does coasting save petrol

00ec25

When my VED renewal come through recently one of the leaflets expressely stated that coasting used more fuel than leaving the car in gear - can anyone explain why an engine uses more fuel when idling than when the in gear but not under load, eg: decelerating approaching a stop line in gear versus coasting to a halt out of gear
Dad used to coast the last 1/4 mile right up to the garage door as it was all down hill getting home

(BTW - before you point it out , I know that coasting means you are not in control of the car as it is out of gear, so is not allowed for that reason, but does it save petrol, if you do so anyway?)

daveyjp

In modern fuel injected engines when no gas is applied the fuel flow is shut off. When you coast you need fuel to keep the engine ticking over.

00ec25

OK , I have an old car , no idea if it's fuel injected (1996 Ford Mondeo 1.8) but the engine certainly does not cut out when I do the 1/4 mile coast home - I can't swtich off the engine as there are 2 bends and the loss of power steering would be "inconvenient" in that case, so I'm out of gear with engine running at idle to keep the steering going

LincolnshireYokel

I frequently knock it into neutral and coast in preference to braking.

flashg67

Yes,your Mondeo is injected. It doesn't 'cut out' completely, rather allows enough fuel through to run the engine, ie over come the internal 'drag' on the engine - probably a close call though and difficult to actually check either way?

Notmyrealname

00ec25 wrote: »

OK , I have an old car , no idea if it's fuel injected (1996 Ford Mondeo 1.8) but the engine certainly does not cut out when I do the 1/4 mile coast home - I can't swtich off the engine as there are 2 bends and the loss of power steering would be "inconvenient" in that case, so I'm out of gear with engine running at idle to keep the steering going

Yes it is fuel injected.

So you're using more fuel than if you just took your feet off the pedals and it could be argued that you are not in full control of the vehicle.

You misunderstand what is meant by shutting fuel off. The injection system turns off fuel to the engine but the engine is still turning as the wheels turn the gearbox which in turn turns the engine and then when you need power again, you press the pedal and fuel is injected into the engine once more.

When you put it in neutral, the injection system sends fuel to the engine to keep it running which is why it uses more fuel.

jaydeeuk1

What do you mean 'internal drag'? If car is in gear, engine will continue spinning/driving belts even if you could rip out the fuel lines. ECU detects that no fuel is required until its hits a point where the engine labours and then squirts some in to keep it going. Coasting constantly uses fuel to keep engine at 800rpm or whatever your is, even more so if there is a load like aircon, braking etc.

vikingaero

The vast majority of cars on sale within the last 10-15 years are fuel injected. And most of them are designed with "fuel cut-off on overrun."

forgotmyname

As mentioned although it may not fully shut off all the fuel.
If no throttle is used it can use a tiny amount of fuel to keep it running and not risk stalling.

Coasting is pointless. And takes control away from you.

00ec25

Notmyrealname wrote: »

When you put it in neutral, the injection system sends fuel to the engine to keep it running which is why it uses more fuel.

jaydeeuk1 wrote: »

. ECU detects that no fuel is required until its hits a point where the engine labours and then squirts some in to keep it going. Coasting constantly uses fuel to keep engine at 800rpm or whatever your is, even more so if there is a load like aircon, braking etc.

forgotmyname wrote: »

If no throttle is used it can use a tiny amount of fuel to keep it running and not risk stalling..

thanks all - Ok I think I understand
so the effect of engine braking, ie foot off the accelerator but engine still running in gear, means the engine is more efficient because part of the force required to turn the engine comes from the momentum of the road wheels turning it via the gearbox rather than having to constantly burn fresh petrol if it were instead idling in neutral
But when the gradient changes you'd have to go back on the accelerator and pump more fuel into the engine to get up the one upslope on the last leg home. Whereas when coasting, the absence of engine braking means you initally speed up so do have enough momentum (still doing 30mph at that point) to get up that hill without ever having to touch the gas in the whole of that 1/4 mile

forgotmyname wrote: »

Coasting is pointless. And takes control away from you.

As I said I KNOW you are classed as not in control but this is about understanding which is the best way to save money. Dad drove like that for 50 years and never caused or was involved in an accident so you can stuff the control argument, I want the money, not the control

Anyhow.
I'm persuaded - I'll have to learn to drive differently from now onwards and see if the MPG changes.

Dave_C_2

Going down a very long hill. In gear with no throttle you get engine braking and only travel at say 20mph. Comparing this with an engine at tickover is fair and engine braking uses less fuel

However on some long hills it is possible to coast at a much higher speed, say 40 mph. To maintain this with the engine engaged requires a fair bit of throttle - more than just tickover. So coasting uses less fuel to maintain the higher speed.

At some point in the middle is the break-even point.

What about automatic cars in drive - where there is no engine braking?

Dave