In studying the conditions affecting the performance of the engine proper, the amount of lead, outside lap, and inside clearance must be taken into consideration.
LOCOMOTIVE BOILERS AND ENGINES
THE LOCOMOTIVE ENGINE
Lead. By lead is meant the amount the steam port is open when the engine is on dead center or when the piston is at the beginning of its stroke. This amount varies from 0 to ¼ of an inch in practice. By having the proper amount of lead, a sufficient amount of steam behind the piston is assured at the beginning of the stroke and assists in maintaining the steam pressure until the steam port is closed and the steam is thereby cut off. It also serves to promote smooth running machinery. Any admission of steam behind the piston before the end of the stroke results in negative work, hence the amount of lead should be limited and largely controlled by the speed of the machine.
Outside Lap. By the term outside lap is meant the amount the valve overlaps the outside edges of the steam ports when it is in its central position. One of the effects of increasing outside lap is to cause cut-off to take place earlier in the stroke, other conditions remaining unchanged. If, however, the amount of lap is increased and it is desired to maintain the same cut-off, the stroke of the valve must be increased. Within certain limits, outside lap increases the rapidity with which the valve opens the steam port, resulting in a freer admission of steam. The range of cut-off is decreased as the lap is increased, other conditions remaining the same.
When the cut-off is short, the exhaust is hastened, an effect which diminishes as the cut-off is lengthened. The amount by which the steam port is uncovered by the exhaust cavity of the slide valve is increased as the cut-off is shortened. Other things remaining constant the changing of any one of the events of stroke causes a corresponding change to a greater or less degree of each of the other events.
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