Pistons may be cast iron or, on higher speed engines, aluminium alloy. The use of alloy pistons reduces weight and therefore bearing loading, and the loading on the cylinder walls. Allowance must be made for the larger expansion ratio of the aluminium alloys, and so large piston to cylinder clearances (cold) are adopted. As the engine warms up and expansion of a piston occurs this clearance reduces. For this reason, care should be taken to avoid overheating such a piston. Aluminium also suffers from carbon build up with possible burn out when heated above 3000C.
Cooling of these pistons may be achieved by spraying oil onto the under side of the crown. This spray is taken from the lubricating oil fed to the top end. However, if bearing wear occurs, the oil leakage there reduces the flow so that the piston cooling supply also suffers. Improved and more easily regulated cooling can be achieved through coils cast in to the piston crown. These may be fed from the oil supply to the gudgeon pin or may be individually supplied via telescopic pipes. A further problem associated with aluminium type pistons is that of rapid ring groove wear, which can be overcome by casting in ‘ring inserts’ of a harder wearing material and which, if correctly shaped and cared for last the life of the piston. The crown of the piston may be shaped to accept the open valves as the piston passes over TDC on the exhaust stroke. The shaping may be further arranged to ‘squeeze’ the air into the centre of combustion chamber as the piston reaches TDC on the firing stroke. This provides a compressed volume of rapidly moving air into which the fuel can be injected. The air movement ensures adequate mixing between fuel and air so that complete combustion is more easily achieved.
Piston rings will be similar to those in slow speed practice; cast iron alloyed with various proportions of one or more of manganese, molybdenum, or chromium. The top rings may have inserts of chromium for extended life, or possibly bronze inserts to facilitate running in. Running in may also be enhanced by using taper faced rings (1 deg). This limits the contact surface during the first hours of running so that a seal is more quickly established, the ring gradually bedding in to provide the full running face as a seal. On one type of engine the rings are pegged to the piston so that they rotate with it. The piston is made to rotate by an ingenious rack and mechanism, incorporated at gudgeon pin lever, being activated by the swing of the con rod as it completes a cycle. Instead of a gudgeon pin, as such, a spherical end to the connecting rod allows complete and continuous rotation of the piston. The effect of rotation of the piston is to distribute oil around the cylinder wall much more evenly arid accurately than can be achieved with the normally accepted procedures.