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Aviation History
1916
1916 - 0083.PDF
JANUARY 27, 1916. I/XIGHT] THE 160 H.P. MERCEDES AERO ENGINE. Continued from page 42.) REVERTING to the disposal of material, each of the lower halves of the five central bearings for the crankshaft is supported by a bridge integral with the casting, while in the upper half of the crankcase each top half bearing is housed in a partitioning wall—or rather in two, since each is in duplicate. Each of them, though very thin, is con nected to its neighbour by a solid portion in such a way as to give strength to the whole, while to further reduce weight large circular holes are left in each wall parallel with the crankshaft. Moreover each lateral wall, which is, of course, integral with the main casting, is webbed at its junction with the body. Through the duplex partition " Flight " Copyright. The double ball-thrust washer at the front end of the Mercedes crankshaft to take the pull of the propeller. long steel bolts, two to each, pass downwards, through the bridges in the lower half of the crankcase, beneath which they are held by a castellated nut and split-pin. The upper end of the bolts emerge through the top of the base-chamber between each cylinder, and by means of a yoke and single nut serve as holding down bolts to the cylinder base flanges, no yokes, of course, being used for holding down the outer corners of the flanges of the two extreme cylinders, the bolts here passing through the flanges and taking the nut direct. These long bolts, therefore, take the weight of the bottom half of the crank case, and serve to bind crankcase and cylinders into one ringed unit. Naturally the two halves of the crankcase are further connected together by bolts passing through the faced flanges of each. By this construction, of course, a great deal is lost in accessibility, since the engine has to be removed bodily from the aeroplane, and completely dismantled to give access to the main and big end bearings, while even to remove a cylinder the whole of the overhead valve gear has to be first detached. Owing to the infrequency with which attention to the crankshaft bearings should be necessary, however, the first named drawback is perhaps not so important as it appears at first sight, while it gives the advantage, as we have said, of immense stiffness with light weight. Likewise the cylinders, of which there are six separately mounted, are, considering their size, remarkably light. Apparently these are of cast steel, or perhaps a particularly tough grade of cast iron, since one was considerably dented—but not cracked or fractured— by shots. In thickness the walls are approximately -^ in., and the water-jackets, which are separate, are also of steel, welded on to the cylinders. A complete cylinder with water-jackets, valves, valve springs, two ignition plugs and two water connections, weighs 22J lbs., and of this the two valves alone account for exactly the odd 2% lbs. Each valve without its spring or spring retainer weighs § lb., but seeing that the diameter of the head is no less than 72 mm. this may be reckoned as quite light. From our illustration showing the shape of the valves it will be seen that no attempt has been made to streamline the under face, or to strengthen it at the junction of head system, and it might be thought that the valve in consequence was anything but reliable, especially in view of the great tension on the valve springs. However, since maximum revolution speeds are probably no more than 1,200 to 1,250, they are no doubt fully equal to their work, more particularly as the cam profiles are not designed to give more than a moderately quick opening and closing and comparatively small lift. The stem of the valve is 12 mm. in diameter, and is threaded at its end, a retaining collar and nut screwing on to this for placing tension on the spring, these being retained in position by a split-pin pressing through stem and nut. The coned face of the valve is narrow, and the stem works in a guide of the maximum length allowed. Bore and stroke is 140 mm. by 160 mm. so that the valve diameter is slightly more than one-half the bore, it will be observed. This is made possible, of course, by the domed head of the cylinder, and also by slightly recessing the walls adjacent to the head for the accom modation of the valves. No cages are fitted for the valves, the seatings being cut in the top of the cylinder, and the latter do not appear to be particularly well cooled, since there is little room for else than the valve ports, the water spaces being here confined to a narrow channel set transversely between the ports. Perhaps the most interesting novel feature of the Mercedes engine is in connection with the pistons, which are built up of two parts. Crown and gudgeon pin bosses constitute one of these and the piston walls the other. The former is apparently of steel and the latter of cast iron, the two parts being united by autogenous welding, there being sufficient depth to the " skirt" of the crown plate for the purpose. The piston heads are slightly concave; there are three compression holding piston rings and one scraper ring at the base. These are of the usual eccentric type, of cast iron, the depth of the ring varying from 3 mm. at the gap to 4^5 mm. at a point opposite. Each ring is 4*95 mm. wide and rides free in its groove, i.e., the position of the gap is not determined by pins. From top to bottom the piston measures 121 mm., its diamenter at top being 139^44 mm. and at the bottom 13974 mm., while it weighs 5jibs, complete with its rings. Inside and out the piston is beautifully finished, apparently having been subjected to the sand blasting process. The gudgeon pin is hollow, of large diameter, slightly tapered in section, and is secured in place by a simple set screw passing through one of the bosses. Though of hardened steel, the pin does not necessarily take the wear of the connecting rod's oscilla tion ; instead there is a floating steel bush, freely per forated, riding between the connecting rod little end and the pin itself. The perforations not only allow oil to pass freely from the inside to the outside bearing faces, but also serve to hold a quantity of oil in reserve and to distribute it. (To be continued. 83
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