and shows the work of a perfect cycle in an engine working between the received temperature + absolute temperature (T1) and the initial atmospheric temperature + absolute temperature (T0). II. The second is the actual heat efficiency, or the ratio of the heat turned into work to the total heat received by the engine. It expresses the indicated horse-power. III. The third is the ratio between the second or actual heat efficiency and the first or maximum theoretical efficiency of a perfect cycle. It represents the greatest possible utilization of the power of heat in an internal-combustion engine. IV. The fourth is the mechanical efficiency. This is the ratio between the actual horse-power delivered by the engine through a dynamometer or measured by a brake (brake horse-power), and the indicated horse-power. The difference between the two is the power lost by engine friction. In regard to the general heat efficiency of the materials of power in explosive engines, we find that with good illuminating gas the practical efficiency varies from 25 to 40 per cent.; kerosene-motors, 20 to 30; gasoline-motors, 20 to 32; acetylene, 25 to 35; alcohol, 20 to 30 per cent. of their heat value. The great variation is no doubt due to imperfect mixtures and variable conditions of the old and new charge in the cylinder; uncertainty as to leakage and the perfection of combustion. In the Diesel motors operating under high pressure, up to nearly 500 pounds, an efficiency of 36 per cent. is claimed.
Fig. 12.—Graphic Diagram Showing Approximate Utilization of Fuel Burned in Internal-Combustion Engine.
The graphic diagram at [Fig. 12] is of special value as it shows clearly how the heat produced by charge combustion is expended in an engine of average design.
On general principles the greater difference between the heat of combustion and the heat at exhaust is the relative measure of the heat turned into work, which represents the degree of efficiency without loss during expansion. The mathematical formulas appertaining to the computation of the element of heat and its work in an explosive engine are in a large measure dependent upon assumed values, as the conditions of the heat of combustion are made uncertain by the mixing of the fresh charge with the products of a previous combustion, and by absorption, radiation, and leakage. The computation of the temperature from the observed pressure may be made as before explained, but for compression-engines the needed starting-points for computation are very uncertain, and can only be approximated from the exact measure and value of the elements of combustion in a cylinder charge.
TEMPERATURES AND PRESSURES
Owing to the decrease from atmospheric pressure in the indrawing charge of the cylinder, caused by valve and frictional obstruction, the compression seldom starts above 13 lbs. absolute, especially in high-speed engines. Col. 3 in the following table represents the approximate absolute compression pressure for the clearance percentage and ratio in Cols. 1 and 2, while Col. 4 indicates the gauge pressure from the atmospheric line. The temperatures in Col. 5 are due to the compression in Col. 3 from an assumed temperature of 560° F. in the mixture of the fresh charge of 6 air to 1 gas with the products of combustion left in the clearance chamber from the exhaust stroke of a medium-speed motor. This temperature is subject to considerable variation from the difference in the heat-unit power of the gases and vapors used for explosive power, as also of the cylinder-cooling effect. In Col. 6 is given the approximate temperatures of explosion for a mixture of air 6 to gas 1 of 660 heat units per cubic foot, for the relative values of the clearance ratio in Col. 2 at constant volume.
Table III.—Gas-Engine Clearance Ratios, Approximate Compression,
Temperatures of Explosion and Explosive Pressures with a Mixture
of Gas of 660 Heat Units per Cubic Foot and Mixture of Gas
1 to 6 of Air.
| Clearance Per Cent. of Piston Volume. | Ratio
| Approximate Compression from 13 Pounds Absolute. | Approximate Gauge Pressure. | Absolute Temperature of Compression from 560 Deg. Fahrenheit in Cylinder. | Absolute Temperature of Explosion. Gas, 1 part; Air, 6 parts. | Approximate Explosion Pressure Absolute. | Approximate Gauge Pressure. | Approximate Temperature of Explosion, Fahrenheit. | ||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||||||
| Lbs. | Deg. | Deg. | Lbs. | Lbs. | Deg. | |||||||||
| .50 | 3. | 57. | 42. | 822. | 2488 | 169 | 144 | 2027 | ||||||
| .444 | 3. | 25 | 65. | 50. | 846. | 2568 | 197 | 182 | 2107 | |||||
| .40 | 3. | 50 | 70. | 55. | 868. | 2638 | 212 | 197 | 2177 | |||||
| .363 | 3. | 75 | 77. | 62. | 889. | 2701 | 234 | 219 | 2240 | |||||
| .333 | 4. | 84. | 69. | 910. | 2751 | 254 | 239 | 2290 | ||||||
| .285 | 4. | 50 | 102. | 88. | 955. | 2842 | 303 | 288 | 2381 | |||||
| .25 | 5. | 114. | 99. | 983. | 2901 | 336 | 321 | 2440 | ||||||