Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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28 Chapter Two
Voltmeter
200 300 400 500 600
Pyrometer
Exhaust Temp. ºF
Thermocouple
Voltmeter
Figure 2-14.
Pyrometer thermocouple.
Lemon Battery
alloy, which is then connected to a voltmeter. As
the temperature at the connections of the two
metals increases, the reading on the voltmeter
increases. The voltmeter can then be calibrated in
degrees.
Figure 2-13.
Lemon powered battery.
shows a very simplified version of a battery powered
by a lemon. The availability and amount of
electrical energy that can be produced in this manner
is limited by the active area and weight of the
materials in the plates and by the quantity of sulfuric
acid in the electrolyte. After most of the
available active materials have reacted, the battery
can produce little or no additional energy, and the
battery is then discharged.
Thermoelectricity
Applying heat to the connection point of two dissimilar
metals can create electron flow (electricity),
which is known as thermoelectricity (Figure 2-
14). This affect was discovered by a German scientist
named Seebeck and is known as the Seebeck
Effect. Seebeck called this device a thermocouple,
which is a small device that gives off a low voltage
when two dissimilar metals are heated. An example
of a thermocouple is a temperature measuring
device called a pyrometer. A pyrometer is commonly
used to measure exhaust gas temperatures
on diesel engines and other temperature measuring
applications. A pyrometer is constructed of
two dissimilar metals, such as steel and a copper
Photoelectricity
Light is composed of particles called photons that
are pure energy and contain no mass. However,
when sunlight contacts certain materials, such as
selenium and cesium, electron flow is stimulated
and is called photoelectricity (Figure 2-15).
Photoelectricity is used in photoelectric cells,
which are used in ambient light sensors. Solar
energy is light energy from the sun that is gathered
in a photovoltaic solar cell.
Piezoelectricity
Some crystals, such as quartz or barium titanate,
create a voltage if pressure is applied. A change in
the potential of electrons between the positive and
negative terminal creates electricity know as
piezoelectricity. The term comes from the Greek
word “piezo,” which means pressure. Figure 2-16
shows that when these materials, quartz or barium
titanate, undergo physical stress or vibration, a
small oscillating voltage is produced.
Piezoelectricity is the principle used in knock
sensors (KS), also called detonation sensors. The
typical knock sensor (Figure 2-17) produces
about 300 millivolts of electricity and vibrates at
a 6,000-hertz (cycles per second) frequency,
which is the frequency that the cylinder walls
vibrate at during detonation.