Transcript Powertrains

Powertrains
The Goal of a Transmission
► The
transmission allows the gear ratio between
the engine and the drive wheels to change as
the car speeds up and slows down
Components
► Clutch/manual
transmission
or
► torque converter/automatic transmission;
and
► differential,
► driveshaft.
transmission is connected to the engine
through the clutch. The input shaft of the
transmission therefore turns at the same
rpm as the engine
General information: gears
► Purposes
 Gear reduction
 Overdrive
 Directional change
► Gear
ratio: output
radius/input radius
Helical gear
How Clutches Work
Clutch
► Purpose:
connect or
disconnect two shafts
► Pressing pedal pushes spring;
pulls plate away from flywheel
► Releasing pedal allows spring
to contact flywheel
http://www.autopartsonline.co.uk/images/feb01/cl
utch.jpg
 Friction  wear
Animated basic
Clutches
Automobile Clutch
► flywheel
is connected to the engine, and
the clutch plate is connected to the
transmission
► the springs push the pressure plate
against the clutch disc, which in turn
presses against the flywheel.
► This locks the engine to the transmission
input shaft, causing them to spin at the
same speed.
Clutches explode
Clutches engage
What Can Go Wrong?
► friction
material on the disc wears out
► similar to the friction material on the pads
of brake
► When
most or all of the friction material is
gone, the clutch will start to slip
► wears
while the clutch disc and the flywheel
are spinning at different speeds
Clutches wear
Other Clutches in Your Garage
► automatic
transmission contains several
clutches, used to engage and disengage
sets of planetary gears
► air
conditioning compressor has a
magnetic clutch, allows compressor to shut
off even while the engine is running
Compressor
Manual Transmission
Manual transmission
► Purpose:
optimize engine speed with varying
wheel speed
► Ratios (ex.): 2.3:1, 1.6:1, 1.2:1, 1:1, 0.9:1
► Parts






Input shaft
Layshaft
Gears (incl. idler)
Output shaft
Dog collars
Synchronizers
Manual transmission efficiency
► Range:
87 to 99% overall
► Factors





Time in each gear
Speed and torque values in each gear
City vs. highway
Fluid type
Mounting angle
► Duty
cycles
 % efficiency = (input speed * input torque *
100)/(output speed * output torque)
► Improvements
five-speed transmission applies one of five different
gear ratios to the input shaft to produce a different
rpm value at the output shaft
RPM at Transmission Output Shaft
Gear
Ratio
with Engine at 3,000 rpm
1st
2.315:1
1,295
2nd
1.568:1
1,913
3rd
1.195:1
2,510
4th
1.000:1
3,000
5th
0.915:1
3,278
Very Simple Transmission
First Gear
five-speed manual transmission
Reverse gear is handled by a small
idler gear
Synchronizers
► to
allow collar and
gear to make frictional
contact before the dog
teeth make contact.
