Thanks for the explanation Sir.
Zoltan
----- Original Message -----
From: "pensioner" <al_knoll@PACBELL.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, June 01, 2001 9:52 AM
Subject: Re: Oil Temps, Ullage, fill level

> 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