Variations in piston structure
The piston is at the heart of the reciprocating internal combustion engine – it draws in the fresh charge, helps to expel the burnt combustion products and forms an important part of the combustion chamber itself, fundamentally affecting flow and combustion.
The structure of the piston has also been the subject of a lot of work by various suppliers in recent years, and this comes on top of a great deal of evolution in the previous three decades although, with few exceptions, aluminium is the material of choice for race engine pistons. Originally pistons were basically cylindrical, with a full skirt extending 360° around them. So-called ‘pot’ pistons are still produced for some classic applications, but ‘slipper’ pistons, with much smaller skirts and limited to bore contact on the major thrust faces of the piston, are very much lighter.
Lighter pistons are very good for engine performance, as they allow an engine to run to higher speed because inertia forces are kept as low as possible; for a given crank stroke, inertia forces increase with the square of crankshaft speed. The bearings will have a limiting load/pressure, and this is reached at lower engine speed with a heavier piston. Reduced bearing loads also mean lower frictional losses, and the smaller skirt area leads to reduced friction at the piston-bore contact.
Slipper pistons remain the norm in race engines, and their evolution has headed towards ever-smaller skirts. However, a few new developments have come to light in the past couple of years.
One US piston manufacturer has a range of pistons that aim to restore some of the rigidity of the pot piston while maintaining the low mass of the slipper type. The pistons it has developed could be said to be similar in principle to the pot piston, but with skirt relief over much of the diameter and then much of the relieve area machined for lightness, leaving a number of struts.
A British piston producer has developed a different new concept, which aims to divorce the crown and skirt in terms of structure, and this was studied in the February 2014 issue of Race Engine Technology. In a slipper piston, the skirt is joined to the crown, but the new concept allows the skirt to flex without affecting the crown, and vice versa. Usually, as the piston crown flexes under combustion loads, the skirt contact is significantly affected, and this is partly why the piston skirt is machined in the way that it is. By the deliberate uncoupling of the piston crown from the skirt, the crown can flex without deforming the skirt, which can then be optimised independently.
I saw a similar concept tested in a high-speed engine some time ago, but it lacked the structural support for the piston crown which exists in this new concept. There is a forthcoming focus article on pistons in issue 78 of Race Engine Technology, which will discuss new structural concepts among other subjects.
Written by Wayne Ward