does coasting save petrol

Wongsky

mikey72 wrote: »

If you lift off, the throttle will close.
But the ECU will open the idle control valve to keep air supplied to the engine if it hasn't cut the fuelling yet, even if it's above idle.

I get what you're saying - I just doubt the IACV reacts specifically to the overrun scenario, that's what I'm saying.

I believe it has a stepped position it either maintains, or reacts to when the road speed is above a certain low threshold.

Which is why I asked for examples, specifics.

mikey72 wrote: »

There must always be an option to keep air supplied, either by a butterfly, by the idle air control valve, or whatever is on the system to do it.
Otherwise everytime you lift off to overtake, change gear, the engine would be starved of air and die.
Even if you lifted off in gear, you've notice a sudden stall if the air was restricted.

With you on that - just not buying that the IACV reacts specifically to overrun, I think it just maintains a slightly stepped position - either maintained (relevant input probably CTS, ambient air, possibly, or other air / temp related sensors) or reaction to lifting the throttle, whether overrun, or simply lifting but no connection betwixt transmission and engine.

Road speed is normally a factor, as idle tends to be a little boosted until almost stopped.

mikey72

tbourner wrote: »

I'm arguing that for any slope where you would increase speed in neutral, if you keep the car in gear and maintain the same speed as you would be in neutral, you'll save fuel.
That's the optimum proof of the concept, rather than saying you'll get more speed out of coasting etc. So how do we prove that without lots of arguing over how cars work in theory?

What about on the level, on a quiet road, approaching a set of red lights, that'll still be on red when you arrive, so you will be coming to a complete halt, from fourth gear?

And if you maintaining speed using the throttle in your scenario, you won't achieve fuel cut off anyway.

mikey72

Wongsky wrote: »

I get what you're saying - I just doubt the IACV reacts specifically to the overrun scenario, that's what I'm saying.

I believe it has a stepped position it either maintains, or reacts to when the road speed is above a certain low threshold.

Which is why I asked for examples, specifics.

With you on that - just not buying that the IACV reacts specifically to overrun, I think it just maintains a slightly stepped position - either maintained (relevant input probably CTS, ambient air, possibly, or other air / temp related sensors) or reaction to lifting the throttle, whether overrun, or simply lifting but no connection betwixt transmission and engine.

Road speed is normally a factor, as idle tends to be a little boosted until almost stopped.

What are you defining overrun as?

tbourner

mikey72 wrote: »

What about on the level, on a quiet road, approaching a set of red lights, that'll still be on red when you arrive, so you will be coming to a complete halt, from fourth gear?

Same. If you maintain the speed you would achieve when coasting, but in gear, you'll use less fuel.
I was just trying to think of it in a way of an experiment someone could do if they had an OBD reader and iPhone app or something.

Wongsky

mikey72 wrote: »

If you're using brakes, don't coast.
Use engine braking as well.
You want to stop.

Eh?

Some people clearly have contributed to this thread, that they coast more, then have to use their brakes more.

mikey72 wrote: »

If you want to travel further, coast.

Why?

Why would that be clearly favourable to a bit of overrun, and a bit of light throttle, bit of overrun, the less braking than you'd probably have to do if just coasting?

If either controlling / maintaining or eventual deceleration is desired, the more you use your brakes, the more you're converting kinetic energy to heat.

(that's not me arguing for changing down through the gears to do most of the deceleration due to compression, though...)

mikey72 wrote: »

No one is disputing modern cars do it.
Anything with a cat must, otherwise it'll destroy the cat.
You don't have a choice in it.
It's advertised as economy as people buy that.

You mean they lie?

Why?

Are you disputing it being true?

I doubt it's largely advertised as anything.

mikey72 wrote: »

And I doubt a car manufacturer would ever advise coasting.

You know what a lot of modern autos (I'm talking about torque converter / planetary geared autos) do when lifting off when on the move?

Engage lock-up, if they're not locked-up already. Wonder why?

I doubt they advertise that, either.

Wongsky

mikey72 wrote: »

What are you defining overrun as?

I'm defining overrun as being completely off the throttle pedal, with road speed, in gear, with revs above, say, 1300.

mikey72

tbourner wrote: »

Same. If you maintain the speed you would achieve when coasting, but in gear, you'll use less fuel.
I was just trying to think of it in a way of an experiment someone could do if they had an OBD reader and iPhone app or something.

