Heat Engines, Then and Now

The heat engines of interest to Sadi Carnot were steam engines applied to such
tasks as driving machinery, ships, and conveyors. The steam engine invented by
a Cornishman, Arthur Woolf, was particularly admired in France in the 1810s
and 1820s. Operation of the Woolf engine is diagrammed in figure 3.1. Heat Q2

was supplied at a high temperature t2 by burning a fuel, and this heat generated
steam at a high pressure in a boiler. The steam drove two pistons and they provided
the workoutput W1. (In this chapter and elsewhere in this part of the book,
keep in mind that the symbol t represents temperature and not time, as in chapters
1 and 2.) The steam leaves the pistons at a decreased pressure and temperature.
Heat Q1 was then extracted in a condenser where the steam was further
cooled to a still lower temperature t1 and condensed to liquid water. Finally, the
liquid water passed through a pump, which restored the high pressure by expending
work W2, and low-temperature, pressurized water was returned to the
boiler. This is a cycle of operations, and its net effect is the dropping of heat
from the high temperature t2 to the low temperature t1, with workoutput W1
from the pistons and a much smaller workinput W2 to the pump.
The Woolf steam engine and its variations have evolved into a vast modern
technology. Most contemporary power plants operate similarly. The scale is
much larger in the modern plants, the operating steam pressures and temperatures
are higher, and the working device is a turbine rather than pistons. But the
concept of heat falling between a high and a low temperature with net work
output again applies.