How does a DSG work?
Born out of motorsport, the Direct Shift Gearbox (DSG) is in widespread use in everything from Volkswagen to Audi, Skoda, Porsche, and even Hyundai. Here’s how a DSG works.
ANYONE WHO’S considered buying either a Volkswagen or Skoda, or even some Hyundai models will have come across the term DSG, or DCT although both refer to the same thing; a dual-clutch gearbox/transmission. This is a newer type of automatic transmission that had its birth in motorsport via Porsche, but it was Volkswagen that made DSG a household name.
Like a conventional automatic transmission, a DSG uses fixed gear ratios to ensure an engine is running at its most efficient rpm (revolutions per minute) depending on the speed. But, unlike that ‘normal’ automatic, a DSG effectively mates two gear sets (one with odd gears – 1-3-5 and the other with even gears – 2-4-6) and two clutches to allow for pre-selection of gears and a seamless swap between them.
It offers a half-way between the ease of driving you get form a normal automatic transmission (because there’s only a brake and accelerator pedals) and the ‘control’ of a manual transmission just without the physical action of depressing a clutch. And because of how a DSG works there’s no loss of power or momentum during up shifts thanks to the pre-selection of the next gear in the other gear set, and its manually controlled shift is much faster than a driver shifting gear in a conventional manual.
All DSGs allow for both manual and automatic modes. With gear selection, up or down, happening in around 8 milliseconds most drivers will be virtually unaware of what’s going on. And, despite what your mates might tell you, most DSGs can skip gears from, say, sixth straight to fourth in auto (or D for Drive) mode or by a double-pull of the shift paddles.
A DSG is also known as… and who else uses one?
Following in the footsteps of the VW Group, many car makers have introduced dual-clutch gearboxes; often hoping for improved fuel efficiency because of the reduced energy loss in transferring power to the wheels compared with a traditional torque-convertor automatic transmission. Other brands and names include:
- BMW – DCT;
- Citroen and Peugeot – DCS;
- Ford and Volvo – Powershift;
- Mitsubishi – SST;
- Porsche – PDK; and
- Alfa-Romeo – TCT.
Other notables include the Lamborghini Huracan, Nissan GT-R and Ferrari California which run a dual-clutch transmission.
This article explain how a DSG works and why driving a car with one isn’t like driving a normal automatic.
What is a DSG, and how does it work?
A DSG, or dual-clutch gearbox is, in simplistic terms, a manual transmission with two clutch packs to control odd and even gears. Inside the car the driver only sees a brake and a throttle with the gear selection via the dual clutches, one controlling odd gears (one, three, five and reverse, for example) and the other one controlling even gears (two, four, and six).
The gear change is based on engine speed, wheel speed, brake application, throttle position and hydraulic actuators which, depending on the information gleaned by the sensors, will be commanded to select either a higher or lower gear.
If you pull apart a dual-clutch transmission, you’ll see the ‘drive’ shafts are sleeved, meaning that the shaft that holds one set of gears (odd or even) sits inside the other one, and that once together it ends up looking like a single drive shaft; almost like you’ve upturned two glasses and stacked one on top of the other. Almost, but you get the idea. Similarly, one of the clutches sits inside the other, the inner clutch activates the outer drive shaft, and the outer clutch activates the inner drive shaft.
Just like a manual or automatic transmission, a DSG has different sized gears, with the largest gear providing drive to the lowest gear, so, first gear. As Robert’s explanation of an automatic transmission describes HERE, the big gear in the transmission meshes with a smaller gear connected to the engine and so no matter how hard it’s revved the car will only drive slowly.
There are two gear selectors which straddle the gear sets and one will control gears going up, and the other one controls the gears going down. These selectors have magnetic position sensors on them to tell the ‘mechatronics unit’ in VW speak what gear has been selected, with pistons controlling the selectors’ movement.
You’ll often read about how a DSG offers a near seamless shift and virtually no let-up in drive as the transmission shifts from one gear to the next. And that’s because when you switch from, say, second to third gear, there’s a momentary overlap meaning that as third gear kicks in, second gear is still providing a smidgen of drive until third gear has fully engaged.
So, how does the transmission’s computer know I need a higher or lower gear? Simple-ish. Basically, it’ll determine if the throttle is buried in the carpet and the revs are rising that a higher gear is needed and so the ‘mechatronics’ unit will go ahead and pre-select the higher gear. Similarly, if it determines that revs are dropping and the car is slowing down, it’ll go ahead and pre-select a lower gear.
What are the advantages of a DSG?
A DSG tends to weigh less than a conventional automatic transmission because it doesn’t have a torque convertor, but is usually slightly heavier than a manual transmission, because of the additional weight from the mechatronics unit.
Because of the way it works, with ‘the next’ gear always pre-selected, DSG transmissions offer uninterrupted power delivery as you accelerate (similar to a CVT but with a greater torque capability) and a quick response time as well as manual control. And, no matter what your mate might say, there’s no way he/she can shift his/her manual faster than a DSG, and torque-convertor automatics, no matter how clever, are also slower to shift than a DSG.
And, because of this pre-selection of gears, a DSG tends to be more fuel efficient as there’s no wasted power losses as in a traditional automatic with torque convertor. In brief, a torque convertor on a traditional automatic transmission does the job of a clutch on both a manual transmission and DSG; it’s a fluid coupling that allows the engine to spin independently (mostly) of the transmission. And so it’s because an automatic-equipped car is never fully disconnected, like in a manual or DSG that you get improved fuel efficiency from manual and DSG cars.
