Fig. 249.—Sunbeam Eighteen-Cylinder Motor, Viewed from Pump and Magneto End.
In the eighteen-cylinder overhead valve Sunbeam-Coatalen aircraft engine of 475 brake horse-power, there are no fewer than half a dozen magnetos. Each magneto is inclosed. Two sparks are furnished to each cylinder from independent magnetos. On this engine there are also no fewer than six carburetors. Shortness of crank-shaft, and therefore of engine length, and absence of vibration are achieved by the linking of the connecting-rods. Those concerned with three-cylinders in the broad arrow formation work on one crank-pin, the outer rods being linked to the central master one. In consequence of this arrangement, the piston travel in the case of the central row of cylinders is 160 mm., while the stroke of the pistons of the cylinders set on either side is in each case 168 mm. Inasmuch as each set of six-cylinders is completely balanced in itself, this difference in stroke does not affect the balance of the engine as a whole. The duplicate ignition scheme also applies to the twelve-cylinder 350 brake horse-power Sunbeam-Coatalen overhead valve aircraft engine type. It is distinguishable, incidentally, by the passage formed through the center of each induction pipe for the sparking plug in the center cylinder of each block of three. In this, as in the eighteen-cylinder and the six-cylinder types, there are two cam-shafts for each set of cylinders. These cam-shafts are lubricated by low pressure and are operated through a train of inclosed spur wheels at the magneto end of the machine. The six-cylinder, 170 brake horse-power vertical type employs the same general principles, including the detail that each carburetor serves gas to a group of three-cylinders only. It will be observed that this engine presents notably little head resistance, being suitable for multi-engined aircraft.
Fig. 250.—Propeller End of Sunbeam Eighteen-Cylinder 475 B.H.P. Aviation Engine.