An irreversible process is a process that cannot return both the system and the surroundings totheir original conditions. That is, the system and the surroundings would not return to theiroriginal conditions if the process was reversed. For example, an automobile engine does not giveback the fuel it took to drive up a hill as it coasts back down the hill.There are many factors that make a process irreversible. Four of the most common causes ofirreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finitetemperature difference, and mixing of two different substances. These factors are present in real,irreversible processes and prevent these processes from being reversible.