does coasting save petrol

GolfBravo

tbourner wrote: »

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.

And on diesel powered cars this actually kills the DMF (where fitted).

Tobster86

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

First situation, bumper-to-bumper almost stationary downhill traffic, I have the engine off completely. Ignore the people behind who think they'd get somewhere quicker if you raced up to the next bumper; they're idiots.

If there is no downhill component, I use 1st gear and no accelerator (other than perhaps to initially start moving in the petrol 3-series; but not at all in the diesel 306) and let the car walk along at idle revs, perhaps changing to second if there is enough flow. For any stops, I switch off the engine, and don't move again until there's enough of a gap to warrant another move.

I also use the handbrake if necessary to hold the vehicle stationary so whoever is behind me isn't dazzled by my brake lights. I think I'm literally the only person on the road that gives a s*** about this, despite the increasing proliferation of blindingly bright LED light clusters.

In this low-speed situation the biggest fuel saving you can make is having the engine off when stationary, but to answer your first situation; if the hill is steep enough to freewheel down, you will use idle fuel to coast down and virtually no fuel to go down in gear. Infact, you may as well make use of the idle fuel to get the car moving in the first place.

As for the Range Rover, with insufficient operational knowledge of carburetors I reserve comment, other than that any possible saving through coasting would not be worth compromising control. I've heard of people fitting the 3.9l (I think?) EFI heads to those engines, for the purpose of improved performance and economy; although this also tended to be followed by no end of problems afterwards.

Tobster86

GolfBravo wrote: »

And on diesel powered cars this actually kills the DMF (where fitted).

Good point to make;

If moving off with no throttle, on a bigger petrol/diesel engine, causes a lurching/shuddering, you have a dual mass flywheel and aren't doing it any good!

tbourner

Pulling away with no throttle doesn't do anything to a DMF.

Tobster86

tbourner wrote: »

Pulling away with no throttle doesn't do anything to a DMF.

I've heard a few times, although never heard a technical reason why. I could speculate that the effect of DMF wear (said juddering at low rpm) maybe led to the suggestion that these conditions are bad for them?

tbourner

Tobster86 wrote: »

I've heard a few times, although never heard a technical reason why. I could speculate that the effect of DMF wear (said juddering at low rpm) maybe led to the suggestion that these conditions are bad for them?

Harsh acceleration is bad for them, which is where you get juddering. Pulling away with no throttle by nature is very gentle, you can't do it hard or it just stalls, it's the smoothest clutch action you'll ever do in your car.
Maybe people who can't drive very well would damage them attempting it.

mikey72

"never let the clutch pedal out at idle revs."

http://www.honestjohn.co.uk/askhj/answer/33965/limiting-dmf-damage-

Frequent stalling of the engine/driving at extremely low engine speeds.

http://www.understeer.com/pdf/dualmass.pdf

Both bad.

mattyprice4004

tbourner wrote: »

Harsh acceleration is bad for them, which is where you get juddering. Pulling away with no throttle by nature is very gentle, you can't do it hard or it just stalls, it's the smoothest clutch action you'll ever do in your car.
Maybe people who can't drive very well would damage them attempting it.

Wrong.

At idle RPM the engine is at its most un-smooth, and it's exactly this that causes DMF wear. It's the same reason you don't 'bog down' the revs with a DMF equipped car, the vibrations from the engine being at low RPM put a lot of pressure on the DMF.

At high RPM there are more ignition cycles per minute, meaning smoother engine running and less roughness for the DMF to smooth out.

Remember, the DMF is there to smooth out the fact your engine is essentially just a series of explosions happening hundreds of times a minute. The more explosions per minute, the smoother the engine operates. You probably don't notice the roughness at low RPM because the DMF is straightening it out, but when it fails you will.

tbourner

I stand corrected!
I used to do it in my Supra, the torque meant I could get into 4th with no throttle on a level surface! That was when I was going into town twice a day and spending lots of time in traffic. It had a new clutch when the thrust bearing went but that was pretty standard on them when running higher power.
On the diesel Honda I do have a DMF, so probably a good thing I never go into town any more, very rarely get stuck in traffic nowadays and don't remember ever using that 'technique' on this car (only had it a year). I do tend to change gear early though and sit at 1100rpm or thereabouts, although I'm very careful about the amount of throttle I use at those revs, change down if I need to accelerate.

Hadrian

worried_jim wrote: »

Once on holiday in Lanzarote I managed to coast for 38km down a mountain in a Seat Ibiza- I saved fuel!

I'd like to bet you anything you like that the petrol you used in getting UP the mountain added to what you used getting down, halved, equaled what you'd have used on a flat road.
PS 'Coasting' illegality was a wartime measure for PSV's, don't think it applied to any other vehicles. Also NEVER turn your engine off to coast, the steering will lock.