OT Auto boxes and engine revs

[Dave Liquorice]

Hi,

Have a loan auto version of my own car whilst that is having lots of money spent on repairs. Not really driven many autos in the past and never the same make/model/engine.

What does an auto box, in conjunction with the engine management, get up to? I find that the revs go up to 2,500 +/- 100 rpm (ish) and stay there over quite a speed and load range. "Load" either being ordinary acceleration or maintaining speed going up hill (with/without kickdown).

Is the system going for torque or power? Trying to work out the RPM range that I ought to aiming for in my manual or cruise at on long motorway runs, I normally sit at the low 60's and 2,250 ish. I'm wondering if 2,500 and a few MPH faster will get a noticeable improvement in MPG. Even 1 MPG better would be over 3% ...

[newshound]

In these days of engine management systems I *guess* they may be trying to economise on fuel.

Think about a manual; you can maintain a given speed up hill with low revs high gear more throttle or higher revs lower gear and less throttle. It isn't immediately obvious which is more economical. But the EMS knows.

I've just got a Jazz hybrid which is supposed to think about economy all the time. This has continuously variable transmission and I havn't driven it far enough to figure it all out. It has an "eco" mode as well as Drive or Sports on the transmission lever. In Eco, cruise control does *not* maintain speed up steeper hills, it decides it would be better to slow down.

[Dave Plowman (News)]

Just wondering what auto it is? The old and now luckily defunct CVT (rubber band) transmissions did that, but these days most are multi-ratio types. Either conventional epicyclic gears with torque convertor, or robotised synchromesh boxes with a conventional but automated clutch. And neither tends to stay at constant revs. Most conventional ones now lock up the torque convertor when possible to save energy - it's only really used at low engine revs and to cushion a change. However, with 5-8 gears there is more chance of keeping the engine at its most efficient speed, if not driving hard.

But with a decent modern box it can be hard to tell when it changes gear - especially if you're not used to an auto.

[Andy Burns]

My car is 3.0 v6 diesel with 7 speed auto, I had a 2.0 v4 diesel with CVT as a loaner, the smaller engine wasn't particularly underpowered but it *sounded* like it was labouring, fairly constant high revs, like a taxi with a clattery engine.

I assumed it was going for economy ...

Not that uncommon, the Audi multitronic boxes are CVTs, granted they use a metal belt unlike the old DAFs. Depending on the mode the user selects, it can be make to feel "notchy" so the engine changes revs co-ordinated with steps in the ratio, to feel like a traditional auto box, rather than a CVT.

Mine's the robobox, with twin clutches, so behind the scenes it predicts and pre-selects the next gear.

[David]

We recently had an A class Merc (mkII) hire car that had a cvt, and some Audis use CVT.

But then, in the last decade, Audi have used *many* types of "auto" box:

Multitronic; tiptronic; CVT; S-Tronic; R-Tronic, and normal automatic, to name a few....

:-)

nor driving hard.

Modern autos are programmed to change up early, due to the CO2 reducing maps now used.

David

[newshound]

The Jazz hybrid CVT has seven notches, and paddles which seemed silly to me at first, but they do let you use engine braking going down hill. Curiously, though, this does not show significant recharging of the battery: you need to use the brake pedal to achieve that.

[Capitol]

If it's a modern auto, then it's programmed for fuel economy. It will hunt through the gears, up to 7 I believe in some versions and use the engine speed/ fuel mix ratio that seems appropriate. Some engine management systems program themselves to suit the driving style of the owner over a period of time. Generally using cruise control IME gives best fuel economy but the results can vary with speed, ie a lower speed can give worse fuel economy. Mine is most efficient at 55mph. 70mph is about 10% worse.

[Dave Liquorice]

Define "modern" it was a W reg Land Rover Discovery II with TD5 engine. The TD5 is a completly "fly by wire" engine. The auto box is a 4 speed ZF4HP22.

