All About Engines

The Modern Reciprocating Engine 117 is capable, theoretically, of performing 778 ft.-lb., the total energy to begin with was 10,892,000 ft.-lb., of which perhaps not more than 7,780,000 reach the engine; and as this exists in the form of hot steam the engineer has to arrange the engine so that it will do its work with the least possible quantity. For this purpose he makes the steam do work, not by exerting the full pressure as it leaves the boiler, but by expanding and pushing back the piston in order to occupy a larger volume. During this ex- pansion the steam cools, and some of it is condensed to water, while the heat given up corresponds roughly to the work done on the piston. The steam then escapes from the cylinder at a lower temperature than that at which it entered, and the greater this difference the greater will be the amount of work which it has accomplished. If, on the other hand, the full boiler pressure were allowed to act during the whole stroke of the piston the exhaust steam would still be in the same condition as it was when it entered, hence its capacity for doing work at the expense of its own heat would remain unaltered. Compared with an engine of the same size cutting off at quarter-stroke, it would have used four times the amount of steam and done very little more work. In order to admit and release steam it will be remembered that Watt used a complicated valve gear which we did not attempt to describe. This form survived for over a century for pumping engines of the Cornish type, but for other engines it was re-