"Saab used a freewheel system in their two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency".

This does beg the question of ' If this is an efficient way of saving fuel, why do SAAB & other car manufacturers not use this currently ' ?

Each to his own,

If you want to free wheel then its a choice, the highway code is a combination of strong recommendations & traffic laws & is in that sense confusing as it is frequently seen as being law.

With free wheeling, I personally feel that you loose the added engine breaking when slowing down & coming to a stop, which is currently recommended ( not law) to save fuel. I have found personally as well that it extends the life of brake pads / shoes( if you have them),

Edited 7 February, 201115 yr by lfc958

"Saab used a freewheel system in their two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency".

Older cars had carburettors generally, and a carb continues to suck in fuel even when the engine doesn't "need" it (and increasingly so with engine speed) so freewheeling made sense on those older cars.

On a modern fuel injected engine the on board computer provides just the right amount of fuel the engine needs. So under engine braking no fuel is used at all and whilst travelling downhill the fuel consumption is vastly better for a given engine speed as gravity reduces the fuel requirement to hold a given speed.

Therefore on a constant gradient hill just steep enough to overcome engine braking and other drag it would be possible, travelling at constant speed to use no fuel with the car in gear and some fuel out of gear controlling speed purely on the brakes. In this case it is absolutely clear that freewheeling must use more fuel. (And wear your brakes out more quickly too)

On a constant gradient where the car will freewheel at constant speed but will slow with engine braking a little throttle is required to maintain speed and here its a question of whether the fuel required to overcome the engines braking effect is more than used when freewheeling. This situation is less clear (at least to me). Clearly though you are not using the same amount of fuel to spin the engine at the higher speed in gear as you would be on the flat, you are in fact just topping up gravity. I suspect that in gear is still more economical, but can't put forward a convincing argument like the steeper hill.

I don't want to upset anyone, and I'd be the first to admit I'm wrong if freewheeling was scientifically tested and shown to improve economy.

Maybe Top Gear or one of the magazines might take this up?

Not sure about the legality of freewheeling but I did used to have a mate with a motorbike who used to freewheel round roundabouts and bends. When I went on the back of it that scared the willies out of me!

More generally I think the problem from a safety point of view is that you need to get the car back under control (ie able to accelerate) when something unexpected happens. So say you're coasting down a deserted motorway hill at 70 and a horse runs across it where you can only see it at the last second. You have more chance of avoiding it in gear surely.

Its an extreme example but most accidents are due to things we can't predict.

Edited 7 February, 201115 yr by juan27

I do not know if you are a Physics graduate or Physics teacher or what. I have had heavy elements of fluid mechanics and thermodynamics in my OND and Sc degree I have done, I am confident of my systems analysis and results gained.

As it happens I do have a couple of Physics degrees (a BSc and an MSc) although I don't profess to be an expert by any means!

"Saab used a freewheel system in their two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency".

Older cars had carburettors generally, and a carb continues to suck in fuel even when the engine doesn't "need" it (and increasingly so with engine speed) so freewheeling made sense on those older cars.

Juan hits the nail on the head as far as I'm concerned. The 99 was withdrawn in the early 80's wasn't it? As such it will have had carburettors or maybe a primitive fuel injection system. Under those circumstances I'm sure you would be correct.

However modern ECU's allow for much more accurate control of fuelling and even shutting off the fuel under no load which wouldn't have been possible 30 years ago.

I think the most compelling argument though comes from the fact that no manufacturers have introduced 'free-wheeling' systems since Saab did it. Modern double clutch gearboxes with their ability to pre-select gears and engage them in miliseconds would be ideal candidates for this technology. After all it can't be that difficult if Saab did it in the 70's.

But no-one is doing it. That says to me that it gives no benefit.

Just ordered a new 2.0tfsi vrs with parking sensors and have been told by my dealer that it would be around the end of April. However when phoning SUK I have been told it will be 6 months. Now if it actually is 6 months I am going to be without a car for sure so I am starting to worry. If it does take 6 months is there anything I can do or have I just got to suck it up??

Off topic, where can I go to in Bristol to have my alloys painted black, I ordered a White one you see.

:wonder: Just showing how off-topic things can get.....

BTW, don't argue with rocket scientist, (aka lol), it only wastes pages of Briskoda...

:wonder: Just showing how off-topic things can get.....

Well he might want to freewheel to get his alloys painted black!

Ha Ha, yeah, sorry to Nicky for taking his post off topic!

And I'm a Laser Physicist so I reckon I could have a good fight with a Rocket scientist :p

Ha Ha, yeah, sorry to Nicky for taking his post off topic!

And I'm a Laser Physicist so I reckon I could have a good fight with a Rocket scientist :p

I'm only taking the p by the way, lol!

@Stevoraith-how do you jam them copper's laser speed gun thingies then? Or maybe you are responsible for making them??

Damn, off topic again...

As it happens I do have a couple of Physics degrees (a BSc and an MSc) although I don't profess to be an expert by any means!

Juan hits the nail on the head as far as I'm concerned. The 99 was withdrawn in the early 80's wasn't it? As such it will have had carburettors or maybe a primitive fuel injection system. Under those circumstances I'm sure you would be correct.

However modern ECU's allow for much more accurate control of fuelling and even shutting off the fuel under no load which wouldn't have been possible 30 years ago.

I think the most compelling argument though comes from the fact that no manufacturers have introduced 'free-wheeling' systems since Saab did it. Modern double clutch gearboxes with their ability to pre-select gears and engage them in miliseconds would be ideal candidates for this technology. After all it can't be that difficult if Saab did it in the 70's.

But no-one is doing it. That says to me that it gives no benefit.

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You are quie right that these were cars that were in the main carb'd rather than injected I recall and they were more useful on carb'd engines than injected ones.

The 7 speed DSG is an absolute wonder, particularly on the petrols where it can improve fuel consumption by several percent and lower CO2 by one or two groups, it barely weighs moe than the manaul box but through having more gearing has great acceleration and high top gear for good fuel consumption.

Racing motorcylces use sprag-freewheel clutches and I expect some F1 cars do as fuel is now so critical and I would expect this technology to transfer to the road fairly shortly as fuel cost has gone up by 50% in the last year or so.

I shall publish my fuel figures in my signature perhaps, seems to be the thing to do. 43 mpg combined and 52 on a run on cheap petrol is not too shabby for a car that does 0-60 in 7.5 seconds (same as std 330d E46, quicker than diesel vrs)and cost less than £20K new (even the L&K DSG), even better mpg if you free wheel sometimes with the DSG in N in IMHO.

Edited 7 February, 201115 yr by lol

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Racing motorcylces use sprag-freewheel clutches and I expect some F1 cars do as fuel is now so critical and I would expect this technology to transfer to the road fairly shortly as fuel cost has gone up by 50% in the last year or so.

I hardly think racing bikes or cars would fit a freewheeling device to save fuel. Surely the nature of racing is that you are going flat out all the time when you aren't braking and there aren't that many long/steep hills on race circuits anyway!

This is interesting: http://en.wikipedia.org/wiki/Sprag_clutch#Motorcycle_application

(Nowt about saving fuel)

Edited 8 February, 201115 yr by juan27