Does a modern car really use no fuel when off gas?

[Pistom]

This is something I hear very often and I struggle to understand how it can be right.

Even Clarkson (OK, not exactly a reliable source) repeated it on TG the other night.

The thinking is that if your foot is off the accelerator when the car is still in gear whilst moving forward, the forward motion of the car spins the engine through the transmission and fuel injection shuts off the fuel so no fuel is used.

I think this idea has come about because fuel computers show no fuel is being used.

I say - utter rubbish. The fuel injection is still feeding fuel otherwise you would have a lumpiness when you go back on the accelerator again. Engine breaking would be even greater. Try turning the ignition off with the car still in gear and compare that to when the ignition is on and you will see my point proven.

Can anyone confirm my thinking?

[kambites]

The Lotus certainly feels like it; there is a definitely step in engine braking when the engine drops below about 2000rpm (which I'd always assumed was when the engine reverted to its idle fuel load).

[davepoth]

Pistom said:

This is something I hear very often and I struggle to understand how it can be right.

Even Clarkson (OK, not exactly a reliable source) repeated it on TG the other night.

The thinking is that if your foot is off the accelerator when the car is still in gear whilst moving forward, the forward motion of the car spins the engine through the transmission and fuel injection shuts off the fuel so no fuel is used.

I think this idea has come about because fuel computers show no fuel is being used.

I say - utter rubbish. The fuel injection is still feeding fuel otherwise you would have a lumpiness when you go back on the accelerator again. Engine breaking would be even greater. Try turning the ignition off with the car still in gear and compare that to when the ignition is on and you will see my point proven.

Can anyone confirm my thinking?

Fuel injection is very clever now and cuts off the fuel flow on the overrun. My 12 year old fiesta had this as a feature; it's in the manual. Generally it'll only do it if the car has been on the overrun for a few seconds going down a hill, and you may notice a tiny jerk when you put your foot back on the accelerator. Generally newer throttle by wire cars will not have the jerk because the throttle has been mapped to stop it.

[Dracoro]

Pistom said:

I say - utter rubbish.

Irony is strong with this one

You don't understand it therefore it's "utter rubbish"?

Most *modern* (AFAIK most fuel injected cars from the last 20 year) cars no NOT use any fuel on the overrun whilst in gear. It will start putting fuel in at low revs to stop it stalling, you'll want to change down a gear well before then anyway.

[Willy Nilly]

My tractor shows 0 litres/hour when I close the throttle, it shows 33 litres an hour wide open though

[busta]

We need a mapping expert to tell us what the injectors are doing when rpm is higher than idle speed and the throttle is closed, although I suspect you may be correct.

[300KPH]

Id like to call bu*&^*!t on that claim as well. Slowing down from 5,000rpm to 3,000rpm is gonna use less fuel then knocking a car into neutral and 800rpm??? Find it hard to believe to be honest, if ya remember Clarkson made the same claim when he was driving that Audi A8 diesel up to scotland and back or something on one tank, said the Audi guys told him it used less diesel when in gear slowing down.

[Dracoro]

Tell me, WHY would it want to use any fuel? The momentum of the car, when coasting down (in gear) is turning the engine round, so why waste any fuel.

When you cycle and start coasting down a hill, you don't need to pedal (unless you want to go faster of course!). Take your feet off the pedals and the pedals still go round etc. You pedal when you want to maintain a speed, or go faster. When you are coasting down, braking etc. you don't STILL keep pedalling do you?

Edited by Dracoro on Monday 24th January 20:47

[anonymous-user]

busta said:

We need a mapping expert to tell us what the injectors are doing when rpm is higher than idle speed and the throttle is closed, although I suspect you may be correct.

You called! ;-)



Yes, depending on the degree of "negative" flywheel torque being scheduled by the driver demand, the fuel may be competely shut off. (in fact, due to modern emissions limits and the importance of either being at exactly lambda 1 or zero fuel, modern cars are much quicker to get into DFSO (decel fuel shut off) than they used to be)

This is also compounded by the minimum usable injector pulse width. Below a certain cylinder air flow value, the fuel injection pulse width is so short (typically when <1.5ms) that the fuel delivery quantity becomes non linear, and in order to prevent lean or rich excursions, the injectors have to be turned off competely.

The only thing that might cause the injection quantity to remain above zero during a full dsfo event is catalyst over temperature (where fuelling is maintained at lambda 1 during decel to prevent the lean exotherm from over heating the cat. (but you usually have to be drivinng pretty hard to get this)

As for why the engine doesn't stall or misfire, thats because the fuel quanity control is now very very good (as it has to be for emissions control) and careful transient fuel mass management avoids partial rich or lean misfires.


(however, using DSFO isnt the most economical way of driving!) (but that's a whole new topic;-)

[otolith]

Argument from personal incredulity is not usually convincing

[bullitinhead]

I know mine isnt using fuel on closed throttle at speed as the lpg button flashes to say it's lost injection signal.

so it must be right . 2001 2.3 Galaxy


bullit

[Superhoop]

If you have access to datalogging equipment, it's easy to see/prove.

A typical engine will run an injector pulse width of approx 1.6 - 1.9m/s at idle. When on overrun, the injector pulse width is 0m/s, as in not operating at all.

