A new type of engine can, if at all, probably only supersede the common form when engineers can employ steam of very high pressure, and adopt much greater range of expansion than is now usual. Great velocity of piston and high speed of rotation are also essential in the attempt to make any revolution in steam-engine construction a success.
When a new form of steam-engine is likely to be introduced, if at all, can be scarcely even conjectured. It seems evident that its success is to be secured, if a revolution is ever to occur, by the adoption of high steam-pressures, of great piston speeds, by care and skill in design, by the use of exceptionally excellent materials of construction, by great perfection of workmanship, and by intelligence in its management.
Experiment and experience will probably lead gradually to the general and safe employment of much higher steam-pressures and very greatly increased piston-speeds, and may ultimately reveal and remove all those difficulties which must invariably be expected to be met here, as in all other attempts to effect radical changes, however important they may be.
[115] In some cases, as in the Allen engine, the speed of piston has become very high, approaching 800 3√stroke.
[116] The fact here referred to is easily seen if it is supposed that an engine is supplied with steam at a temperature of 400° above absolute zero and works it, without waste, down to a temperature of 200°. Suppose one inventor to adapt the engine to the use of steam of a range from 500° down to 200°, while another works his engine, with equally effective provision against losses, between the limits of 400° and 100°, an equal range with a lower mean. The first case gives an efficiency of one-half, the second three-fifths, and the third three-fourths, the last giving the highest effect.
[117] This term, though perhaps not familiar to engineers, expresses the idea perfectly.
[118] The author is not absolutely confident on the latter point. It may be found more economical and satisfactory, ultimately, to determine the point of cut-off by an automatic apparatus adjusting the expansion-gear by reference to the steam-pressure, regulating the speed by attaching the governor elsewhere. The author has devised several forms of apparatus of the kind referred to.
