Aviation Engines Part 4

Fahrenheit. Constant Volume.

[E] Heat to Raise One Cubic Foot of Mixture 1 Deg. Fahr.

[F] Heat Units Evolved by Combustion.

[G] Ratio Col. 6/5 [H] Usual Combustion Efficiency.

[I] Usual Rise of Temperature due to Explosion at Constant Volume.

=======+========+======+======+========+======+=======+=====+===== [A] [B] [C] [D] [E] [F] [G] [H] [I]

-------+--------+------+------+--------+------+-------+-----+----- 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._

CHAPTER III

Efficiency of Internal Combustion Engines--Various Measures of Efficiency--Temperatures and Pressures--Factors Governing Economy--Losses in Wall Cooling--Value of Indicator Cards-- Compression in Explosive Motors--Factors Limiting Compression-- Causes of Heat Losses and Inefficiency--Heat Losses to Cooling Water.

EFFICIENCY OF INTERNAL COMBUSTION ENGINES

Efficiencies are worked out through intricate formulas for a variety of theoretical and unknown conditions of combustion in the cylinder: ratios of clearance and cylinder volume, and the uncertain condition of the products of combustion left from the last impulse and the wall temperature. But they are of but little value, except as a mathematical inquiry as to possibilities. The real commercial efficiency of a gas or gasoline-engine depends upon the volume of gas or liquid at some a.s.signed cost, required per actual brake horse-power per hour, in which an indicator card should show that the mechanical action of the valve gear and ignition was as perfect as practicable, and that the ratio of clearance, s.p.a.ce, and cylinder volume gave a satisfactory terminal pressure and compression: _i.e._, the difference between the power figured from the indicator card and the brake power being the friction loss of the engine.

In four-cycle motors of the compression type, the efficiencies are greatly advanced by compression, producing a more complete infusion of the mixture of gas or vapor and air, quicker firing, and far greater pressure than is possible with the two-cycle type previously described.

In the practical operation of the gas-engine during the past twenty years, the gas-consumption efficiencies per indicated horse-power have gradually risen from 17 per cent. to a maximum of 40 per cent. of the theoretical heat, and this has been done chiefly through a decreased combustion chamber and increased compression--the compression having gradually increased in practice from 30 lbs. per square inch to above 100; but there seems to be a limit to compression, as the efficiency ratio decreases with greater increase in compression. It has been shown that an ideal efficiency of 33 per cent. for 38 lbs., compression will increase to 40 per cent. for 66 lbs., and 43 per cent. for 88 lbs.

compression. On the other hand, greater compression means greater explosive pressure and greater strain on the engine structure, which will probably retain in future practice the compression between the limits of 40 and 90 lbs. except in super-compression engines intended for high alt.i.tude work where compression pressures as high as 125 pounds have been used.

In experiments made by Dugald Clerk, in England, with a combustion chamber equal to 0.6 of the s.p.a.ce swept by the piston, with a compression of 38 lbs., the consumption of gas was 24 cubic feet per indicated horse-power per hour. With 0.4 compression s.p.a.ce and 61 lbs.

compression, the consumption of gas was 20 cubic feet per indicated horse-power per hour; and with 0.34 compression s.p.a.ce and 87 lbs.

compression, the consumption of gas fell to 14.8 cubic feet per indicated horse-power per hour--the actual efficiencies being respectively 17, 21, and 25 per cent. This was with a Crossley four-cycle engine.

VARIOUS MEASURES OF EFFICIENCY

The efficiencies in regard to power in a heat-engine may be divided into four kinds, as follows: I. The first is known as the _maximum theoretical efficiency_ of a perfect engine (represented by the lines in the indicator diagram). It is expressed by the formula (T_{1} - T_{0})/T_{1} and shows the work of a perfect cycle in an engine working between the received temperature + absolute temperature (T_{1}) and the initial atmospheric temperature + absolute temperature (T_{0}).

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.

[Ill.u.s.tration: 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 a.s.sumed 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 a.s.sumed 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.

[A] Clearance Per Cent. of Piston Volume.

[B] Ratio (_V_/_V_{c}_) = (_P_ + _C_ Vol.)/Clearance [C] Approximate Compression from 13 Pounds Absolute.

[D] Approximate Gauge Pressure.

[E] Absolute Temperature of Compression from 560 Deg. Fahrenheit in Cylinder.

[F] Absolute Temperature of Explosion. Gas, 1 part; Air, 6 parts.

[G] Approximate Explosion Pressure Absolute.

[H] Approximate Gauge Pressure.

[I] Approximate Temperature of Explosion, Fahrenheit.

=====+======+======+=====+======+======+=====+=====+===== [A] [B] [C] [D] [E] [F] [G] [H] [I]

-----+------+------+-----+------+------+-----+-----+----- 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 -----+------+------+-----+------+------+-----+-----+-----

FACTORS GOVERNING ECONOMY

In view of the experiments in this direction, it clearly shows that in practical work, to obtain the greatest economy per effective brake horse-power, it is necessary: 1st. To transform the heat into work with the greatest rapidity mechanically allowable. This means high piston speed. 2d. To have high initial compression. 3d. To reduce the duration of contact between the hot gases and the cylinder walls to the smallest amount possible; which means short stroke and quick speed, with a spherical cylinder head. 4th. To adjust the temperature of the jacket water to obtain the most economical output of actual power. This means water-tanks or water-coils, with air-cooling surfaces suitable and adjustable to the most economical requirement of the engine, which by late trials requires the jacket water to be discharged at about 200 F.

5th. To reduce the wall surface of the clearance s.p.a.ce or combustion chamber to the smallest possible area, in proportion to its required volume. This lessens the loss of the heat of combustion by exposure to a large surface, and allows of a higher mean wall temperature to facilitate the heat of compression.

LOSSES IN WALL COOLING

In an experimental investigation of the efficiency of a gas-engine under variable piston speeds made in France, it was found that the useful effect increases with the velocity of the piston--that is, with the rate of expansion of the burning gases with mixtures of uniform volumes: so that the variations of time of complete combustion at constant pressure, and the variations due to speed, in a way compensate in their efficiencies. The dilute mixture, being slow burning, will have its time and pressure quickened by increasing the speed.

Careful trials give unmistakable evidence that the useful effect increases with the velocity of the piston--that is, with the rate of expansion of the burning gases. The time necessary for the explosion to become complete and to attain its maximum pressure depends not only on the composition of the mixture, but also upon the rate of expansion.

This has been verified in experiments with a high-speed motor, at speeds from 500 to 2,000 revolutions per minute, or piston speeds of from 16 to 64 feet per second. The increased speed of combustion due to increased piston speed is a matter of great importance to builders of gas-engines, as well as to the users, as indicating the mechanical direction of improvements to lessen the wearing strain due to high speed and to lighten the vibrating parts with increased strength, in order that the balancing of high-speed engines may be accomplished with the least weight.

From many experiments made in Europe and in the United States, it has been conclusively proved that excessive cylinder cooling by the water-jacket results in a marked loss of efficiency. In a series of experiments with a simplex engine in France, it was found that a saving of 7 per cent. in gas consumption per brake horse-power was made by raising the temperature of the jacket water from 141 to 165 F. A still greater saving was made in a trial with an Otto engine by raising the temperature of the jacket water from 61 to 140 F.--it being 9.5 per cent. less gas per brake horse-power.