Continuously Variable Transmission
(CVT) is a type of automatic transmission that can change the "gear ratio"
(gears are not generally involved) to any arbitrary setting within the limits.
The CVT is not constrained to a small number of gear ratios, such as the 4 to 6
forward ratios in typical automotive transmissions. CVT control computers often
emulate the traditional abrupt gear changes, especially at low speeds, because
most drivers expect the sudden jerks and will reject a perfectly smooth
transmission as lacking in apparent power.
An extension to CVT design, sometimes known as the Infinitely Variable
Transmission (IVT), allows the transmission to drive a vehicle backwards as well
as forwards. Transmission input is connected to the engine, then it is split
into 2 shafts with one connected to an epicyclic gear set. The output from the
CVT shaft is connected to another shaft that connects to a different set of
gears in the epicyclic. The gear that does not draw power from engine or CVT
transfers torque to the transmission output. The gear set acts as a mechanical
adding machine to subtract one speed from the other, allowing the car to go
forwards, backwards, or neutral.
Variable-diameter pulley (VDP)
This type of CVT uses pulleys, typically connected by a rubber-covered metal
belt. A chain may also be used. A large pulley connected to a smaller pulley
with a belt or chain will operate in the same manner as a large gear meshing
with a smaller gear. Typical CVTs have pulleys formed as pairs of opposing
cones. Moving the cones in and out has the effect of changing the pulley
diameter since the belt or chain must take a large-diameter path when the
conical pulley halves are close together. This motion of the cones can be
computer-controlled and driven, for example by a servo motor. However, in the
light-weight VDP transmissions used in automatic motorscooters and light
motorcycles, the change in pulley diameter is accomplished by a variator, an
all-mechanical system that uses weights and springs to change the pulley
diameters as a function of belt speed. In higher power types, for example that
produced by Van Doorne's Transmissie (part of the Bosch Group), an oil-cooled
laminated steel belt is used.
In the case of a chain the links bear on the pulleys via tapered sides on the
links. Some such transmissions have been designed to transmit the forces between
pulleys using compressive (pushing) rather than traction (pulling) forces. The
chain driven transmission designed by LuK and VAG/Audi uses a special lubricant
which undergoes a phase change under extreme pressure to form a glassy solid,
enabling the chain to transmit considerable torque through small contact
Roller-based CVT (Traction CVT, Extroid CVT, Nuvinci CVP, or IVT)
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) and hence lose efficiency.
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 are very 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 has been effectively applied
to expensive versions of light duty ridden lawn mowers, garden tractors and some
The Hydristor torque converter is a true IVT in that the front unit connected to
the engine can displace from zero to 27 cubic inches per revolution forward and
zero to -10 cubic inches per revolution reverse. The rear unit is capable of
zero to 75 cubic inches per revolution. The common 'kidney port' plate between
the two sections communicates the hydraulic fluid under pressure and suction
return in a 'serpentine-torodial' flow path between the two Hydristor internal
units. The IVT ratio is determined by the ratio of input displacement to output
displacement. Therefore, the theoretical range of Hydristor IVT ratios is
1/infinity to +-infinity/1.
Not all of that ratio range can be realized in the real world. For example, the
input set at 27 divided into the rear set at zero results in the infinity/1
ratio and won't actually work. The front unit set at zero into the back unit at
75 is 1/infinity; ie: the engine turns forever for one turn of the output. This
is actually achieved but does no work. Once a cruising speed has been achieved
with front and rear Hydristors at some appropriate relative displacements,
hydraulic braking is achieved by first simultaneously reducing both front and
rear to zero displacement, then leaving the front Hydristor at zero (thus hydro
mechanically disconnecting the engine from the torque converter hydraulic
circuit and finally beginning to increase rear displacement as a braking
function with the braking pressure and flow being directed to a hydraulic
accumulator pressure tank. The decaying vehicle speed (kinetic energy), the
rising tank pressure and the desired rate of deceleration determined by the
driver all are variables which are easily managed by the Hydristor system. The
stored braking energy can then be re-used for subsequent re-acceleration.
