How Automatic Transmissions
and Transaxles Work
Collectively over the years, we've
explained how automatic transmissions work hundreds of times to
individuals and audiences having varying degrees of interest in
This knowledge article explains
automatic transmission and drivetrain basics for RWD (rear wheel drive)
and FWD (front
wheel drive) vehicles.
Automatic Transmission and Drivetrain
The primary job of an automatic
transmission and a manual transmission is the same, which is to
transmit the engine's power to the road surface in one of the
transmission's various gear ratios. A manual transmission
uses different sized gears that slide along shafts and mesh with one
another to produce each of the transmission's available gear ratios,
whereas an automatic transmission uses the same set of internal gears
to achieve all its gear
ratios. This is accomplished through a complex and ingenious
innovation called a planetary gear set. The planetary gear
set is the heart of an automatic transmission.
But, before we
get into the internal components of an automatic transmission and how
they work, it helps to have a good understanding and a visual picture
of how the transmission transmits the engine's power to the drive
wheels. This is achieved through the vehicle's driveline (or
The two most
common drivetrain configurations found in passenger vehicles are Rear
Wheel Drive (RWD) and Front Wheel Drive (FWD).
The two other drivetrain configurations, Four Wheel Drive (4WD) and All
Wheel Drive (AWD), are not covered in this write-up.
Wheel Drive (RWD)
the image below, you can see that in a rear wheel drive vehicle the
transmission is located immediately behind and in-line with the engine,
which also sits in-line with the vehicle. This is called a
longitudinally powertrain layout.
configuration, the engine's power is transferred to the transmission
using a torque converter. A torque converter is to an
automatic transmission as a clutch is to a standard (or manual)
transmission. (More about how a torque converter works later).
transmission, different configurations within the planetary gear set
determines the gear ratio in which the engine's power is transmitted
through the driveline and ultimately to the drive wheels and then to
the ground. At take-off, the gear ratio is configured to the
transmission's lowest gear. As the vehicle picks up speed,
the gear configuration automatically changes to produce higher
gear ratios enabling the vehicle to travel faster while keeping engine
RPMs (revolutions per minute) relatively low. The higher the
gear ratio the less the engine has to work to achieve higher
speeds. The result is a more efficient vehicle in terms of
engine wear and fuel consumption.
components in a rear wheel drive vehicle include the engine,
transmission, driveshaft, differential and wheel axles.
engine's rated horsepower (measured at the flywheel) is not the
horsepower that reaches the ground. In a RWD vehicle about 15
to 20% of the horsepower is lost through the drivetrain. The
horsepower loss is slightly less in a FWD vehicle.
Wheel Drive (FWD) Vehicle
majority of passenger cars on the roadway today are FWD. In a
front wheel drive vehicle, the transmission is commonly referred to as
a transaxle because the transmission and drive axles (called CV axles)
effectively function as a single unit. In this configuration,
as shown in the illustration below, the engine and transaxle are
mounted transversely (sideways) directly above the front wheels, which
are the drive wheels.
Like the RWD layout, the torque converter is positioned at the rear of
the engine between the engine and transmission. Also like a
RWD vehicle, the engine's power is transmitted to the
transmission/transaxle through the torque converter. But,
here is where the RWD and FWD drivetrains differ. Instead of
using a driveshaft, rear differential and axles located at the rear of
the vehicle to deliver the engine's power to the drive wheels, a FWD
vehicle transfers the engine's power directly from the transaxle to the
drive wheels using CV axles (also called half shafts).
Working together as a single unit, the CV axles and transmission become
a transaxle. A front wheel drive powertrain is more efficient
in transmitting power to the drive wheels, meaning more of the engine's
power actually reaches the ground.
Except the way
in which the engine's power is transmitted to the drive wheels and
their physical and visual differences, the internal workings of an
automatic transmission and a transaxle are basically the same.
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