RE: PH Blog: freewheeling
Discussion
A number of posters do not seem to have a proper understanding of the “Otto-cycle “ 4stroke petrol engine with regard to closed throttle operation and the consumption of petrol. Many persons seem to feel that if the engine is shut down to tick-over as in freewheeling then no fuel is used while this situation exists, this is not true! If you consider back to basics, when the piston descends in the cylinder on the induction stroke it produces a vacuum which is transferred to the intake manifold and ultimately either the carburettor or throttle position switch (PI). When the engine is in the tick-over mode the venturi butterflies are propped open by the slow running throttle stop adjustment screw to give sufficient air flow to sustain approximately 700 RPM engine speed, this is balanced by the slow running mixture screw (ECU for PI )to maintain the correct fuel/air ratio required to suit this situation . Therefore while the engine rotates it will continue to ingest fuel as a mechanical function of the vacuum created unless some additional management system is introduced to actually stop fuel flow and restart the engine while in the freewheeling mode.
I remember doing some research years ago at BMC Longbridge with regard to auto stop & start to save fuel at traffic stops, gridlocks etc, but that was only with reference to stationary vehicles, after leaving the factory I lost track of that piece of development. From my own experience of driving freewheel cars they can really justify their existence on long distance runs but if you live in any type of built-up metropolis it is a waste of space and in fact I tend think it is an encumbrance unless it can be disabled when not required
I have no experience of “drive by wire “ modern control systems connected to cruise control and “think for themselves” Transmissions so maybe I need bringing up to speed on the latest Porkers
I remember doing some research years ago at BMC Longbridge with regard to auto stop & start to save fuel at traffic stops, gridlocks etc, but that was only with reference to stationary vehicles, after leaving the factory I lost track of that piece of development. From my own experience of driving freewheel cars they can really justify their existence on long distance runs but if you live in any type of built-up metropolis it is a waste of space and in fact I tend think it is an encumbrance unless it can be disabled when not required
I have no experience of “drive by wire “ modern control systems connected to cruise control and “think for themselves” Transmissions so maybe I need bringing up to speed on the latest Porkers
SmartVenom said:
fwaggie said:
An easy 35mpg on a motorway run?
My 2006 987 3.2S can get 38mpg on a motorway run with no problem at all.
Somethings wrong somewhere.
My 987 2.7 definitely can't get close to that!My 2006 987 3.2S can get 38mpg on a motorway run with no problem at all.
Somethings wrong somewhere.
That mpg is at a constant 60.
70 and I see 33~34, 90+ and it's back into the 20s.
My commute to work is 2 miles city roads (60mph, roundabouts everywhere), 8 miles dual carriageway, and doing 70~80 along the d/c and 60 on the city roads I get 23.5mpg ish.
35mpg really isn't anything special. That bit of info in the article is almost meaningless without an average mph speed though.
Next time I go for a drive somewhere in it I'll reset the trip on the mway and take a photo after a while and post it in the Porsche forums
philmots said:
Same system has been used on Volvo trucks for years...
Once you get used to it you drive around it and the benefits are great. Although, if you're in a hurry it's not too helpful.
For example I can be doing 50mph on a flat DC approaching a roundabout. I'll lift off/switch off cruise abouy a mile before the roundabout and it'll coast all the way to the roundabout. Ending up at around 30mph. That and it de coupling on slight undulations can mean free wheeling for 1 in 10 miles easily.
Around towns if it's used correctly it's great.. Just need plenty of practice.
Agreed Phil (was wondering how long it would til someone mentioned this!) but we can program how much overrun we want it to "roll" to before the exhaust brake automatically kicks in and brings things back under control. The Porsche doesn't have that luxury so I can fully understand how it would be bloody annoying to drive. Once you get used to it you drive around it and the benefits are great. Although, if you're in a hurry it's not too helpful.
For example I can be doing 50mph on a flat DC approaching a roundabout. I'll lift off/switch off cruise abouy a mile before the roundabout and it'll coast all the way to the roundabout. Ending up at around 30mph. That and it de coupling on slight undulations can mean free wheeling for 1 in 10 miles easily.
Around towns if it's used correctly it's great.. Just need plenty of practice.
If your top gear is long and your in it, which makes sense if you want economy, is the engine braking really that large any way?
Surely aero drag, tyre drag and all the other forces make up the large percentage while the engine speed is high?
