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
V =
Vc
P + C Vol.
Clearance
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