freewheel or coast for mpg? - Alfa Romeo Forum
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freewheel or coast for mpg?

I was more than a little low on petrol tonight having forgotten my wallet.

As I was doing my best to get home and back to the petrol station, I was wondering which uses less fuel.

Knocking it into neutral and freewheeling down hills.....or
keeping it in a high gear and coming off the gas so the engine doesn't brake your speed on the same hill.

Freewheeling is my 1st choice but I remember on TG once it was mentioned on some diesel or other that it used no fuel as you lift your foot off the pedal as you slow at hunctions.

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Freewheeling/coasting will eat more fuel.

The ECU should cut fuel when it senses going down hill in a high gear
whereas it has to fuel the car for tick-over if you're coasting with the clutch in/
in neutral.

I'm willing to be corrected on this.
 
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My 166 once recorded 99.9mpg, the max it can show, over a 3 mile journey downhill in gear, I'm pretty certain in neutral it would have used more.

My thinking is that modern engines cut the fuel when coasting in gear but in neutral it would need fuel to run at tickover.
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In theory Nev is correct. If you leave the car in gear and have your foot off the gas, then the ECU supplies no fuel at all to the engine - the cars momentum runs it.
Whereas if you take it out of gear, it has to supply enough fuel to make it idle.
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You are right Nev. Using a high gear and lifting off the gas will use no fuel, where as rolling in neutral will use the same amount of fuel as sitting still on tickover.
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I cant see that it can use no fuel whatsoever.
It needs some fuel.... however minimal to ignite and keep the engine turning.
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As said above, when in gear and your foot is off the gas the engine doesn’t consume any fuel. BUT only above certain revs. When the revs are dropping, the ECU slowly restores some limited fuel supply to the engine at about 1200-1500 rpm or below to keep it running and make the transition to idle as smooth as possible.
You can try it for example in third gear; release the pedal at 2500 rpm. Consumption is zero, revs are dropping 2000 - 1800 - 1500 - 1300 and somewhere here you will start to feel slight vibration as the fuel supply is smoothly restored, the car passes into idle and it will actually never stop even though you don’t touch the gas pedal. The idle fuel supply is restored much sooner than revs hit idle to keep things smooth. But above 1500 rpm most cars have zero consumption when off the gas.

And emission control is another problem. Drive in 5th at 4k rpm an highway and shift into neutral. It will take good 4-5 seconds for the engine to drop the revs to iddle. The fuel is not cut off inmediately, but slowly, in the controlled way so that the emissions are as low as possible under any circumstances. The same happens when you lift off - the fuel is cut off, but there is still some very limited injection for a few second to stabilize the emissions. Thats why present cars have so poor throttle reaction. The fuel control and air supply are actually not controlled by your foot but ECU filters your instructions according to its program..... So at the first few seconds after lifting off, there still is some very limited consumtion. But we are talking VERY about very low amounts your board computer cant actually display.

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Last edited by gotcha; 12-10-10 at 19:45.
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Cheers fella's. I find this stuff quite interesting. I need to get out more (with my wallet on board).
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Quote:
Originally Posted by Prof Peach View Post
I cant see that it can use no fuel whatsoever.
It needs some fuel.... however minimal to ignite and keep the engine turning.
Think about it - it's directly connected to the wheels. The motion of the car itself is therefore used to keep the engine turning.
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turn the engine off, Prof
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Quote:
Originally Posted by steveisfrowning View Post
turn the engine off, Prof
Might do. That new fangled PAS is over rated anyhoo.
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and servo assisted brakes.
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and servo assisted brakes.
see....they only slow you down....no good at all for this purpose
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good point.
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Just to make sure just remove the key altogether.

The steering may lock, but then that adds to the fun.
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the PAS will still work with the engine switched off if it's a hydraulic PAS pump, as will your servo... I do suggest leaving the key in the ignition lock though
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At last.... someone who knows what he's talkin about!
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At last.... someone who knows what he's talkin about!
I'm not as daft as I look
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I'm not as daft as I look
Thank Allah for that!
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Turn the engine off. Don't worry about lack of steering or limited brakes, who needs them.
 
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Quote:
Originally Posted by Cuore_Sportivo_155 View Post
the PAS will still work with the engine switched off if it's a hydraulic PAS pump, as will your servo...
The PAS pump is driven by the engine, so if the engine's not running there will be no pressure in the system. The brake servo will work for a few applications until the residual vacuum is lost.
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The PAS will work if you turn the engine off but leave it in gear due to the momentum turning the engine and pump over. If you stick it in neutral the engine and pump will stop so no PAS...

Same with the servo brakes.
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Quote:
Originally Posted by symon View Post
The PAS will work if you turn the engine off but leave it in gear due to the momentum turning the engine and pump over.
Hmm . . . I should have re-read the whole thread!
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