M=-- E.P. ( X We—(H-1-13) X Wh).
which will give the weight in pounds per indi cated horse power per hour. M.E.P. represents the mean effective pressure, which in the case of a multiple-expansion engine is the combined mean effective pressure referred to the cylinder in question. For example: In the case of a com pound engine, the combined mean effective pressure for the high pressure cylinder consists of two items: (1) the mean effective pressure of the high pressure cylinder, and (2) the mean effective pressure of the low pressure cylinder multiplied by the ratio of the piston displace ment of the low pressure cylinder to that of the high pressure cylinder. The sum of these two items is the combined mean effective pressure for the high pressure cylinder.
Similarly the combined mean effective pres sure for the low pressure cylinder consists of (1) the mean effective pressure of the low pressure cylinder, and (2) the mean effective pressure of the high pressure cylinder divided by the ratio already stated. The sum of the two items is the combined mean effective pres sure of the lowpressure cylinder.
In the given formula, C represents the pro portion of the piston stroke completed at points on the expansion line of the diagram near the actual cut-off or release; H the proportion of compression; and E the proportion of clearance; all of which are determinable from the indicator diagram. We represents the weight of one cubic foot of steam at the cut-off or release pressure; and Wh the weight of one cubic foot of steam at the compression pressure.
13. Standards of Economy and Efficiency. —The standard expression for engine economy, as already stated, is the hourly consumption of heat units divided by the indicated horse power or the brake horse power. The standard ex pression for efficiency is the thermal efficiency ratio, or the proportion which the heat equiva lent of the power developed bears to the total amount of heat actually consumed, as deter mined by test. One horse-power-hour represents the consumption of 2,545 B.T.U.'s per hour, therefore, 2,345 B.T.U.•s per horse power per hour expresses the thermal efficiency ratio.
In comparing the standard for internal com bustion engines with that for steam engines, it must be noted that the former usually covers the losses due to combustion, but the latter does not, and therefore, in order to make a direct comparison between the two classes of engines as complete horse-power plants, the losses in generating the working agent must be consid ered in both cases not only on the basis of the fuel used, but on the basis of equivalent fuel used in each case. In comparing a gas engine
plant using producer gas, with a steam plant, the producer should be included in the former, and then the fuel consumption, represented by the weight of coal in both cases, may be directly compared.
14. Heat Analysis.—For scientific purposes, a heat analysis of the indicator diagram, in the case of a steam engine, and a heat balance in the case of an internal combustion engine, should be made, showing the manner in which the total heat of combustion is expended in working the engine.
In the case of a steam engine, the analysis shows the interchange of heat from steam to cylinder walls, etc. For example: the amount of heat supplied to the engine in a given time is represented by the number of pounds of steam supplied multiplied by the total heat of one pound of steam. A portion of this heat is used in the jacket, if one be employed, and the remainder passes through the cylinder. The heat entering the jacket is lost partly by radia tion from the outside surface, and the remainder enters the walls of the cylinder and is absorbed by the steam within it. The cycle of operations within the cylinder consists of the following phases: (1) A portion of the entering heat is transferred into a small portion of the thick ness of the cylinder walls, and heats them to the temperature of the entering steam. This trans ference of heat is more active during the period of admission and up to the point of cut-off than during any other part of the cycle. (2) Beyond the point of cut-off, the transference of heat continues until the tower due to ex pansion causes the temperature of the steam to fall below that of the interior surfaces of the cylinder last uncovered. At this point the inter change of heat is reversed, the metal giving up heat to the steam, and causing the re-evapora tion of the particles of water condensed on the surface of the cylinder walls and piston. The radiation of heat from the small thicknesses of the interior walls, which were heated during admission to the temperature of the entering steam, commences after cut-off or after the pressure begins to lower by expansion, and con tinues to the end of the stroke.