We can repeat this process over and over again. This certainly is a very inefficient engine. It wastes a large amount of heat. He assumed that an ideal engine would be a frictionless engine. It would also be a reversible engine.
Heat always flows from an object of higher temperature to an object with lower temperature.
When a reversible engine causes heat to flow into a system, it flows as the result of infinitesimally small temperature differences, or because there is an infinitesimal amount of work done on the system. A real engine always involves at least a small amount of irreversibility. Heat will not flow without a temperature differential and friction cannot be entirely eliminated. Here T is the absolute temperature, measured in Kelvin and a heat reservoir is a system, such as a lake, that is so large that its temperature does not change when the heat involved in the process considered flows into or out of the reservoir. Carnot's result holds for any reversible engine.