Avoiding BGOL = stall . . .

[TripleS]

Feel free to offer your own alternatives, Dave, and to elucidate the "several counts"...

Fair enough, Nick. I'm going to be out most of the day, but I'll try to do it later. I might turn out to be quite wrong in my thinking, so I'm still keen to know what others think.

Best wishes all,
Dave.

[hanse cronje]

there is nothing wrong with the car, having driven a laguna 2.2 and 1.9 D, saab ttid, A4 3.0D and the current car 330D this is the trait of the modern derv. You just need to slightly adjust your driving style. The laguna 1.9 was used on HPC assessment, the saab on an IAM retake and HPC CPD with no issues, examiners and instructors being fully aware of modern derv and their idiosyncrasies.
Currently, in the 330D, i just H & T or dip the clutch slightly earlier than the 250 cup i have the use of because of the reasons Nick has stated. The saab was used to teach the kids at U17CC with no issues.

i doubt this will be the end as someone will need the last word, i wont be bothering to read the next five pages of pointless posts

[TripleS]

Right then, here are the bits that mystify me about Nick's post:

....and I accept that some of my comments might be seen as nit picking, so ignore them if you wish.

"I suspect that Horse, in sticking to "advanced" methods, is getting down to considerably lower speeds than those posted by the previous two contributors. I drove a Mini Cooper D today (thanks Babs ) and as with most diesels I've driven recently, anything below about 15mph, is hard for them to manage."

What? In any gear, or just in the highest available gear? I don't expect many cars will run happily at 15 mph in their highest gear, whether petrol or diesel powered, but I do know that our 406 HDi will run happily at about 5 mph....which is about idling speed in 1st gear. FWIW it will also climb a moderate gradient on a closed throttle in 2nd gear, and a slight incline on a closed throttle in 3rd gear. Admittedly it won't do much at all on a closed throttle in 4th or 5th gears.

"In a petrol car you can come almost to a standstill in the highest gear."

What sort of speed are you referring to there?

"The natural inclination of the petrol engine to "live and let live" at most engine speeds, is due to its lower compression ratio than the diesel, and the fact that it doesn't have to be driven to keep going. The diesel engine, relying as it does on compression for ignition, needs to be driving the car, to maintain momentum, because without the effect of the heavy flywheel to maintain that compression, it stalls."

I don't see that. The diesel engine certainly relies on compression to activate the combustion process, but it doesn't need to be driving the car to avoid stalling. It will also continue to run on a closed throttle, so long as the car is moving at a reasonable speed in a suitable gear, and the clutch is engaged. It will also idle and continue to run on a closed throttle when we stop, so long as we don't stall it ourselves. I also don't understand the relationship you're seem to be seeing between weight of flywheel and the maintenance of compression.

"So yes, you need to dip the clutch rather earlier than feels appropriate...."

When slowing down in a diesel powered car you do need to disengage the clutch earlier than you would in a typical petrol engined car, but that is only because the higher gearing of the diesel car means you get down to idling speed at a higher road speed than would apply in a typical petrol engined car.

To my mind, from a driving viewpoint, there are no major differences in principle between petrol engines and diesel engines; the differences are only a matter of degree. Petrol engines have a wide speed range, whereas the major moving parts in diesel engines tend to be more robust and heavier, so they can't run as fast. (I also understand the combustion process is slower in diesel engines, so that also limits their operating speed.) On the other hand they tend to pull better, albeit over a narrower speed range, so they have higher gearing....which means you can't get down to low road speeds in high gears as readily as you might with a petrol car. Having said that, I don't see that the driving of a diesel car need be any more difficult than the driving of a petrol powered vehicle, so long as you make minor adjustments to accommodate the different speed ranges that can be used.

I think I'd better leave it at that for now. If anybody thinks I've got it seriously wrong, by all means put me right.

Best wishes all,
Dave.

[Mr Cholmondeley-Warner]

Right then, here are the bits that mystify me about Nick's post:

....and I accept that some of my comments might be seen as nit picking, so ignore them if you wish.

"I suspect that Horse, in sticking to "advanced" methods, is getting down to considerably lower speeds than those posted by the previous two contributors. I drove a Mini Cooper D today (thanks Babs ) and as with most diesels I've driven recently, anything below about 15mph, is hard for them to manage."

