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Piston (additional information)



The piston seems to be getting more and more squat, because e.g., a lower compression height keeps the weight down and reduces the construction height. For the complex mechanism, the opposite way around, with long con-rods, would in fact, be even more favourable. Because the pistons have also lost parts of their shafts, the transferring of the heat coming from the bottom of the piston, onto the cylinder, becomes increasingly difficult. Nowadays, this is mainly taken over by the compression rings. This is why some uncharged engines today also use an oil-spray cooling. In turbo-charged engines, the inter-cooler has a similar effect, although not quite as precise.

Even the experts are perhaps astonished, that in relatively staid Diesel engines, some need extra long hubs for the gudgeon-pin bearings and others don't. The possible hardness of certain materials can, and particularly under the influence of heat, be predicted by using sophisticated procedures. This poses the question, of how thick the piston-crown should be and how far it's, perhaps curved surface, must be carried over to the other side. The Diesel engine has, in addition, a comparatively large piston chamber which also limits the design options.

Piston-friction must certainly be considered. Previously, one didn't take much notice of it, probably because of the artificial roughening through honing. Nowadays, this measure, to maintain the lubricity, is deeply embedded in the planning. E.g., through flame-spraying. The roughness of the sleeve is already specified before the manufacturing, and regardless of how the engine is run-in, it hardly changes at all. The frequently applied protective coating of e.g., graphite, has already been mentioned. It's amazing that all that's necessary, are differences on the surface of a few thousandths of a millimeter.

It is incredible, just how well sealed modern pistons are. If, in theory, 250 cm³ fit into the cylinder, on average, at the most, do only 2,5 cm³ escape, in a turbo-charged engine it's a little more. The low oil consumption of modern engines is also phenomenal. Depending on how the vehicle is driven, 10 - 20.000 kilometers without topping up, are quite possible. In this respect, the stricter environmental regulations have helped, since oil is also lost, particularly through the blow-by gases and in addition, these have to be decontaminated afterwards. Therefore, the better solution is to prevent their escaping in the first place. 09/12

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