Magnesium as a piston material
The racing piston market is dominated by aluminium, as are the roadcar and motorcycle markets, although in general the alloys used in racing are quite different from those in passenger vehicles. As racing engineers we look for increased durability and damage tolerance in our pistons, so we tend to make our pistons either as forgings from high-quality wrought material or machine them directly from billets of wrought aluminium. The exceptions to aluminium pistons in racing are those steel components used notably by some successful diesel engines, and aluminium-matrix composites used by a small number of racers where the regulations allow.
The reason that aluminium is so widely used is its combination of properties and its cost. However, people have looked seriously at magnesium in the past as a piston material. While they have found it wanting when compared to aluminium, it is not beyond reason to expect people to look again in the light of new racing regulations and recent developments in engine and manufacturing technologies.
So when aluminium is such a well understood and relatively cheap material, why would people look towards magnesium as a possible replacement? Well, it’s a question of density – at 1800 kg per cubic metre, aluminium is 50% more dense – but the stiffness of magnesium compared to aluminium is pretty much proportional to their densities, (that is, magnesium is about two-thirds of the stiffness of aluminium). With most of the piston’s stiffness being a function of its behaviour in bending, the required increase in section thickness to achieve the same stiffness as aluminium would still leave the magnesium piston much lighter. It is true that the room-temperature strength of good wrought magnesium alloys is lower than typical aluminium piston alloys, but at higher temperatures magnesium stands up to much closer scrutiny. For example, at 250 C the tensile strength of WE43 magnesium is higher than that of 2618, which is a favoured aluminium piston alloy.
With the combination of high-temperature strength, sufficient stiffness and very low density, shouldn’t magnesium be a popular piston material? The most important reason why we don’t use it is its low thermal conductivity – it has around 60% lower thermal conductivity than 2618, which means that without considerable effort put into cooling, the magnesium piston will tend to run much hotter than its aluminium counterpart. This degrades performance, as some of the heat in the piston will be rejected to the incoming fresh charge, raising its temperature and lowering volumetric efficiency in the process, although there ought to be a significant decrease in the heat rejected from the combustion process, so we could expect fuel conversion efficiency to be improved.
Where a set of racing rules rewards increasing power from a given fixed engine capacity, especially with an imposed maximum engine speed, any loss in volumetric efficiency is going to hurt performance. However, where fuel efficiency is at a premium – and this is an important factor in many forms of racing – improved fuel conversion efficiency is to be highly valued. There are types of racing where engine size, engine speed and breathing capacity are free, but the instantaneous fuel flow rate and total fuel load for the race are limited. In these types of racing, where the engine is constricted in terms of performance by an upper fuel flow limit, then fuel conversion efficiency equals performance. With the fashion for regulation that rewards efficiency, the magnesium piston may yet see a new dawn.
Written by Wayne Ward