Powertrain Transmission Types and Operation
The first automobile transmission was little more than a belt drive and clutch connecting the rear axle to the engine. It was a single-speed transmission and provided a jarring experience when engaging the clutch. Later, the manual transmission featured multiple gear ratios and a reverse gear. Eventually, the automatic transmission no longer required drivers to manage the clutch and shift lever. Today, there are a few basic types of powertrain transmission available.
Traditional Automatic Transmission
In this transmission, the engine and input shaft are always connected, via the torque converter, which is itself a hydrodynamic transmission. When the engine is running, the driver selects forward or reverse. Pressing the accelerator pedal increases engine speed and torque converter power transmission, which drives the transmission input shaft.
Individual gear ratios are determined by a series of planetary gearsets, which are engaged by a series of hydraulic-actuated brakes. Depending on driver demand, speed and load, the transmission controller engages the appropriate gear ratio, or speed. Today’s automatic transmissions can shift as hard and fast as manual transmissions and offer comparable fuel economy.
Some automatic transmissions have a manual mode that allows drivers to change gears with some limitations. For example, this powertrain transmission will not allow you to break by trying to downshift into 1st gear at 60 mph. These transmissions are produced by brands such as Tiptronic and Autostick.
Automated Manual Transmission
This powertrain transmission type is similar to an automatic transmission in operation but more similar to manual transmissions in construction. The driver still selects a direction and may have a certain amount of control over gear ratios, but there is no clutch or traditional shift lever. Instead, using buttons or shift paddles, the driver shifts up or down at will. Internally, the automated manual transmission uses hydraulic, electric or pneumatic actuators to engage and disengage gears. In automatic mode, it performs much like an automatic transmission.
Dual-clutch transmissions shift fast and smooth and are usually fitted to sports cars. These are also sometimes referred to as DSG (Direct Shift Gearbox), SMG (Sequential Manual Gearbox) or PDK (German for Dual Clutch Transmission).
Continuously-Variable Transmission (CVT)
The CVT, sometimes improperly called “constant-velocity” transmission, is a belt-drive transmission that may be connected to the engine via torque converter or not, depending on whether it has a zero ratio. Unlike the automatic and automated manual, the CVT does not have distinct gear ratios. Acceleration is smooth, and the transmission controller keeps the powertrain in the most efficient gear ratio during the whole process.
Currently, continuously-variable transmissions offer the best fuel economy, even better than manual transmissions, which gives commuters something to smile about. Some drivers complain that it doesn’t feel like driving a car, so some CVT have a sport mode that fakes shifting through specific gear ratios.
Like the belt-drive CVT, an electric variable transmission in hybrid cars transmits engine torque to the wheels. Multiple electric motors run at different speeds, generating effective gear ratio changes for acceleration, cruising and even reverse.
Over 96% of the vehicles on the road today are equipped with some type of automatic transmission — some models are simply unavailable with a manual transmission. Knowing how each powertrain transmission works can help you make a good choice regarding performance, drive quality and fuel economy.
For more information on powertrain transmission maintenance, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
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