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. Exhaust
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

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.