Fuel cut doesn't occur at all engine speeds above idle though, as fuel is reintroduced somewhere slightly above idle, normally about 200 - 250 rpm above.

I can probably make a datalogger trace of injector pulse width, engine speed, throttle opening etc if anyone is actually really interested in seeing it.....

[Eddie the Ead]

So are we saying that its more efficient to just take your foot off the accelerator rather then taking the car out of gear and coasting? My trip computer suggests otherwise.

[useyourdellusion]

I remember driving a 1987 mk4 XR3i which had an on-board computer. When I lifted off the throttle (whilst going downhill in fifth) it leapt from 30ish mpg to 100mpg. I think that must have been its maximum possible indicated figure.

[Superhoop]

Eddie the Ead said:

So are we saying that its more efficient to just take your foot off the accelerator rather then taking the car out of gear and coasting? My trip computer suggests otherwise.

Then your trip computer must be on the blink - If you knock it into neutral, it's injecting fuel to keep the engine idling (not much, but it is injecting fuel) whereas lifting off the throttle will initiate fuel cut

[redstu]

My 87 528i shut of the fuel when slowing if the rpm was above 1200 iirc.
My passat immediate fuel consumption shows nil consumption.
On the other hand I think my alfa drinks at all times ! The urban fuel "economy" is quite poor.

[Megaflow]

Max_Torque said:

busta said:

We need a mapping expert to tell us what the injectors are doing when rpm is higher than idle speed and the throttle is closed, although I suspect you may be correct.

You called! ;-)



Yes, depending on the degree of "negative" flywheel torque being scheduled by the driver demand, the fuel may be competely shut off. (in fact, due to modern emissions limits and the importance of either being at exactly lambda 1 or zero fuel, modern cars are much quicker to get into DFSO (decel fuel shut off) than they used to be)

This is also compounded by the minimum usable injector pulse width. Below a certain cylinder air flow value, the fuel injection pulse width is so short (typically when <1.5ms) that the fuel delivery quantity becomes non linear, and in order to prevent lean or rich excursions, the injectors have to be turned off competely.

The only thing that might cause the injection quantity to remain above zero during a full dsfo event is catalyst over temperature (where fuelling is maintained at lambda 1 during decel to prevent the lean exotherm from over heating the cat. (but you usually have to be drivinng pretty hard to get this)

As for why the engine doesn't stall or misfire, thats because the fuel quanity control is now very very good (as it has to be for emissions control) and careful transient fuel mass management avoids partial rich or lean misfires.


(however, using DSFO isnt the most economical way of driving!) (but that's a whole new topic;-)

And they can still be maped to provide no fuel at WOT with cranking spped to clear a flooded engine too... Just to really annoy the OP.

[anonymous-user]

The MOST fuel economical way of driving is to convert the least fuel into the most kinetic energy, and waste the least in friction (and hence to heat)

If you are in gear, then the engine is spinning, and this has a lot of friction. So, if you accelerate to say 30mph, the best way to get the furthest is to knock the car into neutral and key off. That way the fuel energy converted into the KE (at 30mph) is entirely converted into just the vehicles aero and rolling friction. if you leave the car in gear, you loose some energy into the engine friction (which is why you car slows faster in gear than in neutral).


Eventually, all the cars KE will be converted (via the inertia of the car) to aero or rolling friction and the car will coast to a halt. Effectively the cars own interia is the "regen" storing energy and releasing it slowly. (which is why ultimate fuel economy is largely independant of mass).


However, in the real world we often need to stop. The best way is to select neutral and key off far enough away from where you need to stop so the cars just coasts to a halt. In reality (i.e. outside of a mileage marathon !) this is pretty tricky. hence the next best solution is to stay in gear and lift off the accelerator. The worst way is obviously to brake.


The lowest fuel consumpion is gained by minimising the cars loses to friction.

[busta]

Eddie the Ead said:

So are we saying that its more efficient to just take your foot off the accelerator rather then taking the car out of gear and coasting? My trip computer suggests otherwise.

Thats another point which has been covered on here before. It depends how long you are coasting for but it's definitely true that you will maintain a higher speed when coasting than when in gear, and travel a greater distance before stopping, so in lots of scenarios this outweighs the fact you are using a small amount of fuel.

On top gear they where referring to avoiding using the brakes, and obviously engine braking goes someway to substitute use of the brakes. Using just engine braking alone on the approach to a roundabout is quite easy to judge and slows you at a useful rate. Trying to coast up to a roundabout requires far better judgment and because the deceleration is so much slower, is quite frustrating.

Every time I try to coast to a junction I either have to get back on the throttle as I'm doing 30mph with 200 yds to go, or I have to use the brakes as I'm still doing 55mph with 50 yds to go.

But yes, coasting is definitely more efficient if you do it for long enough.

[HellDiver]

Pistom said:

I say - utter rubbish.

I say - you're a fool.

Fuel cutoff has been standard on fuel injected cars since Bosch invented the idea with the original EDC injection system.

Even my old 1.6 8v Astra with single point injection would cut off fuel. There was a 1 second lag between pressing the throttle and anything happening, probably because of the extra length of the inlet tract, and the single injector having to fill the plenum. Modern fuel injection is injecting just before the valve, or is direct injection, so zero lag.