[Sutripta]

Originally Posted by Chethan B G

In case of the dry clutch, it is even worse.

Worse for the clutch, not for the other temperature susceptible components.
In the DSG, it is a component subassembly (mechatronics unit) which fails.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

^^^
The main source of heat generation will be a slipping clutch. If it is a wet one, it will be a pretty effective heating source for everything the oil touches.

Originally Posted by Chethan B G

Yes.

You are talking to a guy who is of the generation who still thinks fully synchronised gearboxes are quite neat. DSGs are not really my thing, other than I have two cars which uses them. Wet or dry I do not know to be frank. And since I know little about them, I like to understand more about them, hence my questions.

But, the topic was slightly different, or at least that is how I understood it:

Chetan wrote:

Yes, it definitely can be mitigated since the DSG issues are mainly around the mechatronic module, which is affected by heat, which is generated when the gearbox has to crawl in stop and go traffic. The heat is made worse when the DSG - in the interest of fuel economy - shifts to 2nd gear as soon as the car is moving. Putting it in M stops this from happening, since the car remains in 1st much longer and helps slightly reduce the load and resultant heat

.

When I questioned him, he answered:

Lower load due to higher gear ratio in the first gear.

Which I took for driving at a relative low speed in 1st gear produces less heat in the DSG than in 2nd. Hence my little amateur model. He did not say it prevents the DSG from shifting and thus less clutching/slip. No, it was about lower load in first gear compared to load in second gear. Nothing to do with clutch or clutch slippage. Or at least that is how I read it / interpreted it. If he meant it differently, or interpreted incorrectly I hope he steps in and corrects/adjusts.

When clutches, wet or dry, start slipping all bets are off heat wise obviously. (Forget about specials such as slipper clutches)

At a steady, slow speed, the clutch wet or dry should not slip at all. Admittedly in start/stop bumper to bumper traffic overall it might be the clutch slip that is generating most of the heat during actual gear shifts, but that is not the case here.

So the question remains:
Is there a difference in heat being generated inside the DSG between 1st and 2nd gear driving at a steady pace (no gear changes).

I do not think wet or dry makes a fundamental difference in this. A wet DSG will have a separate hydraulic fluid. Once a gear is selected and the clutch has done its thing, no heat is generated other than whatever heat comes from bearings and the various bits turning around (friction) in the hydraulic fluid.

To my earlier point, if anything in second gear they will be turning at a lower rpm, then in first gear and thus, at least in theory produce less heat. As I mentioned, this whole issue can also been seen as a question about which gear allows a DSG to run at the highest efficiency. Gearbox efficiency, not engine. Heat is a measure of efficiency on a gearbox.

Last night I read through this whole wonderful thread again. It really has some excellent, very interesting and good information in it, thanks to JoshMachine.

I came across his DSG 10 commandments once again, post 65.

Lets take a look at some:

2nd Commandment-
General driving 2: Thou shall always try to drive in higher gears (e.g. D7) to the extent possible, so as to let the DSG freely operate at its highest state of efficiency

That really says it all. Second gear would be better than first. I do not know why JoshMachine made this commandment, what are his considerations? But clearly, the message is get in higher gear, not lower gear. Higher gears give you higher efficiency from a DSG perspective. Which effectively means less heat!

It should be possible to measure this as well. Most, if not all DSGs do have a temperature sensor. With the correct OBD scanner you should be able to read it during driving. Drive a few kilometers in first, go the second and see if it affects the temperature of the DSG. I will see if my scanner can read the life measurements of the DSG temperature. I doubt it somehow. I would think this needs a dedicated brand/model specific scanner.

It is all a bit theoretical at best. When driving bumper to bumper, up and down shifting is likely to be your biggest concern, not heat build up due to driving for miles in a given gear. But, gearboxes do have different efficiencies per gear. So, from that perspective it does remain a valid question. Which gear is most effective/produces less heat at steady pace driving.

