ADUK Archive

Do automatics slow down by the use engine braking in the same way that a manual does ?

I could do with a simple explanation as I am a bit thick as to what goes on under the bonnet.

Thanks

Autos (especially older ones) generally use a viscous coupling rather than a manual-style clutch which provides very little in terms of resistance when the engine would normally be providing a braking effect.

Add to this that, depending on the setup, the gearbox may decide you really want to upshift to save fuel if you're not accelerating. This would also tend to reduce the engine braking effect somewhat.

I'm sure people with more direct experience of autos will be able to give more specifics, but that's my understanding.

Chris, is the above in agreement with your experience of autos?

Cheers & God bless
Sam "SammyTheSnake" Penny

ROG wrote:Do automatics slow down by the use engine braking in the same way that a manual does ?

The engine characteristics have the same effect, (as in, there is often very little engine braking unless at highish revs if the engine is turbo-charged), but this is sometimes less obvious when there is an automatic gearbox.

In fully automatic mode, there is generally very little engine braking because, as Sammy says, the gearbox tends to shift up when it is under no load. If you over-ride the gear selection, then sometimes you can be surprised at how much engine braking there is.

I was recently very lucky to be allowed to drive a BMW X5. This particular example has a 3.0 litre turbo diesel engine mated to an interesting automatic gearbox that has 6 forward gears.

A friend who also drove it had left it in 'D' and suffered a wallowy barge-like experience, but if the gears were directly selected it was very much like a (very large!) sports car complete with a reasonable amount of engine braking. I did notice, though, that my gear selection was typically one or two lower than that which would have been selected automatically.

I'm well behind the times with my understanding of modern automatic transmissions, but I think Sammy is probably right in what he says.

Obviously a friction clutch arrangement (rather than a torque converter or fluid coupling) provides a more positive connection between engine and the rest of the transmission, but a I think a fluid coupling can still transmit drive well enough to do some engine braking, so long as the speed is not too low.

I imagine the main difficulty is getting the control system or 'brain' to understand what the driver wishes to do, and then have an appropriate gear engaged.

My experience of automatics is mainly with Jaguars from the 1960s to 1980s, and they used the Borg-Warner DG, and the model 35, and the model 65, and the General Motors GM400 unit on later cars. IIRC these would always hold onto the highest gear when running on a closed throttle, even down to very low speeds.

My only experience of fairly recent automatic transmissions has been the type fitted on the Ford Mondeo, and these seemed to change down to lower gears when slowing down in a traffic queue. In terms of engine braking this appeared to be better than the systems used on the old Jaguars.

Best wishes all,
Dave.

The ropsa test guidelines say this about use of auto box:

If you are tested on a vehicle with an automatic gearbox the examiner will watch your starting procedures and your use of ‘kick-down’. He will also note if you override the transmission to keep the vehicle in a lower gear when appropriate. You should not use the automatic gearbox as a manual gearbox.

RoADAR by way of jbsportstech wrote:You should not use the automatic gearbox as a manual gearbox.

Any idea why they say that? How is it justified?

My view is that it is more spirited driving is possible when using full control of the gearbox but I suspect RoADAR isn't looking for that kind of driving. I also know StressedDave talks about having full control of an automatic gearbox just by using the accelerator, (and it's a lot of fun to try doing this), but even he says that modern automatic gearboxes make that extremely difficult if not impossible.

I think it needs rewriting due to there being alot of (SEMI AUTO) tip-tronic/gear tronic boxes in modern cars which are designed to be used as a manual if the user desires.

A standard auto box is supposed to be used in drive 90-95% and ocassionally when conditions dictate you should make use of the 1st and 2nd and in some cases 3rd.

I think what they are getting at is that you should not start in 1st and change up/down through 2nd 3rd etc as you would a manual as this is not what a auto box (non semi auto) is designed for and therefore is not in sympathy with the vehicle or good practice.

