Transmissions and Transaxles SIMPLIFIED
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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 transmissions.

This knowledge article explains automatic transmission and drivetrain basics for RWD (rear wheel drive) and FWD (front wheel drive) vehicles.

Automatic Transmission and Drivetrain Basics

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 achieveplanetary gear set 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 drivetrain).

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.

Rear Wheel Drive (RWD)

Referring to 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. 

RWD Powertrain 

In this 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).

Inside the 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.

The Powertrain components in a rear wheel drive vehicle include the engine,  transmission, driveshaft, differential and wheel axles.

HorsepowerAn 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.

Front Wheel Drive (FWD) Vehicle

The 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.

FWD Powertrain Layout

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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