Auto Fuel Consumption

[Nick Mann]

A couple of questions. I don't really understand the torque convertor. How does it work? People have tried the 'Imgaine a big elastic band' but I can't wrap my head around it. What happens when you are sitting in traffic with your foot on the brake in drive? INSERT INTO post VALUES (I try not to do it, as I don't understand where the power is going.)

Second question - I have heard people talking about the mechanical lock up in 4th and 5th. Does this bypass the torque convertor? If so, then cruising on the motorway should give the same mpg as an equivalent geared manual?

Please word replies using 'stupid english person' language!

Ta.

[Nick VR4]

Found this
If I'm siiting in traffic for along time I tend to put it in N
This mainly helps to prevent heat building up in the autobox
I think Autos will use slightly more MPG mainly due to slippage through the box ????


In most cases, the automatic transmission consists of a torque converter and a set of gears called a planetary or epicycle gear train. These are fitted to the car in place of a conventional clutch and gearbox.

A torque converter consists of an impeller that is driven by the engine, and a turbine that drives the gearbox input shaft. Each is bowl-shaped and contains a number of partitions called vanes. The two bowls are placed face-to-face in a casing filled with oil and they are separated by a small clearance so that there is no friction between them.

When the engine is idling, oil is flung by centrifugal force from the impeller and it enters the turbine, which remains stationary because the force of the oil is not yet sufficient to turn it. When the driver depresses the accelerator, the speed of the impeller increases and the turning effort derived from the fast-moving oil becomes great enough to overcome the resistance of the turbine and this begins to rotate the gearbox-input shaft. If the vehicle is in gear, it starts to move off.

After passing its energy to the turbine, the oil re-enters the impeller and is circulated back to the turbine again. As the engine speeds up some turning effort is transmitted, but there is still a degree of slip in the unit so that the gearbox-input shaft is rotating more slowly than the engine.

The torque converter, as the name implies, converts the torque or turning effort of the engine into the higher torque needed by the car at low road speeds. An increase in torque has the same effect as changing to a lower gear, so a torque converter is also gear reducer, acting like a set of extra gears before the engine’s drive reaches the gearbox. It is able to deliver this higher torque because there is a small vaned wheel, known as a reactor or stator, placed between the impeller and turbine that, according to engine speed, directs the oil along a more favourable path towards the impeller, enabling it to give extra thrust to the turbine blades. At pull-away speeds, the torque converter can be double the turning effort produced by the engine and applied to the gearbox.

As the engine speed increases, this 2:1 increase in turning effort is reduced so that when holding a steady throttle opening and cruising, there is no increase at all and the parts of the torque converter rotate at the same speed.

[Kieran]

Right.

I could explain, but these articles probably do a better job...

http://www.bankspower.com/Tech_under...rqueconver.cfm

http://auto.howstuffworks.com/torque-converter.htm

[Nick Mann]

Thanks guys. I understand a lot more now, even if I am still unsure on the details. I still don't get the rubber band thing!

The thing I have learned today is that the torque converter can actually increase the available torque at low speeds/high revs! Sounds pretty good to me! I think that I therefore know why people were talking about 'launching' their auto's at the pod. I thought at the time it was just to increase the power, but if I understood those articles properly, it also increases the torque transmitted as well. Bonus!

[Kieran]

...I have heard people talking about the mechanical lock up in 4th and 5th. Does this bypass the torque convertor? If so, then cruising on the motorway should give the same mpg as an equivalent geared manual?Ta.

Yep. The mechancial lock does effectively give a direct mechanical link between the engine and gearbox. You often feel it. In the GLS, 4th gear is too tall for 30mph, so it stays in third and provided I keep to a light throttle, the revs will eventually drop by about 500 rpm - this is the lock engaging. You may also have noticed that when you're in 4th/5th, the autobox is sometimes slower to respond - this is because the lock mechanism usually has to disengage before changing gear - though not always.

And when you are sat on the M1, with the gearbox in top and the convertor locked, then the taller gearing of the autobox is indeed a great benefit to MPG figures.

[Kieran]

Thanks guys. I understand a lot more now, even if I am still unsure on the details. I still don't get the rubber band thing!

Now then.. I've just clicked what you're confused about here.

The phrase "rubber band" gearbox is NOT to do with the torque convertor or any sort of 'traditional' autobox INSERT INTO post VALUES (ie: one that uses planetary gears and has a series of predetermined ratios).

The "Rubber Band" phrase refers to CVT INSERT INTO post VALUES (continuously variable transmission) gearboxes, and probably has it's roots with the older DAF 33 type cars.

In a CVT, you have 3 gears. Forwards, neutral and back... but the gear ratios vary contstantly. Think of your mountain bike with it's derailier gears...
You vary the ratio by dropping the chain onto a larger or smaller cog, either at the crank end or the wheel end. A CVT is essentially like this, though there are no steps - just two cones and a steel-reinforced belt running between them.

For more info, plus handy diagrams, try here:
http://www.honda.co.nz/h.nsf/t/t_cvt.html

There's a very simple diagram on the left of this page that should help:
http://www.edmunds.com/ownership/tec...4/article.html

[Nick Mann]

See in a strange twist I actually have a vague idea about the CVT. But two seperate people have tried to explain the auto box with the fluid coupling using an elastic band as a similie. I am still non-the-wiser! I have learned a lot though, so thanks.

Incidentally, have you looked at the *****l Rotary on howstuffworks.com? Fantastic! I now think I understand that, too!

[zedy1]

nice info guys

[neil and bob]

Forget that rubber band thing for an auto box, it does indeed describe a CVT. To help understand how a torque converter works try to imagine two desk fans facing each other, one is switched on, the other off, as the air passes from the fan that is on, the other fan will turn, and that's about it, obviously a torque converter has different shaped vanes and the stator, and oil instead of air, but the idea is the same.

[adam_shaw]

I now wonder why it was such a big deal when the 'stator' on my old cars auto box broke.

If they had just said 'torque convertor' I wouldn't have replaced the whole box, I would've just got a new convertor.

I smell a rip-off. Grrrrr...... :INSERT INTO post VALUES (

[ritch_w]

what car was it and what did it cost? was this at the dealers?