An Improved Engine for Commercial Motors.
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W. H. Dorman and Co., Ltd., Stafford, has carried out several improvements in its engines intended for use on commercial motors. These are now made to comply with all the requirements of the War Office subsidy scheme. The engine is turned out in three sizes, the bore being the only variatidn in each model. The smallest bore is 110 mm., the others being 115 mm. and 120 mm. ; the stroke on all the engines is 140 IMP. The lowest-power engine develops 37 h. p. at 1000 r.p.m. The carburetter is situated o,n the right-hand side of the cylinders. A passage is cast between each pair of cylinders, in order to allow gases to pass to the inlet valves. Hot air is sunplied to the carburetter in an ingenious manner. As the exhaust pipes are at the side of the engine remote from the carburetter, it was _hard to provide an external hot-air pipe pleasing in contour and satisfactory in service. This has been done by casting a passage at the forward end of the sump, an external elbow joint leading the hot air from the exhaust pipes through the sump passage to the carburetter The Bosch magneto is driven through a universal coupling, which may be instantly adjusted and set to any position, independent of the driving gear, by loosening one screw. The magneto itself can be detached by loosening one wing nut. The crankcast„ is of aluminium, all the bearings, etc., being in the top half. Two large inspection covers fitted with handles are provided, and the sump is detachable without disturbing any of the other parts of the en gine. The crankcase contains a governor for the carburetter, and this operates by means of internal rods to a separate throttle situated in the induction pipe. Lubrication is by means of a gear-pump driven from the camshaft, oil being forced through a drilled crankshaft to all the main bearings. Oil is poured into the sump from the top of the fan bracket. Thermo-syphon cooling is employed. Arrangements have been made to fit a pump immediately in front of the magneto, and this is so constructed that, in the unlikely event of a breakdown, the water has still a free passage. The valve gear is enclosed by two aluminium inspection plates, and, owing to the absence of pipe work on the engine, is extremely accessible.
The valves are operated by a solid camshaft, the cams being of a special design and giving the
valves a lift of half-an-inch. The camshaft and the crankshaft are run upon three bearings, and are made of good die-cast, anti-friction metal. The maximum bearing pressure is 250 lb. to the sq. in. The valves are of a diameter almost equal to half the bore1 and are interchangeable. The pistons are of special design and embody a unique type of piston ring. At the top of the piston a very deep groove has been turned, and into this groove fits a single broad piston ring of the eccentric type, Over this ring are sprung two concentric rings half the width of the under ring. It will be seen that these two parts have simply to perform the function of keeping a compression and explosion-tight joint, whilst the under ring provides the spring. The parts on the Dorman engine are all made to limit gauges and are interchangeable.