Automatic gearbox and regular use of neutral

Advocado

neilmcl said:

Agree with the above, coasting in gear shouldn't use any fuel, in fact you should save fuel due to engine braking regeneration, whereas being in neutral wastes fuel because the engine will be idling.

How does engine braking save fuel??  What do you think is being "regenerated"?

In neutral the vehicle will go further than in D, hence improved mpg.

Advocado

Keep_pedalling said:

Advocado said:

daveyjp said:

DSG boxes are so flaky driving it without slipping into neutral and coasting is more than enough to cause serious damage.

So I'm doing the right thing?

What model VW is this? 

Transporter

neilmcl

Advocado said:

BOWFER said:

I cannot for the life of me see why his MPG would be improving dramatically.
It's not like foot off the throttle in gear is using fuel.

My mpg improves because the vehicle goes further without using any fuel.  If it's in D it slows down fairly quickly so I then need to press the accelerator again to maintain a decent speed.

Maybe I exaggerated slightly with "steep hill", but anything less than what you would call steep and it loses speed.  And on the flat, it'll go from 40mph to 30mph in about 10 seconds when in D.  In N it probably takes nearer 45 seconds.

But that's not true for the reasons I explained. In fact I wouldn't rely on the real time MPG figures on your dash whilst in neutral as they probably wouldn't be accurate in this scenario.

Also, have you considered how more dangerous this is to other road users. Your stopping distances in an emergency are likely to much higher trying to stop a rolling vehicle, downhill in neutral than it is with the assistance of being in gear.

itsmeagain

Diesel or Petrol - which engine, which car?

Generally speaking, you should not lose speed down a steep hill unless you have the aerodynamics of a bus and brakes dragging, but the gear selection control strategy may intentionally down shift, in recognition of the gradient (to increase engine braking). Some VW's actually coast themselves (often indicated on the IP as eco/D instead of showing a gear number), but as soon as you touch the brake or throttle it re-selects an appropriate gear.

Ordinarily, an 'in-gear' downhill 'over-run' will trigger Over Run Fuel Cut Off (ORFCO), that will save fuel whilst only air is being pumped through the engine and into the catalyst (unless you have valve disablement - unlikely). This saturates the catalyst with O2, so will require a small amount of enrichment on fuel reinstatement to re-neutralise the catalyst to allow chemical 'reduction' and stop NOx breakthrough. The fuel saving on over-run is always more than the fuel lost from the rich re-instatement, no matter what people might tell you!

Now, if you if you put it in N going down steep hills, chances are that it will idle, and is therefore using fuel instead of over-run wit the fuel cut.

There's also a possibility that if your car is a micro-hybrid (fancy stop/start car, possibly with an ISG and extra battery), that will charge the battery by reversing the starter/generator and using an inverter. This is the highly inefficient 'regenerative braking' where the control algorithm may not actually use a brake pressure as in input, but simply apply a control algorithm to charge during over-run!

It's similar to the alternator load coming on to charge the battery. You may have seen on old cars that the engine speeds dips at idle when you switch the lights on. The alternator charging robs crankshaft torque to do this but regenerative charging is much less efficient due to the differential voltage & inverter used. That could be causing you excessive engine braking feel, especially if the previous 'normal starter motor' model didn't suffer such high perceived engine braking.

The improvement that you are seeing is far more than I'd expect (ten times more). It is normally only possible to see a smaller improvement on gentle gradients, where staying in gear would slow down the car (requiring slight throttle opening to maintain speed), and putting it in N removes some of the engine 'pumping loss', allowing the car to maintain speed, and use a smaller amount of fuel by idling instead.

It should not cause any damage to your gearbox but will wear your brakes out quicker (should you ever need them), and arguably dangerously delay your emergency use of accelerator (if required).

If you have any specific questions - just ask. I have developed many the control algorithms & calibrations for this that are used in the Bosch ECU.

Ebe_Scrooge

Advocado said:

neilmcl said:

Agree with the above, coasting in gear shouldn't use any fuel, in fact you should save fuel due to engine braking regeneration, whereas being in neutral wastes fuel because the engine will be idling.

How does engine braking save fuel??  What do you think is being "regenerated"?

