Fuel efficient driving.

tomstickland

Inactive wrote: »

Since when has driving at 70/90mph been " good practice?

I explained that high cruise speeds are an obstacle to fuel efficient driving and if I lowered them then I'd expect better results. Same applies to Landy Andy comments.

tomstickland

skiddlydiddly wrote: »

A lot of your points are sound but the bit about acceleration is flawed.If you do a 0-60 for example and do it with steady acceleration, you will use nowhere near the amount of fuel you use doing it by mashing the accelerator.The variable is that as you put your foot down more, a LOT more fuel goes into the engine.Its more complicated than using x amount of fuel at 2000rpm and assuming its double that at 4000rpm because it changes with the amount of throttle you use significantly.

Both my examples were steady acceleration. One was double the rate of the other.

I did not assume that fuel consumption would be double at 4000rpm rather than 2000pm. I didn't mention rpm at all.

I said that the amount of work to be done is the same and doubling the acceleration required half the time and that these two things balanced out.

I agreed that at maximum throttle then fuel is wasted and drive train losses are increased. At very small throttle angles the efficiency is poor too. This is because the engine does not have enough load on it. A BSFC (brake specific fuel consumption) map with rpm as the x axis and load as the y axis will show contours of fuel consumption. The minimum bsfc occurs at a load around 50-75% of max load.

I have done a fair bit of drag racing and over a 1/4 mile I use probably 1/4 litre of fuel as the injectors are working flat out.Now if I slowly accelerated over the same distance I would use virtually nothing as the injectors would barely be pulsing.

But your final speed would not be the same. If you continued accelerating at this lower rate for long enough to reach the same speed as you did before then you would use as much fuel. Longer time at a lower rate.


I still stand by what I said and do not see any flaws in it.

I recently had my ECU remapped and asked for an economical zone around 60-70mph for the motorway.I have a wideband lambda fitted(so can accurately monitor A/F ratio)and I am using less fuel at this speed than if I was going slower and can actually see the mixture leaning off, so more revs but less fuel.My motorway mpg jumped from 30 to 38mpg so despite being a boring part of the map, its certainly worth it.

Good point - manufacturers do this too in order to try and obtain good book figures.

If you were to drive at the same rpm zone in the next gear down (lower speed) then I'd expect you to obtain even better mpg.

If you had the full bsfc map available then you could select a combination of gear and speed to get you into the best area.

Changing the engine rpm would allow you to move side to side on the map. As the speed increased the load would increase too. When you changed up a gear then the rpm would drop and the load would increase.

skiddlydiddly

You also need to factor in things like fuel being used to cool the cylinder temps at higher revs, so its not getting burned and wasted whereas at lower revs its not required.Its more complicated than it seems.
Also, you are right about the speed difference over my example of a 1/4 mile but are forgetting that if say I'm doing 110mph at the 1/4 mile mark, if I slowly accelerated up to 110mph I would cover considerably more distance, easily triple or quadruple so is more efficient.If your car has a trip computer with live mpg figures just watch how it drops when you boot it(obviously these aren't 100% accurate but give you an idea).

alexlyne

Hammyman wrote: »

The correct way to do it is to lift off the accelerator at the 200m marker for the exit and use engine braking meaning you use no fuel whatsoever from that 200m marker to the end of the sliproad. You are right about trying to time it so you don't need to stop. That should be the aim for every junction, roundabout, traffic light.

Eh? No!
The slip road is for deceleration/acceleration, not the motorway!
Go read the highway code!
(assuming you mean the 200m marker before a motorway exit)
(for roundabouts /junctions etc that affect everyone, then fine)

Driving is about being considerate to other road users, not purely a quest for best mpg regardless of others.

cyclonebri1

Two observations of my own; Firstly that accepting you don't treat the road as a racetrack, ie, you drive with reasonable not excessive accelaration, braking is the killer to fuel consumption. Try to avoid braking and engine braking whenever possible, changing gear when the speed has fallen naturally to the next downshift point.

