Modern Engineering Thermodynamics

520 CHAPTER 13: Vapor and Gas Power Cycles
CASE STUDIES IN APPLIED THERMODYNAMICS Continued
Note that, if we take the isentropic efficiency of the boiler feed
pump to be 70.0%, then the thermal efficiency of the entire Stanley
power plant is quite respectable at
ðη T Þ Stanley
Steamer
ð
= h 1 − h 2s Þ η s
ð Þ engine − v 3 ðp 4s − p 3 Þ/ ðη s
Þ pump
h 1 − h 4
144 1
ð1289:5 − 999:6Þð0:80Þ− 0:01672ð600: − 14:7Þ
778:16 0:700
=
1289:5 − 180:1 + 0:01672ð600: − 14:7Þ
144 1
778:16 0:700
= 0:207 = 20:7%
In Chapter 7, we learned that the amount of entropy production
inside a system is a key factor in limiting its energy conversion efficiency.
For example, the more moving parts and friction there are
inside an engine, the lower is its isentropic efficiency. Since Stanley
engines used double-acting cylinders with two power strokes occurring
with every revolution of the engine’s crankshaft, they produced
the same power as an equivalent eight-cylinder Otto cycle internal
combustion engine that has only one power stroke occurring in
every two revolutions of the engine’s crankshaft.
Case study 13.2. The drinking bird as a heat engine
Many novelty stores carry a toy called the drinking bird, which
bobs up and down, apparently drinking from a glass of water
(Figure 13.59). This toy is really a small heat engine that uses the
evaporation of water from its head as the power source for its
operation (Figure 13.60).
The head must be lightly covered with something that will hold a
small amount of water (e.g., a light fuzz) and the beak is simply a
wick that keeps the head wet. As the water evaporates from the head,
it cools the vapor inside the head causing a slight vacuum to form.
The liquid in the bottom of the bird is then drawn up into the bird’s
neck, shifting the center of gravity of the bird forward, and ultimately
causing it to tip. When the bird tips, the beak is rewetted and
a vapor bubble passes through the neck equalizing the pressures
between the ends of the bird and restoring the bird’s centerofgravity.
The bird then returns to its original upright position. This cycle
is continuously repeated until the water source is exhausted.
A typical eight-cylinder Otto cycle engine has 50–100 moving parts
with very significant mechanical friction, whereas the two-cylinder,
double-acting Stanley engine had only 15–25 moving parts. In fact, an
entire Stanley automobile had only 37 moving parts. Consequently,
the Stanley automobiles were very effective energy conversion devices.
In 1906, the Stanley brothers set a new world land speed record of
127.66 mph at the Dewar Cup Race in Omond Beach, Florida, with a
steam-powered race car called the Rocket (Figure 13.58). The car’s
body was made by a canoe factory and looked like an upside-down
boat. The engine was the same two-cylinder Stanley steam engine used
in their production vehicles, but it had been enhanced with a 4.50 in
bore and a 6.50-inch stroke. The standard Stanley boiler had been
enlarged to a 30.0 inches in diameter and was 18.0 inches high, and
operated at 1000 psia and about 700°F. Under these conditions the
little two-cylinder Stanley steam engine produced a whopping 250 hp.
FIGURE 13.59
Drinking bird.
FIGURE 13.58
The Rocket race car.
In1907,theStanleysagaintriedtosetanewworldlandspeed
record. This attempt ended in disaster, when the car became airborne
at about 190 mph and crashed. The driver survived the
crash, but a spectator in the crowd had to reinsert the driver’s right
eye, which had been dislodged from its socket by the force of the
impact. At the time of the crash, the boiler pressure had been
increased to an incredible 1300 psia.
(a) (b) (c) (d) (e)
FIGURE 13.60
The operation of the drinking bird.