Hero’s Engine

Introducing Engines and Heat Pumps

Heat Engines

Any device that uses heat to perform work

3 Essential Features

1. Heat comes from hot reservoir.

2. Part of input heat performs work

3. Remainder of heat, QC<QH, goes to a cold reservoir

Efficiency (e) = Energy output/Energy Input

• To be efficient, an engine must produce a large amount of work for as little input heat as possible. HQ

We

Car engines are only about 25% efficient! They output work (rotating) by taking in heat (Q) from some htot “resevoir” (source) of heat.

If a machine takes in 100 J of energy to do 50J of work it is _____ % efficient (or ,we say, e = 0.5)

In Terms of Temperature

H

cH

T

TTe

Calculate the maximum efficiency for an ideal engine running with a hot source at 100 C and a cold source at O C. (Switch to kelvin first!)

Carnot’s Principle

• The most efficient engine operates around a reversible process in which both system and environment can be returned the exact states they were in before the process.

• This can’t happen, but the closer you get to it the more efficient you engine is.

Example

• The water near the surface of a tropical sea is at 298.2 K while 700m beneath the surface it’s 280.2 K. It has been proposed that we can use this difference in temperature to make a heat engine. Find the efficiency of it.

• 6%

The Carnot Cycle1. Isothermal expansion 1-2,

W=Q, heat in is work out2. Adiabadic expansion 2-3,

expanding so temp must drop. Work still positive

3. Isothermal compression 3-4, work is negative so Q is out

4. Adiabatic compression 4-1, work negative so into system, raising temp and pressure

Overall Effect

• Heat has been moved