Automotive Automatic Gearbox

A question for the automotive engineers amongst us :~

Is there any point in time when an automotive gearbox disengages from the drive when automatically chanhing gear, if not how is this achieved.

Regards IDP

For the VAG DSG gearbox it has two clutches under computer control. Look up DSG in Wikipedia for a good description. So yes it does disengage when changing and also stationary. I have one in my skoda and it works very well, also giving good fuel consumption.

Other systems use a hydraulic torque converter when in drive, but have a clutch that disengages in neutral I think.

Cars with a torque converter do not have a clutch for neutral, there is always a bit of drag from the torque convertor so that the driver has to use the brake to stop the vehicle from creeping along if in drive or reverse with the engine idling. There is enough drag to hold against moderate inclines on the road.

Variomatic gearboxes are in constant drive, they use variable pulleys to change the pulley ratios so there are no disengaged points with that type of automatic gearbox.

The advent of engine management computers in cars brought with them the ability to control automatic gearboxes from them. The old style of Borg Warner gearbox that used hydraulic oil pressures in lots of small pipe runs and galleries in the gearbox to change gear were replaced with solenoid operated brake bands. Ford use 4 solenoids, one of them locks the torque converter input to output to get rid of the small percentage of loss often associated with automatic gearboxes. This solenoid only operates when the engine speed is reasonably steady and the engine is up to temperature but can be noticed operating as the engine rpm can be seen to drop up to 500rpm when it operates. The other three solenoids being operated by the engine management computer can be set to operate so that there is the best change from one gear to the other with minimal time out of drive and gear changes being done when torque in one gear matches torgue in the next gear to give a smooth change. It also allows the gearbox to be put into sport mode where gear changes happen at higher revs.

There are still some losses with torque converters that have prompted manufacturers to chose twin clutch systems over Borg Warner types. Since these are also using the engine management system to operate the gearbox they will also minimalise the time the gearbox is disengaged but since there is some drag with clutches before full engagement they may have one clutch being overdriven when the other is underdriven to minimalise the time the drive is disengaged.

There must be some time when the gearbox drive is disengaged between gear selections to avoid a mechanical lock up. It can be too small to notice and under normal circumstances of a car travelling along a road you would not notice this.

Martin

Hi

Why do you ask?

It depends on the type of auto box:

The traditional borg warner type with a torque converter must briefly disengage during gear changes to prevent lock up, but in practical terms they do not. The underlying mechanicals are the same whether they are electronically controlled or not.

The cvt type use a variable pulley system, with either some type of clutch or a torque convertor to disengage it for when the car is stationary.

The double clutch boxes like the VAG DSG use two clutches so they have very quick gear changes, so they are only disengaged for a fraction of a second, unless you confuse them by kicking it down when it thinks the next change will be up, or if it kicks down two gears, then the computer gets all confused and it feels like you loose drive altogether, while it sorts itself out.

The automatic operated manual boxes, use normal clutch and gearbox, but with electric or hydraulic actuators, the feel like they disengage for ages during gear changes.

hope this helps

The traditional automatic gearbox uses a stack of epicyclic gears with band brakes on the outside of each section to select the ratios. Those brakes are effectively clutches so a separate clutch isn't needed. The fluid flywheel also has a lot of slip at low revs so allows the engine to idle whilst in gear but also gives rise to poor efficiency and thus fuel consumption.

The DSG gearbox on my Skoda is effectively a twin parallel gearbox arrangement with twin clutches to select which gearbox is used. Thus it can preselect the next required gear in one box while driving with the other giving an extremely rapid and smooth gearchange.  It can also do automatic hill starts by detecting roll-back using the ABS wheel sensors and giving the clutch controlled engagement to prevent further movement.

Russell.

Edited By Russell Eberhardt on 01/04/2016 10:36:20

Chaps,

The reason for the question :~

On a recent driving cousr the instructor insisted that an automatic never disengages drive but I could not see how this was possible so endeavoured to find the answer to complete my education. Many thanks for the replies to date.

Regards, IDP

Instructors are almost as knowledgeable about cars as taxi drivers. The fact they drive them a lot often simply seems to give some of them a superiority complex.

Modern automated gearboxes are controlled by their own transmission control unit (TCU) which needs to be calibrated rather like (and in conjunction with) the engine management system and of course the two have to talk to each other in the process over the CAN. bus

Automated transmissions are generally hydraulically actuated with the exception of some automated manual transmissions (AMTs) that are electrically actuated. They see to be dying a death due to the awful delays during gear changes and they feel as if somebody is in the car operating the gears. DCTs offer the same mechanical efficiency of direct gearing with almost instantaneous gearshifts which is why they are popular in supercars. There is an extra set of output gears so additional cost and size but that's not usually an issue.

