Why is the efficiency of real heat engines always less than unity?

In real motors, there is always energy loss, for example due to friction. Therefore, the efficiency of real cyclic motors is always less than the efficiency of an ideal one at the same temperatures of heaters and refrigerators. It does not even reach 50%. This is due to the fact that the working fluid, having received energy from the burnt fuel, cannot completely convert it into mechanical energy. Part of the internal energy of the gas is inevitably transferred to the refrigerator, dissipated in the surrounding space, and spent on work against the frictional forces of engine parts.

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