RE: PH Blog: freewheeling
Discussion
Try falling of an icy go-kart track onto frozen grass - you really would swear you accellerate.anniesdad said:
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. There may be an off-switch in the Boxster, but will there be in the next generation of hatchbacks that have the feature?
Pedants corner:
It's not the engines rotational inertia that you are trying to decouple, but it's parastic friction.....
Turning an engine round takes energy, if you want an idea of how much, next time you have the bonnet open on your car, reach down and try to turn the front pulley around by hand! The same mechanism is leveraged in "downspeeding" in modern cars, i.e. fitting the tallest gear possible so that the car travels the furthest per engine revolution.
If you take a car, traveling at some speed, then the kinetic energy of the car is the store of energy that has been produced by burning fuel, in effect the fuel energy is now contained within the mass of the car travelling at the velocity. This energy is lost to various sources, like tyre rolling resistance, aero resistance, rotational drag in the drive train and engine, and of course in your brake system when you brake. The most efficient way of traveling the furthest is to minimise drag, so decoupling the engine, and allowing it to return to idle, whilst the car continues to roll onwards is optimum, assuming that you don't want to stop at any point (Mway for example).
BUT, if you DO want to stop, say at a traffic lights, then the most efficient way is to let the KE of your car be slowly transfered into the parastic losses you can't do anything about (aero drag, rolling resistance etc). In this case, then using the engine to brake you whilst it uses no fuel (it will be in deccel fuel shut off mode on the overrun)will be slightly more efficient, but it's not a big deal because the engine uses so little fuel on idle anyway (you only "waste" the amount of fuel you would use idling for the length of time the decceleration to the stop takes, and a typical enigine only uses something like 1.5l/ hr on idle, so in the say 15sec it takes to stop it aint going to be a big save)
It's not the engines rotational inertia that you are trying to decouple, but it's parastic friction.....
Turning an engine round takes energy, if you want an idea of how much, next time you have the bonnet open on your car, reach down and try to turn the front pulley around by hand! The same mechanism is leveraged in "downspeeding" in modern cars, i.e. fitting the tallest gear possible so that the car travels the furthest per engine revolution.
If you take a car, traveling at some speed, then the kinetic energy of the car is the store of energy that has been produced by burning fuel, in effect the fuel energy is now contained within the mass of the car travelling at the velocity. This energy is lost to various sources, like tyre rolling resistance, aero resistance, rotational drag in the drive train and engine, and of course in your brake system when you brake. The most efficient way of traveling the furthest is to minimise drag, so decoupling the engine, and allowing it to return to idle, whilst the car continues to roll onwards is optimum, assuming that you don't want to stop at any point (Mway for example).
BUT, if you DO want to stop, say at a traffic lights, then the most efficient way is to let the KE of your car be slowly transfered into the parastic losses you can't do anything about (aero drag, rolling resistance etc). In this case, then using the engine to brake you whilst it uses no fuel (it will be in deccel fuel shut off mode on the overrun)will be slightly more efficient, but it's not a big deal because the engine uses so little fuel on idle anyway (you only "waste" the amount of fuel you would use idling for the length of time the decceleration to the stop takes, and a typical enigine only uses something like 1.5l/ hr on idle, so in the say 15sec it takes to stop it aint going to be a big save)
jjones said:
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!))
As kambites said, it is a known feature of almost all modern cars. Anything with a decent trip computer will confirm it for you - roll down a hill in neutral at 30mph and you might indicate 200mpg or so. Select a gear and let the engine be driven by the wheels, no fuel is injected and you'll have infinite mpg as no fuel is being burnt to achieve motion. Look at it from an engineer's point of view - why the hell would you want to inject any fuel at all on the overrun? If the resultant engine braking is too much then the driver will instinctively add a tiny bit of throttle and balance it to what they want, so when their foot's off, zero fuel.So when you are slowing down, particularly over a long distance such as on a dual carriageway approaching a roundabout, you're far better off leaving the car in gear as long as possible. This also gives you better and more instantaneous control, and even helps save your brakes in a minor way too - there is no reason at all to disengage the gear when you're slowing down.
ETA - whitecobra beat me to the first paragraph
dom180 said:
Good review but why do 99% of reviews fail to mention the chassis setup/options fitted when reviewing latest pork.... - let me guess 20 inch alloys with PASM but no diff/PTV or chrono/mounts but only as GT Porsche tested that car!
What the hell has the chassis set-up got to do with a specific piece about the de-coupling system?There must be some fuel injected when rolling down hill in gear as if you turn the engine off the thing stops rapidly and the engine never goes below idle speed. This is what you have to do in a morris minor with overheating brakes or any other drum brake vehicle if your desperate to stop! New cars/motor oils are designed to use less energy for the engine to spin so they save fuel, this is why there is less engine braking.
McSam said:
jjones said:
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!))
