Is it true

[Biguns]

Is it true that on modern cars, when say, going down hill, in gear, not touching the clutch or accelerator, it uses no fuel. ie the fuel is shut off.
If so does my 56 plate Focus have this feature

 

[lymmranger]

surely if the fuel was cut off then the engine would cut out?

 

[Biguns]

Heard Jerremy Clarkson say it on an old Top Gear. You are going down hill, The engine is being driven by the cars weight/momentum, you keep your feet off the clutch and accelerator, the car uses zero fuel. It makes sense, but is it true. Obviously at the bottom you use the accelerator and the engine keeps going. if it is true ,it must work off vacuum in the manifold and micro switches on the pedals

 

[Biguns]

Found it, it is true. but I still do'nt know if it applies to my Focus

http://www.civinfo.com/forum/engines-transmission/12494-fuel-used-overrun.html

http://www.linkecu.com/link-help-centre/glossary/over-run-def

I guess it probably does. So all these years of coasting in neutral down hill has been a waste of fuel. Doh

 

[Burnerman]

Its a fair enough assumption that cars with drive-by-wire throttle pedals switch off the engine injectors when coasting downhill - this applies both to diesel and petrol engines.
John

 

[wotan]

Its a fair enough assumption that cars with drive-by-wire throttle pedals switch off the engine injectors when coasting downhill - this applies both to diesel and petrol engines.
John

This is not true, the engine continues to be fed fuel, it's just a very small amount.

Wotan

 

[Burnerman]

In which case I'm very happy to be corrected.....the injector pulse ceases on my PD diesel car though.
John

 

[Biguns]

WOTAN
Are you refering only to diesel engines ?

 

[JohnD]

my old fuel-injected petrol Audi Coupe turned off the fuel completely in these conditions, and on the overun on ther flat. The drivers manual explained it quite a lot. I was surprised because I had only driven vehicles with carburrettors until then.

seems perfectly reasonable to me.

it is wrong to say it doesn't happen. Of course I don't know if it happens on all other cars.

It was "B" reg, I think built in about 1984.

 

[Burnerman]

WOTAN
Are you refering only to diesel engines ?

I really couldn't comment accurately on this one, but my car had to have a live data check early on in its life and the fuel was certainly shut off at that time.
I believe the combination of speed / movement sensors plus the throttle position sensor gives info to the ECU to shut off power to the injectors on both types of engine - simple enough as all injectors are operated by solenoids these days.
John

 

[Burnerman]

my old fuel-injected petrol Audi Coupe turned off the fuel completely in these conditions, and on the overun on ther flat. The drivers manual explained it quite a lot. I was surprised because I had only driven vehicles with carburrettors until then.

seems perfectly reasonable to me.

it is wrong to say it doesn't happen. Of course I don't know if it happens on all other cars.

It was "B" reg, I think built in about 1984.

I think these cars used the Bosch L Jetronic system.....very crude by todays standards but they worked well then. These systems used to pump fuel all of the time through the injectors - nowadays the injector sits with high pressure fuel inside it, and opens only when the engine ECU opens a solenoid to inject the fuel. In this way the duration of the injector open period is adjustable, so for a cold start the injector is simply held open longer.
John

 

[JohnD]

K-Jetronic I think

There was a big diaphragm controlled by a big weak spring, which was sucked a distance dependent on the mass of air travelling into the engine, which moved a needle valve metering the petrol flow

 

[Biguns]

WOTAN
Are you refering only to diesel engines ?

I believe the combination of speed / movement sensors plus the throttle position sensor gives info to the ECU to shut off power to the injectors on both types of engine - simple enough as all injectors are operated by solenoids these days.
John

It would also have to know the clutch was not depressed, via some sort of micro switcc

 

[wotan]

WOTAN
Are you refering only to diesel engines ?

I believe the combination of speed / movement sensors plus the throttle position sensor gives info to the ECU to shut off power to the injectors on both types of engine - simple enough as all injectors are operated by solenoids these days.
John

It would also have to know the clutch was not depressed, via some sort of micro switcc

I just cannot envisage, for reasons of safety, that the fuel would be cut off completely, be it petrol or diesel, I stand to be corrected on this one if there is a definitive answer.
The economy gauge on my BMW when on the overrun, goes full scale indicating more than 80 MPG is being achieved, at no time do I get the impression that the engine is not producing some power.

 

[Burnerman]

K-Jetronic I think

There was a big diaphragm controlled by a big weak spring, which was sucked a distance dependent on the mass of air travelling into the engine, which moved a needle valve metering the petrol flow

K Jetronic? A la the Escort XR3i? I thought Audi would have moved on.
Don't know though.
John