Somebody explain DCT "Surge"

[mixja]

Can anybody explain how the DCT transmission manages to increase acceleration momentarily during a gear shift?

You can definitely feel the momentary increased acceleration and you even see it on an accelerometer as shown below on the 2nd to 3rd gear shift:

[southlight]

In short, the time it takes to adapt the revvs to the next gear (= lower rpm at same speed) will be used to create additional torque. The clutch of the higher gear doesn't engage at the ideal point for rev-matching but somewhat earlier. This way BMW tries to reproduce the effect MT drivers could use either when they drop the clutch early during a gear change.


Best regards,
south

[Radiation Joe]

Where are swamp2 and footie when you need them?

[Payroll]

it's basically a powershift - using a bit too much engine speed and dumping the clutch, the effect of which is a swift kick in the ass.

[southlight]

Originally Posted by Payroll

it's basically a powershift

That's the term I was looking for.


Best regards,
south

[swamp2]

Ever driven a SMG equipped M3? They surge like hell.

There are mathematical definitions and then practical ones. I'll give both. Surge is jerk and yes "jerk" is a formal mathematical term. It is the fourth derivative of position with respect to time in other words the first derivative of acceleration, i.e. the rate of change of acceleration.

Practically how you feel surge in the DCT unit is by using S4-S6, it is the strongest in S6. Simply shift at or near WOT. That "thump" you feel in your back right during the shift is surge. It is a temporary, short duration of increasing acceleration. Similarly if you use S1 or S2 you will feel very little surge even at WOT. SMG cars, primarily due to having only a single clutch and moving that clutch very fast had a very strong surge.

As to the cause: There has been an extensive debate about how (and why) BMW has made the DCT surge. I believe the best explanation of how they accomplished it is simply by clutch phasing and timing. Just like when powershifting a traditional manual if you keep the throttle fully or mostly pinned and operate the clutch quickly you will better maintain or even build revs while clutching. Then it is just as if you popped the clutch from a start. All of the stored angular momentum of the engine and flywheel get dumped to the drivetrain and viola - a surge. So the DCT unit, by managing the throttle, speed and timing of the clutches can create surge.

The debates around why is basically this:

1. BMW implemented surge ONLY to make it feel more sporty and feel more like the SMG.
2. BMW implemented surge to do this above as well as to extract a small extra bit of performance.

[shchow]

With SMG and DCT, you're able to shift without letting up on the throttle. So when the next gear engages, the engine is still revving up.
Unlike a MT when you have to let up on the throttle when changing gears...

[ersin]

Originally Posted by swamp2

Ever driven a SMG equipped M3? They surge like hell.

There are mathematical definitions and then practical ones. I'll give both. Surge is jerk and yes "jerk" is a formal mathematical term. It is the fourth derivative of position with respect to time in other words the first derivative of acceleration, i.e. the rate of change of acceleration.

Actually, the first derivative of acceleration would be the "third" derivative of position. But what's a derivative or two between friends.

Anyway, I think of it like this. When you upshift the speed of the engine must go down in order to match the revs to the speed in the higher gear. In a manual tranny during shifting you lift off the throttle so the revs drop (momentum in the drive train drops because of this), then you shift, re-engage the clutch and the revs hopefully match the new gear. In DCT both clutches are engaged and an upshift just requires going from one to the other without any disengaging and subsequent throttle lifting. During an upshift then the engine will be going faster than what is needed to match the revs to the higher gear. The momentum that would have been lost is now used and felt for that extra "surge" on upshifting. It probably means some clutch slip too. Just my guess.

Cheers.

[Radiation Joe]

Originally Posted by swamp2

...
The debates around why is basically this:

1. BMW implemented surge ONLY to make it feel more sporty and feel more like the SMG.
2. BMW implemented surge to do this above as well as to extract a small extra bit of performance.

I think there is probably another aspect to the surge. If the engineers had simply let the clutch engage as quickly as possible, there would be a significant shock to the driveline with an associated loss of traction. The "surge" is a kind of safety valve with a benefit. It keeps the car from losing traction (and shocking the driveline) as well as using that excess energy to increase acceleration. All of this at the expense of a tad extra clutch wear.

[swamp2]

Originally Posted by ersin

Actually, the first derivative of acceleration would be the "third" derivative of position. But what's a derivative or two between friends.

Thanks, I can differentiate all day it is sometimes the addition I have problems with.

Do note you also have a typo/misstatement above about the DCT clutches. They are never both engaged.

[swamp2]

Originally Posted by shchow

With SMG and DCT, you're able to shift without letting up on the throttle. So when the next gear engages, the engine is still revving up.
Unlike a MT when you have to let up on the throttle when changing gears...

