The Locomotive Of Today

116 The Engine : Cylinders, etc. admitting steam to the end of the cylinder towards which the piston is travelling before it reaches its extreme position, its velocity is arrested, for the steam forms a cushion, for it to strike upon, thus relieving the axle, etc. The closing of the port to exhaust, as above described, before all the steam has escaped in front of the advancing piston, and consequent com- pression of the imprisoned steam, also assists in cushioning; but it has been proved that the compression alone, without assistance from live steam, admitted by prematurely opening the port by lead, is not sufficient to entirely eliminate “knock.” The exact amount of lead necessary is determined by practice, and will vary from 32 of an inch. in most favourable cases to J of an inch in others; when not otherwise specified, this figure benig taken when the motion is in full gear. Another reason for allowing lead is that it causes the ports to be opened for the admission of steam well before the piston commences its stroke; it is thus enabled to enter at greater pressure and fill the cylinder better than would be the case if the valve only opened just as the piston reached the dead centre and commenced to return, this benefit being specially felt in fast running. Besides allowing live steam to enter the cylinder before the return stroke of the piston commences, it is also necessary to release the steam upon the other side of the piston as much as possible. The cavity of the valve, therefore, opens to exhaust upon the steam side of the piston before the comple- tion of the stroke, so that the port openings, small at the commencement of the valve strokes, shall provide an outlet large enough for the free escape of the steam when the piston commences to return, and thus help to reduce back pressure, which is the name given to the retarding pressure upon the piston, and has to be overcome by the steam upon its other side. The size of the biast pipe opening is an important con- sideration, as if contracted the back pressure is greatly increased, but as the forcing of the fire depends very much upon it, it cannot be very large. The resistance also increases very much when the engine “primes” or “lifts water,” as then the back pressure may be increased as much as 70 per cent, more than would be the case with dry steam. If obliged to pass through passages too contracted for it, the steam will be reduced in pressure or be wiredrawn. The most common causes of this are small ports, insufficient port openings, especially when the engine is notched up close and cutting' off early, or throttling, either in the steam pipes or by having the regulator partially closed. With the ordinary slide valve actuated by link motion there will always be a certain