Otto cycle reversible or not?

Question

In my course it states that the Otto cycle is an irreversible process. I'm not really sure why. I have also found by a quick google search that a lot of people say the cycle is reversible. To my knowledge an Otto cycle is both quasi-static and does not have any dissipating effects. (definition of reversibility in my course)

My course states the following (translated from Dutch):

In an Otto cycle where only two reservoirs are involved, the heat transfers in the isochoric processes with finite temperature differences imply that the processes cannot be reversible. On the other hand, if an Otto cycle is reversible, there must be more than two temperature reservoirs.

Is the Otto cycle reversible or not? Why does my course state it isn't?

Answer 1

Otto cycle reversible or not?

All real cycles, including the Otto, are irreversible. The main reason is all real cycle are not composed of quasi-static processes, meaning not composed of processes that are carried out so slowly as to appear to be static (not moving), which would make them impractical.

That said, the internal combustion engine Otto Cycle can be modeled as a reversible cycle as shown in the PV and TS diagrams below. (Note: intake and exhaust strokes not shown).

The heat addition (spark ignition) process 2-3 is modeled as a constant volume (isochoric) reversible heat addition. In a real Otto cycle the ignition results in a rapid increase in temperature and pressure from 2 to 3. A rapid increase is by definition non quasi-static and therefore irreversible.

In the reversible cycle below the heat addition is provided by an infinite series of thermal reservoirs ranging from T2 to T3, each reservoir infinitesimally greater than the previous. This results in a very slow, quasi-static transfer of heat making the process reversible.

The problem with the reversible cycle is it has to occur so slowly that it would be impractical. If you put a reversible Otto engine in your car you would get phenomenal fuel economy, but you would move so slowly that pedestrians would be passing you by!

Hope this helps.