Jeroen

[Sutripta]

^^^
Full discussion when I have access to a proper keyboard.
You had raised the same issue on another thread. There it eas a TC Auto. And there I agree with you.

[Jeroen]

Originally Posted by Sutripta

^^^
Full discussion when I have access to a proper keyboard.

Looking forward to it

Originally Posted by Sutripta

You had raised the same issue on another thread. There it eas a TC Auto. And there I agree with you.

Yes, in essence the same claim and I did not see a really good explanation there either. So I am trying to make my own mind up! Based on the little knowledge I have on transmission in general and some general knowledge (e.g. friction causes heat, heat is inefficiency etc) Since I do not know much about auto-gear boxes and certainly not on DSGs, I look forward to get some insights into these.

I must admit I find these DSGs fascinating pieces of machinery. I have to find somebody who will show and explain an opened up one.

For those who like to get an idea on the inner bits of a more regular Auto transmission and how those get overhauled:

https://www.team-bhp.com/forum/diy-d...ml#post4180547

If anything it will show some details and what things like friction packs look like.

Always thought these Jaguar/Mercedes autoboxes were pretty complex. That is until I started looking into DSGs!

Jeroen

[Sutripta]

^^^
Have you ever come across a n overheating layshaft type gearbox? (Assuming no lubrication problem s. )

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

^^^
Have you ever come across a n overheating layshaft type gearbox? (Assuming no lubrication problem s. )

You don’t see many of these anymore on modern cars! But of course, still quite a few around on many classic cars, especially rear wheel driven ones.

Over the years I have come across few problems with gearboxes overheating. Never during normal use, even in mountainous terrain. All the problems, irrespective of type, I have seen always had a specific reason such as low fluid levels due to leaks. Once it had been filled with an fluid on an incorrect specification.

In all honesty, not that many of these problems either. If there were problems with the transmission system it was very often clutch related or on auto-boxes the torque convertor. Obviously, both wear and will fail at some time.

Are there specific overheating issues in layshaft gearboxes?

Jeroen

[Sutripta]

Originally Posted by Jeroen

Are there specific overheating issues in layshaft gearboxes?

Not in my experience.

Regards
Sutripta

[Sutripta]

Originally Posted by Jeroen

You are talking to a guy who is of the generation who still thinks fully synchronised gearboxes are quite neat. DSGs are not really my thing, other than I have two cars which uses them. Wet or dry I do not know to be frank. And since I know little about them, I like to understand more about them, hence my questions.

There ere a couple of threads that I emenber on DCTs. One was on whether VWs DSGs were failing because we 'wern't treating it right', and another on the Ford PowerShift. Maybe run your eye through those.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

There ere a couple of threads that I emenber on DCTs. One was on whether VWs DSGs were failing because we 'wern't treating it right', and another on the Ford PowerShift. Maybe run your eye through those.

Thanks, I read them (and I actually posted in the Ford PowerShift)
The Ford PowerShift thread is based on the dry clutch DCT. I think my Focus 2018 has a wet clutch. The company Audi too.

I tried to get a temperature reading from my Ford Focus and Audi A6 boxes, but my scanners show many different (live) parameters, lots of various temperature readouts but not the temperature of the transmission. Like I thought, you need a more dedicated Ford and VAG scanner most likely. Or I might have to update my scanner. Havent done that for some 2 years.

Offtopic, but I did notice something quite remarkable. On the Ford, the total mileage I read via my scanner and what I read on the display are different. Almost 40 km different!

Jeroen

[Sutripta]

A modern gearbox, any modern gearbox (leaving known exceptions like worm drives) is very efficient (irrespective of the 15% loss, 20% loss etc bandied about by people, esp. tuners). The ratio of power transmitted to power consumed is so lopsided that measuring gearbox efficiencies accurately is not a very easy task. Gearbox problems mainly come from wrong selection for the job at hand (esp shock and torsional loads) and badly designed lubrication.
Sometimes power is tapped off the input shaft for certain tasks, like running a hydraulic pump. If we look at the gearbox as a system, it will show up as a loss affecting efficiencies. But it is not a gearbox inefficiency in itself.

