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Date:         Fri, 1 Jun 2001 11:26:27 -0700
Reply-To:     Zoltan <zol@FOXINTERNET.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Zoltan <zol@FOXINTERNET.NET>
Subject:      Pensioner,   Re: Oil Temps, Ullage, fill level
Comments: To: pensioner <al_knoll@PACBELL.NET>
Content-Type: text/plain; charset="iso-8859-1"

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


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