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Test drive of the Torotrak IVT (3)

by François Dovat


An infinite reduction (underdrive) ratio is reached at the "geared neutral "point. This situation is a "singularity" akin to the "big bang", because an infinite underdrive ratio theoretically engenders an infinite torque. A control system managing the ratio would lead to overwhelming difficulties, difficulties which are avoided by controlling not the ratio, but the torque.

In fact, the rollers have a total freedom of angular position, so they slant themselves to the exact angle necessary for a "geared neutral". It springs out from the fact that the mechanical devices are lazy: they grant to work, quite as human beings, only if they're forced to… When the throttle is activated, the software orders the raise of oil pressure on the control pistons. These push then the rollers axis tangentially in regard to the main shaft axis, creating a reaction force. Disturbed in their rest, the rollers look for a new equilibrium composing with this interfering force. So doing, they transmit a torque growing with the increase of the thrust of the pistons.

The change of ratio can be lightning fast: a half rotation of the crankshaft is said to be enough to sweep from an extreme ratio to the other, whatever the engine speed is! The variator thus spontaneously and in some hundredth of a second avoids any dynamic overload, for example during a blockage of the driven wheels.

The electronics integrates the management of the engine to that of the transmission and the prototypes are outfitted with a motorized throttle body. Any behavior can be scheduled, be it that of a hydrodynamic torque converter or of a discrete ratios mechanical gearbox.

(© François Dovat)

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Idée & conception © 1999-2011 van Damme Stéphane.

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