Date: Fri, 1 Jun 2001 09:52:51 -0700
Reply-To: pensioner <al_knoll@PACBELL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: pensioner <al_knoll@PACBELL.NET>
Subject: Re: Oil Temps, Ullage, fill level
Content-type: text/plain; charset="iso-8859-1"
As reported by Keith Patchett many years ago, sometimes the full mark is not
optimum operating conditions for a motor.
To wit, the BMW 246 "boxer" motor is somewhat self equalizing and prefers to
run at 1/3 of the way between the lower and upper marks. Why? The
crankcase volume is fixed. Part of the work done by the motor is to
severely agitate the oil in the crankcase by splashing it around. The
higher the oil level the more oil that is subject to the crankshaft and
piston rod motion. The 246 motor has a crankcase breather system set low
enough in the geometry that excess oil is passed out of the crankcase,
eventually reaching an equilibrium at about 1/3 of the way up from the low
mark.
The motor knows...After this equilibrium is reached there is minimal change
in the oil level AND the oil temperature
at constant highway speed is at a minimum.
With a closed volume, the crankcase has to deal with changes in internal
pressure caused by piston motion, the more air in the volume the less the
pressure changes as the oil volume is essentially incompressible. More work
done by the motor that is dependent on free crankcase volume.
If you look at the limiting cases (crankcase full, no air) (crankcase
empty, all air) from a dynamics point of view, the minimum work is at
maximum air volume.
All models are incorrect, some models are useful. What follows is a model.
At the bearing interfaces, all that matters is oil, under pressure. The
bearing doesn't know if it is being supplied from a "dry sump" system or a
"wet sump" system. The WBX like most automotive motors is "wet sump", with
the majority of oil contained in a crank case below the crankshaft. The
level of the oil, as long as it provides a constant source to the oil pump,
doesn't matter to the pressure fed bearings. Splash lubrication of cylinder
walls can be affected if the level drops too low. But as long as there is
some level of splash the pistons are fine.
In a "dry sump" system the oil is scavenged by a pump into a tank outside
the crankcase and that oil is used to supply the oil pump. This greatly
reduces crankcase size and volume with no detrimental effect on lubrication.
The BSA/Triumph motors were dry sump. Hardly able, son, motors are dry sump.
Most aircraft motors are dry sump.
So why have four quarts sitting around in the system. One quart will do
fine for lubrication. But it will get dirty faster than four, and will not
have the opportunity to dump heat as well as four, and it will get "beat up"
faster than four, or five, or twenty.
So if we can achieve crankcase splash lubrication, no pump starvation, and
reasonable operating time between changes the actual amount of oil in the
crankcase doesn't matter. The bearings don't know about crankcase volume,
only temperature and pressure at the bearing surface.
In most consumer applications, oil temperature is ignored unless it exceeds
some critical value. Most SAE 20-50 oils can effectively lubricate at at
sump temperature of 150C. The bearing areas run hotter but are not
conveniently measured. If we maintain an equilibrium below 150C under most
operating conditions the amount of oil in the crankcase doesn't matter.
BUT, an optimum level will PRODUCE less heat from crankcase splash and still
supply
a volume sufficient for cooling as each oil molecule on the average has
longer to cool in the crankcase before being pumped into the higher
temperature bearing regions.
That's a model of the "why".
The "how" is by experiment for each motor and application. What the bmw
motofolk did was to measure the oil temp at increasing levels of crankcase
oil under essentially the same operating conditions. The lowest measured
temperature occurred in the vicinity of /13 up on the dipstick. For the 246
crowd, this is a valuable measure as the motors are air/oil cooled much like
the "air-cooled" VW motors. For us in the water cooled world it is not as
significant from an engine cooling perspective.
It is left as an exercise for the reader to determine the effect of
"overfilling" above the full mark, with respect to engine temperature and
oil system operation.
"You can't win, you can't break even and you can't get out of the game" ...
Moore's interpretation of the laws of Thermodynamics
cheers,
pensioner