Some interesting points:

The TC Automatic in some comparisons with EPA ratings, potentially gets better mileage.

The fluid coupling has some sort of limitation I once read of somewhere of an engine turning 9000rpm or so.

You can also load the torque converter (brake torque) and it has more flexibility for launching. It also can be aftermarket tunable for firmer shifts, a DCT not so much.

A torque converter swap also offers acceleration and response benefits. A DCT doesn't have that capability/ability.

It is unsuitable for road circuit racing.

Depending upon the gear selector, you can skip gears upshifting, a DCT not so much?

 

Interesting..

In general terms a TC functions as "clutch" i.e. transfer of torque coupling engine to transmission gearing.. The actual gearing is applied subsequent TC.. So I would imagine you could design any kind of gear box (non-sequential etc) and mate it with a TC..

In a DCT, the design associates each clutch with a gearbox (odd and even) with a clutch and gears creates a certain limitation in making it sequential, but you can do random, it just may take a little longer.. It is sort of like having two parallel transmissions.. That said, it takes energy to turn two sets of gears so I guess that might explain how a TC that completely locks could be more efficient than a DCT.

But I could definitely see how a TC might be less advantageous at high RPMs, if for no other reasons the fluid movement inertia and physics would become a problem when quickly changing RPMs over a high range.

When you say unsuitable for road circuit racing, are you referring to the TC? Why is that..? the outward inertia of the turns affecting the TC characteristics?

 

Does this not apply to gear bottons in the Multifunction steering wheels? Tried to do it but never dropped to neutral !!!

 

I can't find any reference in the manuals to shifting into N, on the Malfunction wheel. Only with Paddles. So I suspect you can't.. mainly because the overall operational paradigm with paddles is different than with buttons.

With paddles, pulling both is not a function you would ever do when shifting (other than wanting to go to N).

But on the Malfunction, it is a forward or back (in porsche vernacular) on either or both of the L or R buttons. So there is no real way they could have defined the use the buttons that would allow it.

One of the reasons I didn't like the Malfunction wheel was that I didn't really like the button usage. I would have preferred it to be a L(-), R(+) arrangement just like the paddles. Using the buttons forward/back and being an "or" function I guess could be intuitive, I just always felt like I was stabbing at them.

But then again, up until getting a PDK, I've always only had manual trans so maybe this is a standard way of implementing steering wheel shifting.

 

This is a good discussion, thank you.

A modern TC has a "lock-up" feature that makes it quite efficient for steady cruising speeds. A comparison could possibly be made on a dyno assuming the gear ratios are the same for a given DCT or manual car. The variance could be so small that it may take a more exact type of measurement. For instance, 1 mpg over 30 or 300 miles?

A TC isn't suitable for road racing due to heat management, wear & tear, weight, complexity and possibly other reasons I wish I knew more about to be more specific.

I suspect the sequential manual is by far the most popular due to historically reliable, lighter in weight and least complex, being purely mechanical.

 

Yes, it would be interesting to understand the broad technical nuances of each.. and that would probably give a lot of clues as to the preferred use of one technology (i.e TC vs DCT) over the other. I suspect the efficiency of either technology (i.e. mpg or let's call it HP losses) is probably fairly small and it is related more to the suitable to operational characteristics that are more important. (eg. size, high RPM, ability to change RPM quickly, racing environment).. I would think the mechanics of AWD in a C4 would be much more significant than the difference in a DCT vs TC>

One other aspect that I'm guessing figures into this, is indeed the physical size. Which when you consider a Mid or rear engine design with a transaxle is obviously important than with a front engine/rear wheel drive where a larger bell housing is possible. When I looked at the video showing the tear down of the PDK, I was surprised at just how small the clutch mechanisms were (actually both of them which were concentric).

That entire clutch mechanism looked pretty compact as compared to the big donut looking torque converters that I'm used to (admittedly my experience was with older V8's but even at that it is a comparable HP rated engine to a 400HP Boxer engine). But I suspect TC's can be made smaller. But a TC is always going to need a larger mass simply because they require fluid/oil as a method of transferring engine power to the drive/axle.

The other aspect that I think may play into this is I believe the DCT that is used for Porsche PDK shares a lot of commonality with it's M7 counterpart. So I suspect that drives a lot of the engineering oriented decisions in that regard.

