does coasting save petrol

Wig

tbourner wrote: »

There was also a Scion forum where I found a link to this handy gadget, you could buy one of those if you really want some of your own 'evidence'.

I didn't read the rest of your links yet, but on this one.....unless your car has a fuel flow meter (and this gadget is capable of reading it) it is only estimating the fuel rate, which is what I already have on my Punto's inbuilt fuel display..

What is needed IMHO is an aftermarket fuel flow meter to be installed into the fuel line, but after googling for one I doubt any exist that would be accurate enough to measure down to 0 - 0.5 l/hour - 10 l/hour

Tobster86

Tbourner, you seem to have the wrong end of the stick a bit. Wig, you're still talking nonsense.

In all cars you WILL USE LESS FUEL if you go down a long slope (or even approach a stop; the physics are no different) in gear than in neutral.

An engine takes an amount of fuel to sustain idle which WILL be used by the injection system when idling. If the revs are above idle and there is no demand from the accelerator pedal, the injection system will use an amount of fuel between nothing and significantly less than idle fuel.

If this was not the case, then (A) it would be pointlessly less economical; why inject unneeded fuel? And (B) have less engine braking.

With the old mechanical VE and electronic VP injector pumps (the bottom sections are identical) you can see that this would be the case. Open one up/do a google image search and have a look at how they work, infact I have a couple disassembled in my shed that may get photographed if this rediculous debate goes on much further. Some of the VE pumps were actually fitted with an additional mechanism that dampened the effect of taking your foot off the accelerator to reduce the initial engine braking surge.

You may wonder why I'm getting all uptight about it. I used to live near and frequently use Sutton Bank in North Yorkshire (Wikipedia it) and witnessed several crashes where cretins had thrown the car in neutral at the top, come down on the brake, and by the time they reached the hair pin half way down their brakes had overheated and stopped working. And before the crash, they'd actually used more fuel than if they'd have just stuck it in 2nd/3rd and come down at the same speed!

Idle coasting is absoutely pointless, potentially dangerous, bad practise.

Engine-off coasting is potentially dangerous if the driver doesn't know what they're doing. It uses absolutely no fuel, at the expense of possibly more brake wear.

Neither should be condoned nor recommended.

Wongsky

Wig wrote: »

I didn't read the rest of your links yet, but on this one.....unless your car has a fuel flow meter (and this gadget is capable of reading it) it is only estimating the fuel rate, which is what I already have on my Punto's inbuilt fuel display..

What is needed IMHO is an aftermarket fuel flow meter to be installed into the fuel line.

Why?

Your car's ECU already knows when it's pulsing the injectors, plus it's almost certainly aware of how lean / rich it's running (by monitoring downstream sensors) it's running.

Do you seriously doubt any semi-modern car (say, for example, from the early 90s) petrol engine, with a catalytic converter, that has EFI cuts fueling when it doesn't need it (ie overrun)?

Really?

Most petrol cars from that era onwards will likely use EFI systems from the usual suspects often with common parts (eg injectors, sensors, ECU units). There may be some variance in programming based on specifics to do with engine characteristics but you can often see very similar implementations across several makes and models.

I really don't get this skepticism and denial about fuel being cut on overrun. Ask yourself this, why wouldn't they do it?

Occam's razor does tend to follow.

tbourner

Tobster86 wrote: »

Tbourner, you seem to have the wrong end of the stick a bit.

How so?
You use no fuel on overrun, so when coming to a stop at a set of lights that have just turned red; it is the best option (or, if you're particularly far away judge it so you use efficient engine revs to get you the rest of the way).
If you want to travel as far as possible on a flat with no obstacles then you want the engine to be operating and pushing you along, which means using more fuel than the 'idle' situation, so again you want it in the most efficient rev range as possible. Personally I think this is around 1600 rpm, 5th gear is usually 1:1 but the speed you'd be carrying would be bad for economy, so maybe 2nd or 3rd might be better, I said 4th the first time. If you keep constant throttle you'll go a long long way.

