Dumping the DSG?

[Ryeman]

The rotating mass changes speed in whatever time it takes — as does the flipping of an electronic switch.
Whilst not necessarily the same, both are virtually imperceptible in terms of human comprehension, and moving the tacho needle itsalf also takes time.

Stuff happens is not an answer.
The energy required is not explained anywhere that I can find.

[Umai Naa!!]

Where do you want to start?

At how the tacho motor is slow to respond?

[Ryeman]

Where do you want to start?

At how the tacho motor is slow to respond?

What has a tacho needle got to do with the spectacular change in rotational mass which we are supposed to believe is accurately depicted by it?.

[Idle]

Very little, I should think.

The time — 6ms or whatever the figure is to swap a gear, would be the time take to activate a solenoid (possibly more than one, I've not seen the internals of a DSG and mechatronics,) compress one spring and release the other.

The time to dissipate the kinetic energy should be immaterial — the just-released clutch and its associated shaft could just spin down, but probably has a brake.

[Martin]

I still want to know how the rotating mass of crank, rods and pistons can be moved 500rpm in 6 milliseconds (a fifth of a blink of the eye) unless the tacho is fibbing of course.

Fair question

1) The tacho is most certainly lying
It's damped to appear nice and smooth for consumers
The actual engine RPM changes are much faster

2) Cars with DSG and sequential shifts all run a much lighter flywheel - to reduce inertia and allow faster changes
(faster changes in RPM hence faster gear changes)

3) Cars with DSG and sequential shifts all have an ECU managing the engine,
The ECU cuts the power HARD for a precise period of time - this significantly slows the engine inertia
(if you've ever hit the limiter on a track you know what I mean)

On turbo cars this presents special/additional problems,
Closing the throttle on a turbo engine at 6,500rpm causes a massive pressure spike in the inlet manifold
The ECU triggers the diverter valve to deal with this

4) Track cars with sequential shifts have narrow power bands and close ratio gearboxes
The RPM difference from gear to gear is not huge

Have you ever watched V8 supercars on TV?
Seen them "flat-shifting" - pulling on the sequential gear level at full throttle and not using the clutch?
The gear lever has a stress sensor to detect flex - it triggers the ECU to cut the engine for the gear change
These changes are lightening fast against a 5 litre V8 engine - which manages to change inertia very quickly
If a 5 litre V8 can change inertia that fast then a 2 litre 4 cylinder can also
It's actually been around for 15 years or so (in racing)

In summary, engines are able to change RPM much faster than we expect/anticipate
But it's not the same as blipping the throttle in neutral and waiting for the RPM to decrease,
the ECU cuts the engine hard to make it slow down - then there are other factors (flywheel, etc)

I would also add - don't read too much into VW's 8ms claim which is faster than Ferrari
Never the less, DSG changes are very fast as we all know
(I love it when a Commodore has a go at me, as soon as he puts the clutch down it's all over!)

[stefcio007]

I'm currently driving a car with a Getrag 6DCT450 (wet) and it shifts nicely and very quick, downshifting goes nicely too. Not VW obviously.
The 6DCT450 is used by Ford, Volvo, Chrysler, Dodge.
On the other hand the DCT250 is riddled with problems.

Are the high torque rated DCTs generally more reliable?

[Snail Style]

Timing pull reduces the RPM quickly, the same thing that gives your DSG the fart sounds on shift.

[Ryeman]

I didn't even add the flywheel to the rotating mass equation either......still beats me.

[Umai Naa!!]

It's a dual-mass flywheel (just like a manual!) too, if that helps you at all.

[Ryeman]

I WOULD understand if there were two crankshafts each with their own rods and pistons but a change of 500rpm in six thousandths of a second remains a mystery to me.