What? In any gear, or just in the highest available gear? I don't expect many cars will run happily at 15 mph in their highest gear, whether petrol or diesel powered, but I do know that our 406 HDi will run happily at about 5 mph....which is about idling speed in 1st gear. FWIW it will also climb a moderate gradient on a closed throttle in 2nd gear, and a slight incline on a closed throttle in 3rd gear. Admittedly it won't do much at all on a closed throttle in 4th or 5th gears.

I approached a roundabout in 5th or 6th (can't remember). The speed dropped happily down to about 15mph, but then the engine started labouring, so I dipped the clutch.

"In a petrol car you can come almost to a standstill in the highest gear."

What sort of speed are you referring to there?

Down to, say, 5-10 mph. I'll measure it tomorrow.

"The natural inclination of the petrol engine to "live and let live" at most engine speeds, is due to its lower compression ratio than the diesel, and the fact that it doesn't have to be driven to keep going. The diesel engine, relying as it does on compression for ignition, needs to be driving the car, to maintain momentum, because without the effect of the heavy flywheel to maintain that compression, it stalls."

I don't see that. The diesel engine certainly relies on compression to activate the combustion process, but it doesn't need to be driving the car to avoid stalling. It will also continue to run on a closed throttle, so long as the car is moving at a reasonable speed in a suitable gear, and the clutch is engaged. It will also idle and continue to run on a closed throttle when we stop, so long as we don't stall it ourselves. I also don't understand the relationship you're seem to be seeing between weight of flywheel and the maintenance of compression.

Yep, I didn't say it wouldn't, but it's being fed diesel to keep it going, and the flywheel, along with the small output from the engine itself, are keeping it going between compression strokes. I suspect, although I can't say for certain, that unlike petrol engines which shut off the fuel supply completely when there is no drive required, diesel engines continue to be fed diesel even on a closed throttle. I have a feeling that's where the unwanted pull of a diesel against the brakes comes from. I'm happy to be corrected on that.

"So yes, you need to dip the clutch rather earlier than feels appropriate...."

When slowing down in a diesel powered car you do need to disengage the clutch earlier than you would in a typical petrol engined car, but that is only because the higher gearing of the diesel car means you get down to idling speed at a higher road speed than would apply in a typical petrol engined car.

To my mind, from a driving viewpoint, there are no major differences in principle between petrol engines and diesel engines; the differences are only a matter of degree. Petrol engines have a wide speed range, whereas the major moving parts in diesel engines tend to be more robust and heavier, so they can't run as fast. (I also understand the combustion process is slower in diesel engines, so that also limits their operating speed.) On the other hand they tend to pull better, albeit over a narrower speed range, so they have higher gearing....which means you can't get down to low road speeds in high gears as readily as you might with a petrol car. Having said that, I don't see that the driving of a diesel car need be any more difficult than the driving of a petrol powered vehicle, so long as you make minor adjustments to accommodate the different speed ranges that can be used.

Point out if you will, where I said it was more difficult I agree higher gearing will be a factor also.

I think I'd better leave it at that for now. If anybody thinks I've got it seriously wrong, by all means put me right.

Best wishes all,
Dave.

I think we're in agreement, mostly. It's an interesting discussion, and I don't claim to know all the answers.

[TripleS]

Hello Nick, and thanks for your reply.

No, I accept that you didn't say driving a diesel car was more difficult, but others do seem to be reporting a bit of difficulty in some situations.

With regard to the speeds one can get down to before being obliged to de-clutch, I shall be interested to hear what the actual speeds are for the Mini Cooper D. At the moment the speeds you mentioned sound startlingly low.

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.

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

Best wishes all,
Dave.

[ScoobyChris]

With regard to the speeds one can get down to before being obliged to de-clutch, I shall be interested to hear what the actual speeds are for the Mini Cooper D. At the moment the speeds you mentioned sound startlingly low.

The Cooper D has a 6th gear of around 36 ish mph/1000 rpm (off the top of my head ). As Gareth mentions, sometimes there's a need to take an intermediate gear when slowing from NSL to town speeds, but for the most part de-clutching and "coasting" for a short distance seems to work well for me...

