The gas-engine has the advantage in com parison with the steam-engine, in its higher available temperature range and consequent higher thermodynamic efficiency.
The exact treatment of the thermodynamics of the steam-engine requires the use of the higher mathematics, but the general principles have been given and the following will permit its applications to be understood: A steam-engine is a thermodynamic system in which only thermal and dynamic energies are present and operative. Its action is to trans form as large a proportion as possible of the heat supplied it into mechanical power and work. Each pound of steam from the boiler usually brings over the equivalent of about 0.4 horse-power-hour and each horse-power-hour is the ideal equivalent of the heat-content of about 2.3 pounds, a kilogram, nearly, of boiler steam. Of this heat, a part, which is precisely measured by the area of the indicator diagram, is con verted into useful work and an "efficiency° is attained measured by the ratio of the useful to the supplied energy in common units. Thus: where 23 pounds of steam per hour are de manded per horse power developed, in the case assumed, the efficiency is 10 per cent; the heat supplied being that furnished from the fuel and measured by the difference between the "total heat° of the feed-water at condenser tempera ture and that of the steam in the boiler.
The nine-tenths which fails of utilization is composed of a variety of wastes, including the thermodynamic, that portion of the heat reach ing the steam-cylinder and actually acting upon the piston which is not converted into indicated work, the waste by conduction and radiation ex ternally and the waste by the transfer of heat between the metal of the cylinder and the work ing fluid. These quantities in a good example may be taken as follows, the friction wastes of the machine itself being included: Thermodynamic wastes 70 Internal thermal loss 10 Available heat-energy from External waste 5 the boiler 100 Friction 2 I Useful work 13 100 This corresponds, for the ideal case, to an efficiency of 020, nearly.
'The external waste of the steam-engine is usually considered to be covered by an allow ance of about one B. T. U. per square foot per hour per degree range of temperature, Fahren heit, or about three calories per square metre, although, on exposed metal having a rough sur face, it may attain two to three times these fig ures. The exterior of the cylinder is commonly lagged and the heads, if not thus covered, are polished, thus minimizing the waste. The total
waste, on even small engines, has been found capable of being reduced to less than 3.5 per cent, total, inclusive of engine and boiler, by the use of good non-conducting coverings. This loss is often quite unimportant on large engines.
The internal wastes are produced by heat exchanges between metal and steam. at induc tion and eduction; the steam giving heat to the metal at its entrance into the cylinder and rob bing the metal at exhaust, thus transferring heat often in large quantities from the steam to the exhaust side, very much as leakage car ries the steam itself with its charge of heat. The effect on efficiency is precisely that of leak age. In this action, the cylinder-heads and the sides of the piston, being exposed to the widest range of temperature and for the longest penods, are most fruitful of waste; the cylin der, proper, and especially its middle portion, wastes least. The total loss is a function of the temperature range, the time of exposure to transfer, and the quality of steam, and of the ratio of expansion which measures rudely the quantity of steam per unit weight of metaL In any one engine it may be stated, as a rough approximation, that the condensation is a con stant quantity at all expansions. It may be treated as either a constant leakage or as a constant loss of work measurable by an equiva lent back-pressure. A common value of this leakage may be taken, in pounds, as not far from 0.02 B. T. U., per square foot of surfice exposed at cut-off, per minute per Fahrenheit degree of temperature-range. As a fraction of the steam supplied, it is approximately propor tional in any given engine to the square root of the ratio of expansion. With various types of engine, it ranges from 25 or 30 per cent, with simple engines of moderate size to 10 per cent, in muhiple-cylinder engines of modern con struction as a minimum. In steam pumps and very small engines, it may amount to more than the whole amount of steam taken in, for ther modynarhic action. These machines, demanding 100, and even sometimes 150 or more pounds of steam per horse power hour, waste three-fourths or more by °leakage" of heat. The °record breaking' engines of large size and superior design demand as little as 10 to 12 pounds, ap proximating 200 B. T. U. per horse power hour.