Why is rapid expansion/compression considered a reversible/isentropic process?

Question

I am looking over the Otto Cycle on this MIT website and it says at one point "the processes from 1 to 2 and from 3 to 4 are isentropic" in reference to the expansion and compression of the piston. I understand however that the compression and expansion in an internal combustion engine is quite violent and quick. And according to wikipedia "Throughout an entire reversible process, the system is in thermodynamic equilibrium, both physical and chemical, and nearly in pressure and temperature equilibrium with its surroundings." However this isn't the case for our expansion and compression is it? the pressure and temperature of our expansion/compression is not in equilibrium? It also says "reversible processes are extremely slow (quasistatic)". But once again, our expansion/compression isn't slow at all?

I've noticed, there are many processes that seem to be described as isentropic that from what I can tell are not in equilibrium with their environment throughout and are not quasistatic (my book for example says pumps, turbines, nozzles, and diffusers perform isentropic operations)? So through what justifications are these approximations made?

The main reason I ask this question is because I am attempting to calculate the pressures in a gun barrel, and I have seen the process described as isentropic (adiabatic and reversible) but for the same reasons as the Otto cycle, cannot understand how this would apply. The gun barrel acts similar to a rapidly expanding piston. Ignition, rapid expansion of gases against bullet (piston), then bullet (piston) leaves and the gases diffuse rapidly into the atmosphere. How is this reversible or quasistatic in any way? If anyone has an idea regarding the gun barrel and what process it is that would be appreciated as well.

Answer 1

No real process is isentropic. Thermodynamics, however, works only with reversible processes and so we make the approximation of reversibility in order to perform calculations. The results of this calculation represent a bast-case scenario in terms of the amount of work produced (maximum possible) or consumed (minimum possible) and establishes a baseline against which to compare the real process. For example, in a turbine we calculate the amount of reversible work, then measure the actual work, which is less, and report their ration as the efficiency of the turbine. An efficiency of 85% essentially says that the process is 85% close to true reversibility.

In your gun barrel problem we have no option but to assume reversibility. Otherwise you need to build a rather elaborate model to account for the flow of gases, the flow of energy energy and the chemical reactions that take place.