And especially if you have a turbo diesel the most efficient use of the engine appears to be when the turbo is just fully operational, about 1500 to 1800 rpm in most cases, above and below this fuel comsumption suffers.;)

greenman7

Originally posted by cyclonebri1
Two observations of my own; Firstly that accepting you don't treat the road as a racetrack, ie, you drive with reasonable not excessive accelaration, braking is the killer to fuel consumption. Try to avoid braking and engine braking whenever possible, changing gear when the speed has fallen naturally to the next downshift point.

When I am engine braking I use hardly any fuel, to me it is not to be avoided it is to be encouraged. If I drop to a lower gear obviously I have to put my foot on the accelerator and use more fuel. I love this about the modern engine, I think this is one of the main reasons i get 64.3mpg overall in the last two and half years of measurement.

epninety

Strider590 wrote: »

Your wrong...... Not by much, but you are wrong.

When you put your foot down, the engine management system dumps a load of extra fuel into the inlet manifold.

If you accelerate slowly you can avoid this happening.... But you have to accelerate EXTREMELY slowly to do this.

Going back in time..... With a carburettor driven engine, there's a valve in the carb which pumps more or less fuel into the inlet depending on how fast you push down on the pedal, the harder you push, the more fuel it dumps for the initial "kick" of acceleration. This is why on these older engine's, smooth acceleration is vital, too much fuel and the engine will misfire and could even stall.

You're also wrong...

The EMS typically only puts 'a load' of extra fuel in when the throttle is wide open (pedal on the floor or close to). The rest of the time, lambda controlled systems are still running closed loop (fuelling controlled by exhaust emissions) and since typical narrowband sensors can only read one ratio accurately, that will be the ratio that is delivered. When under wide open throttle, the ECU reverts to an open loop map, delivering fuel calculated to put the engine close to the maximum power A/F ratio.

The accelerator pump you are describing in the carburretor exists to counteract the 'lean stumble' caused by suddenly admitting the A/F mix in the manifold which doesn't contain sufficient fuel for the new demand since the carb cannot instantly change the contents of the manifold, or instantly react to demand. The constant condition fuelling is still controlled by the main jetting. (This is assuming a Weber style carb, the type typically fitted with accel pumps).

birkee wrote: »

Quote:
Older carburreter engines. My memory says that in overun (engine braking) the vacuum in the inlet manifold increases significantly, and sucks in more fuel than driving does.

Again, typicaly the vacuum is generated by the engine pumping air out of the manifold which is closed by the throttle plate. Since the main jets are upstream of the throttle plate, they do not admit fuel during this period - fuelling is provided by the idle jettings which admit enough fuel to prevent stalling and provide a decent 'driver feeling' (no engine stumbling, lurching, backfiring etc.)

cyclonebri1

greenman7 wrote: »

When I am engine braking I use hardly any fuel, to me it is not to be avoided it is to be encouraged. If I drop to a lower gear obviously I have to put my foot on the accelerator and use more fuel. I love this about the modern engine, I think this is one of the main reasons i get 64.3mpg overall in the last two and half years of measurement.

You don't get the point though. By using anything, be it gears or actual brakes, no you aren't using any fuel, it's shut off, but you are wasting the momentum or kinetic energy you had. That cost you fuel to create and will cost you fuel again when you speed up.

Put that into practice and you may manage 70mpg.;)

almillar

Hammyman - myhoose explained my argument perfectly. I gave an example of when *I* think it's OK to freewheel. I understand your point that idling whilst freewheeling is using fuel and engine braking uses none, but I'm not sure I agree that using engine braking is more efficient, it depends on the circumstance, and my basic rule would be, if I want to slow down, engine brake, if I don't really want to slow down, freewheel. Certainly with your example, slowing down and speeding back up again would be less fuel efficient than just idling for a while.
Agree with the above that SLIP ROADS are provided for you to slow down and speed up in, NOT MOTORWAYS. DON'T try to save fuel not accelerating enough or slowing down early. THAT is dangerous!
And freewheeling is NOT an offence BTW.

Limey

None of the above instructions apply when driving my everyday runabout. I've tried and no matter what it gets a stirling 22mpg.





At least you never have to slow down as everything gets out of your way. :eek: