We are using modulus signs to avoid complications with signs: all efficiencies are
positive numbers. The definition implies that, the greater the work output for a given
supply of heat from the hot reservoir, the greater is the efficiency of the engine. We can
express the definition in terms of the heat transactions alone, because (as shown in
Fig. 3.7), the energy supplied as work by the engine is the difference between the
energy supplied as heat by the hot reservoir and returned to the cold reservoir