Aluminium is lightweight and in it's pure form, too soft to be used as a material for making pistons. It's melting point lies at 660°C, however, permanent deformations can occur at about 400°C. This is the temperature that a piston in a the Diesel engine can reach at the edges of the piston bowl (see picture 1). To increase the hardness aluminium is always alloyed with (e.g. 15%) silicon.

To improve the thermal conductivity, small amounts of copper are also added. By this, one can recognize, just how important it is, that the heat is transferred quickly. Most of the heat is not transferred to the shaft, but from the uppermost ring to the cylinder sleeve. The copper is added to increase the speed of the transfer, even though it increases the density and thereby, also the weight of the piston material.

One can see just how important it is for the operational reliability, to adhere to exact temperature tolerances. The more the piston is stressed, the more one transfers the task of cooling to the lubrication system. In this case, it is decisive, how close the splash-cooling is to the areas where the heat is greatest. Material thicknesses, e.g., also play an important part here. Between the simple splash-cooling of the piston crown, the entry of the oil-splash into the ring channel and the direct cooling of the ring carrier, a gradual increasing of the coolant portion through the cooling system is also possible.

The distribution of the heat in the piston itself is also important. Indeed, a number of components in the combustion engine tend to tense up through being unevenly heated. Uneven expansion through heat makes, e.g., the hydraulic valve-play adjustment necessary. Even modern cylinder heads have a hard time, on the one side they are constantly kept cooler through the inlet ports, and on the other side, warmer, through the exhaust ports. If the large number of hollow areas are also included, cracking can occur, which means permanent damages.

Let's get back to the piston, it's upper section is temperature-stressed, the lower section has more to do with mechanics. With increasing stress, e.g., in utility vehicles or even in large engines with more than 250 bar pressure, the piston is divided, the AlSi upper part is joined, mostly by four expansion bolts, to the lower steel part. The connection between the two transports even less heat than a one-piece piston, because here, the splash-cooling takes over to a large extent.

The weight distribution in the piston is now more balanced, it's not quite as top-heavy. This brings us to a second, important subject, i,e, the mass of the piston. It's back-and-forth movement is only compensated to about 50 percent by weights on the crankshaft. Neither do the offset shafts change the inert forces very much, particularly as they are often still only optional and are mounted under oil sump, thus not cooperating optimally with the rest of the engine.The components which make up the piston are also not particularly light-weight, certainly not the gudgeon pin, which is the heaviest foreign part in the piston.

The bushings are made of a bronze alloy, and the piston rings, previously made from special cast-iron are now distinctly, sometimes made up of several individual components. So, weight must be saved. The, for the piston, very important measure of the compression height, now comes into play. It should be reduced, which is less of a problem for the direct injection petrol engines than for the direct injection Diesel engines. Although the piston chamber is becoming more and more shallow, it is still necessary.

Nonetheless, today's pistons look very different than their older counterparts. They no longer have the shape of a tin can (see picture 3), but have clearly cut-out flanks, which are only there where they have to guide the piston effectively. The gudgeon pin has been moved very far backwards, in the direction of the piston crown. The most extreme example is the racing piston in picture 3. Have a look also, at the amount of clefting on the piston crown in picture 4.

Picture 5 provides at least a partial answer to the question of how the oil for the lubrication of the piston gets to the cylinder sleeve, one can easily see the salt cooling hole from the inside of the piston to the outside. By the way, the oil circuit under the piston crown really does have something to do with salt, during the casting process, salt serves as a sort of core which is afterwards, washed out under high pressure. 07/11