New M DCT dual clutch transmission by BMW

[warot]

Thanks EnI.

But my question: here in the example, the car is accelerating and in 3rd, so the gearbox prepares the 4th. It will then swift from 3rd to 4th very quickly, without power interruption.

However, if instead of calling the 4th, the driver calls the 2nd, because there is a corner and he brakes hard... The 2nd is not ready, the gearbox will have to switch to the 2nd instead of only engaging the clutch. Therefore, the gearchange will ba a bit slower, and there will be a little power interruption because the process is not as quick.

Is it right?

And in that case, what happens?

-> the gearbox disengages the clutch immediately after the gear call, and the power is interrupted and the gearchange slower. It means, immediately after the gear call the driver feels the order applied by the gearbox, but the process is slower than usual and there will be power interruption.

-> or the gearbox does not react immediately to the gear call, it means it prepares the gear and only then disengage the clutch. So the gearchange itself won't be longer, but there will be a longer delay between the gearchange order and the effective gearchange?

I'm too lazy to find it but Top Gear tested either a VW or Audi that particularly answers this question. He does the exact same test you are talking about it and it responded very well.

Also, your scenario would give the transmission enough time to change. If you are in 3rd at full throttle but apply the brakes, I'm positive it takes you more time to do that maneuver than for the transmission to engage. These things work at MILLISECONDS...

 

[coolraoul]

Thanks for the tip, I'll search it!

 

[Zafiro]

shifter lights?

I was right, the lights tell you when to shift gear

VIDEOS - BMW E93 M3 Cabrio & DCT Video Gallery

 

[martinbo]

Some of my thoughts on this M-DCT and double clutch gearbox technology discussion.

First and foremost, the M DCT gearbox is not a sequential gearbox. Furthermore it does not feature technology that constitutes a significant mechanical advancement over VW's twin-clutch gearbox. Sure, there are real differences such as the use of wet clutch plates and the longitudinal placement of the transmission but other than that, the twin output shaft, input-shaft-within-another-input-shaft concept is the same as what we've seen from VW/Audi almost five years ago.

Concerning the pre-selection of gears the important thing to take note of is the purpose of preselecting a higher gear during acceleration.

In the purest technical sense, as is the case with any modern synchromesh gearbox all the gears are "preselected" i.e. first gear is directly meshed with the gear on the input shaft and this is the case for 2nd, 3rd, 4th and so on. These gears spin freely on the output shaft until locked to the shaft by a spindly little gear ring called a synchromesh. This is typically done by actuating a selector fork either via a human movement controlling the gearlever or a computerised electro-hydrualic actuator.

The dip in acceleration felt when changing up under acceleration is, obviously, most perceptible in a pukka manual, i.e. driver clutches in, moves gearlever which moves selector fork disengaging synchro in second and engaging synchro in third, driver clutches out and floors accelerator. All of this happens in an automated manual gearbox such as SMG III. It just happens much much quicker (like sending an email compared to a telegraph) and in its haste can deliver big wallops in the process of transition.

In a DSG style gearbox the concepts are the same except that now there are two input shafts and two output shafts. The gear ratios are still in constant mesh and are still actuated in the selector fork / synchromesh method. The reason that the systems preselect a higher gear is to ensure that the power delivery when accelerating is as quick and seamless as possible thus maximising the flow of power in a linear fashion, thereby improving acceleration. The interplay between the clutch on the one input shaft disengaging and the clutch on the other re-engaging with the engine flywheel independently of each other is what makes the whole transition process so smooth.

Down-shifting is different. Modern cars decelerate under braking much faster than they accelerate so the emphasis here is not on the speed of the shift (though of course this is still important) but the quality of the shift. Shifting the selector fork on the pre-selected synchromesh for 4th gear to 2nd gear takes milliseconds - a largely academic event. Here the seamless delivery of power isn't important because the driver is braking and not accelerating. The important process is to match engine speed to the geared speed of the newly selected lower gear - hence the blipping of the throttle to raise engine revs to meet the road speed of the lower gear. This process takes up its own time and co-ordination and is largely down to sophistication of the engine and transmission management software logic.

