(176.)

The proportion of the stroke to the diameter of the cylinder must be determined by the velocity intended to be given to the piston. With the same capacity of cylinder, and the same evaporation in the boiler, the velocity of the piston will augment as the magnitude of its diameter is diminished.

The proportion of the diameter to the stroke of the cylinder is very various. In engines used for steam-vessels the length of the cylinder very little exceeds its diameter. In land engines, however, the proportion of the length to the diameter is greater. It is maintained by some that the proportion of the diameter and length of the cylinder should be such as to render its surface exposed to the cooling of the external air, the smallest possible. Tredgold has maintained that since, during the stroke, the steam is gradually exposed to contact with the surface of the cylinder from the top to the bottom, the mean surface exposed in contact with steam being half that of the entire cylinder, the proportion of the diameter to the stroke should be such that the surface of half the length of the cylinder, added to the magnitude of the top and bottom, shall be a minimum. If this principle be admitted, then the best proportion of the diameter to the stroke would be that of one to two, the length of the stroke being twice the diameter of the cylinder; but since the whole surface of the cylinder is constantly exposed to the cooling effects of the air, and since in the intervals of the stroke there is no sensible change of the temperature of the surface, the loss of heat by cooling will in effect be the same, especially in double-acting engines, as if the cylinder were constantly filled with steam. If this be admitted, then the object should be to give the cylinder such a proportion, that its entire surface, including the top and bottom, shall be a minimum. [Pg296] The proportion given by this condition would be very nearly that which is observed in the cylinders of marine engines, viz. that the length of the cylinder should be equal to its diameter.

If in a low-pressure engine the pressure of steam in the cylinder be taken at 17 lbs. per square inch, then the volume of steam will be about fifteen hundred times that of the water which produces it. For every cubic foot of water, therefore, in the effective evaporation of the boiler, 1500 cubic feet of steam will be passed through the cylinder. If it be intended that the motion of the piston shall be at the rate of 25 strokes per minute, or 1500 strokes per hour, then the capacity of that portion of the cylinder between the steam-valve and the piston at the end of the stroke, must consist of half as many cubic feet as there are cubic feet per hour evaporated in the boiler. If the steam, therefore, be cut off at half stroke, the number of cubic feet of space in the cylinder will be equal to the number of cubic feet of water effectively evaporated by the boiler; and if a cubic foot of water effectively evaporated be taken as the measure of a horse-power, then there would be as many cubic feet in the capacity of the cylinder as is equal to the nominal power of the engine.