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Engines

Engines. Second Law I. Real processes often make sense in only one direction in time. The second law of thermodynamics states this: Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object. Heat Engine.

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Engines

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  1. Engines

  2. Second Law I • Real processes often make sense in only one direction in time. • The second law of thermodynamics states this: Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.

  3. Heat Engine • A device that changes thermal energy into mechanical work is a heat engine. • Requires two temperature reservoirs • Takes heat in and exhausts heat out • Conservation of energy applies. • |QH| = |QL| + |W|

  4. Steam Engine • Steam is produced setting the high temperature. • Expanding steam pushes a piston doing work. • Steam is condensed setting the low temperature.

  5. Steam Turbine • A steam turbine replaces a piston with a paddlewheel. • Most power plants use steam turbines. • Fuel is used to make steam • Coal • Oil • Natural gas • Uranium

  6. Efficiency • The efficiency of a heat engine compares the work done to the heat into the engine. • Measures fuel and work • Efficiency also can compare heat in and out.

  7. An auto engine has an efficiency of 20% and produces 23 kW of mechanical power. How much heat is discharged per second? The efficiency is 0.20. The output work in 1 s is W = 23 kJ. Find the input heat. QH = W / e = 115 kJ Find the output heat. QL / QH = 1 – e QL = QH (1 – e) = 92 kJ DQL / Dt = 92 kW Exhaust

  8. Second Law II • Efficiency can only equal 1 when QL = 0. • Perfect conversion to mechanical work • Not possible with real systems • The second law of thermodynamics can also read: No device is possible whose sole effect is to transform a given amount of heat into work.

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