It's pretty smooth at cog shifting under normal driving but put your toe down going up hill the kick down is detectable or the changes up when starting slow and accelerating well (for a Discovery...).

I got the impression it was sitting in torque convertor mode most of the time and hardly ever locking up but could be wrong.

[Huge]

I just bought a 6 speed, 3 litre Disco4. By Disco standards it's quite quick. Certainly makes my old Disco 2 look like an embarrassment.

(There's a local road near my house we call "Nearly Killed Hill" because of a slight embarrassment trying to overtake a 40-tonner going up it.)

[Dave Plowman (News)]

That is pretty old as regards auto technology. The 4HP22 dates from IIRC the 80s. My '92 BMW had a 5 speed ZF.

Think that box only locks up in 4th. It probably spends most of its time in 3rd on country roads.

At one time, autos and diesels weren't a happy combination. London Taxis being an obvious example. The boxes seemed to be petrol ones just mated to a diesel - so the characteristics were better suited to a revvy petrol engine than a slogging diesel. Things are much better these days.

[Scott M]

(Not seeing Dave L's posts, so pinching Dave P's :-))

When I read the OP I immediately wondered if it was a Disco 300Tdi auto

- I've heard that exact description of its operation.

Slack torque converters strike me as the worse thing about Diesel auto combinations - sort of unable to transfer the lo-down torque from one side to the other. The BMW 330d got given a GM box that was unbelievably slushy and unpleasant.

[Andy Burns]

With cruise control, it will maintain speed up hills and then have to brake once over the crest, so long as nobody is up your chuff you can beat the economy by losing some speed going up and re-gaining most of it again on the way down.

[Bill]

In message , Scott M writes

I have to say that my 300Tdi auto Disco Mk1 suits me pretty well, although it's a shame about the rust. The box does feel slushy, but a regular route involves an entry into an uphill main road where good acceleration is needed to match the traffic speed. Flooring the Disco, it accelerates far better than I can achieve with going through the gears in my manual Octavia. I have achieved 33mpg across North Wales and back, where I've been able to cruise at about 55 the whole way. It locks up at just over 50 in top.

[The Natural Philosopher]

our auto deezil freelander has a superb box. Good as the jaguar used to be.

[bert]

In message , Andy Burns writes

I'm confused. CVT by definition means constantly variable transmission. So where do these "steps" in the transmission come from The old DAF certainly didn't have them.

[bert]

In message , "Dave Plowman (News)" writes

The 200TDi Disco auto being an even better example.

[bert]

Best auto I ever had was a Jeep Grand Jerokee 4.0 litre petrol. My 4.0L petrol V8 Defender auto wasn't bad but at about 11 mpg was a tad expensive.

[Andy Burns]

Since the computer is shifting the input and output cones, it can do it by "jumping" from one point to the next, rather than by "sliding" through the ratios ... it can still do that too, but the driver has the choice.

[Dave Liquorice]

Quite probably, 60 on the straight bits but a lot is 50 or less.

This combo seemed to work well once you'd got used to it just going to 2,500 rpm and more or less staying there if accelarating or going up hill. The drop to idle on over run was a bit of a surprise and I wasn't convinced I had as much engine braking of my manual. Though if it did start to run away on a hill it would up the revs a bit and sort of prevent it.

Anyway it seems that it's probably set up for "economy"/"effciency" so I'll try a few long cruises at 2,500 rpm in my manual and see what affect it has on MPG.

[Dave Liquorice]

AFAIK the cruise on my car has no control of the brake. I shall have to try it on some of the hills that I normally decend using nothing but the engine and 3rd gear to brake. Unlike 90% of other drivers who go down on the brakes. Of course when cruise is using the engine as a brake it's not burning any fuel or very very little.

Having had the brakes fade on a Mondeo 3/4 the way down an "enthusiastic" descent of Hartside (1300' in 4.8 miles) I'm a little wary of over using the brake on a long decents.