Free wheeling...

[imamartian]

I was chatting with a neighbour the other day, about the cost of fuel etc.. and we both said we free wheeled sometimes to save fuel... however, he said his car (a vauxhall zafira) would almost stop after about 400yds on the flat and barely keeps it's speed down a hill, whereas, my Audi will roll forever on the flat, and will pick up speed down any incline..!

Should my neighbour get his car looked at?

 

[Avocet]

Depends on what speed. The Xafira will have a bigger frontal area, so more wind resistance at higher speeds. 400 yards at any speed suggests binding brakes or something else wrong. he should go for a 5 minute run, braking as little as possible, then coast to a stop and feel each wheel to see if one is hotter than the others.

As an aside, pretty much all cars for the last 20 years or so, have had over-run fuel cut-offs. That means they use NO fuel WHATSOEVER on over-run (i.e. in gear with the clutch out, but foot off the throttle). If you coast with the clutch down or the gearbox in neutral, you'll actually be using fuel to make the engine tick over!

 

[imamartian]

Depends on what speed. The Xafira will have a bigger frontal area, so more wind resistance at higher speeds. 400 yards at any speed suggests binding brakes or something else wrong. he should go for a 5 minute run, braking as little as possible, then coast to a stop and feel each wheel to see if one is hotter than the others.

As an aside, pretty much all cars for the last 20 years or so, have had over-run fuel cut-offs. That means they use NO fuel WHATSOEVER on over-run (i.e. in gear with the clutch out, but foot off the throttle). If you coast with the clutch down or the gearbox in neutral, you'll actually be using fuel to make the engine tick over!

Thanks Avocet... i will tell him tommorow to try that !!

As for the "over-run" thing, my Audi has a display on the dash for MPG... and it goes up to 200mpg whether i have it in gear and clutch out, or in neutral.... is this figure made up?

 

[Avocet]

Dunno! How old is your Audi and what sort? (Petrol or diesel).

Some manufacturers (French ones generally) will just send the instantaneous MPG display to a series of dashes on over-run, but that's after they display 999MPG. In other words, it's trying to say "infinity", but runs out of digits. Others just display the biggest number they can. If you check the computer by brimming the tank and doing the sums, you'll probably find it's not 100% accurate anyway. I think they just use the over-run number as a "fudge factor" in the rest of the calculations to get an ovrally average as accurate as they can, but that's just a guess.

Of course, the "coasting" thing (although experts will always say that coasting in neutral or with the clutch down is a false economy), is probably more complex in real life. Whilst a car with an over-run fuel cut-off will really use no fuel when on over-run and in gear, it won't go as far as one in neutral because of the engine braking. There must be a point where it does actually save fuel to coast in neutral. I've never been able to work out when that is! On my car, I generally try to leave it in the highest gear possible on over-run (to minimise the drag from the engine braking), but there have been odd occasions when I've kicked it into neutral because I've thought I'd get further, even though it would be using a small amount of fuel idling. I've no idea whether I ended up any better off though, and even at today's fuel prices, I'm sure the difference would have been fractions of a penny!

 

[Burnerman]

With mine (A1) coasting (foot off but in gear) the MPG shows '---'
Coasting in neutral the engine is idling so you do use some juice there.
John

 

[Peter.N.]

Yep, mine's the same, 406 Hdi reads about 350 mpg coasting and no fuel at all in gear, mind you it depends on how steep the hill is because engine braking will bring the car to a halt much quicker than coasting if the hill is not fairly steep.

Peter

 

[acmedone]

Of course, the "coasting" thing (although experts will always say that coasting in neutral or with the clutch down is a false economy), is probably more complex in real life. Whilst a car with an over-run fuel cut-off will really use no fuel when on over-run and in gear, it won't go as far as one in neutral because of the engine braking.

Yep, to turn an engine over it requires power - either from burning fuel or from reducing the car's sum of kinetic and potential energy. These were raised earlier by burning fuel and which will be raised again by burning fuel.

So, if you want to reduce KE and PE when you are or will be using the brakes, then let it power the engine.

But there is another issue. It takes more power to overcome engine friction at, say, 3000rpm than at idle. eg, at 70mph, the fuel has to overcome the engine's own drag as well as aerodynamic drag, tyre drag etc. So if you went to neutral from time to time while cruising, total drag should be reduced and therefore fuel consumption should improve. Just an untried theory though.