In neutral the vehicle will go further than in D, hence improved mpg.

"Regenerated" is perhaps not the best word.  However, in many cars the ECU will detect when the engine is on the overrun, and will cut off the fuel supply - so in that scenario you're burning no fuel at all.  If you coast in neutral, you'll be burning a small amount of fuel to keep the engine at idle speed.

neilmcl

Ebe_Scrooge said:

Advocado said:

neilmcl said:

Agree with the above, coasting in gear shouldn't use any fuel, in fact you should save fuel due to engine braking regeneration, whereas being in neutral wastes fuel because the engine will be idling.

How does engine braking save fuel??  What do you think is being "regenerated"?

In neutral the vehicle will go further than in D, hence improved mpg.

"Regenerated" is perhaps not the best word.  However, in many cars the ECU will detect when the car is going downhill in gear, as it were, and engine braking is being applied, and will cut off the fuel supply - so in that scenario you're burning no fuel at all.  If you coast in neutral, you'll be burning a small amount of fuel to keep the engine at idle speed.

I think regeneration is exactly the correct word. In most modern cars with stop/start systems energy is recovered by both engine and standard braking. More energy saved equals less fuel used.

Advocado

itsmeagain said:

It should not cause any damage to your gearbox but will wear your brakes out quicker (should you need them), and arguably dangerously delay your emergency use of accelerator (if required).

Interesting post - thanks for the info.  It's a diesel van.

I only put it into N when I know I won't need the brakes.  So I coast for a while if there's a long downward gradient and then into D again once the speed has dropped lower than I want.

Good to know it's not damaging anything.

Advocado

neilmcl said:

Ebe_Scrooge said:

Advocado said:

neilmcl said:

Agree with the above, coasting in gear shouldn't use any fuel, in fact you should save fuel due to engine braking regeneration, whereas being in neutral wastes fuel because the engine will be idling.

How does engine braking save fuel??  What do you think is being "regenerated"?

In neutral the vehicle will go further than in D, hence improved mpg.

"Regenerated" is perhaps not the best word.  However, in many cars the ECU will detect when the car is going downhill in gear, as it were, and engine braking is being applied, and will cut off the fuel supply - so in that scenario you're burning no fuel at all.  If you coast in neutral, you'll be burning a small amount of fuel to keep the engine at idle speed.

I think regeneration is exactly the correct word. In most modern cars with stop/start systems energy is recovered by both engine and standard braking. More energy saved equals less fuel used.

Keeping the engine engaged whilst coasting introduces more friction and so mechanical enery is lost causing the van to slow.  Coasting in N removes that friction, so less energy is lost and the van moves a greater distance.

The only thing that causes my van to move is by burning fuel.  Engine braking doesn't create more fuel, so nothing is being "regenerated". 

neilmcl

Advocado said:

itsmeagain said:

It should not cause any damage to your gearbox but will wear your brakes out quicker (should you need them), and arguably dangerously delay your emergency use of accelerator (if required).

Interesting post - thanks for the info.  It's a diesel van.

I only put it into N when I know I won't need the brakes.  So I coast for a while if there's a long downward gradient and then into D again once the speed has dropped lower than I want.

Good to know it's not damaging anything.

Don't take offence but that only tells me how much of a poor driver you are, and goes back to my earlier point of how dangerous coasting can be. You should ALWAYS be in a position to need your brakes.

Advocado

neilmcl said:

Advocado said:

itsmeagain said:

It should not cause any damage to your gearbox but will wear your brakes out quicker (should you need them), and arguably dangerously delay your emergency use of accelerator (if required).

Interesting post - thanks for the info.  It's a diesel van.

I only put it into N when I know I won't need the brakes.  So I coast for a while if there's a long downward gradient and then into D again once the speed has dropped lower than I want.

Good to know it's not damaging anything.

Don't take offence but that only tells me how much of a poor driver you are, and goes back to my earlier point of how dangerous coasting can be. You should ALWAYS be in a position to need your brakes.

Massively disagree. 

You should drive in a manner that means you don't need your brakes by keeping your distance, looking far ahead, keeping a steady speed, pre-empting other drivers' actions etc.