Gearboxes: Manual or Automatic?

[BMW Power]

Originally Posted by braaf

A bit off topic. Martin help me out here please. The F430 Scud has a SMG and changes gears lightning quick (if not as quick as DCT).

Do you thhink if BMW stuck with SMG it they could have made it faster (possibly faster than DCT) and hence more comfortable to silence the crits. Don't get me wrong I like DCT but prefer SMG.

SMG's biggest criticism/complaint was its pump-action shotgun like gear changes (esp the clunk from 1st gear to 2nd gear on full throttle). Even though I personally loved it, most people could never come to terms with it. Another big problem with SMG was reliability.

[braaf]

Originally Posted by BMW Power

SMG's biggest criticism/complaint was its pump-action shotgun like gear changes (esp the clunk from 1st gear to 2nd gear on full throttle).

Agree,I could be mistaken but it think its cos of the clutch. As far as reliability goes I believe it's got to do with some kind of pump that needed replacing on most gearboxes, a problem that BMW sorted out.

[martinbo]

Originally Posted by braaf

A bit off topic. Martin help me out here please. The F430 Scud has a SMG and changes gears lightning quick (if not as quick as DCT).

Do you thhink if BMW stuck with SMG it they could have made it faster (possibly faster than DCT) and hence more comfortable to silence the crits. Don't get me wrong I like DCT but prefer SMG.

braaf, here's my take on the whole SMG / DCT discussion.

Sequential manual gearboxes as we all know are mechanically close to a regular manual box; this means one clutch, constant mesh gears, selector forks and synchromeshes. In addition to all of this, electro-hydraulic actuation was integrated into the gearbox and tasked with the duties of disengaging/engaging the clutch and gear selector mechanisms. This technology found its first real application in motorsport where the quicker shift times and reduced driver workload made a significant difference in the competitiveness of the whole package. Of course, whilst shift times were an imperative, shift quality wasn't...

The benefits of such technology for road applications became apparent and soon big name motorsport makers like Ferrari and BMW started to adapt the technology for public consumption. What the automated gearbox provided was a means of changing gear more quickly than was possible by human/clutch pedal method but it did not fundamentally alter the mechanical process of changing gear. So, under full-bore acceleration, the process still entailed decoupling of the clutch (as quickly as possible), selection of the next gear (as quickly as possible) and re-engaging of the clutch (as quickly as... aw heck, you get the idea).

Picture a quarter mile drag race or magazine road test where the deliverable is to get the best possible acceleration times... do you think that the racer or road tester's going to worry about their gear shift smoothness? Nope. Rapidity of execution is the order of the day - clunks and lurches are of much less importance. So for all-out acceleration runs, the robotised manual did the job of shaving 10ths off the timed runs. Hence, the clunkiness...
In normal, or even spirited A to B driving, shift speed becomes less important and shift quality more pre-eminent. And in this situation, the skilled driver had something over the automated manual: feel. A good driver softens the moment of clutch and accelerator interaction, doesn't slam the next gear home or sidesteps the clutch and hammers the gas pedal. Smoothness is one of the greatest attributes of a fast, controlled driver.

So, these automated manuals lack the ability to "feel out" this smoothness and so, this had to be programmed in via the gearchange and engine management software. The more sophisticated the programming, the better the interaction between the engine (specifically throttle) and the execution of a gear change (particularly prevalent on down-shifts with the obligatory engine speed matching throttle blip). Now, herein lies the success of the F1 transmission technology in the Scuderia - it comes as a result of immense experience in integrating the engine and transmission management systems with mechanical componentry that is built to a certain standard in order to meet performance needs. This experience can only come from intensive programs in motorsport (F1, GT-series), specialised R&D programs like FXX and a constant road car improvement program which lead to the majority of cars being sold with F1-type transmissions. Pretty much a strategic, core product development program for Ferrari for a decade which yielded results.

BMW's engineers clearly saw the short-term application of robotised manuals (as did Ferrari's) in the light of the new, more sophisticated and better suited to automated shifts, dual clutch gearbox technology. Think of a DCT gearbox as being two robotised manuals superimposed on one another. By having two input shafts, each with its own clutch, each tasked with driving its own set of gears [1,3,5,7 and 2,4,6 respectively] two things are achieved:

1. Under acceleration, the next gear is pre-selected - but still disengaged - in anticipation of being called upon. This dramatically reduces the shift time because the action of engaging selector fork on synchromesh is already done.

2. The interaction of the two clutches with the engine and each other enables a "softening moment" in the gearshift process which can be programmed in to smooth out gearchanges up and down a gear.

So, mechanically-speaking, by having "two" gearboxes working in unison, each being readied before the shift event means that the DCT concept is superior in its shift speed and quality vs a single, automated 'box.

Of course, the proof is in the pudding with none other than Ferrari - the most successful of robotised manual car makers - moving toward a seven speed dual clutch system.

For BMW, even with their expertise, the move to DCT boxes meant that the SMG concept had quickly become obsolete and the pursuit thereof pointless.

[PanterroR]

^

THANK YOU!

[Guibo]

Because the DCT pre-selects the next gear, it will be very tough for any SMG/F1-like transmission to beat the DCT's speed. The advantage of this for DCT is that power is delivered in a near seamless fashion, whereas the SMG still needs to disconnect and re-engage the clutch (at which point the power/wt ratio of the vehicle is momentarily effectively zero). At higher speeds, where the effect of drag and rolling resistance are compounded, the benefits of the DCT should be even greater.
As for the feel, it appears that Porsche have engineered-in some of the brutal characteristics of SMG into its PDK. In "Sport Plus" mode, the shifts are even faster than standard, and there is likely less slipping of the clutch, less dampening effect. This is sometimes what sporting drivers prefer. From R&T's data panel of the Boxster S w/PDK and Z4 w/DKG, one can see the surging effect that Evo mag noticed while testing the GT-R; it appears that each upshift is accompanied by a slight surge in acceleration whereas in a conventional transmission, we expect the opposite. I'm not sure, but I've read elsewhere that this is due to the rotational energy of the engine (at full WOT, remember) as it picks up the next lower gear, much like you can get with SMG at very aggressive settings. (However, with SMG you will not get the pronounced upward step as visible in the PDK graph; there's still the lag when drive is momentarily disconnected).



Notice that the 599, with its very fast shift times, is still leaving some performance on the table; it's clearly losing some time with each shift that it otherwise would not with a DCT.

[martinbo]

As always, nice contributions Guibo...

[Guibo]

Thanks, martinbo.

[braaf]

Martin and Guibo, thanks alot guys. You must forgive me, I'm very intruiged by the engineering that goes into vechicles. One more question. In the graph, are those steps in the 599 graph indicating gear changes?

[martinbo]

^ Pleasure! No worries, though, that's the whole idea of the tech talk forum. We learn from each other.

Yep, exactly, a long first gear run to thereabouts 80 km/h then a shift to second at 120 km/h, 3rd sees a change up at 160 km/h and so on...