Heat and Its Work
By Joule’s law of the mechanical equivalent of heat, whenever heat is imparted to an elastic body, as air or gas, energy is generated and mechanical work produced by the expansion of the air or gas. When the heat is imparted by combustion within a cylinder containing a movable piston, the mechanical work becomes an amount measurable by the observed pressure and movement of the piston. The heat generated by the explosive elements and the expansion of the non-combining elements of nitrogen and water vapor that may have been injected into the cylinder as moisture in the air, and the water vapor formed by the union of the oxygen of the air with the hydrogen of the gas, all add to the energy of the work from their expansion by the heat of internal combustion. As against this, the absorption of heat by the walls of the cylinder, the piston, and cylinder-head or clearance walls, becomes a modifying condition in the force imparted to the moving piston.
It is found that when any explosive mixture of air and gas or hydrocarbon vapor is fired, the pressure falls far short of the pressure computed from the theoretical effect of the heat produced, and from gauging the expansion of the contents of a cylinder. It is now well known that in practice the high efficiency which is promised by theoretical calculation is never realized; but it must always be remembered that the heat of combustion is the real agent, and that the gases and vapors are but the medium for the conversion of inert elements of power into the activity of energy by their chemical union. The theory of combustion has been the leading stimulus to large expectations with inventors and constructors of explosive motors; its entanglement with the modifying elements in practice has delayed the best development in construction, and as yet no really positive design of best form or action seems to have been accomplished, although great progress has been made during the past decade in the development of speed, reliability, economy, and power output of the individual units of this comparatively new power.
One of the most serious difficulties in the practical development of pressure, due to the theoretical computations of the pressure value of the full heat, is probably caused by imparting the heat of the fresh charge to the balance of the previous charge that has been cooled by expansion from the maximum pressure to near the atmospheric pressure of the exhaust. The retardation in the velocity of combustion of perfectly mixed elements is now well known from experimental trials with measured quantities; but the principal difficulty in applying these conditions to the practical work of an explosive engine where a necessity for a large clearance space cannot be obviated, is in the inability to obtain a maximum effect from the imperfect mixture and the mingling of the products of the last explosion with the new mixture, which produces a clouded condition that makes the ignition of the mass irregular or chattering, as observed in the expansion lines of indicator cards; but this must not be confounded with the reaction of the spring in the indicator.
Stratification of the mixture has been claimed as taking place in the clearance chamber of the cylinder; but this is not a satisfactory explanation in view of the vortical effect of the violent injection of the air and gas or vapor mixture. It certainly cannot become a perfect mixture in the time of a stroke of a high-speed motor of the two-cycle class. In a four-cycle engine, making 1,500 revolutions per minute, the injection and compression in any one cylinder take place in one twenty-fifth of a second—formerly considered far too short a time for a perfect infusion of the elements of combustion but now very easily taken care of despite the extremely high speed of numerous aviation and automobile power-plants.
Table I — Explosion at Constant Volume in a Closed Chamber
| Diagram Curve Fig. 8. | Mixture Injected. | Temp. of Injection Fahr. | Time of Explosion. Second. | Observed Gauge Pressure. Pounds. | Computed Temp. Fahr. | ||||||||
| a | 1 | volume | gas | to | 14 | volumes | air. | 64° | 0.45 | 40. | 1,483° | ||
| b | 1 | „ | „ | „ | 13 | „ | „ | 51° | 0.31 | 51. | 5 | 1,859° | |
| c | 1 | „ | „ | „ | 12 | „ | „ | 51° | 0.24 | 60. | 2,195° | ||
| d | 1 | „ | „ | „ | 11 | „ | „ | 51° | 0.17 | 61. | 2,228° | ||
| e | 1 | „ | „ | „ | 9 | „ | „ | 62° | 0.08 | 78. | 2,835° | ||
| f | 1 | „ | „ | „ | 7 | „ | „ | 62° | 0.06 | 87. | 3,151° | ||
| g | 1 | „ | „ | „ | 6 | „ | „ | 51° | 0.04 | 90. | 3,257° | ||
| h | 1 | „ | „ | „ | 5 | „ | „ | 51° | 0.05 | 5 | 91. | 3,293° | |
| i | 1 | „ | „ | „ | 4 | „ | „ | 66° | 0.16 | 80. | 2,871° | ||
In an examination of the times of explosion and the corresponding pressures in both tables, it will be seen that a mixture of 1 part gas to 6 parts air is the most effective and will give the highest mean pressure in a gas-engine. There is a limit to the relative proportions of illuminating gas and air mixture that is explosive, somewhat variable, depending upon the proportion of hydrogen in the gas. With ordinary coal-gas, 1 of gas to 15 parts of air; and on the lower end of the scale, 1 volume of gas to 2 parts air, are non-explosive. With gasoline vapor the explosive effect ceases at 1 to 16, and a saturated mixture of equal volumes of vapor and air will not explode, while the most intense explosive effect is from a mixture of 1 part vapor to 9 parts air. In the use of gasoline and air mixtures from a carburetor, the best effect is from 1 part saturated air to 8 parts free air.
