chap14 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 14, the reader should
be able to:

•
•
•

Add and subtract decimal numbers

Read a chart and graph
Calculate percentages.

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KEY TERMS:
chart
decimal point • diagram • direct drive • drive gear •
driven gear
fractions • gear reduction • graph • overdrive
percentage • scientific notation • variable

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DECIMAL
Decimals are commonly used by services. The placement of the
decimal point indicates the value of the number. The naming of
decimals includes tenths, hundredths, thousandths, and higher.
Decimals are used to represent fractions of a unit by using a dot
called a decimal point to indicate that the number is a decimal.
Tenths A decimal with one number to the right of the decimal
point indicates an accuracy of 1/10 or 0.1. For example, 0.7 is the

same as 7/10 and is pronounced “seven tenths” or “zero point
seven.” A decimal can also include numbers larger than zero, but
has a resolution or accuracy measured in tenths, such as in 14.7.
Continued
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Hundredth Decimals with two numbers to the right of the decimal point indicate an accuracy to 1/100 or 0.01. For example, 0.47
is pronounced “forty-seven hundredth” or “zero point four seven.”
Thousandth A decimal with three numbers to the right of the
decimal point indicates an accuracy to 1/1000 or 0.001. For
example, 0.867 is pronounced “eight hundred sixty-seven

thousandths” or “zero point eight six seven.”
Adding and Subtracting Decimals When adding or subtracting
decimals, the decimal point has to be aligned. This ensures that the
numbers are placed into the correct position of tenth, hundredth,
and thousandth. For example:
0.147
+ 0.02
0.167
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PERCENTAGE

Percentage is the relationship of a value or number out of 100.
Using money as an example, three quarters (25 cents each) equals
75 cents ($0.75) or 75% of a dollar ($1.00). Many examples are
not that easy, for example, 70 is what percentage of 120? To
determine the percentage, divide the first number (70) by the
second number (120).
70 ÷ 120 = 0.58
To convert this number to a percentage, multiply the number by
100 or move the decimal point two places to the right (58) and
then add a percentage symbol to indicate that the number is a
percentage (58%).
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SCIENTIFIC NOTATION
Very large and very small decimal numbers are frequently
expressed using scientific notation, which written as a number
multiplied by the number of zeros to the right or left of the
decimal point.
For example, 68,000 could be written as 6.8 × 104 , indicating the
number shown has 3 zeros plus the 8 to the right of the decimal
point. Small numbers, such as 0.00068, would use a negative sign
beside the number over the 10 to represent that the decimal point
needs to be moved toward the left (6.8 × 10-4).

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ADDING AND SUBTRACTING
Techs are often required to add or subtract measurements when
working on vehicles. For example, adding and subtracting is
needed to select shims (thin pieces of steel) for adjusting valve
clearance or differential preload measurements.
For example, if the valve clearance specification is 0.012 in. and
the clearance is actually 0.016 in., and the shim that is in place
between the camshaft lobe and the valve bucket is 0.080 in. thick,
what size (thickness) of shim needs to be installed to achieve the
correct valve clearance?
Solution: The shim thickness of 0.080 in. results in a valve
clearance of 0.016 in. The specification requires that the shim
needs to be thicker to reduce the valve clearance.
Continued
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Figure 14–1 Valve clearance allows the metal parts to expand and maintain proper
operation, both when the engine is cold or at normal operating temperature.
(a) Adjustment is achieved by turning
the adjusting screw.

(b) Adjustment is achieved by changing
the thickness of the adjusting shim.
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FRACTIONS
Fractions, such as 1/2, 1/4, or 5/8 are commonly found in specifications
for hose inside diameter measurements. A tape measure or machinists rule
can be used to measure the fitting or the original part.

Sometimes, fractions need to be converted to decimal units if the
replacement parts are offered by that measurement method.
When comparing fractions to decimal units, think about the
number of cents in a dollar.
1/2 dollar 50 cents - 1/4 (quarter) 25 cents
1/10 (dime) 10 cents - 1/20 (nickel) 5 cents
Other fractions, such as 3/8, 5/8, and 5/16 are harder to determine.
If a chart is not available, divide the bottom number, called the
denominator into the top number, called the numerator.
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MULTIPLYING AND DIVIDING
Multiplying by a service tech is usually done to determine gear
ratios and to determine the total of many of the same items.
For example, final overall gear ratio is determined by multiplying
the transmission gear ratio by the final drive ratio and is covered
later in this chapter.
Dividing is commonly done when calculating total resistance of
many resistances connected in parallel. In this situation, the value
of the resistance is divided by the number of equal resistances.
For example, if four bulbs with a resistance of 0.4 ohm were
connected in parallel, the total resistance would be just 0.1 ohm.
(0.4 ÷ 4 = 0.1)
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MATHEMATICAL FORMULAS
A formula uses letters to represent values or measurements and
indicates how they are to be acted upon. To use a formula, the tech
needs to replace letters with the actual numbers and perform the
indicated math functions.
For example, a formula to determine engine speed in revolutions
per minute (RPM) is represented by the following formula:

This is used to determine the speed of the engine compared to the
gear ratio and tire size. Wheel and tire sizes are changed at timed
and knowing this is helpful.

