.................................
Automatic
Transmissions
How Automatic Transmissions Work
BY KARIM NICE
If
you have ever driven a car with an automatic transmission, then you know that
there are two big differences between an automatic transmission and a manual
transmission:
1. There is no clutch pedal in an
automatic transmission car.
2. There is no gear shift in an
automatic transmission car. Once you put the transmission into drive,
everything else is automatic.
Both
the automatic transmission (plus its torque converter) and a manual
transmission (with its clutch) accomplish exactly the same thing, but
they do it in totally different ways.
It
turns out that the way an automatic transmission does it is absolutely amazing!
In this article, we'll work our way
through an automatic transmission. We'll start with the key to the whole
system: planetary gearsets.
Then we'll see how the transmission
is put together, learn how the controls work and discuss some of the
intricacies involved in controlling a transmission.
Purpose of an Automatic Transmission
Just like that
of a manual transmission, the automatic transmission's primary job is to allow
the engine to operate in its narrow range of speeds while providing a
wide range of output speeds.
Without
a transmission, cars would be limited to one gear ratio, and that ratio would
have to be selected to allow the car to travel at the desired top speed.
If
you wanted a top speed of 80 mph, then the gear ratio would be similar to third
gear in most manual transmission cars.
You've probably never tried driving a
manual transmission car using only third gear.
If you did, you'd quickly find out
that you had almost no acceleration when starting out, and at high speeds, the
engine would be screaming along near the red-line.
A car like this would wear out very
quickly and would be nearly undriveable.
So
the transmission uses gears to make more effective use of the engine's torque,
and to keep the engine operating at an appropriate speed.
When
towing or hauling heavy objects, your vehicle's transmission can get hot enough
to burn up the transmission fluid.
In
order to protect the transmission from serious damage, drivers who tow should
buy vehicles equipped with transmission coolers.
The
key difference between a manual and an automatic transmission is that the
manual transmission locks and unlocks different sets of gears to the
output shaft to achieve the various gear ratios, while in an automatic
transmission, the same set of gears produces all of the different gear ratios.
The
planetary gearset is the device that makes this possible in an automatic
transmission.
Let's
take a look at how the planetary gearset works.
The Planetary Gearset
When you take apart and look
inside an automatic transmission, you find a huge assortment of parts in a
fairly small space.
Among other things, you see:
· An ingenious planetary gearset
· A set of bands to lock parts of a gearset
· A set of three wet-plate clutches to lock other parts of the
gearset
· An incredibly odd hydraulic system that controls the clutches
and bands
· A large gear pump to move
transmission fluid around
The
center of attention is the planetary gearset. About the size of a
cantaloupe, this one part creates all of the different gear ratios that the transmission
can produce.
Everything
else in the transmission is there to help the planetary gearset do its
thing. This amazing piece of gearing has appeared on HowStuffWorks before. You
may recognize it from the electric screwdriver article.
An
automatic transmission contains two complete planetary gearsets folded together
into one component.
Any
planetary gearset has three main components:
1. The sun gear
2. The planet gears and the planet gears' carrier
3. The ring gear
Each of these three components can be the input, the output or
can be held stationary.
Choosing which piece plays which role determines the gear ratio
for the gearset. Let's take a look at a single planetary gearset.
Planetary Gearset Ratios
One of the planetary gearsets from our transmission
has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of
different gear ratios out of this gearset.
Also, locking any two of the three
components together will lock up the whole device at a 1:1 gear reduction.
Notice that the first gear ratio
listed above is a reduction -- the output speed is slower than
the input speed.
The second is an overdrive --
the output speed is faster than the input speed. The last is a reduction again,
but the output direction is reversed.
There are several other ratios that
can be gotten out of this planetary gear set, but these are the ones that are
relevant to our automatic transmission. You can try these out in the animation
below:
So this one set of gears can produce
all of these different gear ratios without having to engage or disengage any
other gears.
With two of these gearsets in a row,
we can get the four forward gears and one reverse gear our transmission needs.
We'll put the two sets of gears together in the next section.
