Automatic gearbox (Stepspeed Em-CVT) (Description): Description and operation

DESCRIPTION

General

The Electro Mechanical - Continuously Variable Transmission (Em-CVT) is based on a standard CVT unit with electronic components fitted to control the gear ratio. This gives the driver a choice between an automatic gearbox and a semi-automatic Stepspeed manual gearbox.

The gearbox can be operated as a conventional CVT unit by selecting P, R, N or D on the selector lever. Moving the selector lever across the gate trips a microswitch and puts the gearbox into manual/sport mode.

In sport mode, the gearbox still operates as a conventional CVT unit, but becomes more responsive to changes in driver demands. Engine speed is higher in this mode which gives improved acceleration.

When in sport mode, if the selector lever or either of the steering wheel switches are moved to the + or - positions, the system changes to operate in manual mode. Manual gear changes can be performed sequentially using either the selector lever or the steering wheel switches. Movement of the selector lever in a forward direction (+) changes the gearbox up the gear ratios and movement in a rearward (-) direction changes the gearbox down the ratios. Either of the + and - switches on the steering wheel perform the same function as the selector lever when in sport mode.

Gearbox operation is controlled by a Gearbox Interface Unit (GIU) and the ECM which communicate via a dedicated serial link and a hardwired connection. The MEMS3 ECM is programmed with an Em-CVT control strategy to operate the gearbox in conjunction with the GIU.

Stepspeed Em-CVT unit

When in automatic mode, the Em-CVT provides an infinite number of ratios within its operating range. The stepless shifting pattern of the transmission provides a smooth transfer of power to the road wheels whilst allowing full vehicle performance to be available at all times.

In sport mode, the Em-CVT operates as in automatic mode but with a higher engine speed under all driving conditions which gives improved acceleration.

In manual mode, the Em-CVT provides electronic selection of six predetermined ratios. Selection is made by the driver using the selector lever or the steering wheel switches.

The Em-CVT unit comprises mechanical and electrical components which work together to provide the automatic and manual operation of the gearbox.

The following mechanical components comprise the Em-CVT unit (Refer to Em-CVT sectional view):

• Torsion damper
• Planetary gear set
• Clutches
• Pulleys and steel belt
• Pinion shaft
• Differential unit
• Hydraulic pump.