Petrol Question

brettzy

I saw an episode of Top Gear recent where Clarkson drove a diesel Audi from London to Scotland and back on one tank of fuel. At some point he was talking about how driving style makes a huge different in fuel consumption. He said that "most modern cars use no fuel when you are in gear and your foot is off the accelerator". That I never knew!!

So, I wonder is my car one of these "modern cars", doubt it but worth asking. It's a 00 ford focus zetec 1.4

I'd love to know just to know if ya know what I mean:D

.Longshanks.

brettzy wrote: »

I saw an episode of Top Gear recent where Clarkson drove a diesel Audi from London to Scotland and back on one tank of fuel. At some point he was talking about how driving style makes a huge different in fuel consumption. He said that "most modern cars use no fuel when you are in gear and your foot is off the accelerator". That I never knew!!

So, I wonder is my car one of these "modern cars", doubt it but worth asking. It's a 00 ford focus zetec 1.4

I'd love to know just to know if ya know what I mean:D

Does you car give you a read out of your instant fuel consumption? Be very easy to find out if it does

Matt Simis

"No" fuel maybe an overstatement. Mine switches from L/100km to L/Hr (litres per hour) when idling. The engine is ticking over, so it has to be using some fuel. Then there are electrics (alternator) and aircon etc on top of that, all drawing power.

Jimbo

I think you're misquoting him.
When you are slowing down in gear, you use little or no fuel as the revs are dropping and the momentum of the car is keeping your engine running.

Carawaystick

how has a diesel Audi anything to do with a Petrol question??

Diesel's work different to petrol, as the amount of diesel injected into the cylinder can vary in any proportion while the air/petrol mix needs to be in the right proportion so it's not too rich or lean.

When you're idling along at speed in a diesel, only a tiny amount of fuel is injected in as the momentum of the car helps keep the engine turning over, compared to idling while not moving.
in Petrol engines, a similar thing happens only no petrol is injected if you are idling at a high enough speed.

the electrics will all be powered off the battery or the alternator which still gets driven by the belt from the driveshaft which will be turning. It'll only be for a few seconds while you idle to a stop.

This "momentum" will reduce the fuel cnsumption if you allow the car slow from say 120km/h to a stop in gear than coasting to a stop with the clutch down

brettzy

Longshanks100 wrote: »

Does you car give you a read out of your instant fuel consumption? Be very easy to find out if it does

afraid not:p

brettzy

I found this which might clear things up

"Coasting out of gear or in neutral uses more petrol than slowing in gear and using engine as a brake. Most modern car shut of the fuel when engine braking as the car will not stall unlike when is neutral when fuel is required for tick over. "

and this

"Coasting - does it help save fuel?

Coasting - rolling downhill or approaching a junction with the car out of gear - is inadvisable because the driver doesn't have full control of the vehicle, though it used to be quite a common practice to save fuel.

You lose the ability to suddenly accelerate out of tricky situations. You lose engine braking which takes some of the load off the brakes on down hill stretches and helps to avoid brake fade - overheated brakes require harder pedal pressures to stop the vehicle.

These days, coasting is still inadvisable and changes in vehicle fuel systems mean it won't save you fuel either.

Old car with carburettor - take your foot off the accelerator pedal with the car in gear and fuel is still drawn through into the engine. Fuel savings could be made by coasting out of gear.

Modern car with electronic engine management - fuel and ignition systems are effectively combined and controlled by one Electronic Control Unit (ECU). Take your foot off the accelerator and the ECU cuts the fuel supply to the injectors anyway so there's nothing to be gained by coasting.

Modern diesel engines - these also have the ability to shut off the fuel when you take your foot off the accelerator. "

So, apparently if you have a fuel injection system then no fuel is used while in gear with your foot off the accelerator:eek: mine included:D

DUBLINHITMAN

stick @ 58mph in 5th gear on a long journey , you wont go wrong

*Kol*

I have noticed that on my car the instantaneous fuel consumption goes to 999.9 mpg when slowing down in gear with my foot off the accelerator pedal.The fuel must be cut off in that case.

firefly08

So, I wonder is my car one of these "modern cars", doubt it but worth asking. It's a 00 ford focus zetec 1.4

Yes, I'm pretty sure all fuel-injected cars do this. As you mentioned above, there is no need for a fuel injected car to use fuel when the momentum of the car is keeping the engine turning. When the engine slows down to near idle speed, then the fuel supply comes on again to prevent it cutting out. This will also happen if you press the clutch.

If you let it slow down almost to the point of stalling and then press the clutch, you'll hear the engine roaring back to life suddenly as the fuel comes back on.

My '00 Astra does it, and being a newer design I'm sure the Focus does too.

brettzy

It's great to know! The focus is very good on fuel anyway. Does around 450 miles on a full tank (50 liters) and a little more if on a long journey. Might squeeze another couple of miles now that I know this:)

Thanks to all!

roosterman71

My old car would change the readout to 0.0 l/100km when slowing down in gear. As soon as I put the put on the clutch, it would tip back up to 1.0. When stopped, it would idle at 0.7.

So no petrol is being used when slowing down. Just as long as its still in gear and clutch pedal isn't pressed.

Mr.David

DUBLINHITMAN wrote: »

stick @ 58mph in 5th gear on a long journey , you wont go wrong

Why 58?

Seth Brundle

Mr.David wrote: »

Why 58?

There used to be a belief that driving at 56mph (not 58mph!) was the most economical and efficient speed limit. However, this is probably wrong as different cars have different gearings, etc.
This speed was set as the US national speed limit in the 70s during the fuel crisis IIRC.

Tigger

being repetedly overtaken reduces airflow
increases mpg

EPM

roosterman71 wrote: »

My old car would change the readout to 0.0 l/100km when slowing down in gear. As soon as I put the put on the clutch, it would tip back up to 1.0. When stopped, it would idle at 0.7.

So no petrol is being used when slowing down. Just as long as its still in gear and clutch pedal isn't pressed.

Aye my car does the same. I leave it in gear until about 1500 revs as it tends to start using some juice then just before it starts to stutter. Tried telling the father who takes it out of gear going downhill but he doesn't believe me. Them momentum alone is enough to keep the greasy bits going

roosterman71

I know a guy who drives along a t 60, and drops the car out of gear and coasts till it reaches 50, then floors it again to speed up and then coasts out of gear again!! Pure madness. And he won't believe me when I tell him the dangers, and that it uses more fuel