Transcript More from last time

What are Hydraulics?
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Moving something by means of a liquid
(water or oil) under pressure.
What is the MA
of this lift if
A2 = 4 X A1
Pressure (psi) = F1/A1 = F2/A2
http://www.ph.utexas.edu/~itiq/iq/303K/ch15/15.2.3.jpg
Pneumatic Tools
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Moving something by means of
compressed air
How do hydraulic brakes work?
The hydraulic fluid does
not compress very
much. When
pressurized the force
get transmitted
throughout the system.
The brake fluid must
have a very high boiling
point and not freeze in
cold driving conditions.
http://auto.howstuffworks.com/brake3.htm
Click for animation!
Using leverage and hydraulics, the braking
force gets multiplied.
This lever multiplies the force
on the first cylinder by four.
Applied Force is increased 36 times.
The diameter of the brake
cylinder is 3 times larger
than the pedal cylinder. This
is nine times larger in area.
This multiplies the force
another nine times
(although pressure is the
same throughout). The
tradeoff is that the brake
piston only travels 1/9 the
distance of the first pedal
cylinder.
Disk Brakes
http://auto.howstuffworks.com/drum-brake1.htm
Click to animate!
A more modern braking system uses a master cylinder with two pistons
to ensure a fluid leak can be isolated to one of the wheels.
http://auto.howstuffworks.com/master-brake1.htm
Click for animation!
More force is
Required to operate
Disk brakes.
Power brakes are
Used to increase
Forces on the
Master cylinder.
http://auto.howstuffworks.com/power-brake2.htm
What do brakes really do?
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They stop the wheels, not the car.
If the car stops without crashing the
brakes absorb most of the kinetic energy
of the car, turning that energy into HEAT
High performance brakes must quickly
shed the heat that is generated.
Effective braking is determined by the
contact the wheels have with the road.
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Drum brakes tend to have heat build up
inside the rotating drum.
The rotor and brake pads used in disk
brake systems are exposed to the outside
air, making cooling easier.
Drum brakes are less expensive and have
improved, so many cars still have disk
brakes in the front and drum brakes in the
rear.
See article at:
http://www.edmunds.com/ownership/techcenter/articles/43857/article.html
What is Friction?
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An ‘invisible’ force that resists motion when two
surfaces in contact try to move relative to one
another.
Friction is a force which causes the motion
between two surfaces to be reduced.
Because of friction people used to believe before
Galileo and Newton that forces were always
needed to keep something in motion.
There are two types of Friction:
1. Static Friction
2. Kinetic (sliding) Friction
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Static Friction – two surfaces at rest with
respect to each other
Speed of tire at top is 2X speed of the car.
Tire at bottom is not moving relative to the road. Static friction
between tire and road is what controls the car.
Tire tread, road, and driving conditions
determine the coefficient of friction.
A bald tire on a dry roads has a much
higher friction coefficient than on a wet
road.
http://hyperphysics.phy-astr.gsu.edu
Breakaway Point
Kinetic Friction is constant.
Max. Static Frictional Force
http://www.lightandmatter.com
Static friction will
counteract any applied
force up to a certain
threshold at which friction
is overcome and the
object begins to slide.
Static Friction
Static Friction depends
on how squished
together the two
surfaces are.
Once in motion, kinetic
friction resists motion.
Kinetic Friction
FStatic > FKinetic
 is the friction coefficient
Anti-lock braking systems
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ABS keeps the tires from skidding as long as the
driver keeps the brake pedal down.
When slamming on the brakes, wheels can stop
spinning much faster than a car can stop.
Wheels have less kinetic energy than the car.
When a wheel stops rolling it is called “lock up”.
When this happens the car must now skid
because the wheels are not turning.
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Kinetic (sliding) friction is much less than static
friction.
Once a car skids, the tires lose grip with the
road and the driver loses control.
Skidding results in reduced friction between tires
and road.
Lower frictional forces reduces control. Driver
might not be able to turn the car, or car may
spin out if one wheel is skidding and the others
are not.
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Safer only when used properly.
ABS functions only when brakes are not
pumped. ABS creates pedal chatter which
means ABS is working.
ABS doesn’t mean you will stop quicker.