I drive an Audi A3 2.0TDI with DSG gearbox. The engine braking in this even when in D is excellent - better than in many manual cars I have driven (regardless of gear/speed). As others have said earlier in the thread I seldom need to take it out of D but if I ever feel I need a little more engine braking using the S mode holds the lower gears longer and I can descend even quite steep hills under control with no need to brake.

I also drive a classic mini (manual) which has awesome engine braking and whilst I don't have experience of a huge range of cars the Audi DSG is a close second.

Yes the VAG audi/vw dsg gearbox is another exception as it is based on a three-shaft six-speed manual gearbox offering variability in choosing the right transmission ratio. It has twin multi-plate clutch with electrohydraulic control, DSG is able to hold two gears in mesh at the same time. This means it can preselect the next gear and makes for a fast and smooth change.

StressedDave wrote:its predilection to stick it in top gear at low revs...

Not having a great deal of experience driving autos, this was something that surprised me when I drove a couple of modern ones which, in D, seemed keen to get into top gear as quickly as possible. This was fine for the majority of cruising but meant that when you do kickdown for more power, it took a little while to try a few gears before settling on the correct one. This made planning for overtakes in D a little less predictable than I'd have liked!

Switching the cars to manual (or Playstation mode!) was a revelation and made the drive much smoother and more predictable, especially with the steering wheel shift buttons! D was only used for the "boring" sections.

Can't say in either of the cars I really noticed a lack of engine braking, but I guess I was in manual mode for the majority of the time when I would have made use of it.

Chris

StressedDave wrote:At low engine speeds there is a torque converter (NOT a viscous coupling) which uses hydraulic fluid flow to transmit drive.

I sit corrected. Sadly Wikipedia's entry on "viscous coupling" is pretty terse, but still enough to tell me the difference between what I assumed were different terms for the same thing.

Cheers & God bless
Sam "SammyTheSnake" Penny

Gareth wrote:

RoADAR by way of jbsportstech wrote:You should not use the automatic gearbox as a manual gearbox.

Any idea why they say that? How is it justified?

My view is that it is more spirited driving is possible when using full control of the gearbox but I suspect RoADAR isn't looking for that kind of driving. I also know StressedDave talks about having full control of an automatic gearbox just by using the accelerator, (and it's a lot of fun to try doing this), but even he says that modern automatic gearboxes make that extremely difficult if not impossible.

The RoSPA view is that if you want to continually manually change gear, use a manual!

I have driven autos for years now and rarely manually over ride the g/box. I occasionally use the 'S' button, which has the effect of hanging onto gears for longer, but I rarely fully override. I just do not see the point bearing in mind the way I drive. I should also say at this point though, I drive cars with 3ltr engines, it my be a little more beneficial with smaller engines.

I would say that advanced drivers should be able to judge when it is appropriate to use the full auto and when to use the hold functions. On the test the candidatewould need to demonstrate that judgment.

If the candidate uses it as a manual all the time the examiner will have no idea if the candidate is able to to decide when to use the auto and when not to.

I generally find it easier to knock it into 'D' for built up areas, and prefer to use the gears directly on interesting rural roads. I expect I'd use 'D' for dual-carriageways and motorways. Fortunately I have the choice to minimise my use of all kinds of boring roads.

Interestingly, though, when driving the X5 I mentioned earlier, I found that the automatic gear selection was so annoyingly not what I wanted, I ended up selecting gears in built up areas as well.

On another forum I became involved in a debate about whether or not the DSG transmission actually qualified as automatic in the full sense of the term.

Some people argued that it was not a proper automatic, and yet they admitted that it had a mode in which it would cover the full speed range of the car, and make all gearchanges automatically without intervention by the driver. If that is true I would have thought it qualifies as an automatic.

I'm therefore interested to know why the Stressed one seems to (sort of) regard it as a manual gearbox.

One of the arguments elswehere seemed to centre on the method by which the different gears were selected and engaged, but I didn't see that as being relevant. If you can go from zero to top speed simply by pressing the accelerator pedal, that sounds pretty automatic to me.

Best wishes all,
Dave.