<P> A very well designed and built steam locomotive used to get around 7 - 8% efficiency in its heyday . The most efficient reciprocating steam engine design (per stage) was the uniflow engine, but by the time it appeared steam was being displaced by diesel engines, which were even more efficient and had the advantage of requiring less labor for coal handling and oil, being a more dense fuel, displaced less cargo . </P> <P> Using statistics collected during the early 1940's, the Santa Fe Railroad measured the efficiency of their fleet of steam locomotives in comparison with the FT units that they were just putting into service in significant numbers . They determined that the cost of a ton of oil fuel used in steam engines was $5.04 and yeilded 20.37 train miles system wide on average . Diesel fuel cost $11.61 but produced 133.13 train miles per ton . In effect, diesels ran six times as far as steamers utilizing fuel that cost only twice as much . This was due to the much better thermal efficiency of diesel engines compared to steam . Persumably the trains used as a milage standard were 4,000 ton freight consists which was the normal tannage l at that time . </P> <P> The steam turbine is the most efficient steam engine and for this reason is universally used for electrical generation . Steam expansion in a turbine is nearly continuous, which makes a turbine comparable to a very large number of expansion stages . Steam power stations operating at the critical point have efficiencies in the low 40% range . Turbines produce direct rotary motion and are far more compact and weigh far less than reciprocating engines and can be controlled to within a very constant speed . As is the case with the gas turbine, the steam turbine works most efficiently at full power, and poorly at slower speeds . For this reason, despite their high power to weight ratio, steam turbines have been primarily used in applications where they can be run at a constant speed . In AC electrical generation maintaining an extremely constant turbine speed is necessary to maintain the correct frequency . </P> <P> The Stirling cycle engine has the highest theoretical efficiency of any thermal engine but it has a low output power to weight ratio, therefore Stirling engines of practical size tend to be large . The size effect of the Stirling engine is due to its reliance on the expansion of a gas with an increase in temperature and practical limits on the working temperature of engine components . For an ideal gas, increasing its absolute temperature for a given volume, only increases its pressure proportionally, therefore, where the low pressure of the Stirling engine is atmospheric, its practical pressure difference is constrained by temperature limits and is typically not more than a couple of atmospheres, making the piston pressures of the Stirling engine very low, hence relatively large piston areas are required to obtain useful output power . </P>

What percentage of fuel is burned in an engine