Pressure and heat, this is what the piston has to withstand. As a hollow cylinder with a lid on, it seals the combustion chamber and converts pressure into mechanical force. This force is passed on through the gudgeon pin and the connecting rod to the crankshaft. The result is that the to and fro motion (oscillation), is turned into a rotary motion. The force caused by the combustion pressure cannot be applied completely to the con-rod because almost all of the time the con-rod has a slanting position in relation to the piston. The result is, apart from the so-called rod-force, a lateral force is also applied.
The amount of lateral force applied depends on the slant of the con-rod. E.g., it increases on it's path from TDC to BDC, until the con-rod and the crankpin form a right angle. After that, it decreases again until it reaches the other side. This change-over has been deliberately placed in a low pressure-force zone, to enable the change-over to take place with as little strain and as quietly as possible. Thereby of course, the amount of play that the piston has when tipping, is important.
The incredible speed-changes that the piston has to cope with are a problem, in the course of one stroke it moves from a standstill to a top speed of up to 30 m/sec. and back again to standstill. This is the reason why, as far as the piston is concerned, using materials sparingly, and using material combinations to reduce the oscillating mass, were tested very early on. The lubrication also suffers under these speed conditions, because the, abolutely necessary hydrodynamics cannot be built up at a virtual standstill. Todays pistons reach an average speed of up to 20 m/sec., in the racing field they are of course, even higher.
Apart from having the task of making sure that under all temperature-, pressure- and speed conditions, the necessary sealing against gas- and lubricant is maintained, the extra task of influencing the flow of the gases entering the combustion chamber is also added. In particular, the direct-injection, and derived from that, the stratified charging, places high demands. Furthermore, the sleeve-walls are so far developed, that even under unfavourable circumstances, a seizing up, causing the penetration of piston- and sleeve material is prevented. Operational readiness has priority, even under unfavourable conditions.
Of course, in the case of the two-stoke engine, there is also the port-controlling. At least here the pistons are fairly symmetrical through the loop- or uniflow scavenging and they don't have any one-sided accumulation of mass, which, when the demands are higher, can pose a problem. The two-stroke principle is found in very small and also very large engines where the lateral forces being applied to the piston, through the slant of the con-rod, are absorbed by crossheads. Not only here has the service life been extended.
Thereby, the pressure on the modern piston, due to the massive amount of charging, is enormous. The inertial forces, which have to be passed on under this type of pressure, can often amount to several tons. Especially strong are the demands made, e.g., with incorrect injection amounts and -times (Diesel engines) or knocking combustion (petrol engines). Here the sudden pressure increase can be an additional strain and should nonetheless, not lead to a complete breakdown. Such breakdowns are protected against by sophisticated electronics, but also through protective-coated mechanics. 09/12