n smikle wrote:What determines the shape of the skirt?
How do you figure out the rings?
Which parts is the temperature change more critical to?
I will try and fill in the bits I remember...
Skirt:
I'm sorry I don't have figures but certainly the temaperature change is not the only determining factor in designing the barrel shape ( ) of the skirt. The piston goes through a rolling motion as a result of the pressure applied to the thrust face on the power stroke, and anti-thrust face on the compression stroke. The barrel will help reduce wear as the very edge of the crown is pushed into the cylinder wall, but it must keep the piston as close as possble to vertical to help the maintain the ring seal.
There have been over the years, invented many ways of modifying the surface of the barrel and ring land diameters to achaive better sealing and reduced friction, some of these work. One of the most widely used is the sawtooth finish on the skirt.
- A tapered cutting tool is used on the eliptical lathe
/ to cut this pattern in the skirt, it reduces contact
- area and helps drag oil up the cylinder walls towards
/ T.D.C. where it benefits the rings most.
-
/
Also a sprayed coating is quite often used on racing pistons, this is often purported to imporove the break in time, i.e. the time it takes for the piston to 'bed' into the perfect shape and finish for it's particular bore, but different engine builders have different views on it's actual worth.
One of the other notable features to have gained widespread acceptance as a good idea is a series of square nothches cut (turned) above the top ring, developed, I believe by Cosworth originaly they are actually designed ever so slightly larger than ideal so that the top's of the nothches will actuall interfere with the bore and the resultant, very slight micro welding/gaulling will, in effect self seal any microscopic imperfections in the shape/surf. finish at this critical area. The releif in the notches allows flowing material to collect there and prevent seizure.
Rings:
As with the rest of the Piston, something of a black art. There are sumulation programs available to help with this but only a handfull of companies will have them, Mahle and FM for instance, I beleive Ricardo have an off the shelf program to simulate the piston assembly to this level.
The rings are mostly specified by the ring manufacturers, with varying degrees of sucsess. FM/Goetze Germany were very good at this in my experience, some of the North American firms not so much. The amount of extra power that can be released by carefully chosen rings is almost unbeleivable. Taking off the shelf rings from two companies, changing from one top-spec ring pack to another can add as much as 1500rpm to an already very highly revving race motor.
The need to reduce friction must be balanced against the need to mantain a seal, one of the more inventive ways round this is using a more softly sprung ring and using the combustion pressure to force the ring against the cylinder wall with greater pressure on the power stroke only, therby encountering less friction on the other 3 strokes. This is acheived by machining the uppermost face of the top ring groove to allow gas to pas behind the ring, the depth of ring grooves must be carefully considred to keep the volume of the gas here as low as possible. In more extreme cases, i.e. drag racing, gas ports are drilled throgh the crown of the piston feeding in directly behind the ring, a very softly sprung ring can be used but oil loss is high, no problem on a quarter mile race...
The rigidity of the piston is also crucial to aid ring sealing, with the best rings in the world you will not seal a piston that twists/de-forms under load (although they all do to some extent obviously).
Temp change:
The piston always grows more in the direction of the gudgeon pin because the pin boss area is the most material rich area of the undercrown. More metal in a more rigid arrangement, more growth.
Racing pistons with slipper skirts have very little else in the way of solid, blocky areas of metal, the skirts are braced by thin butresses which terminate at the pin boss and have an element of flexibilty meaning the oval turned shape of the piston is at its largest in this area (at room temperature).
In actual fact slipper pistons, due to having no skirt on the sides only have a very small area where the diameter is really smaller, but this is critical.
Think of looking at the top of a 4 valve piston with the valve cutouts vertical, almost like so...
Well, the piston grows more top to bottom than side to side threfore will be smaller in that direction at room temp.
You will also see what looks like a small additional ring groove below the 2nd comp. ring. This is usually a V shaped groove to help allow expansion and therefore de-compression of any gasses which may have escaped past the ring and further help prevent presurisation of the crankcase.
). I worked in a small piston manufacturing outfit for a while and it always seemed to be Mahle Samples we were receiving when translating a road car piston into something more racy. 