Although they suffer from the need to run a hydraulic pump most of the time, the TCU calibration is much more knowledgeable about the optimal shift points than any normal human driver. Thousands of hours of engine and vehicle test data are used to map out the almost infinite number of shift points that result from changing throttle, load and driver desire ("sport" mode).

Most drivers are pretty much talentless at knowing how and when to change gears for maximum economy or performance. Even so, BMWs have 8 speed autos and there isn't a human alive who could possibly know how to control them optimally themselves, even within the team at BMW. We have a peculiar prejudice in the UK against autos but nowadays you are almost certainly going to have better FC with one, even with the slight overhead of the additional hydraulic pumping losses, the main downside being the initial extra cost.

You'll notice that hybrid vehicles aren't generally available with manual transmissions any more. No point developing a hybrid vehicle with 10-15% improvement in fuel consumption only to have the ignorant driver throw it away through a complete lack of driving talent. Honda Civic Hybrid is an example of that change.

Murray

That would be the 2 handles that you pick up to wheel it would it?

..

.

That one has really made my day - thank you

Que 1970s Skoda jokes

1976 Skoda 120 LSE . 35,000 miles in 3 years Cost of bits for annual sevice rose to just over £10 in 1978 ( That was Oil ,filters , plugs .points ) Average mpg 35. Insurace cost less than 50% of the British quotes thanks to the Skoda Baltic and Black Sea insurace Company

Problems . The sun roof leaked , but then that was fitted in England

Depreciation was £500 over three years . As I drove past other cars losing that ,and more , each year i wondered who the joke was really on

> Most drivers are pretty much talentless at knowing how and when to change gears for maximum economy or performance.

I've been driving my car for about six months (my first diesel car although we used to have a diesel camper) and I'm still not 100% sure the best points to change gear for ideal consumption. I have worked out that revving up to 5000 to blow the cack out of the exhaust once a week helps and that wet weather reduces fuel economy by nearly 5% (strange as petrol cars usually do better in the rain?)

Spookiest thing is that choosing the right gear gives the same consumption at 30 and 70, and hardly any different from 55-60. Not what I'm used to.

Neil

Neil.

Back in the early 80's we used to be able to buy a water injection unit that added a small amount of water to the air/petrol mixture from the carburetor. Supposedly to cause a better fuel burn. I also remember something about a similar system being used in early fighter plane engines.

Paul.

**LINK**

**LINK**

In the czech republic the skoda 1000 MB was called the '1000 malé bolesti' - or '1000 small pains'

Edited By pgk pgk on 01/04/2016 23:14:52

Chaps

When I was a boy many moons ago my dad always said that his car ran better in damp or foggy conditions, he put this down to moisture in the air cooling air intake and allowing a heavier intake of air. Looking at the above links seems he was on the right track.

IDP

Water or water/methanol injection works even better on a jet engine. It was frequently used for take-offs from hot/high airfields or for heavily loaded aircraft. It immediately fell out of practice after a BAC 111 crashed on an autobahn on taking-off from Munich in the late 60s or early 70s. Someone had put fuel in the water/methanol tank.

One result of using water, water/methonal injection is that the engine produces a lot more smoke. Early B52 bomber film always shows them taking-off with a lot of smoke - water injection.

JA

I've recently been given a new Renault Trafic van by my work, initial thoughts were old van 2 litre diesel, new van 1.6 litre diesel, Ive now been driving it for three weeks and I am averaging 42 mpg, it has similar HP than the 2 litre and it pulls like a train.

One gizmo it does have is a picture of a gear stick and arrow which comes on as you go up and down the gears, going up the gears it always comes on at certain revs which I assume is roughly were the power band peaks to allow efficient driving and the gear stick appears on the way down through the gears stopping you from letting the van labour in to high a gear, it also has a ECO button which if on improves the MPG and if off makes the engine a bit more lively, the button is on all the time with me

Martin P

When I was at school in the 1950s I seem to remember a farmer's son telling me that one of their tractors had water injection. Can't remember whether the fuel was diesel or TVO. I do remember that in the early 1960s one of the American airlines made quite a big advertising point about the fact that their Boeing 707s had water injection. A magazine article in the era suggested they dropped the feature because the increase in performance meant that servicing had to be more frequent.

Martin, my petrol Renault Megane has the same gearstick and arrow picture to tell me when to change gear, and it advises changing up at surprisingly low revs - usually well under 2,500. At first it annoyed me - I wasn't having a French computer telling me how to drive my car - but changing up when told does make a difference to economy. In fact, the engine does have quite good low-revs torque, which makes me wonder why Renault have found it necessary to fit a six-speed gearbox. This isn't really a criticism, because the car is a delight to drive and its handling is very good.

My Trafic has six gears but the engine has max revs of 5000 so no fear of changing up at low revs. I find the six gears makes towing a loaded trailer easy.

Martin P