Physics Help Forum Is Carnot engine the only reversible engine?

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Jan 20th 2015, 12:01 AM #1 Member     Join Date: Jun 2014 Posts: 67 Is Carnot engine the only reversible engine? Is Carnot engine the only form of reversible engine? Is it possible to have a different form of reversible engine that goes through different processes? For a standard Otto cycle working with ideal gas, theoretically the two processes involving isochoric pressure change can be reversible processes. Does it mean that the Otto cycle can also have same efficiency as Carnot cycle? Since temperature of the working fluid is changing during isochoric pressure change, does it mean there are infinite number of heat reservoirs being used at different temperatures so that there is no finite temperature difference at every point of the isochoric process?
Jan 20th 2015, 12:04 PM   #2
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 Originally Posted by kelsiu Is Carnot engine the only form of reversible engine?
Not really, because it's an idealization. See Rod Nave's hyperphysics:

"In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy".

Entropy sometimes causes issues, just think of it as a measure of "available energy". If the whole universe is at the same energy density, none is available to do any work. Every time you do work, you are levelling out the energy density.

 Originally Posted by kelsiu Is it possible to have a different form of reversible engine that goes through different processes?
I imagine one could propose some different style of idealized engine, but it won't be a real engine.

 Originally Posted by kelsiu For a standard Otto cycle working with ideal gas, theoretically the two processes involving isochoric pressure change can be reversible processes. Does it mean that the Otto cycle can also have same efficiency as Carnot cycle? Since temperature of the working fluid is changing during isochoric pressure change, does it mean there are infinite number of heat reservoirs being used at different temperatures so that there is no finite temperature difference at every point of the isochoric process?
Sorry, I don't know.

 Tags carnot, engine, reversible, thermodynamics

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