Page 264 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 21] THERMODYNAMICS 249

SOLVED PROBLEM 21.6
The four-cylinder, four-stroke diesel engine of a car develops 60 kW at 2600 rev/min. The pistons of this
engine are 100 mm in diameter, and they travel 130 mm. Find the average pressure on the pistons during
each power stroke.

The area of each piston is
πd 2 π(0.1m) 2 2
A = = = 0.00785 m
4 4
In a four-stroke engine a power stroke occurs in each cylinder once every 2 rev, so there are 1300 power strokes
per minute or 1300/60 = 21.7 strokes per second. Because the engine has four cylinders, its total power output is
P = 4P c = 4pL An and
3
P 60 × 10 W 5
p = = = 6.8 × 10 Pa = 6.8 bar
2
4LAn (4)(0.13 m)(0.00785 m )(21.7/s)
SECOND LAWOF THERMODYNAMICS
Internal energy resides in the kinetic energies of randomly moving atoms and molecules, whereas the output of a
heat engine appears in the ordered motions of a piston or a wheel. Since all physical systems in the universe tend
to go in the opposite direction, from order to disorder, no heat engine can completely convert heat to mechanical
energy or, in general, to work. This fundamental principle leads to the second law ofthermodynamics:Itis
impossible to construct a continuously operating engine that takes heat from a source and performs an exactly
equivalent amount of work.
Because some of the heat input to a heat engine must be wasted and because heat flows from a hot reservoir
to a cold one, every heat engine must have a low-temperature reservoir for exhaust heat to go to as well as a
high-temperature reservoir from which the input heat is to come, as in Fig. 21-1.

CARNOTENGINE
An ideal heat engine is one in which every process that occurs is reversible without any loss of energy. Such an
engine is not subject to such practical mechanisms of energy loss as friction and heat conduction to the outside
world. An example of an ideal heat engine is the imaginary Carnot engine which consists of a cylinder filled

T 1 T 2
High-temperature Low-temperature
reservoir reservoir

Fig. 21-6. (Top portion from Modern Technical Physics, 6th Ed., Arthur Beiser, c 1992. Reprinted by permission of
Pearson Education, Inc.)