Volkswagen is combining its diesel Golf Plus 2.0 TDI with its direct shift gearbox (DSG) to deliver up to a 10.6% reduction in fuel consumption and lower emissions compared to a Golf with a five-speed automatic transmission.
A new Golf with the five-speed automatic gearbox would have a fuel consumption of 6.6 litres per 100 kilometers (35.6 mpg US). With the DSG, consumption decreases to 5.9 liters per 100 kilometers (39.9 mpg US).
The DSG is essentially two gearboxes in one, combining the comfort of an automatic with the agility and economy of a manual unit. The six-speed, transversely mounted DSG has two wet clutches (offering a higher thermal load tolerance than dry clutches) with hydraulic pressure regulation. One clutch controls the odd gears plus reverse, while the other operates the even gears.
This dual approach enables the next-higher gear ratio to become engaged but on standby until it is actually selected. In other words, if the car is being driven in third gear, fourth is selected but not yet activated. As soon as the ideal shift point is reached, the clutch on the third-gear side opens, the other clutch closes and fourth gear engages under accurate electronic supervision.
Because the opening and closing actions of the two clutches overlap, a smooth gearshift results and the entire shift process is completed in 40 milliseconds.
Volkswagen is optimizing the DSG for diesel engines, working on features such as pulling more powerfully from low engine revolutions.
In its combination of the new Golf Plus TDI and DSG, Volkswagen refined the fuel injection system to perform more precisely for the initial (pilot) and main injections. This, combined with other enhancements, improves the engine characteristics at idle and at low loads—improvements of which the DSG can take advantage.
The increased pulling power at lower revs ensures that the engine torque required is always available. The commencement of injection is brought forward through “dynamic pilot control” so that up to 15% more torque is immediately available when needed during fast gear changes.
As a result, the engine turns over considerably less than it would with a conventional gearbox to carry out the same work, and decreased engine revolutions reduce fuel consumption.
The Volkswagen engineers also achieved 15% more exhaust gas recirculation in the DSG gearbox load areas to reduce NOx emissions while maintaining a low level of PM emissions. VW claims that the TDI plus DSG combination delivers emissions lower than Euro4 levels.
All together, a good example of the ability of the increasing sophistication of electronic engine management combined with new approaches can deliver further improvements in fuel economy.