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Thermodynamics

Work and Engines

The dominating feature of an industrial society is its ability to utilize sources of energy other than the muscles of men or animals. Most energy supplies are in the form of fuels such as coal or oil, where the energy is stored as internal energy. The process of combustion releases the internal erergy and converts it to heat. In this form the energy may be utilized for heating, cooking, ... etc. But to operate a machine, or to propel a vehicle or a projectile, the heat must be converted to mechanical energy, and one of the problems of mechanical engineer is to carry out this conversion with the maximum possible efficiency.

The energy transformations in a heat engine are conveniently represented schematically by the flow diagram in Figure 07. The engine itself is represented by the circle. The heat Q2 supplied to the engine is proportional to the cross section of the incoming "pipeline" at the top of the diagram. The cross section of the outgoing pipeline at the bottom is proportional to that portion of the heat, Q1, which is rejected as heat in the exhaust. The branch line to the right represents that portion of the heat supplied, which the engine converts to mechanical work. The thermal efficiency Eff(%) is expressed by the formula:

Eff(%) = W / Q2 = (Q2 - Q1) / Q2 ---------- (6)

heat Flow Carnot Cycle The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes (see Figure 08). The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. When the second law of thermo-dynamics states that not all the supplied heat in a heat engine can be used to do work, the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used. In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy.

Figure 07 Heat Engine

Figure 08 Carnot Engine Cycle
[view large image]
Gasoline Engine Diesel Engine Steam Engine The p-V diagrams for the more realistic cases are shown in Figure 09, 10, and 11 for the gasoline, diesel, and steam engines respectively. While the gasoline and diesel engines operate at about 50% efficiency, the steam engine runs at only about 30%. A brief description of the processes can be found in each of the diagram.

Figure 09 Gasoline Engine

Figure 10 Diesel Engine

Figure 11 Steam Engine [view large image]

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