► collar
and gear
synchronize their
speeds before the
teeth need to engage
way to let the engine turn while the wheels and
gears in the transmission come to a stop
► Manual
transmission – engine connected to
transmission by clutch
► automatic transmission - have no clutch,
Instead, use torque converter
Torque converter
► Purpose:
allow engine to run with trans. stopped
► Fluid coupling
► Parts:




Pump
Fluid
Turbine
Stator
► Slippage:
2-8% power loss
 Lock-up torque converters
► fluid
coupling, -- allows the engine to spin
somewhat independently of transmission
► slow
turning or idling, engine torque passed
through torque converter is very small
► step
on padal, the engine speeds up and
pumps more fluid into the torque converter,
-- more torque transmit to wheel
Inside a Torque Converter
- Pump
- Turbine
- Stator
- Transmission fluid
fins - make up the pump of the torque converter are
attached to the housing
pump inside a torque converter is a type of
centrifugal pump
► As
►a
it spins, fluid is flung to the outside
vacuum is created that draws more fluid in at
the center
pump section of the torque converter
is attached to the housing
► fluid
then enters the blades of the turbine,
which is connected to the transmission
► turbine causes the transmission to spin
► blades
of the turbine are curved - fluid,
which enters the turbine from the outside,
has to change direction before it exits the
center of the turbine
► directional
change that causes the
turbine to spin
turbine: the spline in the middle connects to the transmission
► to
change the direction of a moving object,
you must apply a force to that object
► whatever applies the force that causes the
object to turn must also feel that force, but
in the opposite direction
► as the turbine causes the fluid to change
direction, the fluid causes the turbine to
spin
► fluid
exits the turbine moving opposite the
direction that the pump (and engine) are
turning
► If the fluid were allowed to hit the pump, it
would slow the engine down, wasting
power. This is why a torque converter has a
stator
stator sends the fluid returning from the
turbine to the pump. This improves the
efficiency of the torque converter
► stator
resides in the very center of the torque
converter. Its job is to redirect the fluid returning
from the turbine before it hits the pump again
► stator
has a very aggressive blade design that
almost completely reverses the direction of the
fluid
► one-way
clutch (inside the stator) connects the
stator to a fixed shaft in the transmission
► the
stator cannot spin with the fluid -- it can spin
only in the opposite direction, forcing the fluid to
change direction as it hits the stator blades
► when
the car gets moving. There is a point,
around 40 mph (64 kph), at which both the
pump and the turbine are spinning at
almost the same speed
► the
fluid returns from the turbine, entering
the pump already moving in the same
direction as the pump, so the stator is not
needed
allow the engine to operate in its narrow
range of speeds while providing a wide range
of output speeds
Automatic transmission [1]
► Purpose:
to keep engine
within optimal rpm
► Planetary gearsets
 Components:
► Ring
► Sun
► Planet
carrier
 Ratios created through combinations:
► Reduction:
sun input, planet carrier output, ring stationary
► Overdrive: planet carrier input, ring output, sun stationary
► Reverse: sun input, ring output, planet carrier stationary
key difference between a manual
and an automatic transmission
► manual
transmission locks and unlocks
different sets of gears to the output shaft to
achieve the various gear ratios
► automatic
transmission, the same set of
gears produces all of the different gear
ratios
► planetary
gearset is the device that makes
this possible in an automatic transmission
Automatic transmission [2]
► Compound
planetary gearset
 One ring gear, two suns, two sets of planets
► Second
gear
 Input 1: small sun; stationary 1: large sun (functioning
as ring); output 1: planet carrier
► Ratio
1: 1 + R/S, ex. 1 + 36/30 = 2.2:1
 Input 2: planet carrier; stationary 2: large sun; output
2: ring
► Ratio
2: 1/(1 + S/R), ex. 1/(1 + 36/72) = 0.67:1
 Combined ratio: Ratio 1 * Ratio 2, ex. 1.47:1
Planetary Gearsets
► planetary
gearset
► set of bands lock parts of a gearset
► set
of three wet-plate clutches lock other
parts of the gearset
► hydraulic
bands
► large
system controls the clutches and
gear pump to move transmission fluid
around
Planetary Gearsets
Planetary Gearsets & Gear Ratios
Each of these three components can be the
input, the output or can be held stationary
ring gear with 72 teeth and a sun gear with 30 teeth. We can
get lots of different gear ratios out of this gearset
Input
A
B
C
Sun (S)
Planet
Carrier
(C)
Sun (S)
Output
Gear
Ratio
Stationary
Calculation
Planet
Carrier
(C)
Ring (R)
1 + R/S
3.4:1
Ring (R)
Sun (S)
1 / (1 + S/R)
0.71:1
Ring (R)
Planet Carrier
(C)
-R/S
-2.4:1
Animated
Gears
► one
gear set can produce different gear
ratios without having to engage or
disengage any other gears
► two gearsets can get the four forward gears
and one reverse gear for transmission needs
► compound planetary gearset
one ring gear always be the output of the
transmission, but 2 sun gears and 2sets of planets
Planet carrier: Note the two sets of
planets
Inside the planet carrier: Note
the two sets of planets
First Gear
► smaller
sun gear is driven clockwise by the turbine
in the torque converter
► planet carrier spin ccw, but is held still by the oneway clutch (only allows cw)
► ring gear turns the output
Ratio = -R/S = - 72/30 = -2.4:1
► first
planet set engages the second set -- turns the
ring gear; this combination reverses the direction
First Gear
Second Gear
► two
planetary gearsets connected to each other
with a common planet carrier
► first stage of the planet carrier actually uses the
larger sun gear as the ring gear
► consists of the sun (the smaller sun gear), the
planet carrier, and the ring (the larger sun gear).
