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Full Version: Rolling In Gear Uses Less Fuel?!?
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jeebowhite
Hi Guys,

Well the time came yesterday to part with my cash for 6 months MOT, in doing so I decided to enter the prize draw for a Seat somethingorother 1.2 Super diesel with large economy and blah blah blah - if I win, I will sell it and put it towards something nice... but thats beside the point!

In doing so, I learned / read yesterday that if you are driving along in gear, it is better to roll in gear (as opposed to neutral), as apparently doing so will reduce the fuel being fed to the engine to virtually zero... Really?

How on earth is that even possible, there is a vaccum being caused, the engine is still ticking over, and compressing, the valves are still pumpting fuel, but how can it reduce the fuel flow to virtually zero? surely the ignition of the petrol in the cylinders would still need to be sufficient otheriwse the car will just drop pace as the engine slows, and in doing so, slows the wheels (or to the point, the engines compression will reduce the free ride that the wheels are getting).

So really, is this true, or is it just another method to try and keep the air greener, or stop people coasting, because now it "saves you money"?
rojariggs
When you are in gear the wheels turn the driveshaft(s), which then turns the gearbox, which then keeps the engine turning. When the engine management system senses that this is happening it shuts off the fuel supply so you use no fuel.

Conversely, if you are not in gear the engine management system has to put a very small amount of fuel through to keep the engine idling.
jeebowhite
I never thought of it that way... I always thought that you still needed to put something in to aid the combustion as you wouldnt want it trying to spark just Oxygen.

I new that theoretically lifting your foot helped, but I thought that this was because it was more closer to the idling speed injection rate of fuel, rather than say 3000 revs worth of injected fuel...

Learn something new every day!
rojariggs
[quote name='jeebowhite' timestamp='1332850492' post='174613']
you wouldnt want it trying to spark just Oxygen.[/quote]
As it's the ECU that controls the process it will also adjust the spark and the airflow to match the cutting of the fuel.
jeebowhite
again, a point of logic that never crossed my mind! thanks Roger, im just glad that a few quid is saved!


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