Aluminium in valvetrain applications
The pursuit of power in fixed-capacity, naturally aspirated engines has generally driven designers and engineers to increase engine speed. Simply put, engine capacity will dictate maximum torque (making some important assumptions), and the product of torque and speed equals power.
There are several limits on increasing engine speed, but one of those against which engineers continually find themselves battling is the valvetrain, and in particular being able to keep the valvetrain under control. In order to do this, engineers generally like to make the system as stiff as possible between the cam lobe and the valve head. Reduction of reciprocating mass is also extremely important if we are to prevent parts of the valvetrain becoming separated from each other as the engine speed increases.
In seeking to minimise valvetrain reciprocating mass, aluminium is an attractive material. If we look first at the common valvetrain reciprocating components between overhead valve and overhead cam engines – namely the valve, spring, retainer, collets and lash cap – only retainers are regularly produced from aluminium, and these need to be examined regularly. Their bad reputation is mainly a result of people overstressing them or failing to examine them for signs of wear or fatigue. Aluminium metal matrix composites are also used for this purpose. The weight saving achieved through using aluminium retainers is certainly a useful proportion of reciprocating valvetrain mass.
Aluminium collets are sometimes used in race engines, but these offer very little weight-saving potential compared to larger components such as retainers.
Aluminium inlet valves have undergone trials in race engines, and some automotive car manufacturers are very interested in developing these for use.
For overhead cam engines with inverted bucket followers, aluminium has been used with some success by some car manufacturers, but such followers require a ceramic or steel shim against which the cam lobe acts, and can also place a limit on the maximum valve lift velocity that can be achieved. The maximum lift velocity is directly proportional to the maximum eccentricity of the cam-to-follower contact from its zero-lift point. The shim used with aluminium followers requires a ‘wall’ around it to retain it, and so the contact surface is appreciably smaller in diameter than the follower. Since the limit here is often the inability to fit a larger follower owing to the space between adjacent valves, using an aluminium follower with a separate shim can be a disadvantage.
In overhead valve engines, aluminium pushrods have been used with varying levels of success, but where engine speeds and valve accelerations are high, aluminium pushrods can struggle to be stiff enough. They are used for some sporting applications, but even if they were allowed in applications such as NASCAR Sprint Cup, it is unlikely that anyone would choose to use them.
In engines with pneumatic valve return systems, aluminium is a common material for pistons.
The drive to use more aluminium in valvetrains is important to passenger engine manufacturers, as lower valvetrain mass means a shorter, lower-load valve spring can be used. This minimises valve length (and thus mass), and means it is often possible to lower the height and mass of the engine appreciably. This means that a lighter structure is required to mount the engine, and therefore a lighter crash structure. The car, which is now much lighter, performs better in terms of transient response. The valvetrain and cranktrain components (particularly piston assembly mass) are very potent in this respect, allowing significant weight savings to be made in terms of engine mass as well as overall vehicle weight.
Written by Wayne Ward