► input is the small sun gear; the ring gear (large
sun gear) is held stationary by the band, and the
output is the planet carrier
1 + R/S = 1 + 36/30 = 2.2:1
Second Gear
► second
stage, the planet carrier acts as the
input for the second planetary gear set, the
larger sun gear (which is held stationary)
acts as the sun, and the ring gear acts as
the output
1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
► overall
reduction for second gear = 2.2 x
0.67, = 1.47:1 reduction
Second Gear
Third Gear
► automatic
gear
transmissions 1:1 ratio in third
► lock
together any two of the three parts of
the planetary gear
► engage
the clutches that lock each of the
sun gears to the turbine
Third Gear
OverDrive
► lockup
torque converters - the output of the
engine goes straight to the transmission
► shaft (attached to the housing of the torque
converter) is connected by clutch to the planet
carrier
► small sun gear freewheels, and the larger sun gear
is held by the overdrive band
 Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1
OverDrive
Reverse
► similar
to first gear,
► But bigger sun gear being driven by the
torque converter turbine,
► small one freewheels in the opposite
direction
► planet carrier is held by the reverse band to
the housing
►Ratio
= -R/S = 72/36 = 2.0:1
Reverse
Gear Ratios
Gear
1st
Input
Output
Fixed
Gear
Ratio
30-tooth sun
72-tooth
ring
Planet carrier
2.4:1
30-tooth sun
Planet
carrier
36-tooth ring
2.2:1
Planet carrier
72-tooth
ring
36-tooth sun
0.67:1
Total 2nd
1.47:1
2nd
3rd
OD
Reverse
30- and 36tooth suns
72-tooth
ring
Planet carrier
72-tooth
ring
36-tooth sun
0.67:1
36-tooth sun
72-tooth
ring
Planet carrier
-2.0:1
1.0:1
Automatic transmission [3]
► Shifting
 Hydraulic circuits control bands and clutches
 Fluid pressure from two directions controls valve
operation:
►Gear
pump (spinning at engine rpm)
►Governor, spinning with trans. output; spring-loaded
valve to permit more fluid with increasing speed
►ex., accelerating quickly: gear pump pressure high
relative to governor pressure  delayed shift
Clutches and Bands
► how
each of the gear ratios is created by the
transmission
► overdrive - shaft that attached to the housing of
the torque converter is connected by clutch to the
planet carrier. The small sun gear freewheels, and
the larger sun gear is held by the overdrive band.
Nothing is connected to the turbine; the only input
comes from the converter housing
► lots of things have to be connected and
disconnected by clutches and bands
► planet
carrier gets connected to the torque
converter housing by a clutch
► small
sun gets disconnected from the
turbine by a clutch so that it can freewheel
► big
sun gear is held to the housing by a
band so that it could not rotate
Bands
► The
bands in a transmission are steel bands
that wrap around sections of the gear train and
connect to the housing.
► They
are actuated by hydraulic cylinders inside
the case of the transmission
the bands in the housing of the transmission by
removed gear train. The metal rod is connected to
the piston, which actuates the band
two pistons that actuate the bands. Hydraulic
pressure, routed into the cylinder by a set of
valves, causes the pistons to push on the bands,
locking that part of the gear train to the housing
Clutches
►
there are four clutches. Each clutch is actuated by
pressurized hydraulic fluid that enters a piston inside the
clutch. Springs make sure that the clutch releases when
the pressure is reduced
Clutches
► layers
of clutch friction material and steel plates.
