Fig. 212.—The Gnome Engine Cam-Gear Case, a Fine Example of Accurate Machine Work.
Ordinarily the engine is started by turning on the propeller, but for emergency purposes as in seaplanes or for a quick “get away” if landing inadvertently in enemy territory, a hand starting crank is provided. This is supported in bearings secured to the pressed steel carriers of the engine and is provided with a universal joint between the two supports so as to prevent binding of the crank in the bearings due to possible distortion of the supports. The gear on this starting crank and the one on the thrust plate with which it meshes are cut with helical teeth of such hand that the starting pinion is thrown out of mesh as soon as the engine picks up its cycle. A coiled spring surrounds part of the shaft of the starting crank and holds it out of gear when not in use.
Fig. 213.—G. V. Gnome “Monosoupape,” with Cam-Case Cover Removed to Show Cams and Valve-Operating Plungers with Roller Cam Followers.
Lubricating oil is carried in a tank of 25 gallon capacity, and if this tank has to be placed in a low position it is connected with the air-pressure line, so that the suction of the oil pump is not depended upon to get the oil to the pump. From the bottom of the oil tank a pipe leads to the pump inlet. There are two outlets from the pump, each entering the hollow crank-shaft, and there is a branch from each outlet pipe to a circulation indicator convenient to the operator. One of the oil leads feeds to the housing in the thrust plate containing the two rear ball bearings, and the other lead feeds through the crank-pin to the cams, as already explained.
Owing to the effect of centrifugal force and the fact that the oil is not used over again, the oil consumption of a revolving cylinder engine is considerably higher than that of a stationary cylinder engine. Fuel consumption is also somewhat higher, and for this reason the revolving cylinder engine is not so well suited for types of airplanes designed for long trips, as the increased weight of supplies required for such trips, as compared with stationary cylinder type motors, more than offsets the high weight efficiency of the engine itself. But for short trips, and especially where high speed is required, as in single seated scout and battle planes or “avions de chasse,” as the French say, the revolving cylinder engine has the advantage. The oil consumption of the Gnome engine is as high as 2.4 gallon per hour. Castor oil is used for lubrication because it is not cut by the gasoline mist present in the engine interior as an oil of mineral derivation would be.