One result is that the city stop and go fuel economy nearly equals the highway
mileage. The highway mileage is also doubled or more because of lowering the
engine speed to idle in most cases and the resulting huge reduction in engine
losses substantially raises the highway fuel economy. An important result is
that the creation of CO2 greenhouse gas is quartered in existing vehicles in
addition to substantially raising the average fuel economy. Last, the
combination of hydraulic stored energy, the Hydristor's IVT nature, and the
infinite ranges of control result in a vehicle which can zoom up to highway
speed in a few seconds and literally spin the tires at highway speed even though
the engine is idling due to the hydraulic storage. Most important, the Hydristor
eliminates the transmission gears completely and is completely retrofittable
into every vehicle already on the highways thus saving all the existing highway
The E-CVT saw first commercial automotive use in Toyota's THS (Toyota Hybrid
System), first seen in the 1997 Toyota Prius, and subsequently in Toyota's
Hybrid Synergy Drive system. This system is not a true CVT, having a fixed gear
ratio, but behaves very similarly to a true CVT. In this system, the
transmission is an integral part of the hybrid power train and is actually a
torque combiner. The gear train is a permanently-engaged, fixed-ratio, 3-way
planetary gear. The engine is attached to one input (planetary carrier), the
drive shaft and the main electric motor to another (ring gear), and then a
smaller motor-generator controls the differential third input (sun gear) to
create a continuously-variable ratio between engine speed and wheel speed, with
the variation taken by the electric motor and generator. At the extremes, the
vehicle can move under electric power without the engine turning, or it can run
the engine while stationary during engine warm-up or if needed to prevent
discharge of the batteries.
The advantage of the system is its mechanical simplicity - no clutches, torque
converters or shifting gears. A disadvantage is that continuous electrical power
transmission between the two motor-generators is needed even during cruise, with
resulting conversion losses, but the total effect is to increase the net
efficiency through four methods:
The ICE (Internal Combustion Engine) may completely shut down instead of idling
when the vehicle is stopped or driven at slower speeds.
The electric motor operates during high torque demands required to put the
vehicle in motion.
The ICE operates mostly at higher power demands, where it is more efficient.
Energy may be recovered through the generation function (regenerative braking)
when the vehicle is slowing or coasting downhill, with the energy (stored in the
battery) applied to the initial acceleration of the vehicle and/or when high
power demands require that both the ICE and the electric motor operate
The design of the system may be optimized for efficiency or for performance, as
appropriate for the marketing segment for which the vehicle is targeted.
It is a technology invented by Larry Anderson, under US patents 6,575,856 and
6,955,620. Two parallel cones have "floating sprocket bars" mounted in
longitudinal grooves around the circumference of each cone.
A specially-designed chain meshes with the
floating sprocket bars, and is free to slide along the length of cones, changing
the gear ratio at each point. The floating sprocket bars make the A+CVT
positive-drive, non-friction-dependent. Another advantage of the A+CVT is the
simplicity of its design, as it consists of far fewer components than other
transmissions. The technology is also adaptable to a variable diameter
pulley-type CVT, by mounting the floating sprocket bars on the inner face of the
pulley sheaves. A few critics have speculated that noise could be a problem with
the A+CVT. However, Anderson has said that he believes noise will be no more of
an issue with the A+CVT than with other transmissions, as the A+CVT will be
lubricated and encased in a housing.
This is a simple chain-based, mechanically positive linkage technology invented
by Chris Yonge in 2006. A system of sun and planet or bevel gears similar to,
but distinct from, a differential gear assembly creates a phase shift
arrangement whereby the relative angle between linked peripheral and central
circular elements can be adjusted regardless of whether the two are rotating or
not. It is unique in being positively and continually mechanically linked
throughout, simple to manufacture, and having no reliance upon frictional
contact. The basis of this mechanism uses at minimum seven conventionally
machined gears. Potential applications include continuously variable bicycle
gearing through a polygonal sprocket array, power applications for vehicles and
machinery, and propellor/turbine blade pitch adjustment while running under
load. Two CVT units running in parallel to an unmodified power shaft, with both
shafts then combined in an inverted differential gear that acts as a torque
addition device, become an infinitely variable transmission (IVT).
Advantages and drawbacks -
Compared to hydraulic automatic transmissions:
CVTs can smoothly compensate for changing vehicle speeds, allowing the engine
speed to remain at its level of peak efficiency. They may also avoid torque
converter losses. This improves both fuel economy and exhaust emissions.
However, some units also employ a torque converter. Fuel efficiency advantages
as high as 20% over 4 speed automatics can be obtained.
CVTs have much smoother operation. This can give a perception of low power,
because many drivers expect a jerk when they begin to move the vehicle. The
satisfying jerk of a non-CVT transmission can be emulated by CVT control
software though, eliminating this marketing problem.
Since the CVT keeps the engine turning at constant RPMs over a wide range of
vehicle speeds, pressing on the accelerator pedal will make the car move faster
but doesn't change the sound coming from the engine as much as a conventional
automatic transmission gear-shift. This confuses some drivers and again, leads
to a mistaken impression of a lack of power.
Most CVTs are simpler to build and repair.
