All About Engines

The Modern Reciprocating Engine 143 Let it be borne in mind, first, that there is a tremendous advantage in working expansively—that is, in cutting-off at an early part of the stroke. Dur- ing expansion the steam loses heat and falls in temperature, so that it is cooler at the end of the stroke than it was on admission. If the cylinder is not jacketed with steam, there is a great deal of condensation when the working steam enters the cylinder, because the walls of the latter have been cooled by the exhaust steam from the previous stroke; and there is further condensation during expansion. With a steam jacket the initial con- densation is largely, but not wholly, prevented, and the ports and cylinder covers are not affected at all by the jacket. As the working steam expands it becomes cooler than the walls of the cylinder, and some of the steam condensed on the cylinder walls is re-evaporated by heat which flows in from the jacket steam. This is again a loss, but it is at the expense of the boiler, and does not compensate for the loss of the working steam. The loss by heating and cooling is proportionately less in a large engine than in a small one, because the volume (that is, cubic contents) of the interior of a vessel varies more rapidly with its dimensions than does the area. Consider a hollow cube, with edges i foot long. The area, since there are six faces, is 6 square feet, and the volume i cubic foot. If the edge were 2 feet, the area would be 24 square feet and the volume 8 cubic feet, the one being four times and the other eight times greater than the corre-