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The Nissan Jatco "Extroid" CVT


by François Dovat

Since the autumn 1999 a continuously variable transmission (CVT) by means of a semi-toroidal variator named Extroid is optional on the Nissan Cedric and Gloria, cars reserved for the Japanese domestic market and motorized by a turbocharged 3 liters V6 of 280 hp and 388 Nm. From the summer 2002, the Extroïd is also available on the Skyline 350 GT-8, still exclusively in Japan and with the same engine. The cost of this system is higher than that of the metal pushbelt or chain variators, but its torque capacity is somewhat higher and its operation is quieter. On the other hand, its ratio span is limited to 4.36. A hydrodynamic torque converter is thus fitted upstream, but it is locked up once the car has started.

The variator is a variant of the toroidal system which uses only the interior half of a toroid. To obtain an underdrive ratio, the rollers are in contact between the small radius of the driven disk and the largest radius of the driven disk. To get an overdrive ratio, the rollers swivels towards a large radius of the driven disk and a small radius of the driven disk. Thosedisks and rollers are made by the Japanese roller bearing manufacturer NSK from an extremely pure molybdenum-manganese alloy. They are specifically heat treated and finely micro-polished. A traction fluid identical to the one used by Torotrak transmits the torque without metal to metal contact (see our file about the Torotrak IVT for details).

In order to multiply the contact points, there are two half-toroidal cavities, each with two rollers at 180° of each other. The assembly in opposition of these rollers cancels the axial deflection and the contact geometry is somewhat better than within a full-toroidal variator, but on the other hand large and heavy bearings are necessary to balance the strong radial and tangential thrusts generated by this concept (comparable with bevel gears) which tends to eject the aforesaid rollers out of the variator. It is also difficult to set more than 2 rollers in each cavity while 3 of them can be easily located inside a full-toroidal cavity for a better structural stiffness.

A great clamping pressure is necessary between the points of contact when the engine develops a significant torque. In order to limit the losses while driving under part load (low engine torque) a loading cam device automatically raises the contact pressure with the increase in torque.

An encouraging efficiency is announced: 94% for the variator and 90 to 92 % for the whole transmission. The fuel consumption is reduced about 10% compared to a conventional automatic transmission. Alternatively to the continuous mode, 6 or 8 distinct "virtual" ratios are available within the overall span of 2.86 to 0.66.

Pushed or drawn tangentially by hydraulic pistons which create a parasitic force, the rollers change their angle, thus varying the transmission ratio.

To know more:

http://www.nsk.com/eng/journal/mc09/development_cvt.pdf

http://www.nsk.com/eng/journal/mc11/tec200211-02.pdf

http://www.nsk.com/eng/journal/mc12/mc12-16.pdf

The Skyline 350 GT-8

(© François Dovat)
#90

Technical files

Idée & conception © 1999-2011 van Damme Stéphane.


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