Dual Clutch Transmission - resting state?

[Vikas]

Does DCT clutches have same default state like regular clutch? Aka you need to apply force to disengage the clutch. If that is the case, then when the engine is NOT running, both clutch engaged aka transferring power from input shat to its respective output shaft? That can't be right!

I am trying to understand if there is a failure mode in DCT which will cause it to grenade spectacularly if both clutches are engaged.

 

[CKN]

I don't have the answer to your question. However as an observation, as someone who has been looking at the KIA Sorento as a replacement vehicle-their turbo motor with a WET DUAL CLUTCH transmission has been the subject of controversy has to smoothness and reliability. I personally feel that one should not have to think about putting their foot on the gas in a modern automobile-then allow time to creep forward to give the transmission time to engage. Yea-OK.
Some had new transmissions installed due to a major failures.

 

[CST Q]

This is by no means a definitive answer but more of a SWAG. I would think that when the engine is off the transmission is in neutral, both clutches engaged. My 2022 Santa Fe Calligraphy, which has the wet DCT CKN speaks of, will often engage the parking brake even on flat surfaces, since there is no park pawl to hold it still.

Slight highjack... CKN, we love our Santa Fe which has the 2.5T and 8 spd DCT. The shifts are quick, smooth and quiet 99 % of the time. The only time I hear it is when coming to a stop and it goes into 1st/neutral/whatever. at about 5mph. That's with the radio off, windows up etc. Also, when the start/stop is firing back up, there is the slightest slightest of delays before you roll out. I'm kinda OCD about noticing this type of stuff so I may be hyper sensitive.

 

[Cdn17Sport6MT]

I can't offer much help, sorry, but I am keenly interested in knowing more... how dct's contrast with say 8EAT's for the differing circumstances of downshift speed and smoothness, upshift speed, life expectancy, capability of repair (and indeed finding qualified rebuilders), general parts availability, and in-trafffic smoothness. I have prelim ideas on most of these... and I eagerly await comments from more savvy individuals.

 

[ctechbob]

A DCT is going to have the opposite resting state as what you normally think of as a clutch. The DCT clutches are applied by pressure, not released by pressure, so with engine off, both packs will be released. If you think about it, that's the only way it could be, otherwise you'd never be able to start the car once pressure bled off unless you had an auxillary hydraulic pump to pressurize the system.

 

[Vikas]

A DCT is going to have the opposite resting state as what you normally think of as a clutch. The DCT clutches are applied by pressure, not released by pressure, so with engine off, both packs will be released. If you think about it, that's the only way it could be, otherwise you'd never be able to start the car once pressure bled off unless you had an auxillary hydraulic pump to pressurize the system.

Can you explain this? I suspect you must be right or at least you do come across as more knowledgeable here but then you could be faking it
Seriously, the gearbox needs to be in neutral which should be theoretically equivalent to released clutches. So I am not getting the part about never be able to star the car part.

 

[Vikas]

Given current state of processing power, I do not quite understand why DCT can't be better than "average" manual shift driver. I mean it can't beat say Jackie Stewart but you and me? That should be piece of cake. Besides, almost every car these days have all the necessary sensors to anticipate the next move. I mean, it can even detect the red light and instead holding the car by slipping the clutch, it can put in neutral fast than a typical driver and put it back in the gear even faster. It can be programmed to keep the clutch slippage to absolute minimum and it should be able to get the life as good as a good manual shift driver aka over 200K miles. Which I have done in all my manual cars.

 

[CKN]

Given current state of processing power, I do not quite understand why DCT can't be better than "average" manual shift driver. I mean it can't beat say Jackie Stewart but you and me? That should be piece of cake. Besides, almost every car these days have all the necessary sensors to anticipate the next move. I mean, it can even detect the red light and instead holding the car by slipping the clutch, it can put in neutral fast than a typical driver and put it back in the gear even faster. It can be programmed to keep the clutch slippage to absolute minimum and it should be able to get the life as good as a good manual shift driver aka over 200K miles. Which I have done in all my manual cars.

The more I read about the DCTs the more I'm inclined not to be the owner of one.

 

[Vikas]

May be I should just ask Jason from Engineering Explained. He did the best tutorial on DCT

 

[Vikas]

Guess what I found, now all I have to do is to read and understand the following paper!