So where does the heat come from? It comes from the clutches or the coupling fluid.

There are various forms of clutches. In automobiles we use friction clutches. A dog clutch say, has two states - engaged and disengaged. And neither of the states dissipate power. In a friction clutch also, these two states do not dissipate power. (well not entirely true - it does dissipate some power when disengaged, but not important for our discussion). But a friction clutch also has a third state - where it is slipping, transferring only part of the torque. And this generates a tremendous amount of heat. In normal operation this state is transitory, something which it faces when it is shifting between engaged/ disengaged, lasting for a fraction of a second, and so does not cause any problems.

If we have operating modes where this assumption of a slipping clutch lasting only for a fraction of a session does not hold, we are in trouble. So the question is do we encounter such modes? And what happens in the case of manual gearboxes? It is after all the same traffic which the cars are negotiating.

Regards
Sutripta

Rest later. Hoping this evening.

[Sutripta]

So problems occur when slipping the clutch is an operational mode rather than as a transitional occurrence when one moves between clutch engaged and clutch disengaged, and vice versa. There are a number of use cases for this. Think reversing into a tight parking spot.

Basically if we come across a situation where the speed needed is less than that possible with the clutch fully engaged (and so engine at low RPM = low torque), the clutch will need to be slipped. Even if the mathematical calculations show that it is possible, oftentimes the torque produced by the engine at that speed might not be sufficient. Might not be apparent on level ground, but is very apparent trying to start a car on a slope.

Also to be kept in mind is that the first (and dominant) auto transmission (TC + slushbox) can easily handle the use case of creeping along, or starting on a slope. The torque convertor easily handles such cases (if done within reason. The transmission fluid does heat up). You want to creep along - put car in gear, use brakes to modulate your speed.

Any new auto gearbox technology cannot be 'inferior' to that which is prevalent, and that which it is trying to replace. So this needs to be replicated, and is handled by slipping the clutch. But do it for any length of time - temperature problems/ burnt clutch. Any system in which an operational mode uses a slipping friction clutch (DCT, AMT, some implementations of CVTs) will have this problem. A wet clutch will go some distance to handling this heat buildup problem, but not fully. It is not a silver bullet.

A question for readers - what happens in case of manual gearboxes. These are driven in the same condition. And in the hands of a good driver, the clutch normally lasts.

Regards
Sutripta

[Thad E Ginathom]

Those clutch-slipping times (hill start/holding, very slow speed control for manoeuvring, etc) are all part of the deal. We do it because that's how to do it. It is factored in to the cost and wear.

Skilled drivers make clutches last. If I find myself holding the car on a hill longer than I ought to, I actually think, this is time off the life of my clutch plates. We should spend our clutch plates wisely!

[sgiitk]

The DSG at one time was unhappy with synth oil, so people had to shift to mineral oil. I am sure VW has fixed it by now. Sometimes the latest and greatest may be the best.

[Jeroen]

Thanks Sutripta for all the additional information.

It is clearly understood that a slipping clutch produces a lot of heat. In any kind of set up, manual, DSG, DCT. The torgue convertor on classic AT are well suited to the job, even in bumper to bumper traffic. But they will eventually burn out as well when you keep the trannie in D and your foot on the brake. It is not immediate, you would probably need at least 15-30 minutes before it starts happening. As soon as you let the car roll a bit, you are ok again. The state of the transmission fluid, has some influence on the how quickly this might happen.

But back to our DSG/DCT.

I understand the slipping of the clutch under conditions such as bumper to bumper stop / go traffic, parking, hill start. All of this would be done in first gear obviously.

I assume under those conditions is it only one clutch that is engaging/disengaging. The second clutch has no role to play (yet)

Any gear changes upwards will take place very quickly, one clutch disengage as the other engages. So even though it takes place very quickly some friction and thus heat would be generated, but I assume that is very little.

