Compression ratio

“ William Fairbairn gives, as the relative resistances to extension and compression in bars applied as girders, 2 to 1.
As far as practice is any guide, from 8,000 to 12,000 128pounds per inch is the most to be used. The ratio of 90 to 66, seems to express very nearly the action as in the most reliable structures; which will, therefore, be adopted, or 11,000 pounds per square inch nearly. The resistance to compression is very much greater after wrought iron has been somewhat compressed. ”

“ Besides, the more swiftly the globes move, the less are they pressed by the fluid at their hinder parts; and if the velocity be perpetually increased, they will at list leave an empty space behind them, unless the compression of the fluid be increased at the same time. For the compression of the fluid ought to be increased (by proposition 32 and 33.) in the duplicate ratio os the velocity, in order to preserve the resistance in the same duplicate ratio. ”

“ There will be compression, the same at all points, and proportional to the pressure; and the amount of the compression can be expressed as The quantity is the modulus of compression. In this case the linear contraction in any direction is ; but in general the linear extension (or contraction) is not one-third of the cubical dilatation (or compression) .
24. To define the rigidity, we suppose that a solid body is subjected to forces in such a way that there is shearing stress within it. ”

“ Log of the test: The load is applied continuously at a uniform rate of speed. (See Speed of Testing Maching, page 92.) Readings are taken from the scale of the compressometer at regular increments of either load or compression. The stress-strain diagram is continued to at least one deformation point beyond the maximum load, and in event of sudden failure, the direction of the curve beyond the maximum point is indicated. ”

“ But at each alternate bend these outer layers, becoming concave, are subject to increased compression—a compression which they cannot withstand if they have become simply more extensible. To withstand this greater compression they must become harder as well as more extensible. ”

“ Now, if the cycle-pump heats our hands, it will be easily understood that the temperature of the compressors is very high. They are water-jacketed like the cylinders of a gas-engine, so that a circulating stream of cold water may absorb some of the heat. The compressed air is passed through spiral tubes winding through large tanks of water which fairly boils from the fierceness of the heat of compression. ”

“ Compression as a Mixing Means.—It will also be understood, that compression permits the bringing together of a much larger amount of fuel at each charge than would be possible without it, so that the two factors, namely, the volatilizing action of the air, the mixing of the air and vapor, and the compression, all serve to mix together the elements which will produce an explosion when the proper heat is finally applied. ”

“ Compression is beneficial in three ways: (1) it permits a greater range of expansion after ignition; (2) it raises the mean effective pressure, and thus improves the mechanical efficiency and the power in proportion to size and weight; (3) it reduces the loss of heat during ignition by reducing the surface exposed to the hot gases. In the modern gas or petrol motor, compression is employed as in Carnot’s cycle, but the efficiency attainable is limited not so much by considerations of temperature as by limitations of volume. ”

“ We have already explained that, in a medium such as air, a wave of compression is propagated at a speed which is measured by the square root of the quotient of the air-pressure, or elasticity, by its density. In exactly the same way the hump that is formed on a rope by giving one end of it a jerk, runs along at a speed which is measured by the square root of the quotient of the stretching force, or tension, by the density. The propagation of a pulse or wave along a string is most easily shown for lecture purposes by filling a long indiarubber tube with sand, and then hanging it up by one end. ”

“ In internal combustion engines it is a necessary condition of economy to compress the explosive mixture before it is ignited: in the Otto cycle, for instance, the second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke. ”

“ The amount of strain caused by any weight applied in a transverse direction, to a beam supported at both ends, is as the breadth, as the length inversely, and as the 125square of the depth. Whatever depression takes place, tends to shorten the upper, and to extend the under-side; whence the fibres of the top part suffer compression, and those of the bottom extension. The amounts of compression and extension must of course be equal, and therefore if any material resists these two strains in a different degree, the number of fibres opposing each will also be different. ”

“ What he accomplishes here is not all gain, however, for the bounding molecules, thus jammed together, come in collision with one another more and more frequently, and thus their average activity of vibration is increased and not diminished; in other words, the temperature of the gas has risen in virtue of the compression. Compression alone, then, will not avail to enable cohesion to win the battle.
But the physicist has another resource. He may place the cylinder of gas in a cold medium, so that the heat vibrations sent into it will be less vigorous than those it sends out. ”

“ The heat of compression is so thoroughly absorbed that the thermal loss is only 1.6 per cent.; but the loss by friction of the engine is 14.5 per cent., and the net economy of the whole system is no greater than that of the best American dry compressor, which loses about one-half the theoretical loss due to heat of compression, but which makes up the difference by a low friction loss. ”

“ The strength of the enveloping piece is again an element to be considered in determining the amount to be allowed for shrinkage. It is obvious, for example, that a ring of 8 inches thick, and having a bore of, say, 6 inches diameter, would be less liable to crack from the strain due to an allowance of 1⁄50 inch for contraction, than would a ring of equal bore and one inch thick having the same allowance. The strength or resistance to compression of the piece enveloped in proportion to that enveloping it, is yet another consideration. ”

“ If the fluid be much compressed, and the velocity of the moving body much less than that with which the particles of the fluid will rush into vacuum in consequence of the compression, it is clear that the space left by the moving body will be almost instantaneously filled up, (plate 23, fig. 2) ; and the resistance of such a medium would be less the greater the compression, provided the density were the same, because the velocity of rushing into a vacuum will be greater the greater the compression. ”

