MPG Fuel used when on the overrun?

[lobey dosser]

This question has been inspired by two things -

1) Some time ago, Jeremy Clarkson did a London/Edinburgh/London economy run in a Merc of some sort, and I seem to remember him saying that modern fuel injection systems actually cut off the fuel flow when the throttle is shut on the overrun, therefore coasting would not save any fuel.

2) On the Civic, all 6 Eco-lights come on if I take my foot off the gas to slow down to a stop - if I keep the car in gear.
If I slip it into neutral and coast to a halt (probably illegal, but hey-ho) then I seem to get only 4 eco-lights illuminated.

This appears to confirm JC's statement in that, in neutral, there needs to be some fuel supplied to keep the engine ticking over, but, in gear, none is needed as the wheels are driving the engine until such time as you dip the clutch to come to a stop.

For best economy, can any techie gurus confirm or deny that fuel is completely cut off when decelerating?

Ta, Lobey

[TTDegs]

lobey, it was an Audi A8 (I think, might have been an A6)

but I can confirm that I get the same results as you on the eco lights, except that I tend to just push the clutch rather than slipping into neutral.


On my mega eco run (73 mpg) I'm pretty sure I was in gear at all times, even downhills and off the gas - but then there was no actual stopping (in fact no braking at all) in that 120 odd miles, so it doesn't really help to answer the question

[cylon2007]

Yes on the over run with no throttle the fuel is off completely, if however you drop into neutral as you say it needs some fule to tickover. The complete lack of fuel on the over run is for emissions.

[pjr]

If you switch your trip computer to the instantaneous display (the bar graph one), and lift off when decelerating the bar will flick right up to 100mpg - probably more than 100mpg as the fuel is cut off, but the bar graph stops at 100!

[Munro]

On over run the fuel is cut off, but engine braking slows the car down quite quickly. This is the most economical if you have to slow down.

If you coast in neutral the engine uses fuel to tick over and there is no engine braking effect. This is the most economical if you want to maintain speed as long as possible. You need to be very careful though - if the engine stalls you loose braking and steering which can be expensive!!

I think ?

I managed to get 52mpg out of my CTR on one occasion using this as one of the economy driving techniques.

[Pottsy]

Munro - don't forget that Lobey is in a diesel so he has no throttle.

Lobey - in the diesel, the accelerator pedal just controls the amount of fuel sent into the engine. So when above idle speed, with your foot off the accelerator you send in no fuel. In a petrol it's often a different story. Because there is a throttle, the engine is pumping against a closed tube - so you get lots of engine braking. It's quite common for petrol cars to allow a small amount of fuel for a couple of seconds (to prevent jerkiness when coming on and off the throttle), and then cut the flow off (you can sometimes feel this).

[mfosbrooke]

me and my mate had this convo last night about that eppisode of top gear and i thought it were a merc or jag but no it was def an audi.

[Conrad'67]

Doesn't fuel need to be sent into the cylinders at all times to lubricate the piston against the edge of the cylinder. If no fuel was going in on over-run but the wheels were driving the pistons, surely the engine would cease?

[Wander]

Originally Posted by TTDegs

lobey, it was an Audi A8 (I think, might have been an A6)

Definitely an Audi A8.
I know this for sure, because my ex-boss has an A8 which he bought at that time.

He had quite a bit of trouble with it, if I recall correctly. Mostly Software glitches, like lights going on and of at the strangest of times.

[Pottsy]

Originally Posted by Conrad'67

Doesn't fuel need to be sent into the cylinders at all times to lubricate the piston against the edge of the cylinder. If no fuel was going in on over-run but the wheels were driving the pistons, surely the engine would cease?

No - the lubrication from the oil is enough.

[alanwalne]

Originally Posted by Pottsy

No - the lubrication from the oil is enough.

I thought that was the problem with putting petrol in a diesel, or is it only that critical in a BMW diesel.

[Pottsy]

It's only critical in a common rail diesel (most modern diesels, including the Civ and BMWs). It can take just seconds for the petrol to destroy the seriously expensive fuel pump.

[Charles_Harding]

If you want to put petrol into a Diesel, shouldn't you have got a 1.8 in the first place?

"I Just told Lady H. about me wanting to open a theatre in the town.
Are you having me on she asked?
I replied that I'd give her an audition,
But I can't promise anything!"

[alanwalne]

Pottsy

So is the Diesel sitting in the fuel pump is sufficient for lubrication without it being actually pumped through then?

[alanwalne]

Does anyone know the lubrication properties of Chip fat??

[Pottsy]

Originally Posted by Charles_Harding

If you want to put petrol into a Diesel, shouldn't you have got a 1.8 in the first place?

Dim enough to put diesel into a petrol = dim enough to not see the advantages of diesel at purchase time.

Originally Posted by alanwalne

Pottsy

So is the Diesel sitting in the fuel pump is sufficient for lubrication without it being actually pumped through then?

Fuel doesn't lubricate anything in the cylinders - it's all done by oil splashed around from the sump, onto the cylinder walls and controlled by the oil control rings. Diesel fuel does lubricate the fuel pump and injectors - but this is not an issue when on the over-run, because the fuel is still there.

Petrol can damage the inner surfaces of a very high pressure diesel pump - I'm not sure how, but apparently it's best to not start the engine / pump at all.

[lobey dosser]

Originally Posted by Conrad'67

Doesn't fuel need to be sent into the cylinders at all times to lubricate the piston against the edge of the cylinder. If no fuel was going in on over-run but the wheels were driving the pistons, surely the engine would cease?

Spot the former 2-stroke driver, maybe?
Did you ever own a Wartburg http://en.wikipedia.org/wiki/Wartburg_(car), a Trabant? (I like the notion that the Wartburg engine had only 7 moving parts!)

I seem to remember that, with 2-stoke motorbikes on long downhill stretches, you had to open the throttle occasionally to get some oil into the cylinders to avoid a seize.

Thanks to all for the good info.
I hoped it was the case that the fuel was cut off completely.
Off to look for some long hills to run down with zero fuel usage!

[TTDegs]

Originally Posted by lobey dosser

Spot the former 2-stroke driver, maybe?
Did you ever own a Wartburg http://en.wikipedia.org/wiki/Wartburg_(car), a Trabant? (I like the notion that the Wartburg engine had only 7 moving parts!)

I seem to remember that, with 2-stoke motorbikes on long downhill stretches, you had to open the throttle occasionally to get some oil into the cylinders to avoid a seize.

from memory, strokers don't have a seperate oil tank?

In more modern engines (and I'm talking Waaaayyyy general here!), oil is in the bottom of the engine, fuel comes in at the top

The oil is splashed up the insides of the cylinder as the piston goes up, and squished back down as the piston goes back down.


In strokers, the oil and petrol are mixed, so both go in at the top.
When the petrol is ignited, it forces the piston down, but becuase the oil doesn't combust as efficiently as the petrol, there is enough left to lube the cylinder as it does so.
Of course, a huge percentage of the oil does burn, which is why you used to be able to follow your mate from a distance of about 3 miles behind - just watch where the smoke went

[FWH]

Originally Posted by TTDegs

from memory, strokers don't have a seperate oil tank?

Most larger 2 stroke engines do, some more basic one you need to mix the oil into the petrol in the tank.