I really struggle to see how it makes sense, but since it's on a DSG box any way then it's just software any way.
I'm getting fed up of all these pie in the sky figures for economy for big NA engines. Maybe in the euro test cycle which we all know is a bit iffy then cars can do well but I'm fairly certain your average Boxster or 911 driver is gonna use more beans than a Blue Motion driver would so using the same speeds/acceleration rates seems a bit weird in my view.
The test needs to start including some hoonage to cover off the sudden mpg drop you get when you give a big engine some beans, even just for one run away from the lights to motorway speeds.
Dave
Surely aero drag, tyre drag and all the other forces make up the large percentage while the engine speed is high?
I really struggle to see how it makes sense, but since it's on a DSG box any way then it's just software any way.
I'm getting fed up of all these pie in the sky figures for economy for big NA engines. Maybe in the euro test cycle which we all know is a bit iffy then cars can do well but I'm fairly certain your average Boxster or 911 driver is gonna use more beans than a Blue Motion driver would so using the same speeds/acceleration rates seems a bit weird in my view.
The test needs to start including some hoonage to cover off the sudden mpg drop you get when you give a big engine some beans, even just for one run away from the lights to motorway speeds.
Dave
Are you really sure no fuel is injected on overrun? I seem to recall that was the case when fuel injection became common but then they started putting a small amount back in because the catalytic converter would go cold and not work for a while when you got back on the power... No good for the drive cycle emissions test!
Fastdruid said:
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.
Except with two stroke bikes at least, there is lots of braking at motorway speeds, via aero. Unless the traffic backs up, you much never have to hit the brakes on a bike, 2 or 4 stroke.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.
When people buy sports cars with petrol engines do they really look at the MPG figures or the co2 figures??
If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
No arguement from me wether sports cars should me manual auto semi auto etc as personal choice and individuals driving times and places.
If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
No arguement from me wether sports cars should me manual auto semi auto etc as personal choice and individuals driving times and places.
abarber said:
Fastdruid said:
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.
Except with two stroke bikes at least, there is lots of braking at motorway speeds, via aero. Unless the traffic backs up, you much never have to hit the brakes on a bike, 2 or 4 stroke.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.
It's more the feeling I was referring too where, very off-putting when very used to 2 or 4-strokes to jump on the other (not so much if doing it all the time).
KM666 said:
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.
NoYou are less likely actually.
I too used to own Saab 96's both two and four stroke,I always used the free wheel with only a couple of hairy moments,it could be turned off but the control was in the passengers foot well.
I once took the 2t in for MOT and forgot to turn the free wheel off,in those days the brakes were tested on road test and the examiner returned very slowley and very white after flying into a bend faster that he anticipated!!! He passed the car OK commenting how good the brakes were.
garypotter said:
When people buy sports cars with petrol engines do they really look at the MPG figures or the co2 figures??
If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
I disagree.If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
I value these 2 things as important in today's climate. When on today's roads can you exploit the performance of these cars? You end up most of the time sitting behind some lorry or slow moving vehicle so what is the point of having a car that is guzzling huge amounts of fuel under these conditions. One of the advantages of low Co2 levels is that you can escape the high level of road tax, which at about £200 per year is a good saving, plus the extra miles you can do per tank of fuel. Both my M3 and my Boxster do about 30mpg average - whereas my previous car (350Z) could barely manage 24mpg, plus it was over £400 per year road tax and not a patch on both cars performance wise either.
Valveman said:
garypotter said:
When people buy sports cars with petrol engines do they really look at the MPG figures or the co2 figures??
If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
I disagree.If they are worried about these 2 items are they not looking at the wrong sector of the motor trade?
I value these 2 things as important in today's climate. When on today's roads can you exploit the performance of these cars? You end up most of the time sitting behind some lorry or slow moving vehicle so what is the point of having a car that is guzzling huge amounts of fuel under these conditions. One of the advantages of low Co2 levels is that you can escape the high level of road tax, which at about £200 per year is a good saving, plus the extra miles you can do per tank of fuel. Both my M3 and my Boxster do about 30mpg average - whereas my previous car (350Z) could barely manage 24mpg, plus it was over £400 per year road tax and not a patch on both cars performance wise either.
Gary C said:
KM666 said:
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.
NoYou are less likely actually.