Compound Planetary Gearset
This automatic transmission uses a set of gears,
called a compound planetary gearset, that looks like a single
planetary gearset but actually behaves like two planetary gearsets combined.
It
has one ring gear that is always the output of the transmission, but it has two
sun gears and two sets of planets.
Let's
look at some of the parts:
The figure below shows the planets in
the planet carrier. Notice how the planet on the right sits lower than the
planet on the left.
The planet on the right does not
engage the ring gear — it engages the other planet. Only the planet on the left
engages the ring gear.
Next you can see the inside of the
planet carrier. The shorter gears are engaged only by the smaller sun gear. The
longer planets are engaged by the bigger sun gear and by the smaller planets.
Move the shift lever to see how power
is transmitted through the transmission.
First Gear
In first gear, the smaller sun gear is driven
clockwise by the turbine in the torque converter.
The planet carrier tries to spin
counterclockwise, but is held still by the one-way clutch (which only
allows rotation in the clockwise direction) and the ring gear turns the output.
The
small gear has 30 teeth and the ring gear has 72, so the gear ratio is:
Ratio
= -R/S = - 72/30 = -2.4:1
So
the rotation is negative 2.4:1, which means that the output direction would
be opposite the input direction.
But
the output direction is really the same as the input direction
-- this is where the trick with the two sets of planets comes in.
The
first set of planets engages the second set, and the second set turns the ring
gear; this combination reverses the direction.
You
can see that this would also cause the bigger sun gear to spin; but because
that clutch is released, the bigger sun gear is free to spin in the opposite
direction of the turbine (counterclockwise).
Move
the shift lever to see how power is transmitted through the transmission.
Second Gear
This transmission does something really neat in order
to get the ratio needed for second gear. It acts like two planetary gearsets
connected to each other with a common planet carrier.
The
first stage of the planet carrier actually uses the larger sun gear as the ring
gear. So the first stage consists of the sun (the smaller sun gear), the planet
carrier, and the ring (the larger sun gear).
The
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.
For
this stage, with the sun as input, planet carrier as output, and the ring gear
fixed, the formula is:
1
+ R/S = 1 + 36/30 = 2.2:1
The
planet carrier turns 2.2 times for each rotation of the small sun gear.
At
the 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, so the gear ratio is:
1
/ (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
To
get the overall reduction for second gear, we multiply the first stage by the
second, 2.2 x 0.67, to get a 1.47:1 reduction.
This
may sound wacky, but if you watch the video you'll get an idea of how it works.
Move
the shift lever to see how power is transmitted through the transmission.
Third Gear
Most automatic transmissions have a 1:1 ratio in third
gear.
You'll
remember from the previous section that all we have to do to get a 1:1 output
is lock together any two of the three parts of the planetary gear.
With
the arrangement in this gearset it is even easier — all we have to do is engage
the clutches that lock each of the sun gears to the turbine.
If
both sun gears turn in the same direction, the planet gears lockup because they
can only spin in opposite directions. This locks the ring gear to the planets
and causes everything to spin as a unit, producing a 1:1 ratio.
Move the shift lever to see how power is transmitted through
the transmission.
Overdrive
By definition, an overdrive has a faster output speed than
input speed. It's a speed increase — the opposite of a reduction.
In
this transmission, engaging the overdrive accomplishes two things at once. If
you read How Torque Converters Work, you learned about lockup torque
converters.
In
order to improve efficiency, some cars have a mechanism that locks up the
torque converter so that the output of the engine goes straight to the
transmission.
In
this transmission, when overdrive is engaged, a shaft that is attached to the
housing of the torque converter (which is bolted to the flywheel of the engine)
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.
Let's go back to our chart again, this time with the planet carrier for input,
the sun gear fixed and the ring gear for output.
Ratio
= 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1
So
the output spins once for every two-thirds of a rotation of the engine. If the
engine is turning at 2000 rotations per minute (RPM), the output speed is 3000
RPM.
This
allows cars to drive at freeway speed while the engine speed stays nice and
slow.
Move the shift lever to see how power is transmitted through
the transmission.
