Automatic gearbox and regular use of neutral

itsmeagain

born_again said:

Advocado said:

@itsmeagain - as you clearly know what you're talking about, could you just confirm - will I damage my van if I regularly move from D to N and back to D whilst driving at speed?

Thanks.

Transmission spinning fast V engine ticking over... Massive difference in speed. What do you think this is going to do to a auto gearbox. Especially if it's a DCT.

Good question. The boxes that I worked on were 8/9 speed and boringly changed up at the earliest opportunity. When N to D was selected on the move, It chose a high gear with low revs - see shift maps on previous post. Many modern cars cruise at 1500rpm at 80mph these days! Some cars also have an elevated idle in N when the vehicle is moving. This would reduce the differential speed possible. There is also the ability for 'things' in the drivetrain to 'slip'.

Much depends on if the torque structure control of the ECU's have a 'feed forward' on the engine speed controller to match the revs in the time gap between requested D and the realised gear engagement (similar to the manual sustained rev down change concept that Jenni_D refers to). Auto gearbox vehicles do this between every gear change using torque up/torque down functions to make the changes seamless.

The transmission calibrations that I worked on were capable of double gear downshifts on kick-down, (as large as the change in speed than you'd expect from the action we are talking about).

In any event, the FMA should inhibit actions that are likely to cause damage. My expertise isn't on the mechanical integrity of the transmission under such customer usage scenarios, and it may be that the control systems that achieve the effect automatically (as per skoda mentioned earlier) would be inhibit the function under certain scenarios that the OP could override manually.

itsmeagain

Jenni_D said:

Does the OP "blip" the throttle when switching back to D? (Same as you'd do in a manual if dropping down gears to e.g. make an overtake - you'd blip the throttle to align the revs to the trans speed).

You may be surprised that flight recorders show that less than 10% of people do this. Also only 18% of people proactively blend off the brake when stopping (to stop the vehicle jerk)! Maybe it doesn't tell you to do it in the highway code.

itsmeagain

neilmcl said:

BOWFER said:

Advocado said:

basill said:

Not just DSG, Volvo 8 speed non dsg autos coast in eco mode.   Must be something to save.    Emissions if not actual fuel possibly 🙂

If Volvo of all manufacturers have their cars coast, it can't be as dangerous as people on here seem to think.

I think the 'danger' part of it is being grossly exaggerated by some on here, I'm not joining in on that.
My electric car has very little to zero braking effect in D.
I have to select B mode to get any.
I wouldn't call it dangerous at all, it's just people getting all preachy.

Not at all.

Just think for a minute, your coasting downhill in a near 3 tonne vehicle do you really think your stopping distance is going to be the same as a vehicle that's in gear. It certainly won't be and every cm you can save in an emergency stop is critical. Nothing preachy about, it's just a simple fact.

Can you please explain why you think that a modern auto vehicle, auto selecting the highest gear and only changing down each time the revs get low (offering very low engine braking), would stop significantly earlier, than the same vehicle in Neutral. Bear in mind that modern braking systems are perfectly capable of locking the wheels and delivering near perfect cadence braking whether in gear or not?

Also bear in mind that the brakes are relatively 'cold' having not needed them to maintain speed down the hill.

Please explain the technical reason for this fact without saying that it's a well known fact.

shinytop

itsmeagain said:

Jenni_D said:

Does the OP "blip" the throttle when switching back to D? (Same as you'd do in a manual if dropping down gears to e.g. make an overtake - you'd blip the throttle to align the revs to the trans speed).

You may be surprised that flight recorders show that less than 10% of people do this. Also only 18% of people proactively blend off the brake when stopping (to stop the vehicle jerk)! Maybe it doesn't tell you to do it in the highway code.

but nearly all of them are over 65, wear driving gloves and read the highway code a lot. 

Flight3287462

OP a few tips from my time living in Africa and travelling by Taxi Brousse, out of gear, engine off and at night turn the lights off too  

You'll save a fortune but you may kill a lot of people.

Advocado

itsmeagain said:

neilmcl said:

BOWFER said:

Advocado said:

basill said:

Not just DSG, Volvo 8 speed non dsg autos coast in eco mode.   Must be something to save.    Emissions if not actual fuel possibly 🙂

If Volvo of all manufacturers have their cars coast, it can't be as dangerous as people on here seem to think.

I think the 'danger' part of it is being grossly exaggerated by some on here, I'm not joining in on that.
My electric car has very little to zero braking effect in D.
I have to select B mode to get any.
I wouldn't call it dangerous at all, it's just people getting all preachy.

Not at all.

Just think for a minute, your coasting downhill in a near 3 tonne vehicle do you really think your stopping distance is going to be the same as a vehicle that's in gear. It certainly won't be and every cm you can save in an emergency stop is critical. Nothing preachy about, it's just a simple fact.

Can you please explain why you think that a modern auto vehicle, auto selecting the highest gear and only changing down each time the revs get low (offering very low engine braking), would stop significantly earlier, than the same vehicle in Neutral. Bear in mind that modern braking systems are perfectly capable of locking the wheels and delivering near perfect cadence braking whether in gear or not?

Also bear in mind that the brakes are relatively 'cold' having not needed them to maintain speed down the hill.

Please explain the technical reason for this fact without saying that it's a well known fact.

I've already asked @neilmcl to explain his theories, but he goes uncharacteristically quiet.

Advocado

Flight3287462 said:

OP a few tips from my time living in Africa and travelling by Taxi Brousse, out of gear, engine off and at night turn the lights off too  

You'll save a fortune but you may kill a lot of people.

Thanks.  I'll bear that in mind.

ontheroad1970

I don't see how locking the wheels is a desirable thing at all, what with ABS and all.

itsmeagain

ontheroad1970 said:

I don't see how locking the wheels is a desirable thing at all, what with ABS and all.

Just to clarify, no one said that it was desirable to lock-up the wheels. I was saying that "modern braking systems are perfectly capable of locking the wheels". I.e, modern brakes are more than capable of achieving maximum braking, without the additional support of engine braking to do so.

The question... why would additional 'in gear' engine braking make the car stop quicker than using a modern braking system alone? I.e, "with ABS and all"

BOWFER

neilmcl said:

BOWFER said:

Advocado said:

basill said:

Not just DSG, Volvo 8 speed non dsg autos coast in eco mode.   Must be something to save.    Emissions if not actual fuel possibly 🙂

If Volvo of all manufacturers have their cars coast, it can't be as dangerous as people on here seem to think.

I think the 'danger' part of it is being grossly exaggerated by some on here, I'm not joining in on that.
My electric car has very little to zero braking effect in D.
I have to select B mode to get any.
I wouldn't call it dangerous at all, it's just people getting all preachy.

Not at all.

Just think for a minute, your coasting downhill in a near 3 tonne vehicle do you really think your stopping distance is going to be the same as a vehicle that's in gear. It certainly won't be and every cm you can save in an emergency stop is critical. Nothing preachy about, it's just a simple fact.

What you going to do with electric vehicles then?
Like I said, they can have next to zero engine braking effect in standard drive modes.
The driver has to select different driving modes every time they start the car again to obtain any engine braking effect.
In my car it's a set amount, on other cars the driver can adjust the amount.
But, standard, zero engine braking.