It is often said that history repeats itself, to show that we have done it all before. Almost all (new) developments are not really new but mostly improvements of already known concepts. In the business of engine design this seems equally true. Almost all engine concepts have been tried before, some with success, others less so.
We humans are a very conservative species, especially when it comes to engines. What we have been doing on internal combustion engines for the past 100-plus years must be good, we all tell ourselves, and all the money spent on developing them cannot be thrown away just because a new concept could (or would) provide added value. So let’s stick with the piston engine and focus on the area most related to efficiency – the contact zone between liner and piston rings.
Efficiency is the magic word when it comes to engine power and fuel efficiency, both of which are important factors in motorsport. The principle is simple: get as much energy as possible out of the fuel and air being burned in the combustion chamber and transfer it to the driven wheel(s).
As engineers, we learn that the delta between power and efficiency is the friction in the various stages between the combustion chamber and the wheels, as well as the energy lost in the exhaust gas and the coolant. The main area of friction is in the piston package, mainly the friction between piston skirt and liner, as well as between piston rings and liner.
The friction between the rings and the liner depends on many factors. Some are to do with by the chosen engine concept, others are related more to the chosen material specification and geometry of the piston rings and liner. Let us look at both.
The piston rings need to follow the liner in all circumstances. In doing so, they need to have as little blow-by as possible to maintain the compression and performance. Piston ring life will be much longer when the liner’s roundness is very accurate, and this is influenced mainly by how homogeneous the engine concept works out on initial liner deformation – the liner’s deformation after assembly of the cylinder head and in hot running conditions.
An even cylinder head bolt spacing, acceptable cylinder head bolt forces and homogeneous cylinder head gasket loading are key. That way the piston rings can be designed to have lower radial force on the liner without losing contact, all leading to lower friction.
Material specification and geometry
Assuming the ‘right’ engine concept has been chosen, there is still a contact zone between piston rings and liner to be defined in more detail. On the ring it is desirable to have as little contact as possible without actually losing contact, and this can be achieved by properly shaping the contact zone (the so-called barrel shape) and choosing the correct surface finish and treatment. In modern engines almost all rings are treated with different types of coating (PVD, DLC, nitrided and so on) in order to reduce friction and wear. As soon as the ring starts to wear, the contact surface increases and so does friction.
On the liner there are also a couple of parameters we can influence to improve the contact area. From the very beginning, engineers have worked on the surface finish of the liners, and through various steps we have developed state-of-the-art honing procedures. Where at first the roughness was described simply as maximum/minimum values of the profile, we have since learned that the ratio between deep and shallow grooves can also be used to optimise the honing profile, leading to lower wear rates and lower friction and blow-by. Other, more apparent factors such as bore cylindricity and liner straightness remain of equal importance – all very obvious parameters, but still difficult to maintain in all circumstances.
Of course this is a very simplified view of this topic, which describes only the main influences of the contact area between piston rings and liner on friction. And as long as we continue to use internal combustion (piston) engines, developments will continue in this area to further improve the efficiency of our engines, making it difficult for other engine concepts to come to prominence.
Written by Dieter van der Put