When DSG gearboxes were first available, they were claimed to be a great advance on old torque-converter automatics. Apart from faster and smoother gearchanges, the elimination of the torque converter was supposed to make them much more efficient, so that there would be no increase in fuel consumption or emissions.
That promise doesn't seem to have been fulfilled. HJ's test data on the new Skoda Superb shows that with the 2.0 TDI CR 170 engine, the DSG box adds 6g/km to the CO2 output and cuts the combined mpg from 47.1 to 42.2. (see www.honestjohn.co.uk/road_tests/index.htm?id=348&s...e)
There's a similar pattern on other cars, e.g.
Skoda Octavia 1.9 TDI 105PS: manual 57.6mpg 130g/km, DSG 47.9mpg 156g/km (see www.skoda.co.uk/ourcars.aspx)
Volkswagen Touran 2.0 TDI 140PS: manual 47.1mpg 159g/km, DSG 43.5mpg 172g/km (see www.volkswagen.co.uk/#/new/touran/which-model/engi.../ )
So it seems that while the DSG is more efficient than a torque-converter automatic, it still lags some way behind the efficiency of a manual. I don't understand why this should be the case, because surely the DSG as an automated manual has no more inefficiencies than a manual box, apart from a small weight increase which shouldn't account for the more than a tiny proportion of the efficiency reduction.
The only reason I can see for the consistent increase of ~10% in fuel consumption is that the DSG may be changing gear at different points to a manual gearbox on an economy drive. Is this the best explanation, or am I missing something?
Edited by NowWheels on 12/09/2008 at 13:30
|
|
And conversely automated manuals (when left in auto mode) are more efficient than manuals, which is probably one of the reasons they are becoming more prevalent.
|
And conversely automated manuals (when left in auto mode) are more efficient than manuals
So it seems. But since a DSG is just a pair of automated manuals running beside each other, why is it less efficient?
|
Havn't you just answered your own question "pair of automated manuals running beside each other" the key word is "PAIR".
Carse
|
Havn't you just answered your own question "pair of automated manuals running beside each other" the key word is "PAIR".
I thought the point of the design was that only one of them is engaged at any particular time, so that the idle gearbox adds weight but not drag?
|
It's not a pair of gearboxes. It's a dual clutch gearbox.
It weighs more so that will add to CO2 output.
More in depth knowledge of the testing regime is required, but manfacturers probably have more ways of increasing manual mpg than DSG.
What actually matters is real life. From figures given for the A3 I feel the DSG figures are more 'honest' and achieveable.
Extra urban figure for the A3 manual is 62, for DSG it's 57. 62 from the manual - I just don't believe that is possible. 57 from DSG is still high, but I have achieved low 50s.
Manual urban is 36mpg, DSG is 37mpg and I get around this figure consistently.
|
|
|
The 6 speed DSG as used with the higher powered petrols and diesels (170nm torque+) has a wet clutch arrangement, which must add extra drag to the internals.
Whereas the petrols that use the new 7 speed DSG (dry clutch, used on lower powered cars) actually see improvements with fuel economy and emissions in comparison to the manual versions, depending on spec.
|
|
Dont forget these these MPG tests are done in very specific ways, that can flatter some cars and not others.
|
My friend who had a Seat toledo 140td with the dsg box (some of you kindly commented when i asked about fuel consumption on technical) has now changed the car for a jetta with same engine with 6 speed manual.
His figures have now gone from about 40 back up to his usual 55+ ish.
|
If he is genuinely getting 55+mpg on a regular basis from a 2.0TDi 140 manual either he is driving in a way simply not possible in a DSG, doing hundreds of motorway miles on a daily basis at 55ish mph or he needs a new calculator.
|
If he is genuinely getting 55+mpg on a regular basis from a 2.0TDi 140 manual
No to be fair he is going by the cars fuel computer on the new one, but he's a hgv driver of many years and knows how to drive economically whilst maintaining progress, he like one or two here will be anticipating slowing or stopping and will be on overrun far longer than most drivers.
Used to get similar consumption from his previous Bora 1.9 110hp diesel.
And he can drive better than the DSG, he can anticipate the road ahead, something the computer cannot yet do.