DCT does cut the fuel during shifts to manage torque. You don't need to move the pedal but the computer does the same thing as a person (albeit much much faster). However, you're likely correct that is does less fuel cutting when you get the surge similar to powershifting a manual.

[rsmamg]

this is the issue i was talking about when i asked why my DCT suddenly "jolted" forward during shifts which are most noticeable in S4 and S5 modes.

thanks for the responses guys. this helped clarify for me.

[SenorFunkyPants]

I get the impression that the latest software update has softened the "surge" in S4 and above. Certainly the clutch engagement in S4 and above is not as fierce as previously, which often felt too harsh. Altogether a big improvement such that S4 is now my default gearbox mode choice.

[footie]

I have discussed this at length and frankly I can't be arsed to join into a debate on the subject.

Is it better than with no surge? There is some who have found that zero surge is actually quicker at WOT but surge proves to be quicker below this point.

I maintain that the surge is purely there to mimic SMG and give that connect between driver and the actual shift. My only major complaint about the surge was that it still happened during cornering at times which can cause brief lose of traction when least needed or expected.

P.S.
My XF automatic also gives surge in manual mode so I am still stuck with the bloody thing.

[Steveo]

Originally Posted by SenorFunkyPants

I get the impression that the latest software update has softened the "surge" in S4 and above. Certainly the clutch engagement in S4 and above is not as fierce as previously, which often felt too harsh. Altogether a big improvement such that S4 is now my default gearbox mode choice.

Has the OP carried out the software update as i'm finding that the surge in S4/5 has all but gone

[BPMSport]

That's exactly what I thought last time I drove the DCT model. It basically "throws" you, literally, into the next gear.

Kind of cool. Quite different from the *BANG* the M5 used to give me.

[ersin]

Originally Posted by swamp2

Thanks, I can differentiate all day it is sometimes the addition I have problems with.

No prob. They didn't teach me how to add in college either.

Originally Posted by swamp2

Do note you also have a typo/misstatement above about the DCT clutches. They are never both engaged.

Doh!

[rsmamg]

what i have noticed with my old SMG was the jolt from shifting in SMG seemed to momentarily cut power from the engine then immediately reengate it from say 2nd to 3rd.

however, with the new DCT, i get the feeling that between gears, the car doesnt seem cut power and then jump back into the next gear as with SMG, i feel as if the car leaps forward.

i felt more in control with the SMG shifts, but with DCT the jolt forward makes me feel like i am about to lose control

[swamp2]

Originally Posted by footie

I have discussed this at length and frankly I can't be arsed to join into a debate on the subject.

Is it better than with no surge? There is some who have found that zero surge is actually quicker at WOT but surge proves to be quicker below this point.

I maintain that the surge is purely there to mimic SMG and give that connect between driver and the actual shift. My only major complaint about the surge was that it still happened during cornering at times which can cause brief lose of traction when least needed or expected.

P.S.
My XF automatic also gives surge in manual mode so I am still stuck with the bloody thing.

There is no debate going on here at all. You did notice I was objective enough to provide both my theory/opinion and those contrary to it.

[footie]

Originally Posted by swamp2

There is no debate going on here at all. You did notice I was objective enough to provide both my theory/opinion and those contrary to it.

I did notice that swamp. I just didn't think there was any point continuing our difference of opinion on the merits of the surge, that's all.

Some like it and some don't, some see benefits in it and again some do not. I'm only thankful that BMW provided both parties with settings to suit.

One thing I do miss about the M-DCT is how smooth it was on upshifts, smoother than the ZF auto in my XF.

[mixja]

Originally Posted by footie

I have discussed this at length and frankly I can't be arsed to join into a debate on the subject.

Is it better than with no surge? There is some who have found that zero surge is actually quicker at WOT but surge proves to be quicker below this point.

I maintain that the surge is purely there to mimic SMG and give that connect between driver and the actual shift. My only major complaint about the surge was that it still happened during cornering at times which can cause brief lose of traction when least needed or expected.

P.S.
My XF automatic also gives surge in manual mode so I am still stuck with the bloody thing.

Here's some VBox data from M3 DCT vs M3 SMG (courtesy DLSJ5) for 60-130mph runs. The thing to look at is the black line in each graph which shows acceleration...

Looking at these comparisons, the DCT is able to maintain acceleration (even increase it momentarily) during gear shifts, whilst the SMG has a pronounced dip in acceleration.

So how can this "surge" not be faster then a WOT? And it doesn't seem to mimic the SMG at all...

M3 DCT:


M3 SMG (Supercharged):

[mixja]

Originally Posted by Steveo

Has the OP carried out the software update as i'm finding that the surge in S4/5 has all but gone

Yes I have 2.34.0.0.2 and surge is still there...