What is Continuously Variable Transmission?
The continuously variable transmission
(CVT) is a transmission in which the ratio of the rotational speeds of two shafts,
as the input shaft and output shaft of a vehicle or other machine, can be varied
continuously within a given range, providing an infinite number of possible ratios.
The continuously variable transmission should not be confused with the power split
transmission (PST), as used in the
Toyota Prius and other hybrid vehicles that use
two or more inputs with one output, despite some similarities in their function.
A CVT need not be automatic, nor include zero or reverse output. Such features may
be adapted to CVTs in certain specific applications.
Other mechanical transmissions only allow a few different discrete gear ratios to
be selected, but the continuously variable transmission essentially has an infinite
number of ratios available within a finite range, so it enables the relationship
between the speed of a vehicle engine and the driven speed of the wheels to be selected
within a continuous range. This can provide better fuel economy than other transmissions
by enabling the engine to run at its most efficient speeds within a narrow range.
CVTs have been refined over the years and are much improved from their origins
Ratcheting CVT
The Ratcheting CVT is a transmission that relies on static friction and is
based on a set of elements that successively become engaged and then disengaged
between the driving system and the driven system, often using oscillating or indexing
motion in conjunction with one-way clutches or ratchets that rectify and sum only
"forward" motion. The transmission ratio is adjusted by changing linkage
geometry within the oscillating elements, so that the summed maximum linkage speed
is adjusted, even when the average linkage speed remains constant. Power is transferred
from input to output only when the clutch or ratchet is engaged, and therefore when
it is locked into a static friction mode where the driving & driven rotating
surfaces momentarily rotate together without slippage.
These CVTs can transfer substantial torque because their static friction actually
increases relative to torque throughput, so slippage is impossible in properly designed
systems. Efficiency is generally high because most of the dynamic friction is caused
by very slight transitional clutch speed changes. The drawback to ratcheting CVTs
is vibration caused by the successive transition in speed required to accelerate
the element which must supplant the previously operating & decelerating, power
transmitting element. An
Infinitely Variable Transmission (IVT)
that is based on a Ratcheting CVT and subtraction of one speed from another will
greatly amplify the vibration as the IVT output/input ratio approaches zero.
Ratcheting CVTs are distinguished from Variable Diameter Pulleys (VDPs) and Roller-based
CVTs by being static friction-based devices, as opposed to being dynamic friction-based
devices that waste significant energy through slippage of twisting surfaces.
Roller-based CVT
Consider two almost-conical
parts, point to point, with the sides dished such that the two parts could fill
the central hole of a torus. One part is the input, and the other part is the output
(they do not quite touch). Power is transferred from one side to the other by one
or more rollers. When the roller's axis is perpendicular to the axis of the almost-conical
parts, it contacts the almost-conical parts at same-diameter locations and thus
gives a 1:1 gear ratio. The roller can be moved along the axis of the almost-conical
parts, changing angle as needed to maintain contact. This will cause the roller
to contact the almost-conical parts at varying and distinct diameters, giving a
gear ratio of something other than 1:1. Systems may be partial or full toroidal.
Full toroidal systems are the most efficient design while partial toroidals may
still require a torque converter (e.g., Jatco "Extroid"), and hence lose
efficiency.
Hydrostatic CVTs
Hydrostatic transmissions use a variable displacement pump and a hydraulic motor.
All power is transmitted by hydraulic fluid. These types can generally transmit
more torque, but can be sensitive to contamination. Some designs are also very expensive.
However, they have the advantage that the hydraulic motor can be mounted directly
to the wheel hub, allowing a more flexible suspension system and eliminating efficiency
losses from friction in the drive shaft and differential components. This type of
transmission is relatively easy to use because all forward and reverse speeds can
be accessed using a single lever.
This type of transmission has been effectively applied to a variety of inexpensive
and expensive versions of ridden lawn mowers and garden tractors. Many versions
of riding lawn mowers and garden tractors propelled by a hydrostatic transmission
are capable of pulling a reverse tine tiller and even a single bladed plow.
One class of riding lawn mower that has recently gained in popularity with consumers
is zero turning radius mowers. These mowers have traditionally been powered with
wheel hub mounted hydraulic motors driven by continuously variable pumps, but this
design is relatively expensive. A company call Hydro-Gear, a joint venture between
Sauer-Danfoss and Agri-Fab, Inc., of Sullivan, Illinois, created the first cost-effective
integrated hydrostatic transaxle suitable for propelling consumer zero turning radius
mowers. An integrated hydrostatic transaxle (IHT) uses a single housing for both
hydraulic elements and gear-reducing elements. As of May 9, 2007, Hydro-Gear remains
the only company producing integrated hydrostatic transaxles for consumer zero turning
radius mowers in North America.
Some heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural
machinery including foragers and combines, but not anything that works the ground
because the transmission cannot transmit enough torque.