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      09-04-2014, 03:31 PM   #12
42pilot
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Drives: 2014 X5 35d MSport
Join Date: May 2014
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Quote:
Originally Posted by MattBianco View Post
Thank you @42pilot for taking the time to explain at length. If I understand and have simplified this correctly, a small amount of oil burn (not loss/leak) is a byproduct of using better performing metal alloys for each specific component, which in aggregate produce more power/efficiency, but they expand and contract at different rates under temperatures due to the differences in their composition, and some oil enters the cylinders through the piston heads.

Until you correct me I deduce:
  • The oil burned produces slightly more emissions but still maintains their level low enough to comply with standards
  • The burned oil may actually help prolong the life of some components as it provides additional lubrications
  • The burned oil may reduce the life of some emission equipment (catalytic converter, for instance)
  • If one experiences this from the time the engine is young, it is indicative of the design byproduct. If one approaches this at over 50k miles / 80k km, then it is not due to design, but wear/tear/leaks/bad rings/other issues.
  • This is more likely to occur in high-performance engine that operate at higher pressures, with a high red line. And therefore, this should not occur in low pressure engines. And, diesels that have pressurized fuel/air mixtures but not have highly pressurized cylinders, have lower redlines, and rely on the fuel to deliver lubrication.
  • Keep a an extra liter/quart of the appropriate oil handy in the garage

Thank you again.
The oil on the cylinder walls should not be scraped off completely by the rings, thereby seeping into the combustion chamber. The amount is very small and designed this way and where the majority of oil usage stems. You want the thinnest film of oil between piston rings and cylinders to extend the life of the motor.

Burned oil is useless. Oil in its liquid state not only lubricates, it also cools and cleans. I would bet each of our motors has piston squirters - that is, jets that spray oil on the under-side of our pistons thereby keeping them cool after the combustion stroke.

Normal oil usage in our cars will not shorten the life of emissions equipment, including O2 sensors and cats. Catalytic converters really don't start working until high temps - like 900 degrees. This will burn normal oil usage residue out of the honeycomb cat interior. The problem will occur when you have excessive oil usage - then you will have component failure.

Oil usage should be consistent through the life of the motor. A good BMW motor, properly maintained, should last +200,000 miles (this is mechanical parts, not electrical like sensors, injectors, starter, etc). When they wear out, oil usage will increase noticeably and you should perform a compression test or leak-down test on each cylinder.

Consistent high RPM will wear a motor out faster, but regardless of displacement of the motor, wear should be consistent under normal driving. A diesel motor operates under much higher compression ratios than gassers (pressure as you state). I think my 30d is 18:1 ratio where the gas version is probably 10:1 or 12:1. A diesel motor requires high pressure to ignite the air/fuel mixture since it does not use spark plugs. Since diesels provide much more torque than a gas motor, there is no need for higher rpm's to move the car down the street.

In my opinion, it is much more important to change the oil in a diesel more often than a gas motor. Diesels produce soot and have sulfur in the fuel. If you add sulfur and water (humidity), you get a mild acid. Additionally, soot is a carbon by-product and is abrasive - very fine, but still abrasive. I change the oil in my truck tractor and car every 5 - 7,000 miles (tractor every 100 hours). I WILL NOT extend the oil change per BMW's guidelines, but then again, I bought my 30d and did not lease it.

Fuel is not a lubricant. In fact, its one of the worst things you can put in your engine since it dissolves (or what is known as washing cylinders) oil from surfaces so you have metal to metal contact. This is a potential problem I had with the RaceChip. You can't just dump more fuel in a cylinder and expect more power without trade-offs. Any un-burned fuel will wash the oil off the cylinder walls and create higher heat, and accelerated wear. The fuel can even seep past the piston rings into the oil, and you will dilute the oil's lubrication properties and f-up your entire motor. That's an extreme case, but it happens.

Lastly, use the lightest weight oil BMW recommends. Heavy weight (high viscosity) oil robs power, does not increase lubrication and puts strain on the motor when first started from dead cold. I like 0w30 synthetic because it is like water when the motor starts (ensuring lubrication throughout the motor very quickly). This oil also creates a perfect thin film of oil between all rotating or reciprocating metal parts for the best lubrication versus power loss. Again, NASCAR and Formula 1 use a straight 5w or 0w oil for races. We need a little weight (the 30 in the 0w30) to our oil when we are at idle and need pressure and a consistent film of oil that 0W struggles to produce.
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