SOLUTION

In a steam locomotive, work output can be measured as “indicated” or “cylinder” horsepower, or as “drawbar” horsepower. “Indicated horsepower” refers to an indicator diagram, plotting piston displacement against pressure, a device invented by James Watt early in the history of the steam engine. “Drawbar horsepower” is the power that the locomotive applies to its trailing load, in other words, indicated horsepower minus the internal friction and rolling resistance of the locomotive.

The thermal efficiency of the locomotive is the work output, specified as drawbar horsepower or indicated horsepower.

Of the power developed in the cylinders, only 75% reached the drawbar, the rest being consumed in friction of moving parts and rolling resistance. It is easy to see how the total thermal efficiency of a steam locomotive was 4% to 8% at best. Typical coal consumption was 4.5 pounds per drawbar horsepower-hour, burning bituminous coal with a calorific value of ( 13,700,000 joules) per pound. A horsepower hour is 2,685,600 joules. Therefore the thermal efficiency of a typical steam locomotive was no more than 4% to5%, calculated as (2,685,600/(4.5 x 13,700,000)