DSG wet vs. dry clutch...

[dasjetta2.0t]

With the recent announcement that Ford will begin using a 6 speed, dual clutch "DSG" style transmission with a DRY clutch I started thinking that maybe the wet clutches are not a good idea. VW's own 7 speed is based on DRY clutch technology which leads me to believe that eventually the 6 speed wet clutch version will be phased out. According to Ford, the 6 speed is a sealed unit requiring no expensive maintenance. I'm starting to feel like a lab rat for VW helping them to perfect the technology. Ford's unit is manufactured by Borg Warner which I believe is the same supplier to VW for the 7 speed unit and probably the 6 speed too.

Any thoughts?

[racerbunny24]

i've got a thought

**** FORD

but to answer your question it seems as though (from an uninformed ousiders perspective) that in order to show advancement, vw may have prematurely released its wet Dsg system, with later intent on perfecting. yes sort of like testing with people's 26-38 k property.

but i havent heard from the designers of the wet clutch version. maybe it needs more maintenence but costs less for/less laible for a need to replace?

[curvedinfinity]

"Wet" means the clutch is immersed in oil. The oil helps cool the parts, but adds more friction. Basically, the dry ones are more efficient, but more technology went into making them.

That said, despite our first generation DSGs requiring regular oil changes, they are still great pieces of hardware.

[Gandalf]

FIRST, lets get the facts straight.

The recent announcement is no great surprise - it was announced by getrag over 6 months ago:
http://www.getrag.de/media/0000001060.pdf

And here is the transmission in question - the 6DCT250
http://www.getrag.de/en/231

The dry clutches in the DQ200 (VW 7-speed dry DCT) are made by LUK, not BW. I don't recall who is making the Getrag (dry) clutches.

The reason for the switch to dry DCT's is purely for economy. The DQ250 is simply not a very efficient transmission - sure it's better than a conventional slush box, but it is still way off a manual. The dry DCT's are as described - likely to be just as efficient as a manual, potentially more if the ratios are optimised, potentially slightly less if the churning losses of that many more gears / shafts is truly included...

Why are wet DCT's good? Simply put, a whole lot of energy can be absorbed through a wet clutch. Note that the dry DCT's have very short 1st gear ratios compared to wet DCT's. This is so they can get the thing moving (putting little energy into the clutch in the process), then quickly shift to second - and one reason why the VW has got 7-speeds, to get more ratio coverage.

Wet DCT's - contrary to what many say / believe - are not cooled by the oil 'dipping' but by cooling oil being fed into the centre of the clutch, where it is flung outwards by centrifugal force out through the plates (which have grooves in them), cooling the plates in the process. There are grooves in the outer diameters of the clutch baskets to let the oil out. This oil also acts as a lubricant - meaning it is always switched on, the flow is merely modulated depending on demand (i.e. the more energy the clutch is absorbing, the greater the flow).

Co-incidentally, the DQ250 oil fill level is actually just over the outer clutch basket - it is dipping by less than 1" however. This though, is one of the reasons for the low efficiency as the clutch is always churning.
Other reasons for the low efficiency, are the fixed displacement pump, meaning the same pump has to provide both low through high flow for cooling (at low pressure), and low through high pressure (at low flow) for clutch clamping & gear engagement. In short - you can't tune / size the pump to deliver both high pressure and high flow, and expect it to be truly optimal at cruise.

Another efficiency issue is the slip-ring seals that deliver oil to the clutches. The seals in the DQ250 are not the most efficient.
The DQ250 clutch cooling is also fixed - i.e. you can't just cool one clutch, you have to cool both as they are a concentric design, which means you need to provide a little more cooling flow than would be really needed as well.

As you can see - there are two clear options - either provide a more efficient clutch and hydraulic system, or switch to a dry clutch, and lose the performance capability.

By choosing the 6DCT250, Ford has clearly indicated it iss targeting small cars with what could politely be termed 'average' engines. Nothing over 200lb ft, and potentially slightly poorer performance than a modern 6speed auto. But, they will be dammed efficient, and a good match for the sub 2.0 litre eco-boost engines we keep hearing about.

Finally - The DQ200 was initially developed - I understand - at a similar date to the DQ250 - like approaching a decade ago... It was quickly realised that getting the launch quality of a dry-plate system right was very difficult, to the extent they left it alone, and concentrated on the wet-clutch DCT. A few years later, processors had improved, experience had been gained, but it was (is) still HUGELY more difficult to configure a dry-clutch launch than a wet - two reasons being that the dry clutch locks up completely (the wet never does, hence you always have controlled 'microslip to monitor so you know the characteristics at all times), and secondly, the dry clutch friction co-efficiency changes with temperature - something that is not monitored, unlike a wet system which has temp sensors monitoring the fluid leaving the clutch.

So don't be afraid of dry DCT's taking over, but do accept that the DQ250 will be dropped at some point in the future, replaced by much more efficient wet DCT's, which will do everything the DQ250 does, but a lot better as well.



Modified by Gandalf at 11:16 AM 1-30-2009

[cmosentine]

Damn Gandalf, you either designed this thing or talk a good game. Excellent explanation.

[wannagofast]

Quote, originally posted by cmosentine »

Damn Gandalf, you either designed this thing or talk a good game. Excellent explanation.

X2

[Gandalf]

You're welcome...

No I didn't design it - but I talk/walk DCT all day long

[wwwbecker]

Gandalf, one side point you didn't mention....as a 7spd, the DQ200 box cost as much as the 6spd DQ250. Also, if a new version of the DQ250 was designed today with a smaller pump (with new friction clutches that require less lube flow & lower leakage solenoids) it would probably be more efficient than the current DQ200....and it would definitely be if it was a seven speed.

[karmic_flop]

In my experience the wet clutch DSG is a much smoother drive than the dry clutch. I have a Golf 118TSI 7spd (dry clutch) DSG, the gearbox behaves like a learner who's still getting the hang of using the clutch. The time between accelerator changes and engine/gearbox reaction can be anywhere between instant and 3 seconds... and 3 seconds is an eternity when you're driving a car. Then it sometimes drops the clutch from a standing start for no apparent reason.

The car spent a lot of time in the workshop last year getting a replacement gearbox, water pumps etc. Between the dealerships loan car system and the VW track day I've had a pretty good sample of the VW range. The wet plate gearboxes are much better to drive. I'd gladly spend the extra 7% fuel consumption for the smooth drive of the wet clutch DSG and not wearing 2 gearboxes out (rattling odd gear shaft) in 12 months.

Then there's the full time job of getting VW to care about your problems... ... but that's a matter for another thread.

[Issam Abed]

Gandalf, one side point you didn't mention....as a 7spd, the DQ200 box cost as much as the 6spd DQ250.

No where close.
think of the 02E has an 02Q but with a "computer" for shifting. The gears are of similar diameter and construction.
The 0AM (7-Speed Dry clutch DSG) or more like an LA7 VW Gearbox than anything.

Gandalf sounds like someone that develops clutches for the 02E. Say it aint so Gandalf?