# Heat Engines

1. Feb 24, 2004

### UrbanXrisis

Can someone explain to me how Heat engines work?

2. Feb 24, 2004

### NateTG

There are *lots* of different types of heat engines. Some - like Peltier junctions - involve complicated mechanisms.

Stirling cycle engines rely on using gas expansion. You should be able google and find something much better than what you will get from me in a post.

Any fuel burning engine that you've seen qualifies as a heat engine.

3. Feb 24, 2004

### UrbanXrisis

you see, I really don't get it. My physics teacher told me that heat engines formed a PV diagram...looked like a rectangle. There was an increase in pressure...he said it was abiabatic. Then increase in volume, he said it was isobaric. Then abiabatic, then isobaric. And now we are back at step one. My question is, in a PV diagram, how can you have a change in pressure when there is no heat added to it? What causes that change?

4. Feb 24, 2004

### HallsofIvy

Staff Emeritus
PV= NRT. Change the volume of course! (You do it slowly so that any temperature change conducts to the outside air.)

5. Feb 24, 2004

### NateTG

Halls - heat and temperature are not the same thing. What you're describing allows heat to flow in from the outside environment if the volume is increasing. (That's why, for example, rapidly expanding gasses cool things.)

Urban - Do you know what type of heat engine your teacher was trying to describe? The typical theoretical device is a Carnot Cycle engine, but there are many others like the Otto Cycle or the Rakine Cycle which describe automotive and steam engines.

A Carnot cycle involves Adiabatic compression and expansion, and Isothermal compression and expansion. There are no isobaric phases in a Carnot cycle.

6. Feb 24, 2004

### UrbanXrisis

Yes, he is talking about the Carnot cycle. I just dont understand. You say that there is a change in volume, but what outside force causes it?

7. Feb 24, 2004

### NateTG

The idea is that you get work out of transporting heat from one reservoir to another.

For the carnot cycle, the outside force that you describe is an 'implementation detail.' Its more of what an ideal engine *should* do.

In practice, engine design is an attempt to create a mechanism for introducing these outside forces that you describe.

8. Feb 25, 2004

### UrbanXrisis

Could you please explain the isothermal compression and expansion of the carnot cycle?

9. Feb 25, 2004

### NateTG

In a carnot cycle you've got temperature reservoirs at temperatures $$T_1$$ and $$T_2$$. What happens is that the working gas is compressed or expanded while it stays in thermal equilbrium with one of the temperature reservoirs.

Where power needs to be applied depends on what the carnot engine is being used for, and the carnot engine.

10. Feb 25, 2004

### turin

A rectangle on a PV diagram is inconsistant with these strok descriptions. Two opposing strokes (the isobars) will be parallel to each other on the PV diagram, but the adiabatic strokes will not be parallel (at least, I can't think of a case in which they both would be). In order to make a rectangle, you would need to replace the adiabats with isochors.

11. Feb 26, 2004

### UrbanXrisis

is there any good sites where they give a detailed description of the heat engine?