 Welcome back Mechanical AI, did you know that almost all processes that occur in the real life are completely irreversible? This makes us wonder, what is irreversibility? Before we jump in, check out the previous part of this series to learn about what availability is. Now, first let's care out the small concept of availability applied to a non-flow process. Let P1 be the ambient pressure and P1 and V0 be the initial and final volumes of the system respectively. If, in a process, the system comes into equilibrium with the surroundings, the work done in pushing back the ambient atmosphere is P0 into V0 minus V1. Availability in such cases is the difference between F0 and F1, where F is equal to internal energy at the state added to the product of final pressure and the state volume and the product of Fp0, with entropy of that state subtracted from it. The actual work done by the system is always less than the idealized reversible work. The difference between these two quantities, that is, the ideal work and the actual work of the system, is called irreversibility. It is often referred to as degradation or dissipation. Efficiency, according to the second law, or exogefficiency as the name suggests, imputes the efficiency of a process, taking the second law of thermodynamics into account. The exergy balance equation of a process is given as exergy intake equals the sum of exergy output, exergy lost, and the exergy destroyed. Efficiency of exergy is defined as exergy output upon exergy input. Rearrange the exergy balance equation and substituting, we get exergy efficiency eta B equal to 1 minus the sum of exergy lost and exergy destroyed, divided by the exergy intake. For all engineering systems, this can be expressed as eta B equals w net upon m dot fuel into standard Gibbs free energy of reaction at temperature T and pressure P0, which is usually 1 bar. Hence, we first saw what availability applied to a non-flow process is, then found out what irreversibility is, and then defined efficiency according to the second law.