# Reversible engine and entropy

1. Dec 17, 2008

### KFC

I learned Thermodynamics long time ago, forgot most of the content. I just want to ask I question.

For a reversible engine, no matter how many complete cycles it done, the total change of entropy of the engine will always be ZERO, right? What about the change of entropy of the universe in this case? Also ZERO?

2. Dec 17, 2008

### p.tryon

Would this be a combustion engine?

3. Dec 17, 2008

### KFC

I don't know, it is a conceptural question, no other detail. I read some book to review therodynamics and stat. mech. , some material said for reversible process, to complete one cycle, nothing change, the change of entropy is ZERO. Just want to confirm that.

4. Dec 18, 2008

### LURCH

That depends no whether the universe is reversable (oscilating nuiverse) or non-reversable (heat-death). Current observations suggest that the latter is the case, so entropy just contiues increasing 'till it reaches maximum, when no more energy can flow from one place to another. At this point, the universe will be a fairly homogenous low-density cloud of energy.

5. Dec 18, 2008

### KFC

I completely have no idea of this. So what about the system itself, will entropy change?

6. Dec 18, 2008

### p.tryon

This seems reasonable. If there is no change in the engine, there will be no change in entropy of the engine system. However, I am struggling to imagine an engine that does not involve some type of change.

7. Dec 18, 2008

### KFC

Like carnot engine? But this is a very ideal case, textbook said there is no engine with efficiency larger than that of carnot engine, does that mean practical engine will never be reversible?

8. Dec 18, 2008

### p.tryon

Yes and saying that a real engine can not be thermodynamically reversible is the same as saying it can not be 100% efficient. In theory it is possible but in practice it is impossible because in a working engine there will always be friction...etc...