planes just blow air
> horizontally accross the engine with no vertical separation,

What airplanes?

> some just
> have the cylinders stuck out in the airstream.

Yes, like a radial in a Stearman, for instance.
--
Dan
C172RG at BFM

Dan Luke
August 12th 03, 10:30 PM
"mikem" wrote:
> The reason I got into this thread in the first place it to try to
> dispell the OWT that you cited above: "Don't underestimate the
amount
> of heat the oil removes from the engine.". If an aircraft has no oil
> cooler, then the amount of heat removed from the engine by the oil
is
> ZERO! (Unless you have a BIG leak).

You are just splitting hairs. No, oil is not dumped out of my
airplane's engine taking excess heat with it. Neither is water dumped
out of my car's engine, yet the car engine is still a "water cooled"
design.
--
Dan
C172RG at BFM

mikem
August 12th 03, 10:56 PM
On Tue, 12 Aug 2003 16:30:14 -0500, "Dan Luke"
> wrote:

>You are just splitting hairs. No, oil is not dumped out of my
>airplane's engine taking excess heat with it. Neither is water dumped
>out of my car's engine, yet the car engine is still a "water cooled"
>design.


But there are not many cars without a radiator, either! (except
dragsters). I haven't seen a "water cooled" car for more than fifty
years. Nowadays, they use a witches brew of Silicates to carry heat
from the block to the radiator. The cooling takes place when air blows
through the radiator...

MikeM

Dennis O'Connor
August 12th 03, 11:10 PM
Mike, you live in a different universe than I, apparently... Most
everything you say here violates what I know to be engineering fact, and
thermodynamic necessity, after a lifetime of building up and burning down
engines...

Oil absorbs internal heating of engine parts from both combustion and
friction and transfers it to the external metal of the engine (primarily but
not limited to; oil pan, rocker covers <the big two>, crankcase, and
propellor hub, and a cooler if present) and that metal then transfers the
heat both convectively and conductively to the external world...
Up to 40% of waste heat is transferred out of the engine by the oil
according to this source http://www.uccc.co.uk/when/hycwoil.htm
- actually that looks a bit high and I tend to size my calculations on a 33%
basis...
See also
http://www.usace.army.mil/publications/armytm/tm5-691/chap9.pdf

http://www.perma-cool.com/Catalog/Cat_page02.html

Engine cooling is a science, not guesswork, and there has been millions of
dollars spent on research since the invention of the internal combustion
engine as a prime mover... In ten minutes I could amass dozens more
authoritative references, but I suspect it will be wasted effort...
Have a good un...

Denny

"mikem" > wrote in message

August 12th 03, 11:56 PM
Doug > wrote:
: paths for cooling and radiator). Also consider the FAA certification,
: the FAA may require the engine to run without the anti-freeze coolant
: (leaks out), but FAA doesn't require engine to run without oil. So oil

Actually, I believe that a certified aircraft engine has to run
for a certain period of time without oil, and still produce (albeit
decreasing) power.

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

Chris Kennedy
August 13th 03, 01:04 AM
Doug wrote:

[snip]

> I don't know about aircraft engines but
> motorcycle engines power to displacement went up dramatically when
> they started doing water cooled (oil in Suzuki's case).

[snip]

Liquid cooled (be it water, antifreeze, oil or liquid sodium)
powerplants tend to produce more power per displacement than air cooled
(using the common semantics of "no working fluid to transport heat from
hot spots to something that will reject it") because it's far easier to
control temperatures and thus easier to maintain tighter tolerances.

One of my cars has a normally aspirated 3.4L "air cooled" engine that
generates a bit over 350HP at sea level. Some fairly exotic stuff was
used by the engine builder to try to match the expansion rates of studs,
heads, cylinders and cam towers, but ultimately it was necessary to give
up on head gaskets and run cylinder and head metal-to-metal. It suffers
from morning sickness when cold -- it runs on random combinations of
four, five and six cylinders until things expand into place. I wouldn't
have to deal with this if I could limit the range of operating
temperatures for the cylinders and heads, and could be more aggressive
with leaning the mixture and advancing the timing (which would result in
increased output).

To tie this into the more hair splitting part of this thread, this "air
cooled" engine is oil cooled in the sense that piston coolers help keep
crown temperatures manageable and much of the heat rejected by the
sodium filled exhaust valves and the heads is transported by the oil to
a pair of largish oil coolers that reject to the air. The cylinder
walls, however, have no direct cooling other than forced air. If I ever
get myself in gear and replace the existing heads with liquid cooled
ones I'll probably describe it as "an air cooled engine with liquid
cooled heads", since the only real heat rejection path for the cylinders
will continue to be direct contact with the air stream.


--
Chris Kennedy

http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97

Ron Natalie
August 13th 03, 02:34 PM
> wrote in message ...
> Doug > wrote:
> : paths for cooling and radiator). Also consider the FAA certification,
> : the FAA may require the engine to run without the anti-freeze coolant
> : (leaks out), but FAA doesn't require engine to run without oil. So oil
>
> Actually, I believe that a certified aircraft engine has to run
> for a certain period of time without oil, and still produce (albeit
> decreasing) power.
>
Nope. It has to run all the requirements on 1/2 oil capacity. There are no
"no oil" tests nor any "run to destruction" tests. About the closest thing
is that the engine should not catch fire, burst, or exceed the loads allowed
by the engine mounting in the event of such a situation. This is shown
by analysis rather than a specific test.

August 13th 03, 02:46 PM
Ron Natalie > wrote:
: Nope. It has to run all the requirements on 1/2 oil capacity. There are no
: "no oil" tests nor any "run to destruction" tests. About the closest thing
: is that the engine should not catch fire, burst, or exceed the loads allowed
: by the engine mounting in the event of such a situation. This is shown
: by analysis rather than a specific test.

I stand corrected then. Just for grins, ever seen a document
describing this certification procedure? Also, is that 1/2 oil
requirement the reason for the ludicrous (and very messy) maximum oil
capacity of 8 quarts for a 4-banger and 12 for a 6? IIRC, the POH says,
that any level at or above 2 quarts is "acceptable," although certainly
not by me.

-Cory


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

Google