Automatic Cut-Off Engine

One in which the valve gear which distributes the motor fluid from the generator to the cylinder is so constructed that without human intervention the engine itself will alter the point at which the supply of fluid shall be cut off from the cylinder. The object is to cause the engine under wide variations of load to make the same number of revolutions in a given time. In attaining this, it is sought to supply to the cylinder at each stroke only so much fluid as is nece ary to overcome the load at the specified speed. Automatism is secured in two general ways: the cutoff is varied directly, by the load, or indirectly, by the speed as affected by the load. In the first system, the effort on the engine shaft is transmitted to the load through a series of springs of requisite strength and stiffne on that shaft. The springs are therefore more or le strained as the load increases or diminishes. The positions of levers attached to these springs are made to adjust the point of cut-off in some of the methods alluded to in article ADJUSTABLE CUT-OFF (q.v.). In the second system, a governor driven by the engine shaft is so arranged that the plane or position of a revolving ma shall be varied by variations in the speed of the engine. This revolving ma is so linked to the distributing gear that the valve shall remain open for admi ion of steam a shorter or longer fraction of the piston stroke, as the speed of the governor is faster or slower. This latter is the most usual system, but has the theoretical disadvantage that the variation of load must precede the variation in speed, which must occur before the variation in cut-off can occur which is to offset the first variation and keep the speed constant. The cut-off must therefore always be hunting the load, and cannot get nearer to it than a certain number of revolutions. But, practically, by running at a high number of revolutions per minute and with a sensitive governor the time becomes very small during which such irregularity may exist, and the percentage of irregularity correspondingly low.