POWER. The word "power," as used by the engineer, indi cates energy under human control and available for doing mechanical work. The principal sources of power are the muscular energy of men and animals; the kinetic energy of the winds and of streams; the potential energy of water at high levels, of the tides and of waves; the heat of the earth and of the sun; and heat derived from the combustion of fuels. Of these sources of power the winds, waves and solar heat suffer the disadvantage of being essentially intermittent and therefore requiring some method of storage of power if the demand for power is continuous. From the point of view of the size and cost of the power plant, when large amounts of power are required, windmills, wave motors, and solar engines are not adaptable to large-scale power-generation; tidal power, while it may be developed in certain places for large power, usually entails excessive first cost; volcanic power or natural steam has been used in Italy and experimented with in California; hydraulic turbines and heat engines alone permit the construction of compact plants of practically unlimited capacity and of moderate first cost. The commonly accepted unit of power is the horse-power, which was defined by James Watt in 1783 as the equivalent of 33,00o ft.lb. of work per minute. This is about ten times as much work as can be done per minute by a labourer working eight hours per day.
The use of domesticated animals was the first enlargement of the power of man and the beginning of his civilization. The use of the wind for sailing vessels was an early development but its use in operating windmills dates from about the 12th century. Water-wheels were known in Greek times and are described by Vitruvius, but their capacity was very small. To the end of the 18th century the largest water-wheels for industrial use did not exceed ro horse-power. The earliest operative heat engine is the cannon, used first at the end of the 13th century. The social consequences of its invention were momentous; it had a great part in the destruction of the feudal system. It represented a greater concentration of power than had been possible previously. Its indirect influence in stimulating the development of the art of cutting metals is of prime importance in the history of the heat engine.
The special incentive which gave birth to the steam engine was the desire to remove water from mines (particularly the tin mines of Cornwall). In 1698 Capt. Savery's engine was pat ented and a number of his engines were built. They were found
to be extravagant in their use of coal. Four years later the first steam engine using a cylinder and piston was devised by New comen. It was while repairing a model of this engine that James Watt made the improvements that resulted in the modern steam engine. In 1782 Watt patented a double-acting rotative engine which, for the first time, made steam-power available for driving all kinds of mechanism. The result of this invention was the factory system and the industrial revolution. It became possible also to apply steam-power to navigation and to railroads.
The next important advance in power generation was the invention by Fourneyron of the hydraulic (reaction) turbine in 1827, for utilizing the energy of water available under high heads. Impulse water-turbines of the Pelton type, adapted to use the highest heads, were developed in California about 186o. Hydraulic turbines have now reached a high degree of perfec tion giving efficiencies in excess of 90%. The largest unit built up to 1928 has a capacity of Ioo,000 horse-power.
The thermal efficiency of a heat engine is a function of the maximum and minimum temperatures of the working substance and also of the cycle of operations. The cycle of maximum effi ciency for given temperature limits is the Carnot cycle. Com bustion, which is the source of heat in heat engines, either may occur outside the engine (external combustion) or may occur inside the engine (internal combustion). In external-combustion engines the working substance is distinct from the products of combustion and heat travels to it by conduction through con taining walls such as boiler heating-surface. The maximum tem perature of the working substance is then limited by the strength of the containing walls at high temperature; in 1928 the practical limit of temperature was about goo° F. With internal combustion the products of combustion are used as the working substance and there is no maximum temperature limit since the containing walls, piston and valves can be water-cooled. The theoretical thermal efficiency of the steam turbine, operating under the limiting con ditions of 1928, is about 36%, of the Diesel engine about 50%. The brake thermal efficiencies actually realized are considerably lower. The internal-combustion engine is compact, of light weight, instantly available for use, has low labour cost and no stand-by losses. Its principal disadvantage is that it uses a fuel more costly than coal except in Diesel engines utilizing the cheapest grades of oil.