Reverse Gear
Reverse is very similar to first gear, except that
instead of the small sun gear being driven by the torque converter turbine, the
bigger sun gear is driven, and the small one freewheels in the opposite
direction.
The
planet carrier is held by the reverse band to the housing. So, according to our
equations from the last page, we have:
So
the ratio in reverse is a little less than first gear in this transmission.
Gear Ratios
This transmission has four
forward gears and one reverse gear.
Let's summarize the gear ratios, inputs and outputs:
After reading these sections, you are probably wondering how the
different inputs get connected and disconnected.
This is done by a series of clutches and bands inside the
transmission. In the next section, we'll see how these work.
Clutches and Bands in an
Automatic Transmission
In the last section, we discussed how each of the gear
ratios is created by the transmission.
For
instance, when we discussed overdrive, we said:
In
this transmission, when overdrive is engaged, a shaft that is attached to the
housing of the torque converter (which is bolted to the flywheel of the engine)
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.
To
get the transmission into overdrive, lots of things have to be connected and
disconnected by clutches and bands.
The
planet carrier gets connected to the torque converter housing by a clutch. The
small sun gets disconnected from the turbine by a clutch so that it can
freewheel.
The
big sun gear is held to the housing by a band so that it could not rotate. Each
gear shift triggers a series of events like these, with different clutches and
bands engaging and disengaging. Let's take a look at a band.
Bands
In this transmission there are two bands. The bands in
a transmission are, literally, 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.
In the
figure above, you can see one of the bands in the housing of the transmission.
The gear train is removed. The metal rod is connected to the piston, which
actuates the band.
Above
you can see the 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.
The
clutches in the transmission are a little more complex. In this
transmission 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. Below you can
see the piston and the clutch drum.
Notice
the rubber seal on the piston -- this is one of the components that is replaced
when your transmission gets rebuilt.
The next
figure shows the alternating 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. These
clutch plates are also replaced when the transmission is rebuilt.
The
pressure for the clutches is fed through passageways in the shafts. The
hydraulic system controls which clutches and bands are energized at any given
moment.
When You Put the Car in
Park
It may seem like a simple thing to lock the
transmission and keep it from spinning, but there are actually some complex
requirements for this mechanism.
First,
you have to be able to disengage it when the car is on a hill (the weight of
the car is resting on the mechanism).
Second,
you have to be able to engage the mechanism even if the lever does not line up
with the gear. Third, once engaged, something has to prevent the lever from
popping up and disengaging.
The mechanism that does all this is pretty neat. Let's
look at some of the parts first.
The 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.
Above you see the 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 -- the force from the weight of the car
helps to push the parking mechanism out of place because of the angle of the
taper.
This rod is connected to a cable that
is operated by the shift lever in your car.
When
the shift lever is placed in park, 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.
If
the mechanism is lined up on one of the high spots on the output, then the
spring will push on the tapered bushing, but the lever will not lock into place
until the car rolls a little and the teeth line up properly.
This
is why sometimes your car moves a little bit after you put it in park and
release the brake pedal -- it has to roll a little for the teeth to line up to
where the parking mechanism can drop into place.
Once
the car is safely in park, the bushing holds down the lever so that the car
will not pop out of park if it is on a hill.
Automatic Transmissions:
Hydraulics, Pumps and the Governor
Hydraulics
The automatic transmission in your car has to do
numerous tasks. You may not realize how many different ways it operates.
For
instance, here are some of the features of an automatic transmission:
· If
the car is in overdrive (on a four-speed transmission), the transmission will
automatically select the gear based on vehicle speed and throttle pedal
position.
· If you accelerate gently, shifts
will occur at lower speeds than if you accelerate at full throttle.
· If you floor the gas pedal, the
transmission will downshift to the next lower gear.
· If you move the shift selector to
a lower gear, the transmission will downshift unless the car is going too fast
for that gear. If the car is going too fast, it will wait until the car slows
down and then downshift.
· If you put the transmission in
second gear, it will never downshift or upshift out of second, even from a
complete stop, unless you move the shift lever.
