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Piston Test

Not only the size and shape of the piston is going through a changing phase. In some cars the all-time, highly praised aluminium piston, now has to make way for steel components. In trucks, the aluminium piston has long lost it's place. Thus, the previous differentiation between auto-thermionics and auto-thermatics is now only a question found in outdated journeyman's examinations. What has remained, is a certain amount of ovalness and an uneven amount of play in the gudgen pin and the counter-direction caused by a certain material accumulation when warming up. The less rigid shaft can also adapt itself better to changed clearance. A little newer is the deliberately slanting form of the piston-skirt, which now has contact, only in a narrow zone almost right at the bottom. Due to the higher strain on the pressure-side, it is no longer quite symetrical.

If one remains with aluminium, e.g., in the petrol engine, the more highly strained pistons are forged. Apparently, the forged piston can be held to narrower tolerances, even when high demands are made of it. Of course, one is no longer as free when designing the interior of the piston below the crown, which does play a very important part when high demands are made. After all, the compressive form must be able to enter and exit. In this case, unsupported surfaces are impossible. All in all, one can observe an incredible amount of development, particularly in the racing field, with average piston speeds of up to 25 meters/sec.

The choice of material for the piston depends largely on the material that was used for the sleeves. After all, they are partners as far as friction and heat-expansion are concerned and they must be compatible during all possible temperatures, pressures and operating conditions such as mixed- or dry-friction. The demands here are high, e.g., up to 120 bar combustion pressure in the petrol engine and 200 in the Diesel motor car, in truck engines it's even higher. The peak temperatures for aluminium are 400°C, for steel they are even higher, 500°C. This is why a steel piston needs external cooling more than an aluminium one does.

Another point to consider, is the piston acceleration. The last remaining sports cars with natural aspiration engines, could manage up to 10.000 RPM, in the meantime, Formula 1 cars achieve about twice that amount. The result is, piston speeds of way above 25.000 m/s², despite the engines having an extremely short stroke. Pistons for these engines have incredibly low compression-heights and virtually no shaft. The danger of tilting and the noise development seems to be of secondary importance. In some Diesel engines the compression-height can not be reduced any further because the piston chamber, together with sufficient sleeve thickness and the, compared with the petrol engine, much thicker gudgeon pin, require sufficient room.

The sliding quality of the piston is influenced by the piston rings and their pre-tension, and sometimes also their separately processed surfaces, the ring-zone itself however, is not influenced. On the contrary, one tries to avoid any contact between this and the cylinder sleeve. At least, as a rule, one only needs to worry about two compression rings and one oil-ring. The only important ring-zone measurement is the distance between the top piston ring and the piston crown, which is determined by the pressure and the temperature. The gap to the following rings is then, distinctly smaller. Because the rings have become thinner, the ring- grooves have also been reduced in width.

Apparently contradictory, is that which has happened to the shaft. On the one hand, it should guide the piston, on the other hand, it has become smaller. There, where it does no guiding, e.g., the gudgeon pin area, it has disapeared almost completely. To make the contact surface changeover more acceptable, apart from the axial offset, there is now also the reduced piston clearance, which is compensated for by the seemingly elastic sleeve-walls. Thus, a part of the piston guides and nevertheless, adapts itself to certain conditions. Another incredible feature is, what the tight guidance of the, in the meantime, almost always coated pistons, has meant for the noise reduction in our engines. 09/12