As kambites said, it is a known feature of almost all modern cars. Anything with a decent trip computer will confirm it for you - roll down a hill in neutral at 30mph and you might indicate 200mpg or so. Select a gear and let the engine be driven by the wheels, no fuel is injected and you'll have infinite mpg as no fuel is being burnt to achieve motion. Look at it from an engineer's point of view - why the hell would you want to inject any fuel at all on the overrun? If the resultant engine braking is too much then the driver will instinctively add a tiny bit of throttle and balance it to what they want, so when their foot's off, zero fuel.So when you are slowing down, particularly over a long distance such as on a dual carriageway approaching a roundabout, you're far better off leaving the car in gear as long as possible. This also gives you better and more instantaneous control, and even helps save your brakes in a minor way too - there is no reason at all to disengage the gear when you're slowing down.
ETA - whitecobra beat me to the first paragraph
Engine braking is as Mr Harris knows very important when balancing a car on the throttle, is it wise to use such a system on sports cars (even if you can switch it off)?
Either way if it means Porsche can make large capacity normally aspirated engines for a while longer it can't be all bad.
Oddly enough I tried freewheeling only yesterday on the M74, on a downslope approaching Abington services exit - it did leave me feeling that some elements of car control were diluted.
The Rover P4, as well as 2- and 4-stroke Saab 96/95s, had a selectable freewheel function. On my Saab 96 V4 it provided a quieter cabin! No idea about economy coz back in the day fuel was (relatively) cheap!
The Rover P4, as well as 2- and 4-stroke Saab 96/95s, had a selectable freewheel function. On my Saab 96 V4 it provided a quieter cabin! No idea about economy coz back in the day fuel was (relatively) cheap!
Max_Torque said:
Except the best way of slowing down, in terms of absolute max distance travelled per unit fuel, is to coast with the engine off. the next best is to coast with engine decoupled and at idle, and worst way is to overun with the engine being spun up by the cars KE.
Yes, you're quite correct - but only in terms of absolute max distance travelled per unit fuel. Compared to my example, you could have backed off a hundred yards sooner, burnt less fuel beforehand, disengaged and gone further at coast. But in the real world this is approaching infuriating for anyone behind you and just isn't practical if you want to get anywhere this week - when I'm rolling down a big hill at a cruise, I'll knock it out of gear because this will help me maintain my speed efficiently. But when trying to slow down, the most reasonable compromise is backing off a bit early and letting the engine spend some time without fuel (as opposed to backing off hugely early and coasting to slow down, well, more slowly!).My main point, though, is that under no circumstance should you have the engine decoupled and be braking, because any resistance benefit you might have gained from decoupling is irrelevant anyway so why waste the idling fuel?
dom180 said:
It's nice to see the whole - and the car pictured is in fact a manual!!!
Since Porsche are so configurable, if you're raving about the latest model but failing to tell us the spec, we can only guess on options/what works and, since no-ones done a what options work best on a Boxster yet or a PASM and/or PVT and/or/chrono v non, be nice to get as much detail as possible.
Oh dear.Since Porsche are so configurable, if you're raving about the latest model but failing to tell us the spec, we can only guess on options/what works and, since no-ones done a what options work best on a Boxster yet or a PASM and/or PVT and/or/chrono v non, be nice to get as much detail as possible.
Edited by dom180 on Thursday 26th July 13:10
What the hell?
Effectivly putting it nuetral will only save fuel in really old cars, like carb'ed cars because they fuel all the time!
In any half modern fuel injection engine, you lift off, the injectors turn off and the inertia of the car/drivetrain turns the engine over, so effectively in that coast you are getting infinite MPG. if you dip the clutch or put it in nuetral the injectors will have to start up again, and keep the engine running at tick over, thus using more fuel.
There must definetly be some crossed wires in what the technology is trying to achieve or how it works, because it certainly cant be to decouple the wheels from the engine to save fuel.
You only have to look at what other manufactures are doing. Making sure they are couple is exactly how things like BMW's effecient dynamics work, you lift off, the inertia then turns the enginer over for "free" turning the alternator and charging the battery and so on.
Effectivly putting it nuetral will only save fuel in really old cars, like carb'ed cars because they fuel all the time!
In any half modern fuel injection engine, you lift off, the injectors turn off and the inertia of the car/drivetrain turns the engine over, so effectively in that coast you are getting infinite MPG. if you dip the clutch or put it in nuetral the injectors will have to start up again, and keep the engine running at tick over, thus using more fuel.
There must definetly be some crossed wires in what the technology is trying to achieve or how it works, because it certainly cant be to decouple the wheels from the engine to save fuel.
You only have to look at what other manufactures are doing. Making sure they are couple is exactly how things like BMW's effecient dynamics work, you lift off, the inertia then turns the enginer over for "free" turning the alternator and charging the battery and so on.
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