Chris

[fungus]

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.

[Babserella]

As the Mini Cooper D is mine that Nick was driving I will try and find out the lowest speed I can get to in 6th gear - I'm on a course all day today so won't be able to post till later, so watch this space.

[ScoobyChris]

As the Mini Cooper D is mine that Nick was driving I will try and find out the lowest speed I can get to in 6th gear - I'm on a course all day today so won't be able to post till later, so watch this space.

My guess is around 30/31mph

Chris

[Mr Cholmondeley-Warner]

I tested the Alfa this morning. Once the engine is warm, it will drop to about 600rpm before juddering. In 6th gear that's about 12 or 13 mph. To put that into context, it's maybe one and a half car lengths before coming to a complete stop, at traffic lights, for example.

Chris's figures suggest that on Saturday I was probably in a lower gear than I remembered, perhaps 4th.

[martine]

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

That makes sense - according to the spec. it is geared for 30.6mph/1000rpm in 5th. As has been suggested it seems an early change into 4th or 3rd as you approach a roundabout for example might be best.

[Mr Cholmondeley-Warner]

Where did you find the specs, Martin?

[Horse]

Thanks for al the comments!

'My' car (as opposed to the 'family' car) is a Vauxhall Astra 1.7TD, '02 version.

Typical hazard approach involves slowing, then selecting a lower gear - the sort of thing I've been doing on cars and bikes for 35 years.

Until I drive a work pool car, a later model Astra diesel:
Slow in high gear, stall, rolling restart, lower gear, continue . . .

I can't tell you exactly what speed/gear/revs combo it stalls at, but slowing from 60-70 needs a downshift earlier than I would otherwise do - so either brake-gear-brake-gear or BGOL is needed.

Erm, adapt to accomodate the specifics of any vehicle.
"Past performance is no guide to the future", as is said elsewhere.

Indeed, if I didn't adapt I would have spent far more time restarting the engine My concern is that such as fundamental change in style is needed such that I have to either coast with the clutch in or take an intermediate gear mid-braking.

For all my posts on here, I'm fairly old skool Blue Book

My point/question is that two similar cars vary so much that a dramatic change in driving style is necessary.

I suspect that Horse, in sticking to "advanced" methods, is getting down to considerably lower speeds than those posted by the previous two contributors.

You saying I'm slow?

I rather think that Gareth's point is about approaching hazards quickly, where flexibility seems to be sacrificed earlier in the process, and his way of dealing with this, and maintaining the flexibility he wants (e.g. for a swift "looking to go" outcome at a roundabout) is to adopt a lower gear earlier in the process. No doubt he will correct me if I'm wrong.

Yup, fine taking a lower gear 'for flexibility' prior to braking - what about those 'firm' braking situations - that option just won't be available.

I do try to drive in a fairly economic way, so slowing early for lights and trying to keep the car rolling - but I [usually] work on the basis of select speed then select gear. I'm interested that declutching is suggested - is that to just let the car coast until - eventually - a lower gear is selected?

What? In any gear, or just in the highest available gear? I don't expect many cars will run happily at 15 mph in their highest gear, whether petrol or diesel powered, but I do know that our 406 HDi will run happily at about 5 mph....which is about idling speed in 1st gear.

Thinking about it, the sort of situation is leaving a motorway having travelled in top gear, then slowed on the slip road. One junction in particular springs to mind, M4 J13 Wbound exit, where there's a works exit with its own traffic lights followed by the main set of lights, so I'm probably down to 35-40 in top when . . . stall. My Astra copes no problem!

I suspect that Horse, in sticking to "advanced" methods,

PS Haven't forgotten I owe you a PM Working on it

[Gareth]

I'll be doing some experimenting (eventually) but in the meantime some figures and thoughts.

Our diesel car appears to do 33.5 mph per 1000 rpm in 6th gear, (or, at least, the nearest comparable car for which I could find figures does). I also found a Seat Ibiza PD130 TDi is geared at over 37 mph per 1000 rpm in 6th.

With our car, if I want to use the accelerator I try to keep the engine speed above a minimum of 1500 rpm, (although that doesn't work for 1st or 2nd), and I try to keep it to gentle acceleration only below 2000 rpm. It seems to give a nicer result than accelerating (or accelerating more firmly) from lower engine speeds.

