<P> The efficiency of an engine cycle can be calculated by dividing the energy output of mechanical work that the engine produces by the energy input to the engine by the burning fuel . </P> <P> The historical measure of a steam engine's energy efficiency was its "duty". The concept of duty was first introduced by Watt in order to illustrate how much more efficient his engines were over the earlier Newcomen designs . Duty is the number of foot - pounds of work delivered by burning one bushel (94 pounds) of coal . The best examples of Newcomen designs had a duty of about 7 million, but most were closer to 5 million . Watt's original low - pressure designs were able to deliver duty as high as 25 million, but averaged about 17 . This was a three-fold improvement over the average Newcomen design . Early Watt engines equipped with high - pressure steam improved this to 65 million . </P> <P> No heat engine can be more efficient than the Carnot cycle, in which heat is moved from a high temperature reservoir to one at a low temperature, and the efficiency depends on the temperature difference . For the greatest efficiency, steam engines should be operated at the highest steam temperature possible (superheated steam), and release the waste heat at the lowest temperature possible . </P> <P> The efficiency of a Rankine cycle is usually limited by the working fluid . Without the pressure reaching supercritical levels for the working fluid, the temperature range the cycle can operate over is quite small; in steam turbines, turbine entry temperatures are typically 565 ° C (the creep limit of stainless steel) and condenser temperatures are around 30 ° C . This gives a theoretical Carnot efficiency of about 63% compared with an actual efficiency of 42% for a modern coal - fired power station . This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is often used as a bottoming cycle in combined - cycle gas turbine power stations . </P>

Who designed a steam engine to power machines