chap13 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 13, the reader should
be able to:

•
•
•
•
•

Explain kinetic energy and why it is so
important to brake design.
Discuss mechanical advantage and how it is
used in a vehicle.
Explain the coefficient of friction.
Describe the difference between heat and
temperature.
Describe the methods used to identify plastic,
iron, steel, and aluminum.

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KEY TERMS:
acid material • alkaline • brake • brake horsepower (bhp) •
BTU (British Thermal Unit)
caustic material • Celsius (centigrade) • conduction •
conductor • convection • dynamometer (dyno or dyn)
energy • Fahrenheit • first-class lever • force • fulcrum
horsepower • hypothesis • inertia • insulator
kinetic energy • leverage • mass • mechanical advantage
Newton’s laws of motion
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KEY TERMS:
pedal ratio • pH • potential energy • power • propagation
radiation • root cause
scientific method • second-class lever
third-class lever • torque
weight • work • wrought alloys

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SCIENTIFIC METHOD
The scientific method is a series of steps taken to solve a problem. It 
help eliminate errors and achieve an accurate result. A scientific method 
involves:

Step #1  Observe the conditions or problem; define or describe.
Step #2  Formulate an explanation that could be the cause.
Step #3  Use the explanation (hypothesis) to see if it matches the 
existing problem. If not, return to step 2.
Step #4  After the explanation has proved to be a possible solution 
to a problem, additional tests should verify the method.

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Using the Scientific Method  While a service technician will not 
perform research, a scientific approach to problem solving is very 
important.
This means that every fault should be investigated to determine 
the root cause, the true cause of the failure, rather than solving 
what at first is thought to be the problem or fault.
Many techs ask themselves “why” when they discover a fault. 
Often this leads to another possible problem and then the 
technician should ask another “why.”
This scientific method of finding the root cause of an automotive 
problem is often called the “five whys.” By the time the tech has 
asked “why” five times, the root cause is usually discovered.
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Examples of the Five Whys  As an example, an owner may state  
that the vehicle does not start and the battery appears to be dead. 
Applying the five whys:
First why—What caused the battery to become discharged? To 
answer this question requires observation and creating of a 
hypothesis, such as “is the battery defective” or “did the 
customer leave the lights on?” This requires questioning the 
owner and testing the battery.
Second why—Assume the battery was in good condition but 
discharged. Now the technician should ask the second why. 
“Why did the battery become discharged?” A battery ignition 
off drain test and testing of the charging system needs to be 
performed. Assume the battery drain test was OK, but  the 
charging system was not working OK.
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Third why—The charging system was not working correctly. A 
visual inspection found that the generator (alternator) drive belt 
was not tight enough. The third why: “Why is the accessory 
drive belt still loose?”
Fourth why—“Why was the accessory drive belt loose?” The 
cause could be a defective tensioner. If the tensioner was not a 
problem, then another “why” needs to be asked.
Fifth why—If the accessory belt and tensioner were okay, 
further investigation would be needed to find the root cause. 
For example, “Is one of the tensioner retaining bolts loose, 
maybe from a previous repair?” This could be the root cause.

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ENERGY PRINCIPLES
Energy is the ability or the capacity to do work. Chemical, 
mechanical, and electrical energy are the most familiar kinds 
involved in automobile operation.
Energy is called kinetic energy if
it is in the form of a moving object. 
An example is a moving vehicle.
Potential energy is capable of being 
changed to useful energy, such as 
energy stored in a battery or a
vehicle at the top of a hill.
In both of these cases, there is no 

energy being released.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 13–1 Energy, the ability to
perform work, exists in many forms.

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TORQUE
Torque is the term used to describe rotating force that may or may 
not result in motion, measured as force multiplied by the length of 
the lever through which it acts. If a one­foot­long wrench is used to 
apply 10 pounds of force to turn a bolt, then you are exerting 10 

pound­feet of torque. 

The metric unit for torque is Newton­meters.
1 pound­foot = 1.3558 Newton­meters
1 Newton­meter = 0.7376 pound­foot
See a conversion chart on Page 97 of your textbook
Figure 13–2 Torque is a twisting force
equal to the distance from the pivot
point times the force applied
expressed in units called pound-feet
(lb-ft) or Newton-meters (N-m).
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WORK
Work is accomplishing movement when force (torque) is applied. 
It is calculated by multiplying the applied force by the distance the 
object moves.
If you applied 100 pounds of force to move an object 10 feet, then 
you accomplished 1,000 foot­pounds of work.

Figure 13–3 Work is calculated by
multiplying force times distance. If
you push 100 pounds 10 feet, you
have done 1,000 foot-pounds of work.

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What is the Difference Between Torque and Work?
The designations for torque and work are often confusing. Torque is
expressed in pound-feet because it represents a force exerted a certain
distance from the object and acts as a lever. Work, however, is expressed
in foot-pounds because work is the movement over a certain distance
(feet) multiplied by the force applied (pounds). Engines produce torque
and service technicians exert torque represented by the unit pound-feet.

