Simple heat engine

Figure 4.3-1 (a) Schematic diagram of a simple heat engine, (b) Schematic diagram of a fluid flow engine. 

A simple heat engine, (a) The engine is hkially at T,. (b) When heated toT, the piston is pushed up due to gas expansion, (c) When cooled to T, the piston returns to the original position. 

The work, W, can range from 2ero, if the engine is completely ineffective, to the limiting negative value attained for reversible operation. If IT = 0, then the process degenerates to one of simple heat transfer, for which... 

Automotive air conditioning systems provide simple heating and cooling/dehuniidification functions. There is no provision for filtration of the air. All current automotive air conditioning utilizes vapor-compression refrigeration systems coupled to the automobile s engine. 

These properties are made use of in many applications ranging from domestic cookers to linings which must withstand the heat from jet engines. There is simple heat resistance, i.e. the ability of the enamel to protect the... 

Simple pumps for circulating gas or liquid can be made using two valves with a chamber between them. One ingenious pump uses a heat engine (A. R. Pearson and J. S. G. Thomas, 1925). The hard glass or silica bulb A (Figure 80, T) contains air initially at atmospheric pressure. This is heated with tap T closed and when the... 

The power versus efficiency characteristics of the endoreversible Carnot heat engine is a parabolic curve. The endoreversible heat engine is a simple model, which considers the external heat-transfer irreversibility between the heat engine and its surrounding heat reservoirs only. 

Carnot s cycle of Fig. 4.3 is certainly a remarkably simple realization of a heat engine. Can any other engine do better Surprisingly, Carnot concluded that the answer must be No Carnot s general conclusion can be summarized in the following statement ... 

Heat engines that are potential candidates for coupling a solar heat source include thermoelectric, thermionic, lliermochemical, magnelohydro-dynamic, Rankine, Brayton (simple or recuperated), and cascaded cycles. 

The reversible work tUfSystrev of a coupled fuel cell-heat engine system is independent of the state of the cell and is always equal to the Gibbs free enthalpy of the reaction ArG° at the ambient state [4], It is assumed that the standard condition is equal to the ambient state to keep the argumentation simple. This result indicates the necessary equipment to utilise the exergy of the fuel. 

The direct conversion of the energy of chemical reactions to electricity in fuel cells (Chapter 13) rather than in heat engines will double the energy available and provide clean and relatively simple devices fuel cells contain no moving parts. 

The classical approach to the second law is based on a macroscopic viewpoint of properties independent of any knowledge of the structure of matter or behavior of mblecules. It arose from study of the heat engine, a device or machine that produces work from heat in a cyclic process. An example is a steam power plant in which the working fluid (steam) periodically returns to its original state. In such a power plant the cycle (in simple form) consists of the following steps ... 

Scientists also noticed that when they tried to convert heat into work, the matter was not as simple as converting work into heat. A machine, which would take heat and convert into work (a heat engine), would necessarily have to discard some (lower quality) heat (Second Law of Thermodynamics - Sec. 5.1). In other words, difference between heat and work (and other forms of energy) was emphasised by this law. 

Since all reversible heat engines working between the same two temperatures will have the same efficiencies, we can conclude that their efficiencies depend only upon the two temperatures between which they work. For further thermodynamic consideration it is, therefore, sufficient that we consider that type of reversible machine, which will lend itself to simple thermodynamic treatment. A machine employing Carnot s cycle is of such a type. 

The figure below shows a simple form of a heat engine. A cylinder fitted with a weightless piston is initially at temperature T]. Next, the cylinder is heated to a higher temperature T2- The gas in the cylinder expands and pushes up the piston. Finally the cylinder is cooled down to T] and the apparatus returns to its original state. By repeating this cycle, the up-and-down movement of the piston can be made to do mechanical work. 

A very simple solar engine absorbs heat through a collector. The collector loses some of the heat it absorbs by convection, and the remainder is passed through a heat engine to produce electricity. The heat engine operates with one-half the Carnot efficiency with its low-temperature side at ambient temperature and its high-temperature side at the steady-state temperature of the collector, T. The expression for the heat loss from the collector is... 

Initial concepts of temperature came from the physical sensation of the relative hotness or coldness of bodies. This sensation of warmth or cold is so subjective relative to our immediate prior exposure that it is difficult to use for anything but simple qualitative comparison. The need to assign a quantitative value to temperature leads to the definition of a temperature scale. The concept of fixed points of temperature arises from the observation that there exist some systems in nature that always exhibit the same temperatures. The scientific or thermodynamic definition of temperature comes from Kelvin, who defined the ratio of the thermodynamic or absolute temperatures of two systems as being equal to the ratio of the heat added to the heat rejected for a reversible heat engine operated between the systems. This unique temperature scale requires only one fixed point, the triple point of water, for its definition. 

From the characteristics of a particularly simple kind of heat engine, the Carnot engine, and from universal experience that certain kinds of engine cannot be constructed, we concluded that all reversible heat engines operating between the same two heat reservoirs have the same efficiency, which depends only on the temperatures of the reservoirs. Thus it was possible to establish the thermodynamic scale of temperature, which is independent of the properties of any individual substance, and to relate the efficiency of the engine to the temperatures on this scale ... 

We begin by comparing two expressions for the efficiency of a simple reversible heat engine that operates between the two reservoirs at the thermodynamic temperatures 6 and 02 We have seen that... 

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