Fuel economy.

[PhilMary]

On a slight downhill in top gear, 2017 Fiat based pvc, I can see the indicated mpg is 99mpg.
I could also slip it into neutral and let it roll down the hill.
Which uses least fuel? In other words, when the van weight is pushing the engine, is it using any fuel?

 

[kevenh]

Modern vehicles stop using fuel when coasting downhill or coasting to a red light, etc.
When putting the vehicle in to neutral there is fuel used to keep the engine running.

edit: so, coating IN gear is best for fuel economy

 

[The Wino]

I think on most modern engines you use less fuel in gear as the management system cuts off the fuel and lets gravity turn the engine over wheras out of gear your using fuel to idle the engine. At the end of the hill you could use less fuel to let the speed increase out of gear to coast up the next hill if you use less fuel idling the engine for the last bit downhill and up the next than climbing the next hill.
I think your fuel computer would show the difference.

 

[pappajohn]

Which uses least fuel? In other words, when the van weight is pushing the engine, is it using any fuel?

The throttle is effectively closed so shouldn't use any more than at idle.
Definitely not advised to put in neutral going downhill.
You won't get much engine braking from a diesel but any is better than none at all.

 

[Stealaway]

As above but you just wont roll as far.
It is better using it on overrun.

^{_{Subscribers  do not see these advertisements}}

 

[TinaLes]

Don't it just break your heart when you climb the next long hill & the MPG meter drops from some unrealistic figure to 18/20.
Since having the van, I have always amused myself by trying to keep the average MPG above 27.4MPG, if I get stuck in traffic, I have been known to re-set it! Always the optimist eh?
LES

 

[cmcardle75]

The throttle is effectively closed so shouldn't use any more than at idle.
Definitely not advised to put in neutral going downhill.
You won't get much engine braking from a diesel but any is better than none at all.

It will use zero fuel coasting in gear, whilst will use some fuel coasting in idle.

However, despite this there are some cases where being in neutral will use less fuel overall.

This is because you will get engine braking with the engine engaged to the wheels. The engine at idle will likely have lower revs than it would be engaged in gear. This will use more energy overall as more engine compressions will occur. If at the end of the coasting period, drive is required (rather than stopping), then the overall energy usage will be lower by idling at lower revs than by coasting at higher revs.

I wouldn't actually propose driving in neutral to take advantage of this, but it may be technically more fuel efficient under certain conditions, even with overrun fuel cutout on a modern engine.

BTW, most diesel engines don't have throttles and, instead, control power by varying fuel flow alone. Some modern ones may have a flap to increase vacuum for EGR operation, but it isn't really a throttle in the conventional sense.

 

[DaveFromBarrow]

good read - https://www.lightfoot.co.uk/news/2018/02/02/bad-technique-coast-driving-downhill

d

 

[Jamesh]

Don't it just break your heart when you climb the next long hill & the MPG meter drops from some unrealistic figure to 18/20.
Since having the van, I have always amused myself by trying to keep the average MPG above 27.4MPG, if I get stuck in traffic, I have been known to re-set it! Always the optimist eh?
LES

I take my bike computer off before I climb the large hill to my house!

 

[Landy Andy]

Better fuel economy is turning the engine off for the down hills. If you really needs to save fuel, and don't care about the risks.

^{_{Subscribers  do not see these advertisements}}

 

[Fdhadi]

Best advise I can give you is move or just holiday in the Netherlands.

 

[kevenh]

Better fuel economy is turning the engine off for the down hills. If you really needs to save fuel, and don't care about the risks.

Lol. You’re joking of course - rereading I am sure you are but I’ll bite
how do you get better fuel economy than the engine controller recognising the fuel can be cut off while the vehicles coast?

 

[Landy Andy]

When you coast the engine will inject small amount of fuel to lubricant the cylinders. It will also cause drag. No drag in neutral, but engine provides fuel to allow it to tick over. So engine off, and in neutral, equals no drag, and no fuel use.

 

[Two on Tour]

So engine off, and in neutral, equals no drag, and no fuel use.

You will need some muscles, engine off = no power steering, and likely a heavy brake pedal when you lose the brake servo.

 

[Landy Andy]

You will need some muscles, engine off = no power steering.

And leg muscles as no servo assist on the brakes.

^{_{Subscribers  do not see these advertisements}}

 

[Two on Tour]

And leg muscles as no servo assist on the brakes.

Just editing when you posted.