I have an engine cycle in mind looking for it's name

1. Jul 10, 2011

shanesworld

This is a simple question I have because I am looking at a particular engine cycle, and I can't seem to think of its name...thus it is very difficult to search for discussions of something you don't know the name of. I'll describe the cycle, if any one knows the name of the cycle please let me know, or remind me of what it is called. This is neither the Carnot cycle, nor the Otto Cycle,( aguable even more simlpe minded)

In four steps on a Pressure-Volume Diagram it goes

1) Gas in piston (or the likes) expands at a constant pressure P1
2) Heat is allowed to escape, but volume is held fixed at V1
3) Gas in piston contracts at constant lower pressure P2
4) Heat is added while volume is held fixed at V2. (V2<V1)

It looks like a square drawn on the P-V diagram. What is it's name?

2. Jul 11, 2011

EWH

Sounds similar to the Ericsson cycle.
isothermal compression - isobaric heat addition - isothermal expansion - isobaric heat rejection
so it's a P-V square.

3. Jul 11, 2011

shanesworld

...well, I don't actually think that is a P-V square though, but it's interesting in itself.

Last edited: Jul 11, 2011
4. Jul 11, 2011

EWH

If all the phases are isobaric or isothermal, then it can be a square on the P-V diagram. If any phase is other than isobaric or isothermal, it can't be a square on the P-V diagram.

5. Jul 11, 2011

shanesworld

Isothermal usuall does not generally correspond to a straight line on P-V diagram. For instance for an ideal gas nRT=PV. At a constant temperature (i.e isothermal) this means that PV=constant; if the volume changes, the pressure must also change. So in a step where pressure is changed and the volume is held fixed, this would not be isobaric in general... there is at least one major case where your statement is not true. That is why I said the cycle you mentioned is not a square on the P-V diagram.

Last edited: Jul 11, 2011
6. Jul 11, 2011

jack action

I don't think someone bothered naming such a cycle as its existence is pointless.

The problem lies in your isobaric expansion phase followed by a isochoric cooling phase. The only way to achieve the isobaric expansion is by constantly increasing the temperature (like in a brayton or a diesel cycle). And right after that, you are cooling the gas ... without a «work retrieving» phase in between! You absolutely need an adiabatic phase, worst case scenario, an isothermal phase to convert the heat you added into work.

The same goes for your isobaric compression where you need to constantly drop the temperature in order to achieve that, hence removing heat ... followed immediately by a heat addition!

If I didn't make any mistake, the maximum efficiency ($\eta$) you can get with the cycle you described is:

$\eta = \frac{k-1}{k}$

where k is the specific heat ratio for the gas. For air, the max efficiency turns out be 29%. And that is assuming infinite pressure and compression ratios!

7. Jul 12, 2011

EWH

"Isothermal usuall does not generally correspond to a straight line on P-V diagram. "

Quite right, my mistake.

8. Jul 18, 2011

shanesworld

Wow, hadn't checked on this thread in a while but thanks for the post. ...appreciate it.