The friction material is splined on the inside,
where it locks to one of the gears. The steel plate
is splined on the outside, where it locks to the
clutch housing
When You Put the Car in Park
► output
of the transmission: The square notches
are engaged by the parking-brake mechanism to
hold the car still
► parking-brake mechanism engages the teeth on
the output to hold the car still. This is the section
of the transmission that hooks up to the drive
shaft -- so if this part can't spin, the car can't
move
parking mechanism protruding into the housing where the
gears are located. Notice that it has tapered sides. This helps
to disengage the parking brake when you are parked on a hill
This rod is connected to a cable that is operated by
the shift lever in your car
the rod pushes the spring against the small tapered bushing.
If the park mechanism is lined up so that it can drop into one
of the notches in the output gear section, the tapered
bushing will push the mechanism down
Hydraulic System
The Pump
located in the cover of the transmission. It draws fluid from a
sump in the bottom of the transmission and feeds it to the
hydraulic system. It also feeds the transmission cooler and
the torque converter.
The Governor
► clever
valve that tells the transmission how
fast the car is going. It is connected to the
output, so the faster the car moves, the
faster the governor spins
► Inside
the governor is a spring-loaded valve
that opens in proportion to how fast the
governor is spinning
faster the car goes, the more the governor valve opens and
the higher the pressure of the fluid it lets through
Valves and Modulators
►
Throttle Valve or Modulator
►
To shift properly, the automatic transmission has to know
how hard the engine is working.
►
throttle valve linkage in the transmission. The further the
gas pedal is pressed, the more pressure is put on the
throttle valve.
vacuum modulator to apply pressure to the throttle
valve. The modulator senses the manifold pressure, which
drops when the engine is under a greater load
►
► Manual
Valve
► what the shift lever hooks up to.
► Depending
on which gear is selected, the
manual valve feeds hydraulic circuits that
inhibit certain gears.
► For
instance, if the shift lever is in third
gear, it feeds a circuit that prevents
overdrive from engaging
► Shift
Valves
► supply
hydraulic pressure to the clutches
and bands to engage each gear.
► The
valve body of the transmission contains
several shift valves. The shift valve
determines when to shift from one gear to
the next
► The
shift valve is pressurized with fluid from
the governor on one side, and the throttle
valve on the other
► speed
increases - pressure from governor
forces shift valve over until the first gear
circuit is closed, and the second gear
circuit opens.
► Since
the car is accelerating at light
throttle, the throttle valve does not apply
much pressure against the shift valve
► accelerates
quickly - throttle valve applies
more pressure against the shift valve pressure from the governor has to be
higher before the shift valve moves over
far enough to engage second gear
How Differentials Work
► To
aim the engine power at the wheels
► To
act as the final gear reduction in the
vehicle
► slowing
the rotational speed of the
transmission one final time before it hits the
wheels
► To
transmit the power to the wheels while
allowing them to rotate at different speeds
Differential [1]
► Directs
power from
trans. to wheels
► Allows differences in
wheel speed
► Open differential
 Trans. output  input pinion  ring gear  cage
w/ pinion gears  side gears (two)
 Turning: pinion gears rotate; one side gear spins
faster
Why You Need a Differential
► Car
wheels spin at different speeds
► For
the non-driven wheels -- spin
independently
► If
no have a differential, the wheels be
locked and forced to spin at the same
speed, one tire would have to slip when
turning
►
differential is a device that splits the engine torque two
ways, allowing each output to spin at a different speed
all-wheel-drive (full-time four-wheel-drive) vehicles. These
all-wheel-drive vehicles need a differential between each
set of drive wheels
Spinning at Different Speeds
Characteristic
► input
pinion is a smaller gear than the ring
gear; -- last gear reduction in the car.
