If a Diesel engine piston, because of the high strain, needs an extra bushing for the hub, then a further material is taken into consideration, namely steel. It has asserted itself in the entire truck region. It's target is now the field of high-torque Diesel car engines. The increased amount of cooling for this material is achieved by intensifying the already available oil-spray cooling. In addition, because aluminium, also as an alloy, must have greater dimensions than steel, the weight disadvantage is no longer that serious.
If we assume different tasks for different areas, we may as well also divide the piston and make the parts from various materials. In the field of heavy truck-engines the piston is either divided or the skirt area is almost completely separate from the upper part. The area between is strongly waisted and if looking from a cut-away point of view, the ring-zone will only be perceived as an appendage. Through this, the cooling channel will basically, be accessible from the outside. It has to be closed at the bottom by a sheet-metal collar.
In the two-piece piston the cooling channel is also free for working on from the outside. Thus the thicknes of the walls between it and the piston chamber can be made somewhat thinner, without having to worry about inadmissible weakening. The result is an excellent heat transfer almost directly to the engine oil. By the way, the piston must not always be divided into an upper and a lower part, a ring-like aluminium casing in the skirt area is also possible. The two parts must not necessarly be screwed together, they may also be welded.
As long as we have such a nice picture like the one above, of the cold-starting and test-running of an engine, we should perhaps once more, deal with one of the most important measurements, the installation clearance. For todays lightweight pistons it amounts to 0,4 mm, in aluminium cylinder blocks it's less, and in Diesel engines it's more. Thereby, the measurement, which was previously necessary for regulating pistons with cast steel strips, is achieved. By the way, apart from the regulating piston, the shrinking on of the gudgeon pin has also disappeared, at least in Diesel engines it has. Normal sleeve-bearings are used, they are of course, secured against unwanted shifting.
Something not at all fitting to the above picture, is the observance of the temperatures. In this case, one must also differentiate between petrol- and Diesel engines. The former heats the piston crown quite evenly, the latter places particularly high demands on the rim-edge. Astonishingly, it's not so much the actual high temperature that is decisive, it's the effect on the properties of the piston material when it is permanently exposed to these temperatures. In addition to this, although modern engine-oils can withstand higher temperatures, oil-carbonizing does still occur in the region of the upper piston-rings, which again, increases the temperature sensitivity.
The performance of a perhaps available oil-spray cooling is amazing. It is the undisputed leader as far as heat transfer is concerned, more effective even than the transfer through the first piston ring or even the skirt, which is much lower down. This can be done with a more complicated oil-spray nozzle on the end of the cylinder or more simply, through a small drilling in the big-end. Indeed, in the first case, the oil- pressure and circulation must be respectively adjusted. Should the upper ring be fitted with an insert, this can be cooled directly through a channel. 09/12
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