Page 267 - Schaum's Outline of Theory and Problems of Applied Physics

252 THERMODYNAMICS [CHAP. 21

gasoline vapor and air from the carburetor is sucked into the cylinder through the intake valve as the piston
moves downward. In the compression stroke, both valves are closed and the fuel-air mixture is compressed to 1/7
or 1/8 of its original volume. At the top of the stroke the spark plug is fired, which ignites the fuel-air mixture.
Gases from the burning fuel expand and force the piston downward in the power stroke. Finally the piston moves
upward again to push the waste gases out through the exhaust valve.
In a diesel engine, only air is drawn into the cylinder in the intake stroke. At the end of the compression
stroke, diesel fuel is injected into the cylinder and is ignited by the high temperature of the compressed air. No
spark plug is needed. Diesel engines are more efficient but also heavier and more expensive than gasoline engines.

SOLVED PROBLEM 21.12
Describe the operating cycle of a gasoline engine with the help of a p-V diagram.

In Fig. 21-8 the events that occur during each cycle of the engine are as follows:
ab: intake of fuel-air mixture
bc: compression stroke
cd: ignition of fuel-air mixture; the heat Q 1 is released into the cylinder
de: power stroke
e: exhaust valve opens, which starts the release of the waste heat Q 2
ba: exhaust stroke

p
d

Q 1

c
W
e
Q 2
a b

V

Fig. 21-8. (From Modern Technical Physics, 6th Ed., Arthur Beiser, c 1992. Reprinted by permission of Pearson
Education, Inc.)

REFRIGERATION

A refrigerator is a heat engine that operates backward to extract heat from a low-temperature reservoir and
transfer it to a high-temperature reservoir (Fig. 21-9). Because the natural tendency of heat is to flow from a hot
region to a cold one, energy must be provided to a refrigerator to reverse the flow, and this energy adds to the
heat exhausted by the refrigerator.
Figure 21-10 shows a refrigerator whose working substance is an easily liquified gas called a refrigerant.
The operation of this refrigeration system proceeds as follows:
1. The compressor, usually driven by an electric motor (or by the car engine in the case of an automotive
air conditioner), brings the refrigerant to a high pressure, which raises its temperature as well.
2. The hot refrigerant passes through the condenser, an array of thin tubes that give off heat from the
refrigerant to the atmosphere. The condenser is on the back of most household refrigerators. As it cools,
the refrigerant becomes a liquid under high pressure.