CVT torque handling capability is limited by the strength of their belt or
chain, and by their ability to withstand friction wear between torque source and
transmission medium for friction-driven CVTs. CVTs in production prior to 2005
are predominantly belt or chain driven and therefore typically limited to low
powered cars and other light duty applications. More advanced IVT units using
advanced lubricants, however, have been proven to support any amount of torque
in production vehicles, including that used for buses, heavy trucks, and earth
Leonardo da Vinci, in 1490, conceptualized a stepless continuously variable
transmission. The first patent for a toroidal CVT was filed in 1886. From the
1950s, CVTs have been applied to aircraft electrical power generating systems.
A CVT, called Variomatic, was designed and built by the Dutchman Huub van Doorne,
co-founder of Van Doorne's Automobiel Fabriek (DAF), in the late 1950s,
specifically to produce an automatic transmission for a small, affordable car.
The first DAF car using van Doorne's CVT was produced in 1958. Van Doorne's
patents were later sold to Volvo along with DAF's car business and CVT was used
in Volvo 340.
In the 1980s and 1990s, the Subaru Justy was offered with a CVT. While the Justy
saw only limited success, Subaru continues to use CVT in its keicars to this
day, while also supplying it to other manufacturers.
Nissan first introduced CVT in the 1992 Nissan March with a unit sourced from
Subaru. In the late 1990s, Nissan designed its own CVT that allowed for higher
torque, and includes a torque converter. This gearbox was used in a number of
Japanese market models. Nissan is also the only car maker to bring roller-based
CVT to the market in recent years. Their toroidal CVT, named the X-troid, was
available in the Japanese market Y34 Nissan Gloria and V35 Skyline GT-8.
However, the gearbox was not carried over when the Cedric/Gloria was replaced by
the Nissan Fuga in 2004.
After studying pulley-based CVT for years, Honda also introduced their own
version on the 1995 Honda Civic VTi. Dubbed Honda Multi Matic, this CVT gearbox
accepted higher torque than traditional pulley CVTs, and also includes a torque
converter for "creep" action.
Toyota introduced the E-CVT in the 1997 Prius, and all subsequent Toyota and
Lexus hybrids sold internationally continue to use the system. Although sold as a CVT it is in fact not such a
device as the gear ratios are fixed and the transmission is actually a torque
blending device, allowing either the electric motor or the ICE (internal
combustion engine) or both to propel the vehicle. The response of the complete
system (under computer control) is similar in feel to a CVT in that the ICE
speed is relatively low and constant under low power or high and constant under
BMW used a belt-drive CVT as an option for the low and middle range MINI in
2001, forsaking it only on the supercharged version of the car where the
increased torque levels demanded a conventional automatic gearbox. The CVT could
also be manually 'shifted' if desired with software simulated shift points.
General Motors designed a CVT for use in small cars, which was first offered in
2002. After just three years, however, this transmission will be phased out in
favor of conventional planetary automatic transmissions. Audi has, since 2000,
offered a chain-type CVT as an option on some of its larger-engine models, for
example the A4 3.0 L V6.
The 2005 Ford Freestyle and Five-hundred use a new chain-driven CVT allowing
engine torque to go up to 300 N•m. The transmission was designed in cooperation
with the German Company Sachs - ZF and is currently produced in Batavia, Ohio.
The CVT is computer controlled and combines fuel efficiency and smooth riding.
Ford also sold Escort (European version) and Orion models in Europe with CVT
transmissions in 80's and 90's.
The 2007 Dodge Caliber and the related Jeep Compass employ a CVT using a
variable pulley system as their optional automatic transmission. Sachs - ZF
supplied its belt drive CVT unit to many car manufacturers including BMW and MG
Rover. Contract agreements were established in 2006 for the first full toroidal
system to be manufactured for outdoor power equipment such as jetskis,
ski-mobiles and ride on mowers.
Many small tractors for home and garden use have simple hydrostatic or rubber
belt CVTs. They can
deliver a lot of power but can also build up speed to 10-15 MPH, all without
need for a clutch or gearshift. Most snowmobiles use CVTs. Most new
motorscooters today are equipped with CVT. Virtually all snowmobile and motor
scooter CVTs are rubber belt/variable pulley CVTs.
Some combine harvesters have CVTs. The machinery of a combine is adjusted to
operate best at a particular engine speed. The CVT allows the forward speed of
the combine to be adjusted independently of the machine speed. This allows the
operator to slow down and speed up as needed to accommodate variations in
thickness of the crop.
CVTs have been used in SCCA Formula 500 race cars since the early 1970s. More
recently CVT systems have been developed for karts, and have proved to increase
performance, and engine life expectancy.