► rear axle ratio or final drive ratio -- gear
ratio in the differential or final drive ratio
about 4.10
► always applies the same amount of
torque to each wheel
► In
dry conditions, -- most traction, torque
applied to the wheels is limited by the
engine and gearing;
► in
a low traction situation, -- driving on
ice, torque is limited to amount that will
not cause a wheel to slip
►A
car may be produce more torque -needs to be enough traction to transmit
that torque to the ground
Differential [2]
► Problems
with open differential
 Same torque to each wheel
 Low-traction situations
► Solutions




Clutch-type limited slip differential
Viscous coupling
Locking differential
Torque-sensing (Torsen) differential
► open
differential -- same torque to both wheels. If one of front and one of back tires comes off the
ground, they will just spin helplessly in the air
► limited
slip differential (LSD), -- positraction
- use various mechanisms to allow normal
differential action when going around turns.
► When
a wheel slips, they allow more torque to be
transferred to the non-slipping wheel
Clutch-Type Limited Slip
► same
components as an open differential, -- adds
a spring pack and a set of clutches.
► cone
clutch like the synchronizers in a manual
transmission
► clutches
step – while one wheel spin faster than
the other, as in a turn.
► The
clutches want both wheels to go the same
speed. If one wheel wants to spin faster than the
other, it must first overpower the clutch
Viscous Coupling
used to link the back wheels to the front wheels
► when one set of wheels starts to slip, torque will be
transferred to the other set
► 2 sets of plates inside a sealed housing filled with a thick
fluid
► One set of plates connect to each output shaft. Under
normal conditions, both sets of plates and the viscous fluid
spin at the same speed
► When one set of wheels tries to spin faster (slipping) -- the
set of plates spins faster than the other.
► The viscous fluid, (stuck between the plates) catch up with
the faster disks, dragging the slower disks along
►
Locking and Torsen
► locking
differential -
adds an electric,
pneumatic or hydraulic
mechanism to lock the
two output pinions
together
► Torsen
differential -- purely mechanical
device; no electronics, clutches or viscous fluids
► Torsen (Torque Sensing) works as an open
differential when same torque going on wheel
► one wheel starts to lose traction, the difference
in torque causes the gears in the Torsen
differential to bind together
► Torsen is superior to the viscous coupling
because it transfers torque to the stable wheels
before the actual slipping occurs
Driveshafts
► Connect
wheels to differential
► Axle connections (CV joints):
 Allow vertical motion (suspension)
 Allow wheels to turn
► Torque
steer
http://www.autosite.com/garage/subsys/05-04f.asp
continuously variable
transmission (CVT)
► Ideally,
the
transmission would
be so flexible in its
ratios that the
engine could always
run at its single,
best-performance
rpm value.
New technology: CVTs
► Continuously
variable transmissions
 Vary gear ratio  engine always at best rpm
 Reduced fuel consumption, better performance
 Popular types: friction, hydrostatic, ratcheting
► Frictional




CVTs
Small radius: low torque, high speed
Large radius: high torque, low speed
Differential slip
Tmax = ukFNR
continuously variable
transmission (CVT)
►A
CVT has a
nearly infinite
range of gear
ratios
continuously variable
transmission (CVT)
Advantages of CVTs
Feature
Benefit
Constant, stepless acceleration
from a complete stop to cruising
speed
Eliminates "shift shock" -- makes for
a smoother ride
Works to keep the car in its
optimum power range regardless of
how fast the car is traveling
Improved fuel efficiency
Responds better to changing
conditions, such as changes in
throttle and speed
Eliminates gear hunting as a car
decelerates, especially going up a
hill
Less power loss in a CVT than a
typical automatic transmission
Better acceleration
Better control of a gasoline engine's
speed range
Better control over emissions
Can incorporate automated
versions of mechanical clutches
Replace inefficient fluid torque
converters
Torodial CVT
Hydrostatic CVTs