Diesel Automatic Fuel Economy - Cardew
There is a thread in 'Discussion' asking advice about diesel automatics. Somebody wrote:

"In theory diesel engines should be better suited to automatic boxes than petrol, because of the way they deliver power and torque."

I would also have thought that high torque at low revs - like the big lazy V8s in American cars - would be ideal.

However it doesn't seem to work out like that in practice on many cars. I owned a Merc 300D(non turbo) auto and the fuel consumption was poor - much less than two 240Ds I owned. Living in Germany I questioned several Taxi drivers who had the same experience.

I also note that in the Car by Car breakdown HJ states;

"Steptronic auto increases fuel consumption of 330D by around 25%."

Obviously an Auto will normally have a slightly higher fuel consumption than a manual, but not of this magnitude.

Has anyone got a technical explanation?
Diesel Automatic Fuel Economy - RogerL
I don't know about diesel automatics but although the overall consumption of petrol automatics is slightly lower than the equivalent manual, the consumption is actually better in certain circumstances.

Motorway cruising consumption can be slightly better in an automatic as the overall gearing is usually higher and the torque converter is locked out.

Towing consumption can also be slightly better in an automatic, I suspect due to the box selecting the correct ratio and changing more frequently than the driver of a manual.

The consumption down side of an automatic is around town where it is considerably worse, but this is just where an automatic is most relaxing.
Diesel Automatic Fuel Economy - Cardew
I agree with most of that, except the around town bit. Mercedes published an article some while ago that said research suggested the opposite was true in town. Their reasoning was that most of us will hang on to an inappropriate gear if we can see ahead and know we are having to slow down. For instance if we are in 3rd gear and see traffic lights a couple of hundred yards ahead we(some of us) might stay in 3rd rather that change up and down again. An automatic won't.

However the reason I put this in the technical section is that I am puzzled by the adverse fuel consumption figures of diesel automatics when it would appear that their characteristics would suit an auto.

Diesel Automatic Fuel Economy - 659FBE
I suspect that the reason many diesel automatics do not show good fuel consumption figures is due to the high engine torque at low revs, which maximises torque convertor slip. Coupled with the weight penalty of a diesel, this is a sure-fire formula for bad figures unless the box has a torque convertor lock.

Although the later cars have this (usually on the upper ratios only), the amount of slip in the lower gears during town driving is appreciable.
Diesel Automatic Fuel Economy - RogerL
Around town, an automatic gearbox is using the torque converter as lock-out doesn't occur until 50 mph, or thereabouts. The gear ratio might be slightly more efficient but the torque converter efficiency is quite poor.

Conventional gearboxes with auto controls, like Alfa-Romeo "Selecta?" or Vauxhall "Easytronic" may prove more economical around town as they have no converter power losses.
Diesel Automatic Fuel Economy - Dizzy {P}
I suspect that the reason many diesel automatics do not show good fuel consumption figures is due to the high engine torque at low revs, which maximises torque convertor slip. >>

This reason hadn't occurred to me but I think you are spot on. It does seem that the higher the torque output, the greater the mpg impact. This would explain why the BMW 330 and 530 diesels in particular lose a lot of their mpg advantage when mated to an automatic box.
Diesel Automatic Fuel Economy - Dave_TD
Pity the Octavia diesel auto only comes in turbo guise, I find the non-turbo has more torque from idle up to 2000rpm, which would (if other posters here are right) make it less liable to loss of economy with an auto 'box. Hmmm.
Diesel Automatic Fuel Economy - Cardew
That sounds like a plausible explanation - thanks.

Although by the same reasoning a big 'lazy' V8 with masses of torque at low revs should suffer from the same differential in consumption between Auto and manual versions around town?

Diesel Automatic Fuel Economy - Torque
The biggest difference between engine rpm and transmission input shaft speed occurs when the vehicle pulls away (due to torque converter slip). The difference gradually decreases as the vehicles speed increases. This is one main area that fuel economy is lost.

Earlier generation vehicles the torque converter clucth (if fitted) was not applied until a speed of around 45 mph was achieved.

The Mercedes car did not have a lock up clutch in their torque converters until the electronic 5 speed (722.6) unit came out around 1996.

Some later vehicles now apply the lock-up clutch at much lower speeds to try and improve fuel economy, on certain vehicles this can be in 2nd gear and at speeds of less than 20 mph.


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