A specific type of CVT is the infinitely variable transmission (IVT), which has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined "higher" ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output/input which have high input/output ratios that are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit. Most continuously variable transmissions are not infinitely variable.

Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system) that facilitates the subtraction of one speed from another speed within the set of input and planetary gear rotations. This subtraction only needs to result in a continuous range of values that includes a zero output; the maximum output/input ratio can be arbitrarily chosen from infinite practical possibilities through selection of extraneous input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. Importantly, the IVT is distinguished as being "infinite" in its ratio of high gear to low gear within its range; high gear is infinite times higher than low gear. The IVT is always engaged, even during its zero output adjustment.

The term "Infinitely Variable Transmission" does not imply reverse direction, disengagement, automatic operation, or any other quality except ratio selectabilty within a continuous range of input/output ratios from a defined minimum to an undefined, "infinite" maximum. This means continuous range from a defined output/input to zero output/input ratio.

Hydristor IVT

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 but real-world ratios are constrained by physics.

What is an IVT

The IVT moves away from the constraints of stepped ratio automatic transmissions and unlike Continuously Variable Transmissions there is no torque limitation. The ratio range is provided not by a system of gears, as found in a conventional automatic transmission, but by a variator which consists of a set of discs and rollers and is termed a 'full toroidal' variator (more information on how the variator works can be found here).

The IVT technology allows control of the complete powertrain enabling optimisation of the efficiency of the engine. In conventional transmissions the engine and transmission are controlled separately.
The IVT is torque controlled rather than ratio controlled. The software determines the torque required at the road wheels and then requests torque from the transmission rather than setting a specific ratio. The engine can then be used to deliver the power at its most efficient operating point. In conventional automatics where the gear ratios are fixed the engine only runs at its most efficient operating points for short periods of time.

This optimisation of the entire powertrain brings about a fuel economy benefit in three ways:

• Due to the geared neutral feature of the IVT the engine can be directly connected to the road wheels whilst the vehicle is stationary. There is no need for a separate starting device such as an inefficient torque converter.
• Due to its wide ratio spread the IVT provides the ability for the engine to run at a high overdrive of 74mph/1000rpm.
• Because the IVT does not have discrete ratio steps it can run the engine at optimum conditions for fuel economy and emissions.

Core IVT benefits
The full toroidal traction IVT technology gives the following generic benefits. To learn about the specific benefits the IVT brings to different markets and applications, click on the appropriate section of this website.

Running an engine at optimum conditions
The exceptional ratio range of the IVT provides an almost total decoupling of engine and vehicle speed. For any given power demand the IVT will operate the engine at a pre-determined torque and speed, largely independent of vehicle speed. This ensures that the engine always operates at minimum specific fuel consumption, i.e. at maximum efficiency. Typically, this means that the engine operates at low speed (reduced friction losses) and high torque (reduced throttling losses).

Geared neutral
The IVT has a feature termed ‘geared neutral’ where the engine can be connected to the road wheels whilst the vehicle is stationary.This feature delivers a number of benefits including:

• No need for an inefficient starting device such as a torque converter.
• Continuous torque at the wheels.
• Accurate control of the vehicle at low speeds.

See also our CVT section for more information on gearboxes.

Click for more information on IVT Gearboxes