Re: why is oil consumption so common?
Changes in engine design.
A common offender is PCV and breather systems. Under loads, excessive blow by overpressurizes the crankcase and drives oil vapour into the intake tract. In this case, the PCV may appear to be a problem, but on second thought, the blowby should never be that excessive even under load.
With the rings next in line for interrogation, we look at the industry wide move to lower ring tension, and strutted pistons with short compression heights. If done correctly it should not increase oil consumption. At the risk of sounding like a fanboy, a prime example of this is the Skyactive engines as they use extreme examples of both, but generally tend to keep their oil- trust me, I'm on the lookout for Skyactiv offenders. LT rings have been in use for many years across the board. The problems arise IMO from manufacturing variances and/or lack of diligence.
To design an engine with proper ring action throughout it's life requires balancing a few design parameters- all relating to ring seating.
-piston-cylinder wall clearance
-wall finish (hone)
-piston crown design
-piston ring design
-cylinder wall contact pressure (BMEP effected on ring)
Piston-Cylinder Wall clearance
The greater the clearance, the more effect a given BMEP can act on the rings. Great for ye olde days, dedicated race engines and small engines, however industry trend has been reducing this clearance.
Wall Finish (hone)
Variances are largely manufacturer dependent according to tooling preferences and/or intentional like Cadillac's NorthStar racing hone which did not bode well with tepid senior style driving, or even normal operation. Directly affects rate and quality of ring seating.
Piston Crown design
Aside from wall-piston clearances, the shape of the crown's circumference affects how well BMEP can affect the ring. A tapered design will allow more effect on the ring with same BMEP. A shorter distance to the top ring land producing a shallow crevice will have the same effect, and vice versa.
Piston ring design
Ring tension aside, thicker or thinner rings will require more or less BMEP respectively to exert the same contact pressure. Also, ring face design ie tapered, cutaway, gapless will effect contact pressure. Wiper rings tend to be tapered to act as a 'check' for oil allowing oil to be deposited on the wall on the upstroke, and scraped off on the down stroke. We've seen assembly defects where at least the wiper ring is installed upside down and operating in reverse. Oil control rings contrary to popular belief are not intended to 'seal oil out', they literally control the amount of oil supplied to the wiper (2nd) ring, so that oil deposited onto the wall is not uncontrolled.
Cylinder Wall Contact Pressures during wear in
This is affected by all other the other design variables. The required Contact Pressure to seat the given ring on the given hone.
When some designs changes are implemented without adjusting the other ones or revising procedures, we see increased oil consumption.
Regarding break-in, there is debate about how, when and who is responsible for it. It's undeniable that this varies wildly among manufacturers, as well as their break-in recommendations. Personally, I do what I can to ensure the best break-in possible and initially operate my engines hard (around the torque peak is the target). No matter how precise machining is becoming, fitting a pre-formed ring to a pre-machined bore will always have produce variances. Causing a ring to contract or expand just slightly beyond it's perfectly round shape (the specified ring gap), will cause a leak virtually all around because it no longer (or never) follows the wall exactly. Larger bore exasperates the effect. It's true that machining is getting very precise even in volume production and engines are coming off the line tighter, but the rings in particular still must mate to the bores. The consequences of failure for that to happen, results in unhappy motorists and outrageous normal consumption limits.
