CVT and dual clutch transmission

(a) Continuously variable transmission (CVT)

A CVT provides an infinite number of ratios between two limits, without fixed gear steps.

 Engine -> [Primary pulley]==steel belt==[Secondary pulley] -> wheels
            (moves apart/close)           (moves close/apart)
 Low speed: primary small radius, secondary large radius
 High speed: primary large radius, secondary small radius

• Two conical (V) pulleys, each with one movable half, are connected by a steel push-belt or chain.
• Hydraulic pressure (controlled by the ECU) changes the width of the pulley grooves. When the primary pulley closes, the belt rides at a larger radius on the primary and a smaller radius on the secondary, so the ratio decreases (higher road speed).
• The engine can therefore stay near its most efficient speed.
• Advantage: smooth, no shift shock, better economy.
• Limitation: belt slip and wear limit the torque capacity; the "rubber-band" feel with engine drone; costly to repair.

(b) Dual clutch transmission (DCT)

A DCT has two clutches, one for the odd gears (1, 3, 5) and one for the even gears (2, 4, 6, R), each with its own input shaft (a hollow shaft over a solid shaft).

 Engine -> [Clutch A: odd gears 1,3,5 ]
        -> [Clutch B: even gears 2,4,6 ]
 One gear engaged, next gear pre-selected

• While one gear carries the power, the next gear is already selected on the other shaft. At the shift, clutch A opens as clutch B closes, so there is almost no interruption of power (shift time under 100 ms).
• It is actuated electro-hydraulically by the transmission control unit (TCU); wet clutches are used for high torque and dry clutches for light cars.
• Advantage: fast shifts and good efficiency, like a manual with automatic convenience.
• Limitation: jerky at low speed and clutch overheating in traffic; complex and costly.