snow chains

even if they are still turning, but not at road speed then the effect is identical to being locked: no grip.

That surprised me when I came across it on a vectra diesel, I was in heavy traffic and eased the clutch up on idle, the car accellerated for itself! As you say the idle increases as soon as the car is rolling slightly.

Some cars do cut the fuel completely on the over-run and bring it back in at a low speed.

Especially petrol ones; even the last of the cars with carbs did this, by means of a little non-return valve in the bottom of the float chamber.

Chris

It's not peculiar to diesels or even Vauxhalls in general. I'd wager any modern car is capable of this. Even my old carburettor cars supplied enough fuel at idle to run the car in gear, although you had to be very gentle engaging the clutch. It does work much better with modern cars with fuel injection & electronic engine management and of course more torquey engines.

No diesels do this, the reason being that the engine breaking effect on a high compression engine when there is no fuelling is immense. As an experiment, I compared decelerating my car from 40mph to 20mph in 4th gear, first just by taking my foot off the accelerator (which took 22 seconds) and second by turning off the engine (which took less than 5 seconds). Actually I'm pretty confident no modern engine management systems (petrol or diesel) ever *completely* cut the fuel, as it makes for very unpleasant deceleration and risk of stalling.

The level of grip when your wheels are losing traction, but still turning, is greater than if you're wheels are actually locked. There is a loss of traction but it can't ever be described as no grip whatsoever as there will always be some resistive force between your tyres and the ground, unless your car is somehow magically floating on air.

All old diesels pre electronic control did it, there's no throttle so you get less engine braking on lift off, quite a few small engines have a throttle plate that closes at the last minute to give some braking, it's less common nowadays in order to avoid failing the emissions requirements when it starts fuelling again.

Which also applys when they're locked. But it's still sod all.

Remember we are talking about descending a slippery hill.

I live on an estate that has a short but steep drop onto a roundabout. It never used to get salted, so when conditions were very icy, cars used to regularly slide down the hill and hit the roundabout. I always managed to get down, but when driving rear wheel drive vehicles, if I lifted off completely, the rear would fish-tail enough that I would also have crashed.

The key was to use just a whiff of throttle in 1st gear; this would allow me to successfully get to the bottom every time.

The fact that there is always *some* grip is irrelevant; what matters is if there is enough grip to retain control.

Chris

a diesel without any throttle restriction and no fuel is just a compressor and it will give massive engine braking compared to a petrol since its compression ratio is so high (compared to petrol)

The point is that many petrol cars reduce fuelling on the over-run to the degree that there is sufficient effect on the driven wheels to cause loss of control. I've had this happen many times with a whole range of two and four wheeled vehicles over more years than I care to remember.

Don't know about all diesels, but my son's Transit would most definitely do it very easily when being driven unloaded.

Chris

Indeed, but it can be a significant factor when trying to stop!

Under some circumstances it may be better to use a higher gear when going down a slippery hill, particularly in a rear wheel drive vehicle.

Chris

It's essentially a lossy air spring, unlike a compressor it doesn't let the compressed air out at peak pressure.

try turning one over by hand and you will get an idea of how and why diesels have so much engine braking.

If one has to slow down, one has no choice but to slow down. If the grip is so bad that engine idle overcomes it - say you have built up speed on a good surface and entered a low grip region - you have much bigger things to worry about. But if you're only doing 6mph to start with, a sudden lift in 1st doesn't cause a big slippage and the car will not spin.

If the application of the brake is so light, allowing the engine to prevent full lock up may be better - and it doesn't cog like ABS.

Yes it does.

At that particular moment the engine is being fueled. Take your foot off the accelerator when you are moving and there will be no fuel being injected until the engine is at close to idle.

Nope, you simply don't understand how a modern EMS works.

Actually, I do. On my diesel, at no point is the fuel ever completely cut off when you lift off the accelerator while in motion.

I don't know what you drive, but I can only assume it's an ancient piece of s*it if it isn't cutting the fuel when you lift off whilst in gear and moving.

No you don't.

Then "your" diesel is an antique.

Or you're a liar.

I call the second one.

Actually if it's an antique it'll definitely cut off all fuel on overrun.