All About Engines

The Oil Engine 241 for a longer or shorter part of the stroke through the linkages shown in the diagrams. Diesel engines are constructed with trunk pistons or with cross heads and connecting rods. In the latter case they are usually enclosed, high-speed engines with forced lubrication. As the system of forced lubrication has been described in connection with steam engines in Chapter VI., it need not be referred to again. But there are interesting facts about the construction which deserve record. The pistons are made in two parts—the head and the skirt. The former has to resist rapid changes of tem- perature and the latter has to resist wear. The head is made of a special iron, low in phosphorus, which will stand temperature changes without crack- ing, and the latter is made of a hard, close-grained cast iron of good wearing qualities. Further, a cavity in the head is filled with asbestos, which prevents some of the heat from reaching the pin upon which the upper end of the connecting rod turns. Now while the presence of lubricating oil above the piston is not attended with the risk of premature explosion, as in gas and petrol engines, it is never- theless objectionable, because it may permit over- heating to occur. And it is extremely difficult to prevent oil creeping past the cylinder rings—that is to say, more oil than is needed to lubricate the rubbed surfaces. The lower ring does not scrape off the excess of oil splashed up from the oil bath, because it cannot push before it a thin film of oil of consider- able area. But in the Mirrlees Diesel piston there Q