ok all you engineers and brainiacs, enlighten with some ponderings...
or even some formulas.
i have been wondering about cylinder wall friction. supposedly it is the no. one robber of power via friction. i read in "Desktop Dynos" some time ago that it is a far greater loss of power than valve train friction/weight. i also understand the benifits of low tension rings with or without a dry sump system. my real question is one of oversquare vs. undersquare. cw says that oversquare has a lesser known benifit of less cylinder wall friction because of shorter stroke producing a shorter swept area for the rings. however, someone in the sr forum was saying that the larger bore produces more friction because of the higher contact surface area. this of course is completely neglecting to mention the other many benifits of going oversquare.
any ideas guys?
chris the ignorant conversation starter
cylinder wall friction
-
s13sr20chris
- Posts: 4148
- Joined: Tue Jun 10, 2003 9:32 am
- Car: '89 Nissan S13 w/redtop running 13psi and not leaking fuel anymore
- Contact:
i would think that a ratio of ring outer surface area to swept length would give you a good estimate.
friction coefficients can be considered equivalent, so the normal force is what is really important in your friction force equation. In this simplified view, a longer stroke will induce a smaller normal force on the opposing cylinder wall.
friction coefficients can be considered equivalent, so the normal force is what is really important in your friction force equation. In this simplified view, a longer stroke will induce a smaller normal force on the opposing cylinder wall.
-
s13sr20chris
- Posts: 4148
- Joined: Tue Jun 10, 2003 9:32 am
- Car: '89 Nissan S13 w/redtop running 13psi and not leaking fuel anymore
- Contact:
i would help more, but i dont have the time with all my projects. this racecar just isnt going to build itself.
you can do your own theoretical analysis fairly easily.
take a 3 litre 6 cylinder for example, which is 0.5 litres per cylinder
experiment with the different bores and strokes that will give you 0.5 litres for one cylinder, remembering that
D= pi*(bore/2)^2*stroke=displacement, 0.5 in this case
your swept length is obviously the stroke,
and your ring contact surface area will be approx.
SA= pi*bore*7.5mm
dont forget to keep your units consistent (use 7.5E-3 meters instead of 7.5 mm)
this is all very rough, but should give you an idea of what happens in real life.
you can do your own theoretical analysis fairly easily.
take a 3 litre 6 cylinder for example, which is 0.5 litres per cylinder
experiment with the different bores and strokes that will give you 0.5 litres for one cylinder, remembering that
D= pi*(bore/2)^2*stroke=displacement, 0.5 in this case
your swept length is obviously the stroke,
and your ring contact surface area will be approx.
SA= pi*bore*7.5mm
dont forget to keep your units consistent (use 7.5E-3 meters instead of 7.5 mm)
this is all very rough, but should give you an idea of what happens in real life.
A few thoughts:
The chief type of friction experience by the piston assembly should not be rubbing friction, but viscious friction from hydrodynamic lubrication, where the assembly rides on a film of oil on the cylinder walls.
However, the strength of the oil film in hydrodynamic lubrication is dependent on both piston velocity and normal pressure. A good visualisation is a hydroplaning tire: When sliding velocity becomes low, or when footprint pressure is high, hydroplaning stops. If hydrodynamic friction breaks down, rubbing friction results. Imagine the difference in magnitude between the 2 types of friction.
The bad thing is that rubbing friction will occur - the piston slows down and stops quite a bit in a reciprocating engine.
In addition to the loads on the piston skirt already discussed previously, compression rings are designed to seal harder with increasing combustion chamber pressure - thus ring contact pressure is a function of cylinder pressure in addition to ring tension.
My thoughts are that the ratio of contact pressure to piston velocity (as functions of bore and stroke) can be optimized in order to minimize the overall amount of friction for various engine loadings and speeds.
Does this sound reasonable?
The chief type of friction experience by the piston assembly should not be rubbing friction, but viscious friction from hydrodynamic lubrication, where the assembly rides on a film of oil on the cylinder walls.
However, the strength of the oil film in hydrodynamic lubrication is dependent on both piston velocity and normal pressure. A good visualisation is a hydroplaning tire: When sliding velocity becomes low, or when footprint pressure is high, hydroplaning stops. If hydrodynamic friction breaks down, rubbing friction results. Imagine the difference in magnitude between the 2 types of friction.
The bad thing is that rubbing friction will occur - the piston slows down and stops quite a bit in a reciprocating engine.
In addition to the loads on the piston skirt already discussed previously, compression rings are designed to seal harder with increasing combustion chamber pressure - thus ring contact pressure is a function of cylinder pressure in addition to ring tension.
My thoughts are that the ratio of contact pressure to piston velocity (as functions of bore and stroke) can be optimized in order to minimize the overall amount of friction for various engine loadings and speeds.
Does this sound reasonable?
-
s13sr20chris
- Posts: 4148
- Joined: Tue Jun 10, 2003 9:32 am
- Car: '89 Nissan S13 w/redtop running 13psi and not leaking fuel anymore
- Contact:
yeah, but it lends to supposing that longer stroke is more efficient in terms of friction than a bigger bore. thats ok, but i kind of came in to this with a prediliction towards bore. not very scientific i know. i never would have thought about the viscous friction thing. this definitely puts a whole new slant on things. so, now we have the ring to bore contact area as one strike against bore and lower piston speed(could only be a bad thing is this discussion) as another.