in which
| T1 | = | absolute temperature of steam at initial pressure. |
| T2 | = | absolute temperature of steam at exhaust pressure. |
Example:—The temperature of the steam admitted to the cylinder of an engine is 340 degrees F., and that of the exhaust steam 220 degrees F. What is the thermal efficiency of the engine?
| (340 + 461) - (220 + 461) | ||
| Thermal efficiency = | ——————————— | = 0.15 |
| 340 + 461 |
The mechanical efficiency is the ratio of the delivered or brake horsepower to the indicated horsepower, and is represented by the equation:
| B. H. P. | |
| Mechanical efficiency = | ——— |
| I. H. P. |
| in which | B. H. P. | = | brake horsepower, |
| I. H. P. | = | indicated horsepower. |
All engines are designed to give the best economy at a certain developed indicated horsepower called full load. There must, of course, be more or less fluctuation in the load under practical working conditions, especially in certain cases, such as electric railway and rolling mill work. The losses, however, within a certain range on either side of the normal load, are not great in a well designed engine. The effect of increasing the load is to raise the initial pressure or lengthen the cut-off, depending upon the type of governor. This, in turn, raises the terminal pressure at the end of expansion, and allows the exhaust to escape at a higher temperature than before, thus lowering the thermal efficiency.
The effect of reducing the load is to lower the mean effective pressure. (See [Figs. 38] and [39].) This, in throttling engines, is due to a reduction of initial pressure, and in the automatic engine to a shortening of the cut-off. The result in each case is an increase in cylinder condensation, and as the load becomes lighter, the friction of the engine itself becomes a more important part of the total indicated horsepower; that is, as the load becomes lighter, the mechanical efficiency is reduced.