Thursday 26th July 2012

PH Blog: freewheeling

Quick, Harris thinks he may actually have found something wrong with the new Boxster!

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Decoupling. Get used to it, it's a phrase you're going to read with increasing frequency over the coming years.

Decoupling is a simple concept - quite simply disengaging the gearbox input from the motor and allowing a car to coast without the interia of engine internals. It's nothing new, our ancestors spent endless hours coasting their early motor vehicles down slopes in an effort to reduce fuel consumption, but it does have a marked effect on efficiency.

Clever, yes, but it ruins motorway flow

I've just spent a week decoupling in a new Boxster, and I bloody hated it.

I hadn't realised just how much store I set by being able to accurately judge a car's coasting characteristics with a gear engaged - normally the longest ratio on a motorway. You know the stuff, maintaining progress without using the brakes by anticipating and keeping a view as far ahead as possible. Well, now I do, because de-coupling kind of wrecks normal progress using this method.

On level ground, or a slight downhill section, you feel the powertrain release pretty much instantaneously as you back off the throttle. The trouble is, your brain expects a certain amount of residual deceleration at this point, and it never arrives - it's a bit like falling of a track onto wet grass, that sensation of almost accelerating again as you hit the green stuff. Of course you can adjust for this, but it takes time and distance, which isn't always available on a British motorway. Also, if the car(s) ahead of you are coasting, you have so much less friction that you inevitably catch them and have to brake. Irritating.

Re-engagement is a little sloppy too: there's a definite jolt in the car, and you have to judge the reapplication of throttle very carefully - too much and you'll encourage the gearbox to kick-down, which is most unseemly.

Thankfully you can decouple the decoupling

The economy gains are undeniable. I was averaging an easy 35mpg in the 2.7 litre Boxster PDK, a staggering achievement from a 170mph roadster, but the cost to serene progress on the Motorway was simply too great.

The upside was I thought I'd found a real gremlin in the otherwise impregnable Boxster package, but that proved to be unfounded. By hitting the Sport button, you kill the decoupling antics and the Stop-Start function - and you get faster shifts and a raised rev-limiter. On balance, I'd live with the latter to avoid the former, but the Boxster would be pretty much perfect if there was simply a button to kill the coasting alone.

By now you know the rest about the Boxster. It's the best car Porsche makes.

Chris

[JerseyS2000]

Interesting legal question - is driving whilst decoupled still deemed to be in full control of the vehicle by the BiB? I was under the impression that its verging on illegal to freewheel in a car??

Ben

[kambites]

I'd imagine it'd be horrible when driving hard because you don't have engine braking to get the nose to tuck in; you'd have to use the brakes to steer the car instead.

There may be an off-switch in the Boxster, but will there be in the next generation of hatchbacks that have the feature?

[Fastdruid]

No different to anything 2-stroke I'd have thought.

Jump on a two-stroke and the lack of any engine braking can seriously put you off but you soon get used to it.

Not quite sure if decoupling would be quite the same but would have to try it to tell.

[thewheelman]

This Decoupling idea, I really hope it doesn't catch on. It sounds terrible.

[anonymous-user]

JerseyS2000 said:

Interesting legal question - is driving whilst decoupled still deemed to be in full control of the vehicle by the BiB? I was under the impression that its verging on illegal to freewheel in a car??

Ben

Good question.
When I was learning to drive coasting was a HUGE no no.

[kambites]

Didn't some old Saabs used to do this; although I think they actually called it "free-wheeling".

[Mr_Sukebe]

I did think that this type of freewheeling was actually LESS efficient.

I was given the impression that in most cars, that when you back off the throttle and coast, but in gear, that the momentum of the car not only keeps the car moving, but also turns the engine over, and that whilst in that mode, that no fuel was actually being fired into the cylinders, meaning that you're burning even less fuel that you would with the car at "tickover" as it's not under load.

Did I miss something?

[SirSamuelBuca]

wonder how many people are going to crash in massive balls of flame because they drop a gear and end up rolling faster than they were driving :O








probably none but

[900T-R]

kambites said:

Didn't some old Saabs used to do this; although I think they actually called it "free-wheeling".

