IanWatson

Hello all!

Not sure if anyone recalls, but a few weeks ago there was a short item on TG about fuel efficiency which has been playing on my mind now and again.

Although it seems slighty counter-intuitive, a (Scandanavian?) study suggested that rather than feathering the throttle when driving for economy, petrol engines are actually at their most fuel-efficient whilst under load, and the suggestion was to keep engine revs under 2k and use three-quarters throttle for maximum economy.

This doesn't 'sound right' to me (although I don't actually know anything about the subject!) - and perhaps the effectively dual-engine nature of a turbocharged engine complicates things further.

any views? (and I know it's not really a paradox!)

Ian Griffiths

Well, that is how I drive out of habit and I've been rather pleased with the fuel consumption figures that I can return. Not in a Scoob mind, and not very scientific either but TG certainly sounded plausable.

millband

IIRC, the study said it was best to use peak of torque curve for acceleration to cruise speed, then be gentle when you got there.

I think the gist was you should avoid acceleration as much as poss for economy, but when you do accelerate, do it using the most efficient part of engines rev range.

Steve

[This message has been edited by millband (edited 18 July 2001).]

Diablo

Does anyone really care

LMAO at this one.....

D

sasman

Yes that report was sponsored by BP i think.

GaryC

From experience I can say it is true. Granted I have been trying it in a 330d, but mpg is better if I (near) floor it to say 60, then sit at 60 rather than gently feather it to 60. I have a instant mpg guage on the dash so can see when fuel consumption drops. Flooring it sends the needle off the scale, but 6-7 seconds later when up to speed the neddle drops back top 50+

SDB

bare in mind here, I know NOTHING about engines , but...

it may be that whilst under load, every bit of fuel that is burnt is used to propel the car forwards (as the engine in under load), but while feathering, some of the energy is wasted in just keeping the engine at the speed of the wheels???

get what I mean?

I could be talking out of my bottom though

IanWatson

Simon - isn't that load always present? (again not sure which orifice I'm using!)

brickboy

Keeping it under 2K and using lots of throttle isn't fuel efficient, as most petrol engines typically don't work well until spinning above 2K ....

But it's certainly more fuel efficient to squirt it on acceleration to reach the desired cruising speed, than to accelerate gently to that speed.

All to do with gaining momentum -- imagine you're at the bowling alley. You can either give the ball a swing to quickly accelerate it to the desired speed then let go, or you can walk all the way down the lane pushing it gently. The first requires a larger initial effort, but over a much shorter time so is overall more efficient. The second requires a smaller initial effort, but over a much longer time = more fuel used.