However control of the car is maintained,
because skidding is avoided.
The ABS controller (computer)
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Ensures that the tires slows down at the same
rate as the car, but keeps the tires’ forces just
under the point where sliding would occur –
just under Fstatic-max
This maximizes the braking power.
Demo of conventional brakes vs. ABS:
http://www.youtube.com/watch?v=uq4DDMMoomU
How is this done?
The system uses a computer to monitor
the speed of each wheel. When it detects
that one or more wheels are turning
slower than the remaining wheels, the
computer sends a signal to momentarily
remove and reapply the pressure to the
affected wheels to allow them to continue
turning. This "pumping" of the brakes
occurs at ten or more times a second.
http://www.familycar.com/brakes.htm
The system consists of an electronic
control unit, a hydraulic actuator*, and
wheel speed sensors at each wheel.
Information from wheel speed
sensors is fed to the controller.
http://www.aa1car.com/library/abs1.htm
* An actuator is a device in a car that uses electricity to do some sort of
mechanical movement- Such as power door locks or a remote trunk opener.
In this case the actuator releases pressure in the brakes to pulse the
brakes.
If the controller determines that one tire is
rotating more slowly than the other tires, the
controller sends a signal to the wheel’s actuator to
lower the brake fluid pressure to prevent skidding.
Solenoid valve closes,
preventing more fluid
from getting to the brake.
It then is reopened, thus
pumping the brake.
www.lake-link.com
Do ABS brakes work in the snow or
on icy roads?
What is the primary advantage of
ABS brakes?
Heat – what a waste!!!
Regenerative braking
absorbs some of the car’s
kinetic energy to generate
electricity instead of
producing wasteful heat.
The electric motor operates like a
generator during braking.
This puts energy back into the
battery.
How a Parallel Hybrid car works
http://www.youtube.com/watch?v=F19qeag-o_I
What are benefits of
Regenerative Breaking?
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Have wheels turn a generator connected to a
battery to capture some of the Kinetic Energy of
the Car
Allows the Pruis to use a much smaller gas
engine since high power demands can be
supplemented by electric motor and battery.
Transfer unwanted KE into Potential Energy to
be converted back into KE at a later time.
Regenerative Breaking Demo
How the battery and engine work
together is determined by a
microcontroller
Scenarios:
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All the engine power is not needed, so excess engine
energy is used to charge the battery
Engine power is not sufficient, so battery operates an
electric motor boosting power
Unwanted Kinetic Energy is used in braking to charge
battery
Car only needs low power provided by electric motor,
then gas engine stays off.
While at rest, all systems are off; no idle waste.
Scenario 1
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Engine charges a low battery
This happens if battery is low, usually at the beginning of a trip
Scenario 2
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When waiting at lights, engine and
electric motor are off – no idling
Scenario 3
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Engine power exceeds car’s demand, therefore engine
turns generator.
Generator (i.e. electric motor) charges the battery
Scenario 4
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Energy demand can be satisfied by battery only
Scenario 5
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When braking, wheels turn generator that in turn charge the battery.
This also happens sometimes when coasting.
Scenario 6
Engine only – energy demand equals that of gas engine.
Scenario 7
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Engine and Battery working in consort to drive wheels
Demand exceeds power of gas engine
Hybrid Benefits
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Low emissions
Car performs well under most cases with a much
smaller engine, due to reliance on the battery
and electric motor for boosts in power
Very sophisticated embedded microcontrollers
that manage energy demands
Captures excess energy of motor and during
breaking
Flywheel Storage System
(FES)
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Another technique for storing unwanted
KE of the car
Energy is transferred to a flywheel as KE.
Flywheel turns freely and independently of
until KE is needed again to propel the car.
Thought question….
What are some reasons the Prius might
get better mileage in stop and go
conditions than on the highway?
http://www.getrichslowly.org/blog/2007/05/30/how-to-improve-yourgas-mileage-23-top-tips-for-better-fuel-economy/
http://www.consumerenergycenter.org/transportation/consumer_tips/ve
hicle_energy_losses.html
Good sources of information.
Losses in Conventional Car
http://www.consumerenergycenter.org
Typical Car
Prius
Simulator
http://priuschat.com