Yes, a leftover from the two-stroke days when going off an incline with no throttle and the engine spinning away at a gazillion rpm would mean no lubrication (because of 'idle' fueling) and a seized engine. The feature survived for a few more years when the lawnmower engine was replaced by Ford V4 and Ricardo/Triumph 4 inline units.

Big difference is the freewheel in the Saabs was manually operated...

[loudlashadjuster]

Mr_Sukebe said:

I did think that this type of freewheeling was actually LESS efficient.

I was given the impression that in most cars, that when you back off the throttle and coast, but in gear, that the momentum of the car not only keeps the car moving, but also turns the engine over, and that whilst in that mode, that no fuel was actually being fired into the cylinders, meaning that you're burning even less fuel that you would with the car at "tickover" as it's not under load.

Did I miss something?

That was my impression also.

I know what Mr Harris means though, my C-Class sheds speed at a very slow rate compared to any other car I've driven. It might be eco-gubbins, it might just be the 7-speed 'box, but whatever the cause it is disconcerting and I'm still adjusting after 8 weeks in it.

[Charge99]

Mr_Sukebe said:

I did think that this type of freewheeling was actually LESS efficient.

I was given the impression that in most cars, that when you back off the throttle and coast, but in gear, that the momentum of the car not only keeps the car moving, but also turns the engine over, and that whilst in that mode, that no fuel was actually being fired into the cylinders, meaning that you're burning even less fuel that you would with the car at "tickover" as it's not under load.

Did I miss something?

Freewheeling uses some fuel as it takes petrol to keep the engine ticking over, whereas you are correct that no fuel is used when no throttle is applied in gear at speed. However, you travel further whilst coasting with the clutch disengaged as there is less deceleration and this more than makes up for the small amount of fuel used.

[filski666]

Lift-Off Oversteer is dead!

[kambites]

Mr_Sukebe said:

I did think that this type of freewheeling was actually LESS efficient.

It uses less fuel (none in fact) to be engine braking but it also slows you down more so which is more economical overall depends on whether you want to be slowing down anyway (or are on a steep down-hill).

One would hope that it automatically re-engages the mechanism when you touch the brakes?

Edited by kambites on Thursday 26th July 12:32

[McSam]

Great. So when an intelligent driver is approaching a junction and wants to back off early and let the engine slow the car down on the overrun - which shuts off fuel completely, uses zero, and does not waste fuel keeping the engine idling for no reason - the car thinks it knows better. Yeah, right..

[breadvan]

So where does that leave PSE? With no overrun - no pops and bangs?

[jjones]

how certain are we of this no fuel when no throttle down hill? takes a lot of force to spin an engine over with no explosions to help it on it's way? guess someone could test he theory by switching off going down a hill and see how far they coast and then retry with ignition (best performed on private land of course because of PAS and servo brakes (and be mindful of the steering lock!))

[AJLintern]

Modern cars seem to have less engine braking these days generally, probably some eco fuelling thing. I miss being able to actually decelerate when I lift off

[kambites]

jjones said:

how certain are we of this no fuel when no throttle down hill?

Absolutely certain in the case of both of my cars. If you're engine braking down to low speed, you can feel them restart fuelling when the revs drop below a certain point (about 2k rpm) as a distinct decrease in engine braking. I can also see it from the data-logging software I have on my laptop, if I plug it into the OBD port.

[anniesdad]

kambites said:

I'd imagine it'd be horrible when driving hard because you don't have engine braking to get the nose to tuck in; you'd have to use the brakes to steer the car instead.

There may be an off-switch in the Boxster, but will there be in the next generation of hatchbacks that have the feature?

Afaik, decoupling does not occur if the car is in the upper rev range in lower gears, so sporty driving is unaffected. This coasting function is for when a car is being driven at normal road speeds using higher gears and little or no throttle.

http://www.porsche.com/microsite/technology/defaul...

Edited by anniesdad on Thursday 26th July 12:41

[KM666]

Wont this tech mean that we'll have more cars spinning at roundabouts? If you've ever entered a roundabout in 5th gear theres a chance the car will spin due to lack of drive and therefore traction, surely when completely coasting the effect will be more pronounced? I wouldn't like to be around the legal dept when the first one goes sideways into a lamp post ala countless barried saxos and the like.