If you ever tried to roll a piston across the floor you have probably noticed that it’s impossible to roll them in a straight line. No it’s not your floor, you can try it on the most level surface on earth and piston will never roll in a straight line. It will always deviate from a straight line and roll in the direction of the crown. If you had enough space you would notice that the piston rolls in a giant arc. So what does this tell you? It tells you that the piston sides are not straight, it’s not a perfect cylindrical shape and you can easily confirm this by putting the piston on a level surface and having a light source behind it.
If you rock the piston from side to side you will notice that the sides are actually tapered. From top to bottom the piston incorporates a taper, with the top being smallest and the taper increasing towards the bottom. So why do pistons have a taper? Why don’t they have straight sides just like the cylinder they are supposed to fit in?
The reason for the taper is the very nature of the internal combustion engine and the heat that it generates. And pistons being made from aluminium do expand under heat, but all parts of the piston are not exposed to the same amount of heat. The crown being closest to the combustion expands the most, and the skirt expands less. This is why most room for expansion must left at the crown to account for this. If the piston had straight sides we would have one of two negative scenarios. We would either have good ring seal at the top at the expense of loose piston skirt and piston slap at the bottom, or we could achieve a properly fitting piston skirt at the expense of a ring pack that is too tight and has potential to seize or bind in the bore.
What happens when we look at the piston from the top? Surely it must be perfectly round in this case because it’s supposed to fit into a perfectly round cylinder. Well, the answer is both a yes and a no. The ring pack is indeed perfectly round because it’s trying to achieve the best possible ring seal with the cylinder, but everything from below the bottom ring land isn’t round. It’s actually egg shaped or oval, and to see why we have to remember what happens to the piston during combustion.
As combustion occurs, combustion pressure forces the piston down the bore, at that point the sides of the piston, which are at 90 degrees to the piston pin, experience the most load. The sides of the piston are the major and minor thrust side. In clockwise rotating engines, which account for the vast majority of engines, the major thrust side of the piston is always the left side of the piston when looking at the engine from the front. On counter clockwise rotating engines the major thrust side is the right side of the piston when looking at it from the front of the engine. The side opposite to the major thrust side is the minor thrust side.
The piston skirt along the major and minor thrust sides must follow a perfectly round contour because these are the load bearing sides of the piston and they need maximum stabilization in the bore. The axis of the piston along which you can find the major and minor thrust sides is of course called the thrust axis. But what about the other axis of the piston, the one along the piston pin? The situation is different here and the skirt along the pin axis does not need to follow a perfectly round contour because the piston pin stabilizes the piston in the bore, and the skirt along the pin axis isn’t load bearing.
This is why many pistons don’t even have any skirt along the bore, and when they do have it they incorporate an egg shaped contour to reduce friction. If the piston skirt had a perfectly round contour along the pin axis it would generate additional friction without any strength or stability benefits.
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