The original question seems to be a bit lost now, because it's not a simple answer and different situations mean different things. In very simple terms overrun in gear uses no fuel and coasting in neutral uses the same amount of fuel as sitting with the engine running. Overrun obviously creates engine braking so you'll stop sooner than coasting, so coasting uses fuel but you'll roll a lot further.

Wig

Tobster86,
You're not arguing with me............. you're arguing with the vauxhall user manual that I quoted, and you are arguing with a lack of specific published information -from vehicle manufacturers- on what their cars actually do under these conditions.

And your categorical statement cannot be justified, because there is always the potential that if you coast in gear your engine braking will cause you to apply throttle sooner than if you coasted out of gear, and it is possible (if 'overrrun' is a feature of the vehicle) that either method may be the better method to use depending on.......
the speed,
degree of slope (incline, flat or decline),
road surface,
presence of other vehicles/traffic lights on red/roundabouts infront of you,
your top speed limit before braking

Tobster86

There is no situation where there is any advantage to coasting out of gear in idle; it is burning fuel to do nothing.

Give me an example of where you think that coasting will save you fuel by not having to accelerate as soon.

And I'm not arguing with the Vauxhall manual. I will however point out that it's over-extravagently describing completely normal functionality of an injection system as a special feature. It doesn't suprise me that Vauxhall would tend to do this because they don't have much else to say about the quality of their cars.

Wig

Tobster86 wrote: »

There is no situation where there is any advantage to coasting out of gear in idle; it is burning fuel to do nothing.

Give me an example of where you think that coasting will save you fuel by not having to accelerate as soon.

Read post #38 & #39, there are lots of occasions where taking it out and idling will allow you to travel further before having to reapply throttle to accelerate again. The fuel used over distance A to B in the out-of-gear idling mode can be less than the in-gear mode, it's basic logic that either method will be more beneficial than the other according to the conditions (assuming that 'overrun' is a feature on the vehicle), knowing and measuring the exact fuel used is IMHO impossible, so where the line is drawn is purely guesswork.

Wongsky

Wig wrote: »

Tobster86,
You're not arguing with me............. you're arguing with the vauxhall user manual that I quoted, and you are arguing with a lack of specific published information -from vehicle manufacturers- on what their cars actually do under these conditions.

And your categorical statement cannot be justified, because there is always the potential that if you coast in gear your engine braking will cause you to apply throttle sooner than if you coasted out of gear, and it is possible (if 'overrrun' is a feature of the vehicle) that either method may be the better method to use depending on.......
the speed,
degree of slope (incline, flat or decline),
road surface,
presence of other vehicles infront of you,
your top speed limit before braking

You, and others, keep talking about driving in gear resulting in needing to use the throttle sooner, well that's probably true, but if the revs are above around 1200, you'll be using no fuel, with the throttle closed, and you'll likely only need a little throttle, using minimal fuel, when you do need to use the throttle again.

Wig

Wongsky wrote: »

You, and others, keep talking about driving in gear resulting in needing to use the throttle sooner, well that's probably true,

It's not probably true, it's definately true.

and you'll likely only need a little throttle, using minimal fuel, when you do need to use the throttle again.

...........Similarly, when resuming acceleration after a period of out of gear coasting, your accelerator position will be the same as after the in-gear coasting, so the only difference between the two methods remains the fuel used over the distance A - B in the out-of-gear coasting mode

post #48............knowing and measuring the exact fuel used (A-B) is IMHO impossible, so where the line is drawn is purely guesswork.

Wongsky

Wig wrote: »

It's not probably true, it's definately true.

...........Similarly, when resuming acceleration after a period of out of gear coasting, your accelerator position will be the same as after the in-gear coasting, so the only difference between the two methods remains the fuel used over the distance A - B in the out-of-gear coasting mode

post #48............knowing and measuring the exact fuel used is IMHO impossible, so where the line is drawn is purely guesswork.

Thing is, when driving, the most economical periods are going to be when you're travelling at a reasonable speed, your revs aren't too low, and your foot completely off the throttle. Next to that, will be when only the lightest use of the throttle is sufficient to maintain the desired speed.

So to get the best economy, is a combination of maximising those periods and using good observation, planning and anticipation to reduce periods of braking (ignoring new fangled regenerative braking for a second) and acceleration.