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POWER AND HORSEPOWER
Power means the rate of doing work, and equals work divided by 
time. If the object is moved in 10 seconds or 10 minutes does not 
make a difference in the amount of work accomplished, but it does 
affect the amount of power needed. Power is expressed in units of 
foot­pounds per minute.
An engine produces horsepower (hp). One horsepower is the 
power required to move 550 pounds one foot in one second, or 
33,000 pounds one foot in one minute (550 lb 60 sec 33,000 lb). 
This is expressed as 500 foot­pounds (ft­lb) per second or 33,000 
foot­pounds per minute.
Horsepower = 
torque times RPM divided by 5252
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The actual horsepower produced by an engine is measured with a 
dynamometer, (abbreviated as dyno or dyn). It places a load on 
the engine and measures the twisting force the crankshaft places 
against the load.
The load holds the engine speed, so it is called a brake. The horse 
power derived is called brake horsepower (bhp) and calculated 
from torque readings at various engine speeds (in revolutions per 
minute or RPM).

Figure 13–4 One horsepower is equal
to 33,000 foot-pounds (200 lbs 165 ft)
of work per minute.

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NEWTON’S LAWS OF MOTION
Sir Isaac Newton (1643–1727) was an English physicist and 
mathematician who developed Newton’s three laws of motion:
1. The first law of motion states that an object at rest tends to 
stay at rest and an object in motion tends to stay in motion 
unless acted on by an outside force.
For example, it requires a large force to get a vehicle that is 
stopped into motion. It also requires that a force be applied to 
slow and stop a vehicle that is in motion.

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2. Newton’s second law states that force needed to move an
object is proportional to the mass of the object multiplied
by the acceleration rate of the object.
This means it requires a great deal more force to accelerate a 
heavy sport utility vehicle (SUV) than a small economy car.
The rate of acceleration depends on the amount of force applied.
3. The third law states that for every action, there is an opposite 
and equal reaction.
For example, when the air­fuel mixture is ignited in an engine, 
force is exerted on the piston, forcing it downward, causing the 
crankshaft to rotate. The opposite action is applied to the 
cylinder head of the engine and applies the same force,
although this part is designed not to move.
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KINETIC ENERGY
Kinetic energy is a fundamental form of mechanical energy, the 
energy of mass in motion. The greater the mass of an object and 
the faster it moves, the more kinetic energy it possesses. The job 
of the brake system is to dispose of that energy in a safe and 
controlled manner.
Engineers calculate kinetic
energy using this formula:

Another way to calculate this:

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If a 3,000­pound vehicle traveling at 30 mph is compared to a 
6,000­pound vehicle at 30 mph, the equations for their respective 
kinetic energies look like this:

Results show that when vehicle 
weight doubles from 3,000 to
6,000 pounds, kinetic energy

also doubles from 90,301 to 
180,602 foot­pounds.

Figure 13–5 Kinetic energy increases in direct
proportion to the weight of the vehicle.

In mathematical terms, kinetic energy increases proportionally as 
weight increases. If weight quadruples, so will kinetic energy.
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If a 3,000­pound vehicle traveling at 30 mph is compared the same 

vehicle traveling at 60 mph, the equations for their respective 
kinetic energies look like this:

The vehicle traveling at 30 mph 
has over 90,000 foot­pounds of 
kinetic energy. At 60 mph the 
figure increases to over 350,000 
foot­pounds. 

Continued
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Figure 13–6 Kinetic energy increases as the
square of any increase in vehicle speed.

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At twice the speed, the vehicle has exactly four times as much 
kinetic energy. If the speed were doubled again to 120 mph, the 
amount of kinetic energy would grow to almost 1,500,000 foot­
pounds!
In mathematical terms, kinetic energy increases as the square of 
its speed.
In other words, if the speed of a moving object doubles (2), the 
kinetic energy becomes four times as great (22 = 4).
And if the speed quadruples (4), say from 15 to 60 mph, the 
kinetic energy becomes 16 times as great (42 = 16).
This is the reason speed has such an impact on kinetic energy.
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What is the Difference Between Mass and Weight?
Mass is the amount of matter in an object. One of the properties of mass is
inertia. Inertia is the resistance to being put in motion and the tendency to
remain in motion once it is set in motion.
The weight of an object is the force of gravity on the object and may be
defined as the mass times the acceleration of gravity.
Therefore, mass means the property of an object and weight is a force.

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Kinetic Energy and Brake Design  The relationships between 
weight, speed, and kinetic energy have significant practical 
consequences for the brake system engineer.
If vehicle A weighs twice as much as vehicle B, it needs a brake 
system that is twice as powerful.
If vehicle C has twice the speed potential of vehicle D, it needs 
brakes that are, not twice, but four times more powerful.

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INERTIA
Although brake engineers consider weight and speed capability 
when brake systems, these are not the only factors involved. 
Another physical property, inertia, affects the braking process 
and the selection of brake components. Inertia is defined by 
Newton’s first law.
Brakes Cannot Overcome the Laws of Physics
No vehicle can stop on a dime. The energy required to slow or stop a
vehicle must be absorbed by the braking system. All drivers should be
aware of this fact and drive at a reasonable speed for the road and traffic
conditions.

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MECHANICAL PRINCIPLES
The primary mechanical principle used to increase application 
force in every brake system is leverage.
A lever is a simple machine that consists of a rigid object, that 
pivots about a fixed point called a fulcrum.
There are three basic types of levers, but the job of all three is to 
change a quantity of energy into a more useful form.

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A first­class lever increases force applied to it and also changes 
the direction of the force. The weight is placed at one end while 
lifting force is applied to the other. The fulcrum is positioned at 
some point in between.
If the fulcrum is placed twice as far from the long end of the lever 
as from the short end, a 10­pound weight on the short end can be 
lifted by only a 5­pound force at the long end.

Figure 13–7 A first-class lever increases
force and changes the direction of the
force.

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