Clausius Statement of Second Law of thermodynamics
According to the second law of thermodynamics it is impossible to have a thermodynamics system in which only the transfer of heat energy from cooler to hotter body is done.
Kelvin Planck Statement of Second Law of thermodynamics
According to the second law of thermodynamics it is not possible for any machine which work in close thermodynamic cycle and convert the energy obtain from the single heat source and convert it into work done on to its surrounding.
Heat engine is an energy conversion device which is design to convert the heat energy into mechanical energy. Heat engine convert the heat energy into mechanical energy by bringing the heat energy from high potential reservoir to low energy reservoir. According to this statement of second law that heat cannot flow from colder region to hotter region with the aid to external work, helps to determine the direction of flow of energy in heat engine. According to the other statement of second law that there is no device which only convert heat into energy, helps to determine the efficiency of the heat engine.