Table II.—Properties and Explosive Temperature of a Mixture of One Part
of Illuminating Gas of 660 Thermal Units per Cubic Foot with Various
Proportions of Air without Mixture of Charge with the Products of a
Previous Explosion.
| Propor- tion, Air to Gas by Volumes. | Pounds in One Cubic Foot of Mixture. | Specific Heat. Heat Units Required to Raise 1 Lb. 1 Deg. Fahrenheit. | Heat to Raise One Cubic Foot of Mixture 1 Deg. Fahr. | Heat Units Evolved by Combus- tion. | Ratio Col. 6/5 | Usual Combus- tion Efficien- cy. | Usual Rise of Temperature due to Explosion at Constant Volume. | |||||
| Constant Pressure. | Constant Volume. | |||||||||||
| 6 | to | 1 | .074195 | .2668 | .1913 | .014189 | 94. | 28 | 6644. | 6 | .465 | 3090 |
| 7 | to | 1 | .075012 | .2628 | .1882 | .014116 | 82. | 5844. | 4 | .518 | 3027 | |
| 8 | to | 1 | .075647 | .2598 | .1858 | .014059 | 73. | 33 | 5216. | 1 | .543 | 2832 |
| 9 | to | 1 | .076155 | .2575 | .1846 | .014013 | 66. | 4709. | 9 | .56 | 2637 | |
| 10 | to | 1 | .076571 | .2555 | .1825 | .013976 | 60. | 4293. | .575 | 2468 | ||
| 11 | to | 1 | .076917 | .2540 | .1813 | .013945 | 55. | 3944. | .585 | 2307 | ||
| 12 | to | 1 | .077211 | .2526 | .1803 | .013922 | 50. | 77 | 3646. | 7 | .58 | 2115 |
The weight of a cubic foot of gas and air mixture as given in Col. 2 is found by adding the number of volumes of air multiplied by its weight,.0807, to one volume of gas of weight.035 pound per cubic foot and dividing by the total number of volumes; for example, as in the table, 6 ×.0807 =.5192⁄7 =.074195 as in the first line, and so on for any mixture or for other gases of different specific weight per cubic foot. The heat units evolved by combustion of the mixture (Col. 6) are obtained by dividing the total heat units in a cubic foot of gas by the total proportion of the mixture, 660⁄7 = 94.28 as in the first line of the table. Col. 5 is obtained by multiplying the weight of a cubic foot of the mixture in Col. 2 by the specific heat at a constant volume (Col. 4), Col. 6/Col. 5 = Col. 7 the total heat ratio, of which Col. 8 gives the usual combustion efficiency—Col. 7 × Col. 8 gives the absolute rise in temperature of a pure mixture, as given in Col. 9.
The many recorded experiments made to solve the discrepancy between the theoretical and the actual heat development and resulting pressures in the cylinder of an explosive motor, to which much discussion has been given as to the possibilities of dissociation and the increased specific heat of the elements of combustion and non-combustion, as well, also, of absorption and radiation of heat, have as yet furnished no satisfactory conclusion as to what really takes place within the cylinder walls. There seems to be very little known about dissociation, and somewhat vague theories have been advanced to explain the phenomenon. The fact is, nevertheless, apparent as shown in the production of water and other producer gases by the use of steam in contact with highly incandescent fuel. It is known that a maximum explosive mixture of pure gases, as hydrogen and oxygen or carbonic oxide and oxygen, suffers a contraction of one-third their volume by combustion to their compounds, steam or carbonic acid. In the explosive mixtures in the cylinder of a motor, however, the combining elements form so small a proportion of the contents of the cylinder that the shrinkage of their volume amounts to no more than 3 per cent. of the cylinder volume. This by no means accounts for the great heat and pressure differences between the theoretical and actual effects.