There has been a great deal of evolution in piston design over the years, in which a large number of usually small development steps have given us some really beautiful components. If we are to describe a modern racing piston, we would probably be safe to say it is an aluminium component with a ‘slipper’ skirt, having ribs joining the piston pin bosses to each other and then to the relatively unsupported lateral areas of the crown. This is pretty much the state of the art at the moment, arrived at by improvements in manufacturing technology and by doing lots of hard work testing small incremental changes. Skirts are now very minimal, in an attempt to reduce friction.
However, a couple of new structural concepts that depart quite strikingly from the usual designs have been developed by prominent piston companies, and both are very different from each other. The first, developed by a US piston company, is a skeletal design that ‘ties’ the skirt to the pin bosses more firmly. The middle part of the skirt, which is generally not well controlled relative to the pin boss, is now connected to the pin bosses by bringing the usual webs closer to the piston centreline. The pin bosses are tied to the lateral areas of the crown by very deep ribs, and these in turn are connected to both the skirt and each other by a circumferential ‘stringer’.
It certainly looks a very stiff design, and it is perhaps easier to describe the piston as a very modern take on a slipper piston, but using modern machining techniques to take weight away where it is not required. The company cites improved stiffness and fatigue among the findings of initial testing.
The second concept comes from a UK piston supplier which has gone in the opposite direction, seeking instead to decouple the movement of the crown and the skirt as much as possible. In a conventional piston, the crown and skirt are intimately linked, and the behaviour of one affects the other. By linking both independently to the pin bosses and removing any real stiffness between the crown and skirt, both are free to move without affecting the deformation of the other.
So, under combustion and inertia loads, the skirt is no longer pushed and pulled by the crown. Consequently the skirt profile can be optimised for this, and it has been found that the skirt area can safely be reduced by a significant amount, and this has led to reduced friction. Skirt lubrication is also likely to be improved compared to a standard piston design as there is much more access for oil to lubricate the skirt-to-liner contact as the piston rises from bottom dead centre.
It is interesting to note that both of these suppliers have noted significant, although different, improvements from their new concepts. By straying from the well trodden development path, both have found new areas of performance to explore. We are likely to see both concepts become commercially available in the near future.
Written by Wayne Ward