What are the disadvantages of a DSG?
The most common complaint (or disadvantage) is that a DSG can sometimes feel jerky or that there’s lag between shifts in certain situations. These examples tend to be heavily weighted towards certain scenarios, like when you’re accelerating up to highway speed from an on ramp go to merge but have to back off because a car has moved into your lane… in that situation the transmission would have determined you’re backing off the throttle and so pre-selected a lower gear. Then, moments later, say, the car that moved into your lane, sees you and quickly swerves back into the other lane, allowing you to floor it again to get back up to highway speed, but because the transmission has pre-selected a lower gear it’ll take a moment to catch up with the changed situation and select a higher gear instead.
The other one is from a standing start where people often say a DSG can jerk as it takes off. This is simply because, to preserve the clutches in the transmission, the mechatronics unit will disengage both clutches when the vehicle is stationary. So, just like shifting from neutral into first gear with a conventional manual transmission car there can be a slight jerk as the clutches on the DSG reengage.
DSGs also aren’t the best choice for off-roaders because of the heat build-up in the clutch packs when inching over rough terrain, because the clutch is never fully engaged. For an example, read this test of the Suzuki Vitara.
How to drive a car with a DSG
From a driving perspective, using a DSG is just like using a traditional automatic transmission in that you slot it into D for Drive and then, well, drive off. It’ll change gears for you while you’re driving, allow you to select S for Sport where the system will hold onto gears for longer and even allow manual control via paddleshifts or the shifter itself. All the while you won’t have to worry about a clutch pedal, just brake and throttle. But, there are a few things to bear in mind when driving a car with a DSG…
The key things to remember are around taking off from a standing start, driving slowly up a hill or inching forwards in traffic. All these things can cause wear and tear on the clutch (usually the one engaging first gear) in the transmission, whereas a conventional automatic has a torque convertor which is much better able to handle slow speed crawling.
What you need to remember when driving a DSG is that when you’re moving from a standing start the clutch will have disengaged and placed the transmission into neutral. Then, as you move from the brake to the throttle the transmission is still in neutral and will then engage once the throttle has been applied and, so, just like you would in a conventional manual you can get a slight jerk as first gear engages.
Another issue is when in stop-start traffic and you’re tending to ride the brakes as you inch forwards. The problem here is that because there’s very little speed, the first-gear clutch is never going to be fully engaged and so will be slipping and while the transmission is robust enough to handle these situations, it’s never a great idea to overly stress your car. So, as the owner of a car with a DSG, my advice is to let a gap build ahead of you and then step off the brake and onto the throttle and drive to the next stop, rather than constantly inching forward.
And the same goes for when you’re inching up a hill slowly. Indeed, I have just this situation at home in my Skoda Octavia (which has a six-speed DSG) where if I’m in a rush and jump from the brake onto the throttle the car can jerk as first gear is grabbed. Similarly, if I’m telling off the kids on the school run and just inching up towards the intersection then I’m causing an unnecessary buildup of heat in the clutch as its slipping.
What about if I want to get sporty? No problem, that’s one of the strengths of a DSG in that because a gear is always pre-selected (either up or down based on the driving inputs the system’s reacting to) there’s a near seamless pouring on of power and gear shifting. But, ham-fisted driving can confuse a DSG and cause issues with the way it behaves.
For instance, when you’re shifting gears manually, either via the shifter or flappy paddles if your car has them you don’t want to be touching the brake and shifting up a gear, or keep your foot flat on the throttle and downshift. All this does is confuse the car into grabbing the opposite gear to the one you might want. See, if you’re braking the car will think you’re slowing down and so grab a lower gear, but because you’re trying to shift up, there’ll be a moment’s hesitation while the transmission’s computer determines what you want it to do. And the same is true when you’re accelerating and trying to downshift.
So, while a DSG might seem to be the same as an automatic gearbox because it only has two pedals, it isn’t, and needs to be driven with some car and, essentially, understanding of how it operates to avoid becoming frustrated by the way it behaves in some situations. Bear this in mind the next time you’re reading a motoring writer’s review and they complain about the behavior of a DSG, it’s most likely because they don’t fully understand how the thing works.
History of the dual-clutch transmission (the short version)
Seems odd that it was Porsche that first developed the semi-automatic transmission in the late 1960s. Yep, the company synonymous with manual transmissions offered an automated manual transmission (called the Sportomatic) on the 911 from 1968, which had no clutch pedal (the clutch was vacuum actuated when the gear shifter was moved) and a torque convertor to keep it from stalling while idling. Porsche called it, and take a big breath, the Porsche DoppelKupplang, or PDK for short.
This clever transmission failed to take off and Porsche eventually shelved it in the 1970s. But behind the scenes the boffins kept tinkering with it, throwing it into the 956/962 endurance race car in the 1980s. See, the Porsche engineers saw that there dual-clutch transmission could shift gears quicker than a human and thus make for faster lap times. It could, but Porsche decided not to proceed with the technology, sticking with manual transmission.
Audi, however, took up the idea, adapted it and then dropped its dual-clutch transmission into a Group B rally car. It was a five-speed unit (still referred to as a PDK) fitted to an Audi Quattro S1 E2 driven by legendary rallyist, Walter Rohrl who said: “I could change gear at full throttle, without any decrease in pulling power or any falling off of turbo boost”. And that there was the reason the DSG existed and flourished.