“ But the elasticity is the same thing as the power of resisting compression, which in the case of air we know to be about 15 pounds to the square inch; so that the ether, if equally dense, would balance a pressure of 15 million million pounds to the square inch—that is, it would require a pressure of about 750 millions of tons to the square inch to condense ether to the density of air. On the other hand, its density, if any, must be so infinitesimally small that the earth moving through it in its orbit with a velocity of 1,100 miles a minute suffers no perceptible retardation. ”

“ Mayer, of Heilbronn, measured the work done in compressing air, and the heat generated by the compression, and assumed that the whole of the work done in the compression, and no more, was converted into the heat developed, which was the same thing as assuming that no work was done in altering the positions of the particles of gas. ”

“ There is this great advantage in introducing the definition of the Modulus of Elasticity,—that it applies equally to the flexure of a substance and to the minute vibrations which propagate sound, and the like. And the notion was applied so as to lead to curious and important results with regard to the power of beams to resist flexure, not only when loaded transversely, but when pressed in the direction of their length, and in any oblique direction.
But in the fracture of beams, the resistance to extension and to compression are not practically equal ”

“ Suppose now that at any stage of the compression the partial pressures of the two gases are p1 and p2, and that the volume is changed from V to V − dV. The work of compression is (p1 + p2) dV, and this work will be restored at the corresponding stage if each of the separated gases increases in volume from V − dV to V. The ultimate state of the separated gases will thus be one in which each gas occupies the volume V originally occupied by the mixture.
We may now obtain an estimate of the amount of energy rendered unavailable by diffusion. ”

“ In the case of air, some portion may be compressed more than the rest, and the place of compression may move forward, so that as the compression is released in one place it makes its appearance in an adjacent one. ”

“ The machinery for the compression of air, however, is very different, more simple, and in every way an improvement upon that at the South end. Not finding any convenient means of obtaining a head of eighty-four feet of water sufficient in quantity for working a series of compressors, as at Bardonnêche, there has been established at Modane a system of direct compression, the necessary force for which is derived from the current of the Arc. ”

“ When the air is in a state of compression, we find that the same force with which we compressed it is necessary to keep it in its bulk; and that if we cease to press it together, it will swell out and regain its natural dimensions, which shows its elasticity. This distinguishes it essentially from such a body as a mass of flour, salt, and such like, which remains in the compressed state to which we reduce it. There is something therefore which opposes the compression of air, different from its simple impenetrability, and produces motion, by repelling the compressing body. ”

“ The conditions of speed and discharge corresponding to the greatest efficiency of compression are those ordinarily taken as the best normal conditions of working. The curve marked total efficiency gives the efficiency calculated on the work of compression and kinetic energy of discharge. ”

“ When a gas is compressed suddenly, it has a greater elasticity on account of the rise of temperature which takes place. Liquids are less compressible than gases; water is compressed by about 1 part in 20,000 when the pressure on it is increased by one atmosphere. A knowledge of the elastic properties of solids is of importance in all branches of applied mechanics. Homogeneous solids offer definite resistance to compression, twisting, stretching, and bending, and this resistance is expressed by a number called a modulus. Let the deforming force be reckoned per unit of area, e.g. ”

“ The compression of the surface-elements into a quarter or half their known bulk cannot explain the great weight of the interior, for experiments indicate that a limit to the capacity of reduction of volume is soon reached. So far as we know, the reduction of bulk by pressure becomes less and less in proportion to the pressure exerted. ”

“ The scheme actually proposed only recognizes 21024 distinct states of compression of the water medium, these being sequences of 1024 pulses of two different sizes, one of which will probably be zero. The amplifier at the end of the line always reshapes the signal to bring it back to the nearest ideal signal. ”

“ The accuracy of this mode of measuring the pressure was tested by direct loading of the ram with rails, which was repeated several times during the course of the experiments, so as to guard against any change in the amount of friction of the press. For each increment of weight, the compression of the specimen was measured on its four faces, and its deflection, or amount of bending, on two adjacent faces. The transverse stiffness of long specimens was also tried, by supporting them at each end, and loading them in the middle. ”

“ When a charge is communicated to the fiber, lateral compression results from the attraction of the electricities of opposite name, and, since the volume remains constant, elongation is the consequence—precisely as a band of caoutchouc, having strips of tin-foil upon its sides, may be shown to elongate when charged like a condenser. ”

“ Compression is the life and soul of a modern high-speed internal combustion engine.
When the engine is in proper working order, and being turned by hand, a considerable resistance should be felt at the alternate back stroke of each piston; this back pressure should require a considerable effort to overcome when the handle is being turned slowly. If the compression of any cylinder leaks, that cylinder will not give off its full power. ”

“ Elasticity implies susceptibility of compression, and the susceptibility of compression depends upon the porosity of bodies; for were there no pores or spaces between the particles of matter of which a body is composed, it could not be compressed. ”

“ If a fluid be composed of particles mutually flying each other, and the density be as the compression, the centrifugal forces of the particles will be reciprocally proportional to the distances of their centres. And vice versa, particles flying each other with forces that are reciprocally proportional to the distances of their centres, compose an elastic fluid, whose density is as the compression. ”