Economical driving styles?

Posted by: Top Cat on 05 May 2005

Hi folks.

Have just borrowed an older 535i from my father, who doesn't use it very much. Taxed, insured, and thirsty. Just looking for tips for minimising the fuel consumption as I averaged well under 20mpg over the first 100 miles, which is fine but I'd prefer to keep the visits to the pumps down (and hence cost). Mainly, however, I'm treating it as a bit of a challenge to see just how economical I can get the old girl to run. After all, I like a challenge...

What's more, there's a wee guage below the rev-counter which shows current fiel consumption - and if I'm not careful I find the dial shooting off to the right below 10mpg. Ouch!

A couple of questions:

I've been trying the 'keep the revs below 3000rpm at all times' trick, and easing right off on the accelerator wherever possible - and using as high a gear as early as I can. However, is there a 'happy rpm' figure in which the engine is operating at its most optimal? Sometimes I can get the revs down to 1100rpm in fifth gear, but I wonder if running so low is somehow counter-effective.

Next question is to do with hills. WHat's the best strategy? Build up speed on the straight and use less accelerator up the hill, or try to keep the accelerator constant and let the engine find its own speed (or lack of it, heh heh). The car is quite heavy so its apparent consumption is highest when going up hills almost regardless of the speed.

Trying to see how economically I can get this brute of a car to run has been quite illuminating thus far, and an exercise in restraint. Quite difficult compared to regular driving as well, as the temptation to put the foot down and hoof it along must be suppressed...

Any tips? (no get a bike/scooter/bicycle/walk stuff - of course they're options but not really the point, and recommending an alternative car is all fine and well, but my father has loaned me this car and so it's otherwise costing me nothing)

Ta,

Wannabe-Eco John
Posted on: 06 May 2005 by Top Cat
quote:
a smoke issueing clunker
Well, it passed its MOT less than a month ago, and I know for a fact that its emissions aren't anywhere close to being borderline - especially considering its engine size - though given its age the rules are less stringent than for a new car.

Anyway, just for the record, I need the car for a round-trip of 18 miles, Monday-Friday, until at least mid-July or possibly as long as late-August. So, my estimated mileage would be something in the region of 100/week (allowing for the odd trip elsewhere) - i.e. a couple of thousand tops, until Peru. Might give it back if I don't need a car on my return.

John
Posted on: 06 May 2005 by andy c
What ever you do make a note of the mpg on the trip pc or whatever it has fitted - most helpful and can assist you no end.

When we put our mercedes in for some work at the dealer, the fuel Pc gave up they had thrashed it - I gave eem hell and they had no defence.

Red Face

andy c!
Posted on: 06 May 2005 by JeremyD
quote:
Originally posted by Nime:
Are we certain about this "brisk acceleration" Jeremy?

I was taught that this guzzled fuel.
The so-called "leaden foot syndrome" was much warned against.

Early changing up without labouring the engine was always the order of the day.
Maintain speed without uneccessary braking.
Drive as smoothly as possible.
What qualifies as "brisk" is subjective, from the driver's point of view, and therefore it's not a terribly useful adverb here. However, the point is that accelerating unusually slowly is likely to waste fuel rather than save it, due to the inefficiency involved in running the engine at varying revs for more time. Perhaps I should have said, simply "accelerate normally rather than unusually slowly" but we all have different ideas of normal...

Incidentally, engines' greater efficiency at constant revs explains (mainly) why hybrid petrol/electric cars get such good fuel economy: the petrol engine runs at constant revs as much as possible.
Posted on: 06 May 2005 by Brian OReilly
quote:
Originally posted by JeremyD:
However, the point is that accelerating unusually slowly is likely to waste fuel rather than save it, due to the inefficiency involved in running the engine at varying revs for more time.


Are you quite sure about that ?
Posted on: 06 May 2005 by Nime
quote:
Originally posted by JeremyD:
the petrol engine runs at constant revs as much as possible.


I really wouldn't mind an electric car myself.
I've seen a few about sans exhaust tail pipe in the city.

There are quite a lot of small electric "bubble cars" (fibreglass) from the past (70-80s?)standing about in gardens and yards over here. But I haven't seen a single one on the road. I would imagine with that large, moulded perspex bubble they represent the nearest thing to a mobile mini-greenhouse. Roll Eyes

Nime
Posted on: 06 May 2005 by JeremyD
quote:
Originally posted by Brian OReilly:
quote:
Originally posted by JeremyD:
However, the point is that accelerating unusually slowly is likely to waste fuel rather than save it, due to the inefficiency involved in running the engine at varying revs for more time.


Are you quite sure about that ?
I was until you questioned it... I thought it was well established?
Posted on: 06 May 2005 by Nigel Cavendish
Accelerate slowly, change into high gears early, drive at as low a speed as possible, leave plenty of distance to the car in front, look ahead to anticipate having to reduce speed (both of which reduce the need to use the brakes) and enjoy?
Posted on: 06 May 2005 by Brian OReilly
Heh, heh, heh. Sorry Jeremy, I didn't mean plant seed of doubt..

It's been a long time since I had any involvement in part-throttle mapping, but as I remember, there are contour curves of fuel consumption, which show that an engine is most fuel efficient at about half max rpm and 1/2-3/4 throtttle. Efficiency decreases at full throttle as excessive fuel is added to ensure maximum power and also to provide some cooling of the chamber. Higher engine speeds mean less efficiency due to increasing friction.

There is a difference between engine efficiency and increasing mpg. No matter how efficient your engine is, if you demand more work ie acceleration or speed, then you use more fuel. You might be using it more efficiently, but you will still be using more.

In reality, it's practically impossible to keep to a low consumption strategy without using the gauge or on-board computer. I would suggest using as little throttle as possible whilst accelerating through the gears with max 3500rpm. Watching the gauge will give you a "feel" for the right amount of throttle.

Regards,
BOR
Posted on: 06 May 2005 by Berlin Fritz
Makes yer wanna throttle summfink dunnit ? Smile
Posted on: 06 May 2005 by Rana Ali
All gasoline engine fuel consumption maps I've seen have maximum efficiency points between 2000-2400 rpm at about three-quarter load (approx 3/4 throttle). So that's where you want to aim to cruise. As far as accelerating, do it steadily and slowly.

Peak torque maybe peak volumetric efficiency but that is not the same thing as best fuel efficiency. At peak torque points modern engines need to be fuelled for the best torque versus knock compromise.

Regards

Rana
Posted on: 06 May 2005 by Polarbear
Eek foot to the floor all the time!

Lifes too short to be conservative Razz
Posted on: 06 May 2005 by Berlin Fritz
quote:
Originally posted by Polarbear:
Eek foot to the floor all the time!

Lifes too short to be conservative Razz


No, it's a Gas innit ! Red Face
Posted on: 07 May 2005 by Gianluigi Mazzorana
I've had a 318is for several years and driving fast and sharp was just normal fun.
But those were days when fuel were cheap compared to the liquid gold we're putting in our cars.
Maybe you could think about a methane set which is not dangerous as gpl gas is (it doesn't explode in case of crash) , let you save a lot, keep the engine clean and let your car run as much as a normal fuel feeded one.
I guess you don't need to hear the "roar" but driving comfortable is such a win!
Maybe the nearer bmw dealer can give you advice.
Ciao Graham.