Aviation Engines: Design—Construction—Operation and Repair

by Victor Wilfred Pagé

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.

CONVERSION OF HEAT TO POWER

The utilization of heat in any heat-engine has long been a theme of inquiry and experiment with scientists and engineers, for the purpose of obtaining the best practical conditions and construction of heat-engines that would represent the highest efficiency or the nearest approach to the theoretical value of heat, as measured by empirical laws that have been derived from experimental researches relating to its ultimate volume. It is well known that the steam-engine returns only from 12 to 18 per cent. of the power due to the heat generated by the fuel, about 25 per cent. of the total heat being lost in the chimney, the only use of which is to create a draught for the fire; the balance, some 60 per cent., is lost in the exhaust and by radiation. The problem of utmost utilization of force in steam has nearly reached its limit.

The internal-combustion system of creating power is comparatively new in practice, and is but just settling into definite shape by repeated trials and modification of details, so as to give somewhat reliable data as to what may be expected from the rival of the steam-engine as a prime mover. For small powers, the gas, gasoline, and petroleum-oil engines are forging ahead at a rapid rate, filling the thousand wants of manufacture and business for a power that does not require expensive care, that is perfectly safe at all times, that can be used in any place in the wide world to which its concentrated fuel can be conveyed, and that has eliminated the constant handling of crude fuel and water.

REQUISITES FOR BEST POWER EFFECT

The utilization of heat in a gas-engine is mainly due to the manner in which the products entering into combustion are distributed in relation to the movement of the piston. The investigation of the foremost exponent of the theory of the explosive motor was prophetic in consideration of the later realization of the best conditions under which these motors can be made to meet the requirements of economy and practicability. As early as 1862, Beau de Rocha announced, in regard to the coming power, that four requisites were the basis of operation for economy and best effect. 1. The greatest possible cylinder volume with the least possible cooling surface. 2. The greatest possible rapidity of expansion. Hence, high speed. 3. The greatest possible expansion. Long stroke. 4. The greatest possible pressure at the commencement of expansion. High compression.

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