I have a similar to DSG equipped truck which i drive in manual always, my fuel figures are better than most other identical vehicles who use auto all the time.
Does anyone know exactly the weight difference between a manual and an identical DSG equipped vehicle.
|
Octavia 2.0 TDi 140 Hatch:
DSG box 1560kg
Non DSG 1525kg
|
>> DSG box 1560kgNon DSG 1525kg
I wouldn't have thought 35kg would make that much difference.
The person driving the manual can make infinite judgements on gears needed for the terrain or situation they are approaching, the DSG can only go by the input from various sensors, it can never be as efficient as a good driver, it cannot see the steepening hill, or the junction that is just about to clear.
But its probably better than many drivers who haven't a clue how to drive a diesel.
A question for you regular DSG drivers,
my truck has an automated manual which to me is an exemplary example of useless, when for example approaching a junction/roundabout on overrun but not needing to stop, you wish to go under power again it will without fail select too low a gear and travel literally 10 yards in this wrong gear before changing up again (some other makes to be fair are better at this) the revs being way over what is necessary, this makes for slow progress, poor fuel and danger at junctions, apart from being very annoying.
I always drive the thing in manual to stop this frustrating progress, far smoother progress.
Does the above example bear any resemblance to the way a DSG performs.?
|
|
What we need to know is how 'they' do the tests on DSG cars - auto or manual operation? It's hard to believe that DSG should add >10% to the fuel consumption driven manually with the same change points. The extra weight must be less than an extra bod in the car which wouldn't nearly account for it.
|
|
|
|
|
Andrew English's preview of the new VW Golf says "VW is pushing the more expensive, twin-clutch DSG gearboxes that can act as a full automatic, but with the mechanical efficiency (if not the lighter weight) of a manual."
www.telegraph.co.uk/motoring/main.jhtml?xml=/motor...l
I'd have thought that Andrew English would have done some checking, rather than just reading the press release. He's usually one of the more thoughtful motoring writers.
|
|
He is indeed - here he may just have been calling the DSG efficient in comparison with a 'conventional' torque-converter automatic, which saps power and uses more fuel more noticeably.
|
>>he may just have been calling the DSG efficient in comparison with a 'conventional' torque-converter automatic
Maybe, but if that's what he meant then he is choosing his words very badly.
|
|
Reading the whole sentence, it makes perfect sense to me!
|
Reading the whole sentence it makes perfect sense to me!
English says " with the mechanical efficiency (if not the lighter weight) of a manual."
He didn't say "more efficient than a torque-convertor", or "nearly as good as a manual". He says that it's as efficient, but the figures tell a different story.
|
If the two gearbox in parallel model is correct for a DSG gearbox (I know absolutely nowt about them), is the redundant gearbox always in the next gear up or down with its clutch spinning?
|
|
|
|
>a 'conventional' torque-converter automatic, which saps power and uses more fuel more noticeably.
Not anymore. "Conventional" torque convertor automatics have advanced dramatically over the last few years.
For example, the Mercedes 7G-Tronic is a conventional auto but models equipped with the 7G-Tronic 'box are faster and also more fuel efficient than their manual equivalents.
It's partially due to the extra cog and convertor lock-up in every gear but also because the control systems can ensure that, in normal circumstances, the car is always in the most efficient gear. This helps reduce the fuel wasted by those of us who might be tempted to hold a gear too long or change up too soon.
It is capable and intelligent enough to miss intermediate cogs when required, eg. on a kickdown, but you've also got manual control when you want it.
Very DSG-like but without the complication of twin shafts and fancy clutches.
Kevin...
|
Very interesting - many thanks Kevin. I've always thought that you need at least 2.5 litres to get decent performance from a torque-converter automatic - but if the Mercedes auto becomes the way forward this won't be the case.
My last car was a Mercedes B200 CDI with CVT - the performance was pretty dismal but I think that was due to the agricultural engine rather than the transmission.
|
|
I read something about DSG the other day, I think in Sat's DT, I think the point was that while it is potentially as efficient as manual it is a lot heavier which could effect MPG and CO2 figures.
|
|
|
|
|
|
