Smoke

[Peter M]

Not quite right Luke:

Ring rotation is required in order for piston rings to run-in and seal properly, they must be able to rotate freely in their grooves. The ring rotation is the result of the honing pattern (cross grinding) on the one hand, and the rocking movement of the piston at the top and bottom dead centre on the other hand. Flatter honing angles cause fewer ring rotations whereas steeper angles result in higher ring revolution rates. The ring rotation also depends on the engine speed. 5 to 15 revolutions per minute are realistic figures to get an idea of the dimension of the ring rotation. On two stroke engines, the rings are secured against rotation. As a result, both ring rotation and rebounding of the joint ends into the gas channels are prevented. Two-stroke engines are mainly used in motorcycles, gardening machines and the like. The irregular wear of the rings, a possible coking in the ring grooves and a restricted service life due to inhibited ring rotation is tolerated in this regard. in any case, this type of application is dimensioned for a shorter service life of the engine from the outset. The requirements made on a normal fourstroke vehicle engine that is in road use, are higher by far where the mileage is concerned.

For Mike:
The twisting of the ring joints by 120 ° to each other is only intended for facilitating the start of the new engine. After this period, any conceivable position of the piston ring inside the ring groove is possible, provided the rotation is not inhibited by design (two-stroke engines).

Reference: Piston Rings for Combustion Engines - Motorservice

Interesting comment regarding reduced life of two stroke piston rings being a result of the need to prevent them rotating.  I hadn't made the connection before.  As for the ring joints being set at 120 degrees to aid start up, I can't see that making any measurable difference nowadays with tighter ring end gaps (for improved emission reasons), the bores and rings generally being liberally oiled when first assembled and strong ignition and fuel injection being the norm.  However if we are talking Model T Fords I could see how that could be helpful to have them evenly spaced.  

[Mike D'Silva]

Peter

so this means that on any of our motors, at any time, when we shut the motor off, its possible for one or more oil rings to have the gap at the 6oclock position, and hence we can get oil seep into the combustion chamber. Would this explain why sometimes we can get a small puff off blue smoke on startup occasionally and irregularly?

[StevepGT3]

I have pulled the same motor apart about about 20 times and have never found the rings where I left them, however I have never found them lined up in a row either, so starting at 120 intervals means that they maintain some sort overlap during rotation.

[Peter M]

Yep! 

I don't even think the position of the ring gap would be critical as remember the rings relies on the combustion pressure to make a good seal with the bore so when the engine is stopped there is only the ring tension to make that seal.

Also the bores get doused with oil flung from the main and big end bearings and on the later cars, piston under crown squirters (used for piston cooling) so there is plenty of it around.  The oil doesn't necessarily drain away because of the horizontal cylinders, particularly if the car is parked on a bit of a side slope, giving plenty of time for a little oil to seep through to the combustion chambers.

The only predictor of a smoky startup that I can see is it more likely with an audience! 

Steve,

I don't think the rings would rotate at the same speed as there would be so many factors involved that would vary across each ring that there would be times when the gaps would have to line up occasionally.  Not that I think it matters since the gap is so small and the combustion pressure peak is so quick. 

Let me guess, pulled down a Ducati 20 times? 

[StevepGT3]

 

Let me guess, pulled down a Ducati 20 times? 

You must be psychic