I come out of gear early, and coast on no throttle, until I get to the lights.

Or I need to accelerate further towards the lights, change down, into third, then second, and finally stop.

Assuming I guess the positions right, I'm idling in the first option from as early as possible.

In the second, I doubt I achieve any fuel cut off, as each time I change gear, and declutch, the revs die and the ecu needs to fuel. There is a delay to shut off, so it fuels, then I change again.

Probably never resolve it though.
Mine will idle for about 5 hours on a gallon of petrol.
That's take a few years coasting to lights in my driving.

00ec25

Hi again - OP here - thnks for so many posts, I'll have to read them slowly to follow the arguments but at the risk of offending those giving scientific explanations what I care about is what should I do with my 2 vehicles:

Ford Zetec E 1.8 L engine - I have 2 situations on my journey to work
firstly (as per my OP) the downhill back home of approx 1/4 - 1/2 mile (depending on traffic)
secondly a set of lights , 500m bumper/bumper tail back, Slight downhill. You start from stationary and will take many phases to get through so going from stationary to stationary in 20 -25 metre bounds. My normal is to idle, neutral, brake off, let car roll naturally and then brake to halt. On average this annoys half the people behind who think I should race 20m to stop abruptly so they think they are getting somewhere faster. Its obvious when i have one of those behind, so to avoid aggro I will use 1st and 2nd gear and drive off, coast then brake to stop. As neither involves aggressive braking i don't think additional brake wear is much of an issue, but the second must be less econcomical since I have to rev to pull away

If I have understood the discussion so far it seems to suggest that overrun and braking is best. I have no idea if my engine is fitted with the components you refer to

Second vehicle (37 years old so I'd imagine some of you will have limited experience of such engines) engine = Rover 3.5L V8, twin SD carbs, not injected, not equipped with any fancy gadgets that go wrong. Its just a big V8 in a 4x4 that drinks fuel heavily - would you coast or overrun to get economy (probably impossible is one answer!) in downhill traffic or on an open (downhill) road assuming no traffic

mikey72

00ec25 wrote: »

Hi again - OP here - thnks for so many posts, I'll have to read them slowly to follow the arguments but at the risk of offending those giving scientific explanations what I care about is what should I do with my 2 vehicles:

Ford Zetec E 1.8 L engine - I have 2 situations on my journey to work
firstly (as per my OP) the downhill back home of approx 1/4 - 1/2 mile (depending on traffic)
secondly a set of lights , 500m bumper/bumper tail back, Slight downhill. You start from stationary and will take many phases to get through so going from stationary to stationary in 20 -25 metre bounds. My normal is to idle, neutral, brake off, let car roll naturally and then brake to halt. On average this annoys half the people behind who think I should race 20m to stop abruptly so they think they are getting somewhere faster. Its obvious when i have one of those behind, so to avoid aggro I will use 1st and 2nd gear and drive off, coast then brake to stop. As neither involves aggressive braking i don't think additional brake wear is much of an issue, but the second must be less econcomical since I have to rev to pull away

If I have understood the discussion so far it seems to suggest that overrun and braking is best. I have no idea if my engine is fitted with the components you refer to

Second vehicle (37 years old so I'd imagine some of you will have limited experience of such engines) engine = Rover 3.5L V8, twin SD carbs, not injected, not equipped with any fancy gadgets that go wrong. Its just a big V8 in a 4x4 that drinks fuel heavily - would you coast or overrun to get economy (probably impossible is one answer!) in downhill traffic or on an open (downhill) road assuming no traffic

For the first I do exactly as you.
Downhill, overrun, then you're in gear ready to carry on, unless you find you need to keep touching the accelerator to maintain speed, as the fuel will never cut off, then I coast.
Traffic, it'll make next to no difference what you do, the fuel will never be shut off on overrun, as you won't achieve the revs for long enough.

Large 4x4, I'd keep it in gear and let it suck slighly more petrol through the carbs.
Not for ecomony, coasting would be better, just for better vehicle control.
I had the same engine in an SD1.

tbourner

If its slow traffic I tend to just use the clutch to pull away with no throttle, so you pull away slowly and drive on idle power in 1st. On some cars and if not going uphill you can get into 2nd and 3rd with no throttle.