DSG: what is a Direct Shift Gearbox and how do they work?
A direct shift gearbox, also known as a dual-clutch gearbox, offers an alternative to a conventional automatic
In the not-too-distant past if you were in the market for a new car you would have been faced with two choices when it came to the gearbox – manual, or automatic. These days though there’s another choice of transmission to be considered, the Direct Shift Gearbox, that’s more commonly described using its DSG acronym, and is also known as a dual-clutch gearbox.
From the driver’s seat it’s very likely you wouldn’t necessarily know the difference between a DSG gearbox and an automatic – the controls are virtually identical and a DSG tends to change gears in the same smooth manner as a traditional auto.
DSG vs a conventional automatic
So what are the differences between an automatic and a DSG, and is it better to buy one over the other? A DSG gearbox is effectively two gearboxes in one, connected to the engine by two drive shafts. As well as two gearboxes there are also two clutches and both the gearboxes and clutches are operated hydraulically by a mechatronics system (effectively a combination of mechanics and electronics).
In a traditional automatic the drive from the engine is taken to the gearbox via a torque converter whereas in a manual this is taken care of by the clutch. While a torque converter offers smooth gear changes it uses a thick hydraulic fluid which is less efficient than a clutch-based system.
The advantage of the DSG over a conventional automatic is that the gearbox that’s not currently in use is able to work out which gear you’re likely to want next and will prepare it for use. This allows for both smooth and fast gear changes. In order to do this, an electronic control unit for the transmission uses information such as engine speed, road speed, accelerator position and driving mode to select the optimum gear and to determine the ideal shift point. Each change can be accomplished in less than four-hundredths of a second, although some manufacturers’ systems are faster than others.
Cost of DSG gearboxes
While some cars can only be ordered with a DSG gearbox, this type of transmission is generally offered as a cost option. This cost obviously varies between different car companies but, as an example, Volkswagen charges around £1,400 extra for a DSG gearbox on a Volkswagen Golf hatchback.
In terms of performance and economy there’s not a huge amount to choose between and manual and DSG transmissions. DSGs do tend to be slightly heavier than their manual counterparts and this can lead to cars with them being slightly less fuel efficient.
If we use the example of the Golf again, the 148bhp 1.5-litre TSI petrol model with a manual gearbox returns between 44.8 and 47.9mpg under the new WLTP testing guidelines while the same car with a DSG returns between 42.8 and 44.1mpg. Their CO2 emissions outputs are nigh-on identical at 119g/km and 118g/km respectively. Their 0-62mph times are identical, too.
Driving a DSG-equipped car
For many drivers our increasingly congested roads means that a car with a manual gearbox is becoming increasingly unappealing, especially if you have to contend with a lot of stop start traffic where the constant use of the clutch can become tiring and annoying.
In operation a car with a DSG is effectively the same as a traditional automatic, even if what’s going on with the mechanical items is very different. Like an automatic a DSG-equipped car has just two pedals, the accelerator and the brake, and like a traditional automatic a DSG car is likely to have Park, Reverse, Neutral and Drive settings and you generally need to have your foot on the brake to move between these various drive modes.
When manoeuvring at low speed most DSG cars act in the same manner as an automatic in that they will creep forward or back when in Drive or Reverse when you remove your foot from the brake pedal. This makes it easier to accomplish tasks such as moving the car in and out of parking spaces.
Most modern DSG-equipped cars also come with what is termed a manual mode which allows you to override the computer controlling the car’s gearchanges. You may want to select a lower gear for climbing or descending steep hills, or for when you want to overtake, and this is generally done by shifting the gear lever to the left and moving the lever forwards or backwards to select the gear you want. In some cars this can also be done via paddles mounted behind the steering wheel with plus and minus buttons for changing up and down respectively. Not all cars follow the exact same pattern with their controls though, so it pays to make yourself familiar with how the system works if you’re unfamiliar with the car.
What are the problems with DSG gearboxes?
No mechanical system is 100 per cent bulletproof, but as manufacturers increasingly use DSG transmissions it would appear that their failure rate is very low. On some older models owners have reported faults with DSGs such as noisy bearings or juddering from the transmission but these are generally few and far between.
As DSGs are fully automated they are actually far less open to abuse than a traditional manual. In a normal manual the gears could be ‘crunched’ by an unsympathetic driver or the clutch could wear out prematurely if not operated correctly.
Which companies offer DSG gearboxes?
The Direct Shift Gearbox entered the mainstream in Volkswagen Group cars and are now widely used in VWs, Audis, SEATs, and Skodas. Other manufacturers also offer DSG-style transmissions but many of them use different names. BMW uses them in some models, generally its high performance cars, where they’re know as DCT, while Porsche uses the PDK acronym and in Fords they’re know as Powershift gearboxes.
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