Continued

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FUEL ECONOMY CALCULATOR
Step #1 Fill the tank until the nozzle clicks off.
Step #2 Drive a reasonable distance. Example—220 miles.
Step #3 Fill the fuel tank and record the number of gallons used
For this example, exactly 10.0 gallons.
Step #4 Calculate fuel economy—MPG. Miles driven divided by
the number of gallons used.
MPG = 220 divided by 10.0 = 22.0 miles per gallon
NOTE: Try to use the same station and pump, if possible, to achieve the

most accurate results.

Outside the US fuel economy is measured in liters of fuel needed to
travel 100 kilometers (62 miles), abbreviated L/100 km.
See the metric fuel chart on Page 106 of your textbook.
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GEAR RATIOS
When one gear turns another, the speed the two gears turn in
relation to each other is the gear ratio. Gear ratio is expressed as
the number of rotations the drive gear must make in order to

rotate the driven gear through one revolution.
To obtain a gear ratio, divide the number of teeth on the driven
gear by the number of teeth on the drive gear. Gear ratios, which
are expressed relative to the number one, fall into three categories:
Direct drive
Gear reduction
Overdrive
Continued
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Direct Drive If two meshed gears are the same size and have the

same number of teeth, they will turn at the same speed. Since the
drive gear turns once for each revolution of the driven gear, the
gear ratio is 1:1 and is called direct drive. When a transmission is
in direct drive, the engine and transmission turn at the same speed.
Gear Reduction If one gear
drives a second gear that has
three times the number of
teeth, the smaller drive gear
must travel three complete
revolutions in order to drive
the larger gear through one
rotation.
Figure 14–2 The drive gear is attached or
is closer to the power source and rotates
or drives the driven gear.
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Gear Reduction (cont.) Divide the number of teeth on the driven
gear by the number of teeth on the drive gear, the answer is a 3:1 gear
ratio (pronounced three to one).
This type of gear arrangement, where driven gear speed is slower than
drive gear speed, provides gear reduction. Gear reduction may also
be called underdrive as drive speed is less than, or under, driven
speed. It is used for the lower gears in a transmission.
First gear in a transmission is called “low” gear because output speed,
not gear ratio, is low. Low gears have numerically high gear ratios. A
3:1 gear ratio is a lower gear than those with a 2:1 or 1:1 gear ratio.
These three ratios taken in order represent a typical upshift pattern
from low gear (3:1), to second gear (2:1), to drive gear (1:1).

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Overdrive The opposite of gear reduction condition is overdrive and occurs when
a driven gear turns faster than its drive gear. As shown here, the driven gear turns
three times for each turn of the drive gear. The driven gear is said to over-drive the
drive gear.

For this example, the gear
ratio is 0.33:1. Overdrive
ratios of 0.65:1 and 0.70:1
are typical of those used in
automotive applications.

Figure 14–3 If the driven gear is rotating faster
than the drive gear, it is called an overdrive ratio.

NOTE: Ratios always end in

1 with a colon between. The
first number is less than 1 if
it is an overdrive ratio and
greater than 1 if it is a gear

reduction ratio.
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GRAPHS, CHARTS AND DIAGRAMS
Graph Reading A graph is a visual display of information, used
in auto service to illustrate trends or specs along with some other
variable. A variable is a measurement that changes, such as
engine speed or time.
Graphs displays two variables. One changes from left to right on
the horizontal, or X axis.
The other displays on the Y or vertical axis.

A graph is created by making series of dots at various locations
and connecting the dots with a line.

Continued
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Interpreting a Graph To interpret a graph, select point along the
horizontal (X) axis, then look directly above the point where the
line appears. Mark this spot and look directly to the left along the
vertical (Y) axis to see the value represented by the points on the
graph.

Figure 14–4 A graph showing
horsepower and torque. Notice
that the curves cross at 5252 RPM
or a little bit to the right of the 50,
which is expressed in hundreds
times 10 (thousands) of RPM.

Continued
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Chart Reading A chart is used to represent data, such as
numbers or specifications, along with another variable, such as
model or year of vehicle.
A chart is very useful for showing many different specifications
or other facts in an easy-to-read format.
The next slide displays an example of a specifications chart,
which shows the transmission parts listed along the horizontal
axis (X axis) and gear of the automatic transmission along the
vertical or Y axis.

Continued
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Interpreting a Chart A chart can look complicated but if
studied, is easy to interpret. Look along the horizontal or vertical
axis for information such as model year of the vehicle. Then look
directly above the model year to determine the specs for that year.

Figure 14–5 A typical chart showing what is applied in what gear in an automatic transmission.
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Diagram Reading A diagram

is a graphic design that explains
or shows the arrangement of
parts.
Diagrams are commonly used in
the auto service industry to show
how a component is assembled
and in which order the parts are
placed together.

Figure 14–6 An exploded view showing how the
thermostat is placed in the engine.

Continued
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Interpreting a Diagram A diagram shows the relationship of
many parts. Lines are used to show the centerline of the part and
the identity of the part is often shown as a number or letter.
A separate chart or area of the diagram needs to be looked at to
determine the name of the part. Diagrams are most helpful when
disassembling or assembling a component, such as a transmission.
For best results, use electronic information and print out the
diagram so it can be written on and can be thrown away when the
repair has been completed. This process also helps prevent getting
grease on the pages of a service manual.

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SUMMARY
1.

Decimals are used in many automotive applications and
specifications.

2.

Adding and subtracting decimal numbers are needed for
many automotive service procedures.

3.

A formula is used to determine the volume of something
if other values are known.

4.

A gear ratio is determined by dividing the number of teeth
on the driven gear by the number of teeth on the drive gear.

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end
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