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FUEL DELIVERY....What happens when i lift my foot? 

 

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 Post subject: FUEL DELIVERY....What happens when i lift my foot?
Posted: Thu Apr 19, 2012 7:14 pm 
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Well i hope this is in the correct section.....

If im coasting along at say 100kph, what happens when i lift my foot off the GO peddle in relation to fuel delivery?

*Does it continue to deliver fuel in at a very small amount???

*Or is their a complete fuel cut? I assume this would mean it would stall then!!!

*If it continues to deliver fuel, even in very small amounts, would you not get better fuel economy
if you did not use any engine breaking at all?

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Thu Apr 19, 2012 7:25 pm 
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afaik you shouldn't use a high amount of fuel engine braking or with the throttle shut, the ecu controls fuel, it knows when the throttle is shut so there fore wont be pumping in much fuel.

fuel circulates through the rail and back to tank, the injectors pump in what they need

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Thu Apr 19, 2012 8:49 pm 
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pretty sure you could coast from 100 with a rail full of fuel. de-acceleration uses less fuel than using brakes instead :wink:

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Fri Apr 20, 2012 7:19 am 
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What happens actually depends on whats happening in the drivetrain.

As the ECU knows the engine RPM, and Throttle load, it will cut fuel if you pull your foot off the accelerator until the engine rpm gets below a certain value.

Its called fuel cut on overrun.

The Ford ECU's also do lean cruise - if youve ever watched while your doing 100 km/h - if the Afr's are steady for a few seconds then you can watch them dive right off the lean end and stay there until something changes (speed, rpm, or throttle load).

Nigel
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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Sun Apr 22, 2012 8:17 pm 
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So....for the SAKE OF THE argument.

If was to race around a race track once with heavy breaking only, NO ENGINE BREAKING AT ALL.
And one more time WITH HEAVY ENGINE BREAKING.
Would i burn more fuel on the second lap with engine breaking?
I reckon you would use more fuel for sure.

I'm just wondering that for us that travel LOTS of km's every week (over 1100km), would it not be better (for those of us that have a manual) to a stop with the brakes only. YES you would go through pads quicker...but i just want too know if i would burn a measurable amount more fuel, through constent engine bakeing at regular stops.

Thanks

MITCH

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Sun Apr 22, 2012 9:50 pm 
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Mitch. No, you wouldnt use more fuel using engine breaking - ASSUMING that you accelerated exactly the same way. In fact, you might use less as if you use the brakes you'll have the engine idling, and it will need fuel. The overrun code in the ford EEC is supposed to be very good esp. in later models - and when you do engine braking with *no* input from the accelerator the injectors will be cut for most of the event

However - and this is an argument for street use and those who self-shift auto's -

- are you using a $1500 gearbox to save the $100 brake pads?

Nigel
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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Mon Apr 23, 2012 2:08 pm 
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Nigel,
Taa mate for the info. I do understand how it worx now.
Very much appreciated.
All the best.

MITCHY

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Wed May 02, 2012 2:32 pm 
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In the Ford EEC ECU for the 6-cylinder, the amount of fuel is effectively proportional to RPM, manifold absolute pressure (MAP) and volumetric efficiency (VE). Looking at a typical AU 6-cylinder ECU program, it does not appear to use decel fuel cut-off:

At idle of 750 RPM, assume MAP of about 14 inHg, the code indicates about 65% VE.

Decelerating from cruise at 1900 RPM with the throttle closed, assuming MAP of about 7 inHg, the ECU says about 60% VE.

Multiply the three numbers together to compare the amount of fuel injected:
- Idle: 750 x 14 x .65 = 6825
- Decel: 1900 x 7 x .60 = 7980
Under these RPM and MAP assumptions, engine braking would use about 17% more fuel than idling the engine, clutch in. If my assumptions are off by just a little, there might be no difference at all - eg, at MAP of 6 inHg or RPM of 1600.

But the amount of fuel used in either case is two-fifths of nothing. For comparison, at cruise (where you normally get your best fuel efficiency), you might be doing 2200 RPM, MAP of 17 inHg, giving 80% VE. Multiplied together gives 29920 (The calculated number represents nothing specific, but gives an accurate comparison of the amount of fuel injected). So, both idle and engine braking use only about 25% of the fuel per second that you would use at steady cruise. Put another way, taking your foot off the gas at 100 km/h yields an instantaneous fuel economy of about 100-120mpg or 2.6L/100km. Probably not much saving to be had there.

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Wed May 02, 2012 3:22 pm 
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:shock:

WOW great input. Thanks mate.


MITCHY

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Thu May 03, 2012 2:19 am 
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Other thing to remember is the fuel reg gets its signal from the manifold. The manifold is at its highest vac when you foot comes off, therefore it opens the tank return as the injectors don't need the flow

 

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 Post subject: Re: FUEL DELIVERY....What happens when i lift my foot?
Posted: Thu May 03, 2012 6:21 pm 
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....but the differential pressure between the intake manifold, and the fuel rail remains the same...
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