At 1500 rpm the car would be going at just over 50 mph in 6th ... although in practice I generally find I want to be going close to 60 mph before I'll use 6th.

So, if I'm on the motorway at cruising speed, that's about the slowest I want to be going before I'll change down. I suppose the question is how much flexibility do I want? And at what speed I'm happy to depress the clutch, (and keep it depressed), while I am slowing down.

Comparing this with our petrol engined car, that'll be perhaps half way in it's rev range at motorway cruising speeds -- but I need to check this if I can prise the keys out of Hanna's hands sometime. 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. To be clear, though, I'm not doing this to get increased engine braking, although that is one effect.

I see it as part of choosing the most appropriate gear for the circumstances; I can look ahead and infer that I'll soon be travelling slower, not least because I will be braking, and pick a gear that will be appropriate to that situation, a gear that means I'm prepared for changing circumstances and which will be useful over the range of speeds I'll shortly be using.

I try to do something analogous when driving our diesel car, although I try to tailor how I handle this to the quite different engine characteristics. Staying in 6th really isn't very nice, nor is declutching for an extended time under braking. An alternative approach would be to brake more firmly, so the braking distance and period is shortened, and the amount of time the engine is fighting the brakes is also shortened. But I don't want to have to brake firmly every single time, and while the car is moving I don't like keeping the clutch down for longer than is necessary to change gear.

If, on the other hand, I slow down for a low speed hazard just by lifting off, then the car eventually won't slow down much more and I'll need to brake to overcome the engine being kept going by the electronics. At that point there's no point being in 6th gear with the clutch up, and this seems to happen somewhere between 40 and 50 mph in our car. There's no flexibility to accelerate at this point, so I'm not actually prepared for anything by staying in 6th, so at these speeds, and well before I start using the foot brake, I'll change to a lower gear, 3rd or 4th depending on the circumstances.

[Mr Cholmondeley-Warner]

I suspect that Horse, in sticking to "advanced" methods, is getting down to considerably lower speeds than those posted by the previous two contributors.

You saying I'm slow?

No, I wasn't, and reading your later paragraphs, it seems I misunderstood. I imagined you rolling up to a hazard without declutching, and I hoped you'd be able to achieve lower speeds than that which you have now clarified.

I rather think that Gareth's point is about approaching hazards quickly, where flexibility seems to be sacrificed earlier in the process, and his way of dealing with this, and maintaining the flexibility he wants (e.g. for a swift "looking to go" outcome at a roundabout) is to adopt a lower gear earlier in the process. No doubt he will correct me if I'm wrong.

Yup, fine taking a lower gear 'for flexibility' prior to braking - what about those 'firm' braking situations - that option just won't be available.

I do try to drive in a fairly economic way, so slowing early for lights and trying to keep the car rolling - but I [usually] work on the basis of select speed then select gear. I'm interested that declutching is suggested - is that to just let the car coast until - eventually - a lower gear is selected?

I think you will have to modify that approach, if the speed in your later paragraph is correct, by taking an intermediate gear, perhaps at the point where you identify the fact that you will be in a stop / go situation. So, leave M/way, travelling down slip, see a queue of traffic at the bottom, select (3rd), now back to handling the hazard according to the SOCC.

What? In any gear, or just in the highest available gear? I don't expect many cars will run happily at 15 mph in their highest gear, whether petrol or diesel powered, but I do know that our 406 HDi will run happily at about 5 mph....which is about idling speed in 1st gear.

Thinking about it, the sort of situation is leaving a motorway having travelled in top gear, then slowed on the slip road. One junction in particular springs to mind, M4 J13 Wbound exit, where there's a works exit with its own traffic lights followed by the main set of lights, so I'm probably down to 35-40 in top when . . . stall. My Astra copes no problem!

I would hope any car would cope with 40mph It does sound as if maybe there's an issue with the ECU on that particular car - I'm sure anti-stall should kick in at the appropriate rpm and at least give you an indication that it isn't happy with your gear selection before stalling.

PS Haven't forgotten I owe you a PM Working on it

No worries. We've got a while