snow chains

It's not an "antique" (Astra 1.7 CDTi) and it doesn't cut off all fuel on overrun.

It's not an "antique" (Astra 1.7 CDTi) and it doesn't cut off all fuel on overrun.

This can be verified with a simple test: Measure the time it takes to decelerate from 40mph to 20mph in 4th gear with the engine running and your foot OFF the accelerator, vs the time it takes to decelerate from 40 to 20 in 4th gear by turning the engine off. I tried it - 22 seconds versus less than 5 seconds. The engine braking effect of a high compression diesel is immense, which is why no modern diesel *completely* cuts the the fuel on overrun. Many petrols don't either, but I'm willing to accept that some (newer ones in particular) might.

If a practical experiment doesn't satisfy you, ask any Vauxhall technician who knows the ECUs on these (and many other) diesel cars. I don't know how else to educate you on this subject, since you seem unwilling to accept information from someone who knows better.

None of which alters the fact that under the conditions of descending a slippery hill in a low gear, lifting off completely *can* cause loss of control, especially with a rear wheel drive vehicle.

(BTW, my experience of Vauxhall (and other) "technicians" would lead me to doubt their ability to educate on pretty much any subject...)

Chris

I never disputed that - what I disputed what the claim that the wheels would "lock up". We've established elsewhere in this thread that what the OP of that comment meant was that the wheels were losing traction. There's a difference.

Like anything, depends who you speak to. Midpoint Garage in Middlewich (Cheshire) specialise in Vauxhalls and tuning and have a good working knowledge of the behaviour of the EMS on these and other cars. Got my Astra remapped there.

Which doesn't stop a lot of cars cutting almost all fuel on overrun, you can see the fuel consumption gauge go off the scale. But it's an emmisions control measure which means some carry on fueling, until recently all diesels cut all the fuel on overrun as did many petrol cars.

Vauxhall say different.

They tell you, in the owner's manual, that fuel is shut off on the overrun.

"The fuel supply is automatically shut off during overrun, e.g. when the vehicle is being driven down long gradients or when braking."

It then goes on to explain when fuel is shut off and when it is not in great detail and using language that is simple enough for even you to understand. p199 of the manual. If you had ever bothered to read it, you would have seen the statement.

Page 199 of the Owners Manual is about towing...

If the manual you are looking at does indeed have that in, then it's quite different from my car. Or it's a simplified statement, since the end user doesn't need to know the precise detail of what the engine management is doing. Or perhaps it applies to some petrol vehicles. Whichever it is, my diesel certainly doesn't "shut off" (as in completely) the fuel supply on overrun.

On my diesel (not VX) there is definite fuel cutoff phase. But only above 1200rpm where it comes back in. It is quite noticeable because going down a gentle incline the software can't make up its mind - it cannot do 30mph in 5th smoothly even under gravity.

There is also an inlet valve which closes with "ignition" off, presumably to reduce the problem of running on sump oil. As the management system is Bosch hardware it's possible yours has one and the difference in pumping might explain some difference you see.

The engine needs energy to turn, which includes any compression loss. When pedal is down this energy comes from the fuel, when pedal is up it comes from a reduction in KE & PE of the car. But we know higher compression improves the conversion of fuel energy to mechanical energy so how come it is *believed* that compression loss is also a significant part of the engine's requirements? It can not be both.

Actually it can be, it's an optimisation problem, as your compression increases your thermodynamic effeciency increases, but so do your mechanical losses, at some point the increase in mechanicals losses exceeds the impovement from the thermodynamics, allow for much stronger you have to make th eengine to cope & you've found the ideal compression for your engine.

Further to my other reply, I have been through the Owners Manual and found the section on Overrun under "driving tips" on page 167. The text is much the same as that which you quote above. Of importance, though, is a large asterisk next to the paragraph title "Overrun *". The asterisk is used throughout the manual to signify, and I quote (from the section at the front on making use of this manual), "options not in all vehicles (model variants, engine options, models specific to one country, optional equipment)".

As I have said numerous times in this thread, my car DOES NOT completely cut the fuel on overrun. The asterisk in the manual clearly indicates that this is a feature not present on all model variants. None of the diesel models can be said to "automatically shut off" the fuel supply since they don't completely cut the fuel on overrun.

End of.

Yes, your tendency to bullshit has been noted.

Late carb VWs did it with a vacuum and electronically controlled unit that shut the throttle completely on overrun (and held it open for fast-idle). Sadly, the wires had a habit of breaking...

Diesels do it too, or at least VAG TDIs do: if you roll down a hill in gear, you can hear the combustion clatter stop if the engine turns above idle.

Nor me. At the end of the day, it will take a certain amount of torque to move the car. If the grip is sufficient, the car will move, if it isn't, it won't.

I think it was aimed at people with poor clutch control: using a high gear means that they can't dump so much torque at the wheels by being hamfisted. Of course, this generally just results in them spinning the wheels *faster*.

*Staying* in high gear when moving makes sense for the reason you give, but I've never got the whole 'pull away in 2nd' thing.

Well, you probably wont actually lock them, but they could end up revolving much slower than your road speed, which is pretty much as bad.

I believe that his car has a butterfly valve in the inlet that closes with ignition off to prevent the engine running on it's own oil, and this gives the impression that he states.

By the way, none of my six diesels have had very noticable engine braking on normal overrun, indirect injection nor direct injection. High-ish milage or low milage.

David

Of course the thermodynamic gain must exceed the increase in compression losses (accepting that compression will have some cost) in order to improve the net output. But the thermodynamic gain is in percentages whereas the perceived effect attributed to compression requires factor increases.

When Andrew quoted 5 seconds to go from 40-20 in 4th that would mean the engine having to offer about 180Nm of resistance at the flywheel. Suffice to say that his experiment was faulty.

I don't think the experiment's that aulty, I thimk the assumptions behind it are rather flawed

More to the point, on my diesel car the traction control system will add power if it detects a wheel slipping on the overrun or braking, as distinct from cutting power and/or braking a wheel that is spinning on acceleration.

-- DAS