Why should I coast with the motor off?

[DAN]

when off the throttle, coasting or not, most motors idle. i can't see how any useful fuel is saved by not idleing that little bit. and ever restart is a cold start. so in many cars a extra bit of fuel is used to light off the cat. converters.

[SVOboy]

Well, every restart is not a cold start, particularly if the engine was just on. It might momentarily use more fuel, but this will often be outweighed by the length of the coast.

And the idle in neutral and with 0% throttle is different is most cases from being in gear with 0% throttle. This is easily proven by a Scangauge or SuperMID.

That's why.

[Big Dave]

I can see why some do it although I don't do it myself. Especially when going for very high numbers you have to begrudge every drop of fuel burned. Gassers idle with the throttle closed, that is operating at minimum thermodynamic efficiency.

I need the power steering with my truck. It would take Hercules to turn or stop this monster with power steering pressure. Also starting 444 cubic inches at high compression takes a toll on starters and batteries.

[Daox]

If you bump start the car you don't have the engine dumping fuel in like you would if you use the starter. This eliminates that loss. However, even when using the starter on modern fuel injected vehicles, it doesn't dump in THAT much fuel.

[tasdrouille]

I don't know about an average gasser, but my TDI uses 0.6 L per hour when idling. I don't consider off coasting is really worth it. I coast when possible, but in neutral.

[8307c4]

It all depends on the car for one, and the place you do it on, the time spent coasting I/O is critical, as is consistency, I say at least 20 but really a 30 second glide on a regular basis considerably increases mpg.

There's absolutely no way otherwise, an ICE consumes more than the usual amount of fuel upon starting, even a hot engine injects additional raw fuel into the cylinders as a boost for this process. So a short glide doesn't do anything at all, too short a glide could easily hurt mpg.

Best places to do it is right as soon as I realize I am more than likely going to have to come to a complete stop, such as highly trafficked busy intersection.
Or when pulling into one's driveway, but it depends how the road curves and the hills and other stuff. My parent's driveway I can turn the engine off two blocks before and coast the whole way, but my own it's all uphill so it doesn't work... However, starting out in the morning I can coast out of my driveway and down to the stop sign before even starting the engine, again this varies from one house to the next.
That, or long down hills but at least 1/4 mile long or better.
Speed is important, it makes little sense to lose 20 mph for no good reason if one does not have to stop.

Also works better in a manual IMO, where a miscalculation simply translates to popping the clutch.

[MetroMPG]

Originally Posted by DAN

when off the throttle, coasting or not, most motors idle. i can't see how any useful fuel is saved by not idleing that little bit.

Almost every major manufacturer is rushing to introduce idle-stop as a fuel saving measure in its vehicles. And not just hybrid vehicles.

You typically see figures in the 5-10% range for fuel saved in urban driving from using idle-stop.

EDIT: some, but not all, of these vehicles shut down when the vehicle is coasting or braking at low speeds; others only shut off after the vehicle has come to a full stop.

E.G. BMW is claiming an 8% improvement on its 1 series (non-hybrid).

[Stan]

Originally Posted by tasdrouille

I don't know about an average gasser, but my TDI uses 0.6 L per hour when idling. I don't consider off coasting is really worth it. I coast when possible, but in neutral.

I've measured (with a VAG Com) my TDi's pulse bandwidth at zero while coasting in gear above 26 mph in 5th gear. Fuel cut-off while coasting is a well-documented feature of VW's TDi design.

The only "problem" with this feature is that the car will coast nearly twice as far in neutral (consuming 0.5 l/h on my VAG Com) as it will with the car's inertia turning the engine while coasting in gear, so the question is...which is more efficient? I strongly suspect that it's 6 of one and half a dozen of the other, so like tasdrouille, I simply coast in neutral with the engine running using the pulse-n-glide technique pretty much all the time.

[trebuchet03]

Why?

For the same reason you should turn the faucet off while brushing your teeth...
and
For the same reason you should switch off lights/fans when you leave a room...
and
For the same reason everyone should be recycling...
and
For the same reason you should bring your own bag to the grocery store...

These are all parallel concepts

[MetroMPG]

But if I keep turning the water on & off & on & off when I brush my teeth, I'll wear out my tap!! :P

Also, I only shut off lights if I know I'm going to be leaving the room for more than 30 seconds.