INFLUENCE OF ECCENTRIC THROW ON THE VALVE.
As reducing the travel of the valve diminishes the port opening, a point is reached in cutting off early in the stroke where the port opening is hardly any more than the port opening due to the lead. This is what makes long steam-ports essential for a successful high-speed locomotive. The best-designed engines give an exceedingly limited port opening at short cut-offs, and badly planned motion sometimes seriously detracts from the efficiency of the engine, by curtailing the opening at the point where a very brief time is given for the admission of steam. The magnitude of the eccentric throw exerts a direct influence on the port opening when cutting off early. A long throw tends to increase the opening, while a short throw reduces it. The long-throw eccentric will draw the valve farther away from the edge of the steam-port, when admitting steam for the same point of cut-off, than a short-throw eccentric will move its valve. For an ordinary 17 × 24 inch locomotive, the throw of eccentric should not be less than five inches, unless the engine is intended entirely for slow running. There are many engines running with eccentric throw less than five inches, but they are invariably slow unless the valve-lap is very short. With an ordinary lap, an engine having an eccentric throw of 4½ inches needs so much angular advance to overcome the lap, and provide lead, that the rectilineal motion of the eccentric is very meager at the beginning of the stroke. That is, the center of the eccentric is traveling downward in its circular path, which gives little motion to the valve, just as the crank gives decreased motion to the cross-head when near the centers.