Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGWORKINGSUBSTANCE(MIXTURE OFATMOSPHERICAIR AND FUEL)(LOW PRESSURE ORHIGH PRESSURE SIDEOF THE CYCLE,DEPENDING UPON "POSITION OF PISTONIN THE CYLINDER)HEAT SOURCE(COMPRESSION OR SPARK)HEAT RECEIVER(ATMOSPHERE)HEATED ENGINE(PISTON ANDCYLINDER)147.63Figure 8-9.— Essential elements of open,heated-engine cycle.itself. When a shaft is rotating, we expect a temperaturerise in the bearings; when the shaft hasbeen stopped, we would be truly amazed to observeinternal energy from the bearings flowingto the shaft and causing it to start rotating again.When we drag a block of wood across a roughsurface, we expect some of the mechanical energyexpended in this act to be converted into thermalenergy— that is, we expect a storage of internalenergy in the wooden block and the rough surface,as evidenced by temperature rises in these materials.But if this stored internal energy shouldsuddenly turn to and move the wooden block backto its original position, our incredulity wouldknow no bounds.All of which merely goes to show that we havecertain expectations, based on experience, as tothe direction in which processes will move. Thereasonableness of our expectations is attested bythe fact that in all recorded history there is noreport of water freezing instead of boiling whenheat is applied; there is no report of a lukewarmfluid unmixing itself and separating into hot andcold fluids; there is no report of a gas compressingitself without the agency of some externalforce; there is no report of the heat of frictionbeing spontaneously utilizedtoperform mechanicalwork.Are these actions really impossible? Thefirst law of thermodynamics says that mechanicalenergy and thermal energy are mutually convertible,but it says nothing about the directionof such conversions. If we consider only the firstlaw, all the improbable actions just mentionedare perfectly possible and all processes could bethought of as being reversible. In an absolutesense, perhaps, we cannot guarantee that waterwill never freeze instead of boil when it is placedon a hot stove; but we are certainly safe in sayingthat this or any other completely reversiblethermodynamic process is at the outer limits ofprobability. For all practical purposes, then,we will say that there is no such thing as a completelyreversible process.Nevertheless, the concept of reversibility isextremely useful in evaluating real thermodynamicprocesses. At this point, therefore, let usdefine a reversible thermodynamic process asone which would have the following characteristics:(1) the process could be made to occur inprecisely reverse order, so that the energy systemand all associated systems would be returnedfrom their final condition to the conditionsthat existed before the process started; and (2)all energy that was transformed or redistributedduring the process would be returned from itsfinal to its original form, amount, and location.THE SECOND LAW OF THERMODYNAMICSSince the first law of thermodynamics doesnot deal with the direction of thermodynamicprocesses, and since experience indicates thatactual processes are not reversible, it is apparentthat the first law must be supplemented bysome statement of principle that will limit thedirection of thermodynamic processes. Thesecond law of thermodynamics is such a statement.Although the second law is perhaps moreempirical than the first law, and perhaps somethingless of a ''law" in an absolute sense, it isof enormous practical value in the study ofthermodynamics. 16The second law of thermodynamics may bestated in various ways. One statement, known asthe Clausius statement, is that no process ispossible where the sole result is the removal ofheat from a low temperature reservoir and theabsorption of an equal amount of heat by a hightemperature reservoir. Amongother things, thisThe interested stu(dent will find an excellent (iiscusslonof the second law of thermodynamics in MaxPlanck, Treatise on Thermodynamics . Dover Publications,New York, 1945. (A. Ogg. trans.)180