This most important problem may be stated in its most general, yet definite, form as follows:
To construct a machine which shall, in the most perfect manner possible, convert the kinetic energy of heat into mechanical power, the heat being derived from the combustion of fuel, and steam being the receiver and the conveyer of that heat.
The problem, as we have already seen, embodies two distinct and equally important inquiries:
The first: What are the scientific principles involved in the problem as stated?
The second: How shall a machine be constructed that shall most efficiently embody, and accord with, not only those scientific principles, but also all of those principles of engineering practice that so vitally affect the economical value of every machine?
The one question is addressed to the man of science, the other to the engineer. They can be satisfactorily answered, even so far as our knowledge at present permits, after studying with care the scientific principles involved in the theory of the steam-engine under the best light that science can afford us, and by a careful study of the various steps of improvement that have taken place and of accompanying variations of structure, analyzing the effect of each change, and tracing the reasons for them.
The theory of the steam-engine is too important and too extensive a subject to be satisfactorily treated here in even the most concise possible manner. I can only attempt a plain statement of the course which seems to be pointed out by science as the proper one to pursue in the endeavor to increase the economical efficiency of steam-engines.
The teachings of science indicate that success in economically deriving mechanical power from the energy of heat-motion will, in all cases, be the greater as we work between more widely separated limits of temperature, and as we more perfectly provide against losses by dissipation of heat in directions in which it is unavailable for the production of power.
Scientific research, as we have seen, has proved that, in all known varieties of heat-engine, a large loss of effect is unavoidable from the fact that we cannot, in the ordinary steam-engine, reduce the lower limit of temperature, in working, below a point which is far above the absolute zero of temperature—far above that point at which bodies have no heat-motion. The point corresponding to the mean temperature of the surface of the earth is above the ordinary lower limit.
The higher the temperature of the steam when it enters the steam cylinder, and the lower that which it reaches before the exhaust occurs, the greater, science tells us, will be our success, provided we at the same time avoid waste of heat and power.