You've
probably seen something that looks like this before. It is really the brain of
the automatic transmission, managing all of these functions and more.
The
passageways you can see route fluid to all the different components in the
transmission. Passageways molded into the metal are an efficient way to route
fluid; without them, many hoses would be needed to connect the various parts of
the transmission.
First,
we'll discuss the key components of the hydraulic system; then we'll see how
they work together.
The
Pump
Automatic transmissions have a neat pump, called
a gear pump.
The
pump is usually 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
inner gear of the pump hooks up to the housing of the torque converter, so it
spins at the same speed as the engine.
The
outer gear is turned by the inner gear, and as the gears rotate, fluid is drawn
up from the sump on one side of the crescent and forced out into the hydraulic
system on the other side.
The
Governor
The governor is a 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 -- the faster the governor spins, the more the valve
opens.
Fluid
from the pump is fed to the governor through the output shaft.
The faster the car goes, the more the governor valve opens
and the higher the pressure of the fluid it lets through.
Automatic Transmissions: Valves and Modulators
To shift properly, the automatic
transmission has to know how hard the engine is working.
There are two different ways that this is done. Some cars
have a simple cable linkage connected to a throttle valve in
the transmission.
The further the gas pedal is pressed, the more pressure is
put on the throttle valve.
Other cars use a vacuum modulator to apply
pressure to the throttle valve. The modulator senses the manifold pressure,
which increases when the engine is under a greater load.
The manual
valve is 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. For instance,
the 1 to 2 shift valve determines when to shift from first to second gear.
The
shift valve is pressurized with fluid from the governor on one side, and the
throttle valve on the other.
They
are supplied with fluid by the pump, and they route that fluid to one of two
circuits to control which gear the car runs in.
The
shift valve will delay a shift if the car is accelerating quickly. If the car
accelerates gently, the shift will occur at a lower speed. Let's discuss what
happens when the car accelerates gently.
As
car speed increases, the pressure from the governor builds. This forces the
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.
When
the car accelerates quickly, the throttle valve applies more pressure against
the shift valve.
This
means that the pressure from the governor has to be higher (and therefore the
vehicle speed has to be faster) before the shift valve moves over far enough to
engage second gear.
Each shift valve responds to a
particular pressure range; so when the car is going faster, the 2-to-3 shift
valve will take over, because the pressure from the governor is high enough to
trigger that valve.
Electronically Controlled
Transmissions
Electronically controlled transmissions, which appear
on some newer cars, still use hydraulics to actuate the clutches and bands, but
each hydraulic circuit is controlled by an electric solenoid.
This
simplifies the plumbing on the transmission and allows for more advanced
control schemes.
In
the last section we saw some of the control strategies that mechanically
controlled transmissions use.
Electronically
controlled transmissions have even more elaborate control schemes.
In
addition to monitoring vehicle speed and throttle position, the transmission
controller can monitor the engine speed, if the brake pedal is being pressed,
and even the anti-lock braking system.
Using
this information and an advanced control strategy based on fuzzy logic -- a
method of programming control systems using human-type reasoning --
electronically controlled transmissions can do things like:
· Downshift automatically when going downhill to control speed and
reduce wear on the brakes
· Upshift when braking on a slippery surface to reduce the braking
torque applied by the engine
· Inhibit the upshift when going into a turn on a winding road
Let's talk about that last feature -- inhibiting the upshift
when going into a turn on a winding road.
Let's say you're driving on an uphill, winding mountain road.
When you are driving on the straight sections of the road, the transmission
shifts into second gear to give you enough acceleration and hill-climbing
power.
When you come to a curve you slow down, taking your foot off the
gas pedal and possibly applying the brake. Most transmissions will upshift to
third gear, or even overdrive, when you take your foot off the gas.
Then when you accelerate out of the curve, they will downshift
again.
But if you were driving a manual transmission car, you would
probably leave the car in the same gear the whole time.
Some automatic transmissions with advanced control systems can
detect this situation after you have gone around a couple of the curves, and
"learn" not to upshift again.
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