Now that we have a fluid linkup, how do we accelerate forward and shift
gears? The gears are applied by a series of mechanisms called servos and bands
that are controlled by engine electronics and hydraulic pressure. For example,
when the gas is depressed, the engine control module (ECM) senses the need to
shift, so an electronic signal is sent to the transmission to downshift into passing
gear. Electrical switches called solenoids are tripped and valves within the valve
body divert the flow of oil to the bands or servos, which in turn apply or
disengage gears. Just like magic, the transmission shifts.
Constant Variable Transmission
In recent years carmakers have created a transmission design called the
constantly variable transmission (CVT). In this design, the transmission gear
ratio is constantly changing in order to produce maximum torque, regardless of
the speed of the vehicle. Additional benefits include elimination of harsh shift
points and improved fuel economy.
There are a few different types of CVTs on the market. They include the
following:
Belt Drive System
In this system, the belt rides up and down inside the pulley grooves. The closing
and opening of one of the pulleys produces different gear ratios. As the belt rides
up and down, turning at different speeds, gear ratios are changed. This action
results in a constantly variable gear ratio.
Toroidal CVT
Although the Toroidal CVT system appears to be drastically different than the
belt drive, the operation of all the components of this system lead to the same
results, a continuously variable transmission delivering a constantly variable
gear ratio.
In this transmission, one disc connects to the engine. This is equivalent to the
driving pulley in the belt drive system. Another disc connects to the drive shaft.
This is equivalent to the driven pulley: rollers (or wheels), located between the
discs act like the belt, transmitting power from one disc to the other.
Both the belt-drive CVT and the toroidal CVT are examples of frictional
CVTs that work by varying the radius of the contact point between two rotating
objects. There are CVTs that use a different design, however.
Hydrostatic CVTs