In 2018, which is just around the corner, MINI will offer a third transmission option with the introduction of a double-clutch transmission (DCT). It’s the same thing as a dual-clutch transmission or DSG, which the Volkswagen Group uses. Just as the BMW Group doesn’t say SUV but SAV (Sport Activity Vehicle), they probably use ‘double-clutch’ to seemingly differentiate their transmission from other similar types. It’s the first time for a MINI to have this type of transmission though it is not new to the BMW Group.

The DCT is operated by means of a newly-designed electronic selector lever which always returns to its original position after changing between D, N or R. The parking lock is activated by pressing the P button. S mode can be activated from position D by shifting the selector lever to the left: this supports a particularly sporty driving style as well as allowing manual intervention in the choice of position.

The electronic control of the new transmission for MINI also includes a shift strategy based on navigation data and supports both the engine start/stop function and coasting so as to reduce fuel consumption and emission levels.

The design principle of the DCT has been adapted directly from motor racing. Just like on the race track, it allows drivers to undertake keep the accelerator depressed while the transmission control sets the shift points so precisely within just a fraction of a second that the ideal engine speed is always available after the gearshift to ensure a continuous increase in speed.

This is possible due to the specific structure of the DCT. It combines two partial transmissions in a single housing. The core element of the system consists of two oil-cooled wet clutches: one of these is responsible for the even transmission ratios (2, 4, 6) while the other is for the uneven ratios (1, 3, 5, 7) and reverse. During travel, one of the two clutches is open and the other is closed. They interact when the driver shifts up or down: opening one clutch activates closure of the second at the same time.

If the car is accelerated in 3rd gear, for example, power transmission occurs by means of the relevant clutch and the partial gear for the uneven transmission ratios. At the same time, the transmission control ensures that the next gear required – in this case 4th gear – is already engaged in the partial transmission responsible for the even transmission ratios. All that needs to be done to shift up is to open the clutch of the 3rd gear and close that of the 4th gear. This happens simultaneously without any interruption of pulling power; if anything, the process takes less time than it would to activate the clutch pedal when driving with manual transmission.

In order to have the appropriate gear ready for the next shift, the electronic control system permanently analyses the driver’s intentions and the situation on the road. This analysis incorporates the accelerator position, engine speed, road speed and driving mode. As a result, fast gearshifts can be made to respond appropriately to sudden changes in the driving situation. If acceleration has to be unexpectedly interrupted, for instance, the DCT takes no more than a fraction of second to make the next gear down available instead of the next gear up.

The parallel opening and closing of the clutches makes acceleration and deceleration manoeuvres not just faster but also smoother. Acoustically, the gearshifts can only be perceived in the fast succession of rev changes – also to be seen in the abrupt movements of the tachometer needle. What is more, there are no load change reactions like in manual transmissions.

The 7 gears of the DCT enable a broader spread of gears, thereby reducing the differences in engine speed between the various drive positions. Together with the direct engine link that is characteristic of the transmission, the small rev jumps make for particularly intense driving fun when accelerating.

In automated mode, driving fun can also be enhanced by linking the transmission control to the optional MINI navigation system. The DCT control unit is then able to draw on navigation data to adapt its shift strategy to the given situation on the road. When approaching a sharp bend or junction, for example, a downshift is made early on so as to be able to make use of the engine braking effect for deceleration purposes. When passing through two bends in quick succession, the drive position engaged after downshifting is maintained so as to avoid any unnecessary shifting and ensure the ideal gear is available for powerful acceleration on exiting the bend.

Manufacturing the more complex DCT is probably one reason why MINI is offering it for a limited number of models for now. Perhaps the torque converter automatic transmission will be phased out in due course when the production cost drops further. In the short-term, the Clubman and Countryman, along with the John Cooper Works variants will continue with the 8-speed torque converter automatic transmission.

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[Chips Yap]

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