ACTION OF STEAM IN OPERATING LOCOMOTIVE
Steam Enters Cylinder. The steam, after reaching the steam chest, is admitted alternately to first one end of the cylinder then the other through the action of the valve. The opening and closing of the valve is a continuous process, the amount of opening increasing from zero to a maximum and then decreasing to zero. Because of this fact there will be two periods of wire drawing during each admission, independent of the fact that there may or may not be wire drawing during the period of maximum opening. This action causes a further drop in pressure when the steam finally gets into the cylinder, which loss increases with the speed of the engine.
Steam in Cylinder. After the steam reaches the cylinder it experiences a still further loss caused by condensation due to the comparatively cool cylinder walls, heads, and piston. This loss can be minimized to a limited extent by the use of an efficient lagging but it can never be entirely eliminated. Even if there were no loss in the cylinder due to radiation, there still would be a loss because of the exhaust, which occurs at a temperature much lower than that of the entering steam and which would cool the cylinder walls and parts to at least the average temperature of the steam in the cylinder during the stroke.
When the steam expands in the cylinder in the performance of its work still another drop in pressure occurs, the amount depending upon the point of cut-off. As this can be varied at the will of the operator, it can be seen that the pressure drop can be very great or very small. During this portion of its travel the steam does its first useful work since leaving the boiler. The steam while in the steam chest exerts a pressure on the valve which causes friction and thereby absorbs a portion of the useful work generated by the action of the steam on the piston.
The steam acting on the piston and causing it to move produces rotation of the driving wheels through the medium of the connecting rod, crank-pin, and various other parts, with an effort which varies throughout the stroke owing to the expansion of the steam, the exhaust and compression, which is taking place on the opposite side of the piston, and the angularity of the connecting rod. The pressure on the guides, due to the angularity of the connecting rod, causes friction which reduces the effectiveness of the work done on the piston. The effect of the inertia of the parts at high rotative speeds affects the thrust on the crank-pin to a marked degree. These points and many others which might be mentioned are of much importance in the study of the locomotive and its ability to do useful work in hauling trains.
Steam after Leaving Cylinder. The steam having pushed the piston to the end of its stroke is exhausted on the return stroke, but at a slight back pressure, which opposes the effectiveness of the return stroke and results in a direct loss. The closing of the valve before the completion of the return stroke causes an additional resistance in compressing the steam remaining in the cylinder, but this is not without some advantage. The steam in being exhausted from the cylinder is discharged into the exhaust cavity in the cylinder and from thence into the exhaust passage in the cylinder saddle and out through the exhaust nozzle into the smoke-box. At this point the steam is very much reduced in pressure but, owing to its relatively high velocity, as it leaves the exhaust nozzle and enters the stack, it is still able to do useful work by producing a slight vacuum in the smoke-box in an ejector-like action. The useful work performed is not in the way of moving the machine but in increasing the rate of combustion in the fire-box. The action is such as to cause a rate of combustion unequaled in any other form of steam power plant with the exception, perhaps, of the steam fire engine which a few years ago was so popular.
Table of Contents; Page 35; Page 38; Index