Avoiding BGOL = stall . . .

[Babserella]

Well managed to find out speeds this morning and this was all done without any gas being used.

For 6th gear my car will do 31mph as was thought by previous posters.
Babs x

[TripleS]

Edit: "....unwanted pull of a diesel against the brakes...." What's that about then? I don't recognise the problem you're referring to.

My Ibiza 1.9 tdi was not happy in 5th below about 30 - 35 mph.

A problem that learners would sometimes have, especially in the early stages, was when slowing in 3rd to take a side road to the right, they would change to 2nd, but hold on to the brakes for too long causing the revs to drop just below idle speed. This caused the car to pull against the brakes as the engine management tried to raise the revs up to idle speed. Then when the brakes were released the car would surge forward.

It seems to me likely that this 'unwanted pull' problem is exacerbated by the higher gearing of diesel cars relative to petrol cars, although it may affect all cars due to the generally higher gearing they all use these days: I trust that is the case, isn't it?

Another factor might be that idling speeds are perhaps higher these days than they were before we got fuel injection and fancy engine management regimes. Going back three or four decades I seem to recall that a typical idling speed was about 500/600 rpm, whereas it now seems that 800/1000 rpm is more the order of things, and I don't know why idling speeds should have increased. One might have thought that with the present day regimes of sophisticated engine management systems it should be easier to achieve reliable idling at lower speeds than hitherto. Maybe part of the answer is that with modern cars having much greater electrical loads, a highish idling speed is needed so that the alternator runs fast enough to always be capable of coping with the load, so that the battery charge level is not depleted when idling. Pet hate time: These people who sit around with headlights left on unnecessarily ought to be given a 50 year old car, and made to manage with that during a harsh winter. That'd cure 'em. Right, where were we......?

Ah yes; I also wonder if the 'unwanted pull' factor has increased, not only because of higher idling speeds, but because if an additional load is imposed on an idling engine, the system will detect this and fight back by applying a little more throttle opening or, in the case of a diesel, by increasing the fuelling. In the 'good old days' this didn't happen. The idling speed was more or less fixed by the setting of the carburetter(s) and all that happened when an increasing load was applied was that the idling speed became progressively lower, until ultimately the engine would stall.

There you go, does that make any sense? It it's wrong, I hope somebody can tell us a straight story.

Best wishes all,
Dave.

[zadocbrown]

. Even in that I generally change to 3rd (from 5th) before entering an off-slip in order to make sure that the engine is somewhere in the region that it can be used while I am slowing down, to cover the case where circumstances change.

Does that happen often?

I can't remember the last time I wanted to accelerate on an off-slip, other than in an unusual road layout.

[Gareth]

I can't remember the last time I wanted to accelerate on an off-slip, other than in an unusual road layout.

I often find I want to adjust the speed if there is other traffic. I prefer to be able to adjust up and down, as traffic conditions dictate, rather than just down and less down.

[waremark]

One minor point to consider is that if you allow the electronics to restart fuelling when you are still slowing then fuel is being wasted.

[TripleS]

One minor point to consider is that if you allow the electronics to restart fuelling when you are still slowing then fuel is being wasted.

Presumably what you mean is that if you find you are slowing too quickly (on the over-run) you apply a small amount of throttle merely to reduce the rate at which you're slowing down? Yes, I would regard that as a misjudgement, causing us to use a small amount of fuel that ought not to have been needed. I must mention that to Martin.

Best wishes all,
Dave.

[Mr Cholmondeley-Warner]

I think maybe Mark was referring to the effect we were talking about higher up, where you drag the revs down below the point where the ECU considers it needs to start refuelling to keep the engine going?

[TripleS]

I think maybe Mark was referring to the effect we were talking about higher up, where you drag the revs down below the point where the ECU considers it needs to start refuelling to keep the engine going?

OK, that's another mistake we can make. When that happens maybe it suggests we ought to have been in a lower gear, in which case the engine speed would have been higher, and the closed throttle fuel cut-off would have been effective.

On the other hand, our rate of slowing would then have been greater (than it would have been in a higher gear), in which case we might find we're slowing more quickly than intended......which brings us back to what I thought Mark was referring to. It's quite tricky getting everything right, innit?

Best wishes all,
Dave.

[waremark]

I think maybe Mark was referring to the effect we were talking about higher up, where you drag the revs down below the point where the ECU considers it needs to start refuelling to keep the engine going?

Yes. My approach would generally be to declutch before the ECU starts refuelling - but I have not had a great deal of experience in cars in which this happens at higher speeds.

[TripleS]

On the subject of fuelling in closed throttle situations, I have the impression that all modern engines (i.e. those with fuel injection and engine management systems), whether petrol or diesel, have no fuel supplied to them when on the over-run, so long as the engine speed is above a certain level. The only figure I know relates to a V12 Jaguar we had, and when that car was on over-run, fuelling recommenced when the engine speed dropped to 1200 rpm. The corresponding figure for the 406 HDi looks to be lower still - possibly 1000/1100 rpm, whereas normal idling speed is about 850 rpm. I'll have a go at checking it. The trip computer changing from 999.9 mpg to something a bit less economic should identify the point. I expect the speed will have to be brought down very slowly, otherwise the delay in trip computer functioning will give a false figure for the engine speed at which fuelling recommences.

This morning I found a splendid opportunity to check this on the A169 Pickering to Whitby road: a straight section of road about a mile long and all slightly downhill, but not steep enough to maintain 45 mph (1600 rpm) on a closed throttle in 5th gear.

As the speed reduced very slowly, the instantaneous mpg figure dropped from 999.9 to 942.something when the engine speed dropped below about 1000 RPM. It might even have been as low as 950 rpm, which is lower than I was expecting, given that the idling speed is 800/850 rpm.

I think Peugeot have done very well to get the restart of fuelling as low as that, and as far as I can see it works very well. If anybody else feels able to check the figure on their own car, I would be interested to know the setting on other models.

Incidentally, according to some Jaguar technical information I read some time back, the reason for having fuel cut-off on the over-run is nothing to do with fuel economy; it is because an engine running with a fully closed throttle on the over-run produces very bad emissions. I don't suppose it actually makes very much difference to overall fuel consumption.

Best wishes all,
Dave.

[Horse]

As an addendum / extra info:

In my car, at the particular junctiin where I can experiment, I slow in top down to 1k revs (whuch is the point where it goes square-wheeled) then change down.

The new year will bring more Co car driving, so will update then.

[kfae8959]

Where did you find the specs, Martin?

The top of his head?