- In order that a process may be strictly reversible, it is necessary that the state of the working substance should be one of equilibrium at uniform pressure and temperature throughout.
Mayer entirely ignored the grand fundamental principle laid down by Sadi Carnot - that nothing can be concluded as to the relation between heat and work from an experiment in which the working substance is left at the end of an operation in a different physical state from that in which it was at the commencement.
Just as the working substance which alternately takes in and gives out heat in the steam-engine is water (converted during a part of the action into steam), so in the air-engine it is air.
The practical drawbacks to employing air as the working substance of a heat-engine are so great that its use has been very limited.
One of the chief practical objections to air-engines is the great bulk of the working substance in relation to the amount of heat that is utilized in the working of the engine.
They differ from it, however, in the fact that their working substance is not air, but a mixture of gases - a necessary consequence of internal combustion.
In any heat-engine it is essential that the working substance should be at a high temperature while it is taking in heat, and at a relatively low temperature when it is rejecting heat.
The highest thermodynamic efficiency will be reached when the working substance is at the top of its temperature range while any heat is being received and at the bottom while any heat is being rejected - as is the case in the cycle of operations of the theoretically imagined engine of Carnot.
The relation then between the work expended and the actual cooling work performed denotes the efficiency of the process, and this is expressed by Qt/(Q2-Q1); but as in a perfect refrigerating machine it is understood that the whole of the heat Q i is taken in at the absolute temperature T 11 and the whole of the heat Q2, is rejected at the absolute temperature T2, the heat quantities are proportional to the temperatures, and the expression T,/(T 2 -T,) gives the ideal coefficient of performance for any stated temperature range, whatever working substance is used.
There is this difference, however, between this experiment and the operation imagined by Maxwell, that when the gases have diffused the experiment cannot be repeated; and it is no more contrary to the dissipation of energy than is the fact that work may be derived at the expense of heat when a gas expands into a vacuum, for the working substance is not finally restored to its original condition; while Maxwell's "demons" may operate without limit.
On the familiar indicator diagram the state of the working substance is represented by the position of a point called the " statepoint," defined by the values of the pressure p and volume v of unit mass, as ordinate and abscissa respectively (fig.
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