Simple reasons are:
- suspension geometry esp toe is set based on the driven wheels having power. For example, FWD cars will have toe out on the front to compensate for the wheels natural tendancy to toe in when power is applied.
- Brake balance is set with the driven wheels taken into account. So a FWD car will have proportionately less front bias as it would be assumed that a degree of engine braking was occurring. Sure ABS can help here but even the best systems work better when designed for the car as a whole and not a "get out of jail free" card.
jjones said:
how certain are we of this no fuel when no throttle down hill?
Not very. There are lots of posts on here from people certain this is the case but I'm not so sure.I have an instant mpg read out on my car. If I go down a very steep hill in neutral it reads 99.9 mpg. If I go down in 2nd feet off the pedals it reads 80 odd mpg (3k + rpm).
Also I remember hearing a well known motorbike engineer in MotoGP saying that fuel use was a problem and even when off the throttle some fuel still had to be put into the engine when air was going in. In other words all the time.
If Porsche are spending money on this kind of tech I can see how coasting in gear uses no fuel.
Edited by trickywoo on Friday 27th July 15:26
trickywoo said:
Also I remember hearing a well known motorbike engineer in MotoGP saying that fuel use was a problem and even when off the throttle some fuel still had to be put into the engine when air was going in. In other words all the time.
That's more for ridabilty. Not so much a problem in a car but that moment when you pick up the throttle needs to be smooth, if the engine (effectively) has to restart it can unsettle the bike mid corner. Which as I'm cure you can imagine in MotoGP when on the limit could end up in the gravel.Fuel injection on bikes has been far more problematic than in cars, just see the dual butterfly setups that Suzuki use for example to improve 'feel' and ridability.
kambites said:
There may be an off-switch in the Boxster, but will there be in the next generation of hatchbacks that have the feature?
I met a Ford engineer at a wedding last year and he told me they were developing the same thing. We had a long geeky discussion about it and agreed with the downsides (unnecessary braking etc) but stated the goal was lower CO2 ratings.I think some people are missing the point of this.
When you are on a motorway, ideally you are moving at a steady speed. In the real world, your speed varies due to traffic and road conditions. Where decoupling is intended to save energy is during those fluctuations. In a normal car where the engine constantly engaged, every time you take your foot off the accelerator, energy is being wasted in turning the engine. It's a considerable amount when you consider how strong the engine braking effect can be - try timing how long it takes to lose a certain amount of speed when you lift off to when you put your foot on the clutch.
If you automatically disconnect that parasitic loss, then the car will maintain its speed for longer meaning you use less fuel to speed it up again when you put your foot back on the accelerator. The energy used in the minimal amount of fuel required to keep the engine turning at idle speed will be far less than that required to turn the engine at a higher speed, since frictional losses increase with engine speed in a non-linear fashion.
It is not intended to allow a car to run away down a steep incline, nor is it intended to completely eliminate engine braking where it is actually needed. It is intended to reduce losses where you don't want them. The ideal implementation of this would be alongside some kind of energy recovery system which is able to substitute the engine braking, which loses the energy for good, with one which stores it for later use.
Obviously systems like this will alter what the driver experiences, but then it wouldn't be the first way that people have had to adapt their driving style due to a new innovation.
When you are on a motorway, ideally you are moving at a steady speed. In the real world, your speed varies due to traffic and road conditions. Where decoupling is intended to save energy is during those fluctuations. In a normal car where the engine constantly engaged, every time you take your foot off the accelerator, energy is being wasted in turning the engine. It's a considerable amount when you consider how strong the engine braking effect can be - try timing how long it takes to lose a certain amount of speed when you lift off to when you put your foot on the clutch.
If you automatically disconnect that parasitic loss, then the car will maintain its speed for longer meaning you use less fuel to speed it up again when you put your foot back on the accelerator. The energy used in the minimal amount of fuel required to keep the engine turning at idle speed will be far less than that required to turn the engine at a higher speed, since frictional losses increase with engine speed in a non-linear fashion.
It is not intended to allow a car to run away down a steep incline, nor is it intended to completely eliminate engine braking where it is actually needed. It is intended to reduce losses where you don't want them. The ideal implementation of this would be alongside some kind of energy recovery system which is able to substitute the engine braking, which loses the energy for good, with one which stores it for later use.
Obviously systems like this will alter what the driver experiences, but then it wouldn't be the first way that people have had to adapt their driving style due to a new innovation.
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