
Coasting in Neutral Does Not Save Gas - soundsop
http://www.popularmechanics.com/cars/alternative-fuel/news/coasting-in-neutral-fuel-economy
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noahlt
To be clear, coasting while in gear still saves gas.

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Lagged2Death
I've heard this before, and I understand how cutting the fuel supply to zero
would save gas, but there are a couple of things that I don't understand:

Wouldn't this mean that coasting down a long hill in gear would cool the
engine and in particular the catalytic converter, potentially causing some
operating hiccups and a big emissions spike when eventually reaching the
bottom?

Why would built-in mileage computers, with direct access to actual fuel
injection figures, compute gas mileage from some other measurement that's only
theoretically related under some circumstances? Isn't that sort of going out
of the way for worse results?

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chris_j
> Wouldn't this mean that coasting down a long hill in gear would cool the
> engine and in particular the catalytic converter, potentially causing some
> operating hiccups and a big emissions spike when eventually reaching the
> bottom?

I have observed precisely that. I have coasted down a hill in gear and have
noticed the engine temperature (in my diesel engined car) begin to drop, which
worried me a little at the time. This surprised me a little at the time
because it happened in a matter of minutes, which I hadn't been expecting.

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Lagged2Death
Perhaps a diesel engine is even more prone to the cooling effect, since (it is
my understanding) diesels don't have throttles to limit the amount of air
pumped through the engine on each rotation.

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michaelcampbell
I've been seeing a bit of this type of article lately, perhaps due to the fuel
prices rising, and have wondered; if this is true, why do I continue to hear
exhaust noise when coasting in gear?

Exhaust noise is purely the sound of detonation, which requires fuel. So why
do I hear it if there is no fuel being used?

I'm open to the possibility of my brain faking me out and I'm NOT actually
hearing it, too. But I'm pretty sure I am. (NB: My car is a 1995 model, but it
is fuel injected.)

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orenmazor
what worries me is that the title of the article is 'neutral does not save
gas' and not 'neutral is exceptionally dangerous because you are not in full
control of your car'. if something jumped out at you and you suddenly needed
to gas your car, you just lost 1-2 important seconds (if you're very good at
remembering what gear to hop into for a given speed/rpm). the next stupid
thing you could do is "I'm coasting so I might as well turn my car off". now
you lost power steering.

no, neutral does not save gas. your fuel pump has a idling injection setting
(lets say keeping the engine at 950rpm). when you're in neutral, the injection
mechanism switches to that rate, and you slow down by friction (bearings,
wheels, transmission, weight, etc). whereas if you're coasting in gear, the
engine switches into engine braking, and eventually slows you down without
using fuel. in gasoline engines that works by creating vacuum in the engine
(which slows the movement). in diesel engines that vacuum is not created, so
the exhaust is somehow blocked (again, slowing the movement).

HUGELY oversimplified explanation, I realize. with apologies to actual
mechanics. I'm just a nerd who hammers at his car :)

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JohnnyBrown
I think this idea comes from when most vehicles had carburetors rather than
fuel injectors. If fuel consumption were driven by airflow into the engine
rather than electronically controlled, I imagine there would be some benefit
to coasting in neutral.

