Planetary Gears: Key in the operation of an automatic transmission are planetary gears. In very simple terms, planetary gears consist of several smaller gears rotating around a larger one. An automatic transmission has various sets of planetary gears which vary in size, and can be meshed with each other in various combinations to create various gear ratios.

An automatic transmission’s gear ratios come from selectively engaging and disengaging the sets of planetary gears that are driving the wheels at any time. In any given gear, some of the planetary gears in the transmission are simply idling, while other sets are engaged. By engaging and disengaging specific sets of planetary gears inside of the automatic transmission, the path of the engine’s power through the transmission is changed, as is the gear ratio currently engaged.

The Continually Variable Transmission (CVT): Used by numerous automakers for smoother performance, improved fuel efficiency and a higher degree of refinement, the CVT operates like an automatic, but it does away with pre-set gear ratios and stepped shifting between gears.

The CVT typically has a torque converter, an input shaft from the engine, and an output shaft to the wheels, just like a normal automatic. But, instead of an array of inter-connected planetary gear sets, the CVT creates gear ratios with two pulleys, attached to one another via a steel belt or chain. One pulley is connected to the input shaft from the engine, the other to the output shaft, which connects to the wheels.

The two pulleys inside of this type of transmission do an interesting trick: they can change their diameter. In doing so, the CVT is able to instantly engage a virtually infinite number of gear ratios between an upper and lower limit, by changing its pulley diameters and therefore the effective gear ratio currently in use. Instead of a number of pre-set gears, the CVT’s diameter-shifting chain-linked pulleys allow a nearly infinite number of gear ratios to blend into one another, with no hard transition between ratios.

Noteworthy CVT’s: Key in making best use of CVT technology for refinement, efficiency and driving pleasure is the programming which controls the transmission. Effective tuning of the CVT’s gear ratio strategy against the engine’s power curve lies at the core of every good CVT. Further, to help reduce the strange feel of not shifting gears, which distresses some drivers, many automakers utilize programming that simulates the ‘pause…downshift’ sensation of a regular automatic. Some examples of well set-up CVT transmissions include the Subaru WRX (Lineartronic), Nissan Maxima (Xtronic), and Mitsubishi Outlander.

Connect with Autos.ca