Is it possible for a DSG/DCT to have slippage (other than the normal quick engage/disengage for new gear selection) in other gear than first? From a design point of view, why allow slippage in any other gear than first?

Which does brings us back to the original question:

Which gear is most effective / produces less heat at steady pace driving. E.g. should you manually get your DSG to drop a cog from 2nd into 1st to prevent heat built up?

If there is a difference, why or what contributes to the difference. Same for other gears. (To the earlier statement about a DSG being most efficient in it is highest gear).

You make an interesting point of the clutch also producing some heat whilst fully engaged, i.e. without slippage. But of course the same is true for the various shafts, gears, or rather any component in the box that has a torque applied. (internal torsion) Even if it does not move, no friction, internal friction (molecules?).

I don't think it is much though. I remember having to do calculate radial torque differences (is that the right terms) on shafts at naval college. (how much degrees difference between in- and output on a shaft under load). Nobody seemed to worried about the inherent heat that would generate.

Thanks!

Jeroen

[Sutripta]

Originally Posted by Jeroen

It is clearly understood that a slipping clutch produces a lot of heat. In any kind of set up, manual, DSG, DCT. The torgue convertor on classic AT are well suited to the job, even in bumper to bumper traffic. But they will eventually burn out as well when you keep the trannie in D and your foot on the brake. It is not immediate, you would probably need at least 15-30 minutes before it starts happening. As soon as you let the car roll a bit, you are ok again. The state of the transmission fluid, has some influence on the how quickly this might happen.

Don't quite agree. But as you said,

But back to our DSG/DCT.

not our focus

I understand the slipping of the clutch under conditions such as bumper to bumper stop / go traffic, parking, hill start. All of this would be done in first gear obviously.

I assume under those conditions is it only one clutch that is engaging/disengaging. The second clutch has no role to play (yet)

Second clutch disengaged, so not relevant to our discussion.

Any gear changes upwards will take place very quickly, one clutch disengage as the other engages. So even though it takes place very quickly some friction and thus heat would be generated, but I assume that is very little.

One need not worry about the heat generated in this (transitionary) scenario.

Is it possible for a DSG/DCT to have slippage (other than the normal quick engage/disengage for new gear selection) in other gear than first? From a design point of view, why allow slippage in any other gear than first?

Should not be needed in any gear other than first or reverse.
BUT one never knows.
eg. - In the Maruti AMT (K12 engine), it seems to be programmed to use 2nd as its 'go to' gear. Left to its own antics, often I've found it holding second, slipping the clutch. When sanity says one should drop down to first.

Which does brings us back to the original question:

Which gear is most effective / produces less heat at steady pace driving. E.g. should you manually get your DSG to drop a cog from 2nd into 1st to prevent heat built up?
If there is a difference, why or what contributes to the difference. Same for other gears. (To the earlier statement about a DSG being most efficient in it is highest gear).

If we look at it just from the GB PoV, (ie excluding clutch, and ancillary systems like hydraulic pumps driven off the input shaft), for the same road speed (ie output shaft RPM), a lower input shaft speed (ie higher gear) should theoretically be more efficient. ie less heat generation. However the difference should be so small as to be completely irrelevant. (Ancillaries - a hydraulic pump dumping most of its flow through a pressure release valve is certainly not efficient. However whether the people saying 'keep it in higher gear' had this in mind, or something else I don't know).
If there is a scenario where one needs to slip the clutch in 2nd, I'm pretty sure dropping down a cog will either eliminate the need, or considerably reduce need to slip the clutch.
Anther point I'd like to make is that when you are driving say a manual, you choose the gear appropriate for the then current road conditions and your car. An auto should mimic that as far as possible.

You make an interesting point of the clutch also producing some heat whilst fully engaged, i.e. without slippage.

No, I said a 'disengaged' clutch also produces heat.

(well not entirely true - it does dissipate some power when disengaged, but not important for our discussion)

That is because friction clutches, esp wet ones do not disengage fully. At the first approximation I think we can ignore the heat produced by the disengaged clutch.

Regards
Sutripta