Engagement and control of synchroniser mechanisms in dual clutch transmissions

https://opus.lib.uts.edu.au/bitstream/10453/18348/1/2011001216.pdf

 

[ctechbob]

Can you explain this? I suspect you must be right or at least you do come across as more knowledgeable here but then you could be faking it
Seriously, the gearbox needs to be in neutral which should be theoretically equivalent to released clutches. So I am not getting the part about never be able to star the car part.

A 'normal' clutch uses spring pressure to engage, so its normal resting state is to be engaged. You apply pedal pressure to release the clutch. You wouldn't be able to have that (nor do they use apply springs) in a DCT. If you had springs that applied the plates, you would need either hydraulic or electric to pull them off in order to start the car. I suppose you could use electric actuators (or hydraulic pump), but that would be another system that you would need in order to apply and release the plates.

Far easier to use the hydraulic fluid you already have pumping around the system to do the work for you. So, unless you have an electric pump to pressurize the system and release the clutches (which would add complexity) then you'll need the engine to be turning in order to pressurize the system (far more simple). In order for that to happen, the normal resting state of the clutches needs to be off, or you won't be able to crank the engine since both sets of clutches would be locked on under spring pressure, so, no spings.

Totally different transmission and system, but the Ford 10R80 does use an electric pump to keep the system pressurized so that the auto stop/start system can have the trans in gear and ready to go upon startup since the engine-driven pump is not spinning. So its not impossible for them to have a separate pump to pressurize the system, but its much easier to just have the engine do the work instead of adding complexity to the system.

No, I'm not some DCT engineer, just speaking from what very little DCT diagrams I've looked at. Most of them appear to be multi-plate hydraulically applied clutches. I could always be very wrong.

 

[Cdn17Sport6MT]

What about lasting-power, reliability? Some will say, "here he goes again... " but yes, a DCT can totally manage clutch wear to a better degree than a manual driver (and they are wet clutches... arguably better... versus a dry clutch) but here is the rub: a DCT transmission has double- or triple cone synchronizers, which are bound to be better, quicker, more long-lasting than those in most manual transmissions, but the mechatronics shifts these synchronizers in a brutally-fast manner (at times... certainly not always). If the driver uses manual control of the transmission (and honestly, also in auto - with an aggressive right foot) and he/she downshifts at high roadspeeds and quite aggressively by way of engine rpm, those synchronizers will wear and become disfunctional... Said mechatronic units single-clutch shift... they do not double-clutch shift, whereas a skilled driver who knows how to accurately double-declutch and if he/she drives exactly the same way as the DCT car... then the manual will outlast the DCT.

Lotsa caveats there, I know.

Driven by grandma, likely the DCT will outlast it's manual counterpart.

Driven moderately, and even spiritedly, the DCT will last adequately, at least.

Seems to me, as well, drive 'em gently 'til the box is warm!

 

[Vikas]

What I am not buying is your assertion that clutches needs to be released to start the vehicle. All you need is the vehicle in neutral. Whether clutch is engaged or NOT is irrelevant if the transmission is in neutral. Some of the real cut away photos of DCT shows those telltale springy fingers.

I do see that if the default state of the clutches is locked and gearbox is NOT in neutral and you tried to "bump start" the vehicle, it will 100% grenade the entire DCT! But this gives too much credence to the automotive designer who could be looking after for their customers and we know for sure those engineers probably high five each other when a customers blows up a DCT

 

[CKN]

What about lasting-power, reliability? Some will say, "here he goes again... " but yes, a DCT can totally manage clutch wear to a better degree than a manual driver (and they are wet clutches... arguably better... versus a dry clutch) but here is the rub: a DCT transmission has double- or triple cone synchronizers, which are bound to be better, quicker, more long-lasting than those in most manual transmissions, but the mechatronics shifts these synchronizers in a brutally-fast manner (at times... certainly not always). If the driver uses manual control of the transmission (and honestly, also in auto - with an aggressive right foot) and he/she downshifts at high roadspeeds and quite aggressively by way of engine rpm, those synchronizers will wear and become disfunctional... Said mechatronic units single-clutch shift... they do not double-clutch shift, whereas a skilled driver who knows how to accurately double-declutch and if he/she drives exactly the same way as the DCT car... then the manual will outlast the DCT.

Lotsa caveats there, I know.

Driven by grandma, likely the DCT will outlast it's manual counterpart.

Driven moderately, and even spiritedly, the DCT will last adequately, at least.

Seems to me, as well, drive 'em gently 'til the box is warm!

Interesting thank you for your thoughts. My only comment is that there is hardly any drivers left skilled enough-or even trained in a manual transmission to perform the operation of "double-declutch".