Modern Engineering Thermodynamics

13.2 Part I. Engines and Vapor Power Cycles 451
By 1712, Thomas Newcomen (1663–1729), an English blacksmith, had devised a better steam engine, which could pump
water from very great depths while simultaneously eliminating the need for a high boiler pressure. A schematic of the
operation of his engine is shown in Figure 13.3. He introduced a piston-cylinder arrangement in place of Savery’s pumping
chamber, with the piston attached to one end of a walking beam and a positive displacement piston-cylinder pump,
located deep in the mine, attached to the other end of the walking beam. The pump end was counterweighted so as to
hold the driving piston at the top of its stroke. When low-pressure (3 to 5 psig) steam was introduced under the driving
piston and condensed using a cold water spray, the resulting partial vacuum allowed atmospheric pressure to push the driving
piston down (thus charging the pump with water). Venting the driving piston to the atmosphere allowed the counterweight
to drop, thus pumping the water. These engines typically ran at a speed of about ten cycles per minute.
Opening the manual valve again allowed steam from the boiler to force this water out through the vertical discharge pipe.
A skilled operator could run this engine at about five cycles per minute.
Since both the Savery and Newcomen engines depended on creating a partial vacuum by condensing steam and using atmospheric
pressure to cause the necessary motion, they are called atmospheric engines. Also, since both engines alternately heated
and cooled large metal chambers (Savery’s pumping chamber and Newcomen’s piston-cylinder) during each cycle of operation,
both had very low thermal efficiencies (a fraction of 1%). But, since the concept of thermal efficiency had not yet been developed,
their poor thermal performance went undetected. However, as the technology advanced and more engines came into use,
it became clear to their owners that they required an enormous amount of fuel (wood or coal) to keep them operating.
Walking
beam
Chain
Water
seal
Cylinder
Pump rod
Chain
Piston
Condensing
steam
Water jet
Steam
valve
Haystack
boiler
Hot well
Water
To
mine
pump
Fire
Water pump
FIGURE 13.3
A schematic of the operation of Newcomen’s steam engine. When the cylinder has been filled with steam, the driving piston rises
and the weight of the pump rod forces a stroke on the pump (not shown) at the bottom of the mine shaft. The steam valve is then
closed and the injection water valve opened. The cold water jet condenses the steam in the cylinder, producing a partial vacuum.
Atmospheric pressure (acting on the top of the driving piston) forces the driving piston down, and the walking beam lifts the pump
rod and makes the engine ready for another pumping cycle.