 

An interesting note is the excuse given for the C7 not having a DCT is there wasn't room for one. However, in the 911, McLaren, Ferrari, Cayman there IS room.

I wonder if the PDK is using the same clutch technology that the CGT introduced, allowing for the small size?

 

Yeah, that makes little sense.. I've read GM has invested in dry plate clutches but those are only suitable for lower torque engines. So who knows it may end up just being the way they invest their money. In reality Corvettes are really only a small niche for GM.

The CGTs use a PCCC (ceramic) which is a two plate technology. Looks to be different from what DCTs in the 911's which is multi-ring wet..

 

Lol. I find it hard to believe that Porsches awesome Double Clutch transmission would be stressed when left in D at a stop. SMH.

 

I don't own a PDK, but I understand how it works and though I'm not an automatic fan myself, I really admire the job Porsche did on it. It all comes down to sensors and programing. If you're worried about clutch drag, test it for yourself like this:

1) Stop the car on a level surface.
2) Put the transmission in Neutral.
3) Remove your foot from the brake pedal. The car should not move.
4) With your foot off the brake, pull the "plus" paddle (select 1st gear).

Does the car begin to move? My guess is that it won't. If it doesn't, there is no clutch drag when stopped in Drive. If you now apply the brake, and then release it, and the car begins to move, then the PDK is sensing a change in either brake pedal travel or brake fluid pressure and it then seamlessly and smoothly begins to engage the drive clutch. This would be why it would seem like the clutch was dragging...because Porsche's programing is so good that there's no appreciable delay between brake release and clutch engagement. Other manufacturers have attempted to accomplish this seamless operation with their DCTs, but often with less success than Porsche has achieved.

 

Yeah, I don't believe it is programming. The clutch is still engaged when you are in 1st/D. You can physically feel the "lunk" when you pull it out of 1st/D into N with the paddles. And if it was programming controlling the clutch in this case, then as you reapplied the brake you would feel that "lunk" as well.

With PDK, when you are sitting with your foot on the brake, you can almost feel the gradual pull forward as you gradually reduce brake pressure. Hard to describe.. but it is a pure analog/mechanical feel that isn't related to software control and feedback.

The clutch is simply designed to mechanically slip, drag, whatever you want to call it. The clutch is based on a multi-rig mechanism to transfer torque.

What you are suggesting (i.e. programming sensing brake etc to engage the clutch) is certainly in the realm of technical possibilities. But probably more complex than it would need to be as it is a relatively straightforward thing mechanically to accomplish. And as I mentioned, you can almost feel the pull forward as you are gradually reducing brake pressure, so I'm pretty sure clutch engagement it isn't being controlled by sensors and software.

From a practical engineering standpoint, having sensors and software controlling this I suspect wouldn't be a great technical solution, because a sensor failure would easily lead to a situation where the clutch was fully engaged at stop (which would immediately bog the engine) or keep the clutch from engaging which would leave you stuck.. Its one of those things when you design it, you have to accommodate how things will work with partial failures.

Part of the problem is that most people have a predefined assumption of what a clutch is that is based on that of a traditional manual transmission. So the idea of being in between and having the clutch slip seems like it would be wearing/burning something up. And of course with a manual transmission clutch that is the case.

Clutches can be designed differently such that in the case of sitting, it simply slips (doesn't transfer torque) and it isn't necessarily wearing.. That's sort of the general principal of a mult-ring wet clutch. Some things don't need software and can just be mechanical (or mostly mechanical), lol...

That isn't to say there isn't a lot of programming and sensors involved in PDK.. It uses all kind of information to predict and control the shift changes. At times I'd swear it was reading my mind. For example going down a hill, it seems to know that I want to hold my speed and downshifts.. I suspect it does that by monitoring the increase in speed, reduction in throttle and application of brakes. Its just in the case of idle/stop I don't think that is the case.

 

After looking at the drawing of the pdk clutch I am confused how it works. Is it engaged by centrifugal force. With the dual clutches both on the same plane one sitting inside the other.

 

No it is not centrifical force, it is a slip ring type of design..

I havent looked at the details of the pdk clutch yet, however this type needs some kind of force applied to pinch the slip rings together, which as the pinch force increases gradually transfers torque from the input to the output..

That force is could be magnetic, or generated by hydrolic pressure or manually applied.. depending on the transmission type..

My guess for pdk it is electro magnetically applied somehow based on engine speed..