So in a nutshell, the pre-selection of a higher gear is to maximise the accelerative experience and not to ensure the most rapid downshift possible.

 

[coolraoul]

Okay so you answered my question, the gearbox will more often preselect the higher gear because it is mostly here that speed is required.
Sounds logical.

Awesome post like usual Martinbo, thanks a lot.

 

[martinbo]

^ You're welcome Raoul...

 

[Zafiro]

I just wanna know if the automatic mode is smoother than SMG.

 

[bmer]

I just wanna know if the automatic mode is smoother than SMG.

Of course it is. This is DCT, remember?

 

[klier]

That was pretty much the whole point of developing this whole new gearbox, otherwise SMG would be fine.

 

[S'Pimp]

after n while - back on the top

4 tecnical view

 

[Imhotep Evil]

Some of my thoughts on this M-DCT and double clutch gearbox technology discussion.

First and foremost, the M DCT gearbox is not a sequential gearbox. Furthermore it does not feature technology that constitutes a significant mechanical advancement over VW's twin-clutch gearbox. Sure, there are real differences such as the use of wet clutch plates and the longitudinal placement of the transmission but other than that, the twin output shaft, input-shaft-within-another-input-shaft concept is the same as what we've seen from VW/Audi almost five years ago.

Concerning the pre-selection of gears the important thing to take note of is the purpose of preselecting a higher gear during acceleration.

In the purest technical sense, as is the case with any modern synchromesh gearbox all the gears are "preselected" i.e. first gear is directly meshed with the gear on the input shaft and this is the case for 2nd, 3rd, 4th and so on. These gears spin freely on the output shaft until locked to the shaft by a spindly little gear ring called a synchromesh. This is typically done by actuating a selector fork either via a human movement controlling the gearlever or a computerised electro-hydrualic actuator.

The dip in acceleration felt when changing up under acceleration is, obviously, most perceptible in a pukka manual, i.e. driver clutches in, moves gearlever which moves selector fork disengaging synchro in second and engaging synchro in third, driver clutches out and floors accelerator. All of this happens in an automated manual gearbox such as SMG III. It just happens much much quicker (like sending an email compared to a telegraph) and in its haste can deliver big wallops in the process of transition.

In a DSG style gearbox the concepts are the same except that now there are two input shafts and two output shafts. The gear ratios are still in constant mesh and are still actuated in the selector fork / synchromesh method. The reason that the systems preselect a higher gear is to ensure that the power delivery when accelerating is as quick and seamless as possible thus maximising the flow of power in a linear fashion, thereby improving acceleration. The interplay between the clutch on the one input shaft disengaging and the clutch on the other re-engaging with the engine flywheel independently of each other is what makes the whole transition process so smooth.

Down-shifting is different. Modern cars decelerate under braking much faster than they accelerate so the emphasis here is not on the speed of the shift (though of course this is still important) but the quality of the shift. Shifting the selector fork on the pre-selected synchromesh for 4th gear to 2nd gear takes milliseconds - a largely academic event. Here the seamless delivery of power isn't important because the driver is braking and not accelerating. The important process is to match engine speed to the geared speed of the newly selected lower gear - hence the blipping of the throttle to raise engine revs to meet the road speed of the lower gear. This process takes up its own time and co-ordination and is largely down to sophistication of the engine and transmission management software logic.

So in a nutshell, the pre-selection of a higher gear is to maximise the accelerative experience and not to ensure the most rapid downshift possible.

OK Martin all fine and nice but 2 points to consider:
- the SMGs too had problems with the downshift in gen I and II and only gen. III "nailed it" .
- it isn't VW/Audi's double clutch, it actually 80 years old 1930s technology on witch VW/Audi (well the suppliers really) added some computers and other tweaks to finally make it work.
- I wonder if a 9 speed composite triple-clutch would be the next step forward ?!