| |
|
|
|
|
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 :
• There is no clutch pedal in an automatic transmission car.
• 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 and a manual transmission 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! Let's take a look
inside an automatic transmission.
|
|
|
The Torque Converter |
|
|
|
|
|
|
|
|
The key to the modern automatic transmission is the
torque converter. It takes the place of a clutch in a manual transmission.
A direct descendant of an earlier component called a fluid coupling, the torque
converter offers the advantage of multiplying the turning power provided by the
engine. It is connected to the motor by means of a metal rod known as the
transmission shaft (sometimes called the input shaft), which fits next to but
does not touch the engine crankshaft. Repeat: there is no direct connection
between the engine and the transmission.
Instead, the engine turns the transmission by means of a process called
hydraulic coupling.
Think of two electric fans in a room. Line them up a few feet apart, one in
front of the other, both facing the same direction. Now turn on the rear fan.
What happens? If you've turned the fan up high enough, and if the fans are
close enough together, the front fan will begin to turn as well. In this same
manner does the engine crankshaft influence the transmission shaft, causing it
to rotate.
The identical process occurs in a torque converter, except that transmission
fluid takes the place of air.
But there's more. Inside the torque converter are several components that help
multiply the power. These are the impeller (or pump), the turbine, and the
stator (or guide wheel).
A torque converter is like a giant doughnut. The impeller and the turbine (the
two fans in the analogy above) face one another inside the round metal casing.
The impeller is on the engine side, the turbine on the tranny side. Both of
these components have blades that catch the transmission fluid and cause them
to spin.
Picture two halves of a chambered nautilus, and this will perhaps give you a
better image.
As one fan begins to spin, the other will spin as well. Through centrifugal
force, the fluid moves to the outside of the blades, where it is redirected by
a third fan, called a stator, back to the turbine side. This continual flow of
fluid is what causes the power to be multiplied. |
|
|
The Lockup Torque Converter |
|
|
|
|
|
|
|
|
Because the only connection between two sides of a torque converter is a fluid
connection, there is always a little slippage, running from about 2-8%. To
increase efficiency and gas mileage, most modern automatic transmissions also
have something called a lockup clutch (aka, torque converter clutch).
It works like this. As the speed of the car nears 40 miles per hour, the highly
pressurized transmission fluid is channeled through the transmission shaft and
activates a clutch piston. This metal pin locks the turbine to the impeller, in
effect bypassing the torque converter. It remains this way until the vehicle
slows below 40 mph, at which point the clutch piston disengages and the torque
converter kicks in again. Simple, right? |
|
|
Planetary Gearsets |
|
|
|
|
|
|
|
|
No, we're not talking Mars and Venus here. These are different-sized gears,
just like the ones on the back of your mountain bike. They're called planetary
because they're circular and revolve around a central gear known as a sun gear.
If it sounds like our solar system, you're right -- it's designed on the same
model.
A planetary gearset system has three major elements :
• Sun gear
• Planet carrier, drum, and pistons
• Ring gear and drum
Although there are different variations on the planetary gearset theme (for
instance, some trannies have two sets of planetary gears, two sun gears, etc.),
the essential design has remained unchanged for more than half a century.
One example is the Simpson geartrain. In this design, two sets of planetary
gearsets mesh with two sun gears. The two sun gears are in turn connected
together to form what is called a common sun gear.
The Simpson geartrain is an older, three-speed design no longer in popular use,
but it will give us an idea of the way an automatic transmission works.
In neutral, with the exception of the free-turning input shaft, there is no
motion within the transmission.
In first gear, the front planetary gear is engaged, locking onto the input
shaft. Think of this as the low gear on your bicycle. The gear ratio here is
2.46:1. This gets the car moving.
Once the car is rolling, the transmission shifts into second. The intermediate
band is applied, holding stationary the high clutch drum, the reverse clutch,
and the sun gear. The forward clutch is applied, locking the input shaft to the
ring gear. The gear ratio here is 1.46:1.
In high gear, the ratio goes to 1:1. All planetary gear members are locked to
each other and to the output shaft.
In reverse mode, the reverse and high clutch is applied. The input shaft is
locked to the reverse and high clutch drum, the input shell, and the sun gear.
Gear ratio for reverse is 2.17:1.
These are the basic workings or a Simpson transmission. |
|
|
Clutches, Bands, and Servo Pistons |
|
|
|
|
|
|
|
|
Gear shifting in an automatic transmission is very similar to the action taken
while riding your mountain bike -- except that a series of valves, sensors, and
other components take the place of your brain, deciding when to upshift and
downshift to a larger or smaller gear.
Some cars use a multiple-disc clutch operation. In this setup, a series of
friction discs are placed between steel plates. The clutch also has a piston
and return springs. This whole assembly is known as a clutch pack. When fluid
pressure is applied to the clutch pack, the piston engages, locking the
assembly together and driving the wheels. When pressure releases (for instance,
the engine goes to idle), the piston disengages and the wheels stop turning.
Other cars use transmission bands. In this design, a flexible metal ring fits
around the outside of the clutch housing. It tightens to engage the gears, and
loosens to release them. This type of transmission also uses a servo piston to
activate and deactivate the bands.
The transmission is connected to the wheels via an output shaft, which meshes
to the axles in a variety of ways, depending on your car's design. The
transmission turns the output shaft, which in turn spins the axles, which in
turn make the wheels go. There are a myriad of ways to do this. Since you can
have a front- or rear-wheel drive car, and since engines can be located in the
front, rear, or even middle of the vehicle, the engineering choices are
infinite. Suffice it to say, the axles drive the wheels.
Many modern front-wheel-drive cars use an assembly called a transaxle, which
saves weight (thereby increasing mileage) and space. The transaxle houses the
transmission and the axles in a single unit that weds to the engine. |
|
|
|
|
|
|