The Locomotive Of Today

The Engine: Cylinders, etc. 115 forward gear, then the engine is reversed and similar points found for the backward gear. It is usual to mark the ends of the indicator with the words “fore ” and “ back ” to show the gear the motion is in. Clearance has been described as the distance between the piston and the covers of the cylinders, but when referring to the steam, its action within the cylinders, and its effect upon the working of the engine, as shown by the indicator, the term includes the ports and, in faet, all the spaces filled with steam between the valve face and the piston when at its extreme position. It is not possible in practice to do without clearance as it allows for wear, etc.; but the losses due to it may be decreased by closing the port to exhaust, and compressing a portion of the enclosed steam before the piston has reached the end of its stroke. The steam so compressed into the clearance spaces by the advancing piston forms a cushion useful in bringing it to rest without shock. The amount that can be allowed depends upon the speed of the engine, and can be very mueh more when running fast than when moving slowly, and if carried sufficiently far pennits of the pressure of the steam in the clearance spaces being raised to that of the boiler; then all losses due to clearance are avoided, as the space is full of steam at maximum pressure when the piston commences its return stroke; but cushioning to this extent would entail a loss in another direction by greatly inereasing the back pressure, and reducing the mean effeetive pressure upon the piston. Compression also, incidentally, has the advantage of raising the temperature of the cylinder and port, which had been cooled down to the heat of the expanded steam, and the entering boiler steam is thereby saved from ■condensation with consequent loss of pressure. If all the working joints of the engine affeeted by the reciprocating motion of the piston, particularly the small and big ends of the connecting rods and driving axle-boxes, could be made a perfeet tit, so that when the crank passed the dead •centres no slackness was observable, it would not be alto- gether necessary to open the port to live steam before the piston reached its extreme point, or allow lead; but inasmueh as it is impossible to do this with large and heavy mechanism, such as that of locomotives, there will always be a certain amount of slack; in faet, it is purposely provided in the first instance, and this inereases as wear takes place, until a consi- derable “thump” or “knock” is given, as the cranks pass their dead centres, due to the momentum of the connecting rod, piston rod, etc., being suddenly arrested when these are checked preparatory to commencing the return stroke. By