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      01-19-2012, 11:46 AM   #1
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Tehnical reason for S65 Oil Consumption?


I know there are threads on oil consumption, but what is the technical reason for this? It appears that some cars use no oil between changes and others use a decent amount. My car happens to consume about 1qt every 3,000 miles and I drive it relatively hard. BMW says this is normal, but where does the oil really go? With all of the discussions on rod bearing failures and & proper lubrication this had been on my mind. I would like to hear from some of the engine experts out there.



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      01-19-2012, 12:11 PM   #2
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The oil goes past the piston rings, into the combustion chamber, and out the tailpipe as emissions. Some I suppose might get past the valve guides and then via the intake through the combustion chamber and out the tailpipe but you would notice some blue smoke on a cold start if you had a leaky seal.

The harder you drive it the more you will likely burn. A liter every 3,000 miles is not bad. As I recall my S54 powered M Coupe was similar. All BMW motors seem to consume some oil, of course to varying degrees. I've had to add oil to a 545i sedan, a 530xi wagon and a 128i coupe as well.
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      01-19-2012, 01:37 PM   #3

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      01-19-2012, 02:18 PM   #4
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Oil consumption is caused by a few different things.

One of them is blowby. Expanding gases in the combustion chamber blow past the piston rings and entrain some of the oil lubricating the rings. This oily blowby is then evacuated from the crankcase by the PCV system and returned to the intake system; the oil is then burned in the combustion chamber.

Reverse gas flow is another one. During evacuation of the cylinder during the exhaust stroke, the cylinder pressure is actually lower than the pressure between the first and second piston rings. This causes a reverse flow of gas into the cylinder and it "sucks" some of the oil in liquid and/or vapor from the piston ring system into the cylinder.

Oil is also consumed as acceleration and deceleration of the piston throws oil from the top ring land into the combustion chamber. This is called "throw-off". Throw-off type oil consumption is worst at low loads.

In a similar way, aggressive driving can cause increased oil evaporation from the top ring land. Consumption due to evaporation also takes place at the cylinder liner. It's magnitude is highly dependent upon oil composition.

In older engines, there was some oil leakage from the valve guides but newer valve seal designs have virtually eliminated that as a source of oil consumption.
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      01-19-2012, 02:45 PM   #5
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