 

[big-all]

the actual drag will depend on several things
the load on the engine whilst idling can be quite high it can be ideling but using several times more fuel dependant on load
you need to remember the engine has many uses that all draw energy that needs to be replaced by fuel
assuming an engine is "only idling " when coasting is more often than not going to be wrong

battery charging cooling air conditioning heater power steering will all draw power from the engine
indeed an engine with no fan belt or timing belt will run better but when the electrical load increases to run these items the load will increase

you then need to work out if the occasional motor running off the battery for each item will be less than the mechanical chain running needlessly drawing energy needlessly
a train will quite happily coast for 20 miles with a 1 in 500 or 200m drop

 

[wellsomebodyhastosay]

I notice some cars I have owned will start to roll if I sit with no brakes and out of gear even if the incline is negligable or you think you are on the flat others seem to stay put unless the incline is physically visible.
This also depends on whether the cars are new or have a few miles on them.
This obviously indicates rhe level of resistance within the rolling chasis.

How do the two vehicles in the original post compare in this respect?

 

[Peter.N.]

The ambient temperature also makes a difference, a car will roll more easily when everything is warm, driven gently my 406 Hdi will give around 5 mpg more at 25C than it will at freezing.

Peter

 

[wellsomebodyhastosay]

No energy is 'free' in physics.


You only get out what you put in LESS waste.

So if we consider freewheeling with the engine in neutral at 1000 rpm as a base then (ignoring the slowing effect of wind resistance which will be the same in both scenarios and therefore cancel out) the energy required to keep that engine ticking over is X mpg.

The distance the car will freewheel is distance - less decelleration due to wind resistance.

If you are an a car in top gear instead of out of gear then you will be using more energy to keep that engine spinning at 1000rpm than if you are simply idling because you are not only ticking the engine over you are also rotating the driveshafts and clutch assemblies.

The energy required to keep the engine and associated parts rotating is taken from the potential freewheeling distance the car would have covered without being in gear so you are in effect stealing energy (the cars rolling momentum) that would otherwise have allowed the car to roll further unaided.

At some stage you will have to apply power to maintain the cars speed or accelerate the car up to speed simply to reach the place the car would naturally have freewheeled to in neutral thereby using up more energy than if you had simply freewheeled in neutral due to 1. Frictional losses in the drivetrain and 2. The need to reinject momentum into the vehicle using engine power to accelerate a large mass (the vehicle).

As nothing is for free in physics and keeping your car in gear involves further frictional losses than the engine ticking over it would be possible to prove that leaving a car in gear despite using no fuel will actually use more fuel in the long run as you need to apply energy back into the vehicle to achieve the same rolling distance.

This could easily be proven with a quiet road with a gentle slope followed by a flat section.

Choose a start point and do two test rolls.
One with the engine in neutral passing a start point while rolling at a set speed before releasing the brake and freewheeling down the hill.
Another with the vehicle in top gear and passing the same start point at the same initial speed and then releasing the brake.

If the car does not travel as far in gear then it proves that energy needs to be applied how much depends on how far you then have to drive the car to reach the point where the car rolled to in neutral.

 

[mysteryman]

When freewheeling in neutral with the engine idling, the fuel consumption of the engine is unrelated to vehicle speed, therefore an mpg figure is irrelevant. A car computer shows better fuel economy at higher speeds because the car is freewheeling faster at the same [idling] fuel consumption. You only save this fuel with the car in gear if the gradient is steep enough to keep the engine above idling speed without fuel.

 

[Avocet]

When freewheeling in neutral with the engine idling, the fuel consumption of the engine is unrelated to vehicle speed, therefore an mpg figure is irrelevant. A car computer shows better fuel economy at higher speeds because the car is freewheeling faster at the same [idling] fuel consumption. You only save this fuel with the car in gear if the gradient is steep enough to keep the engine above idling speed without fuel.

...or if you have to come to a stop sooner than you would have coasted to a stop in neutral (which is most of the time in urban driving). In that case, leaving it in gear until the last minute, will provide better overall consumption.

 

[Avocet]

So if we consider freewheeling with the engine in neutral at 1000 rpm as a base then (ignoring the slowing effect of wind resistance which will be the same in both scenarios and therefore cancel out) the energy required to keep that engine ticking over is X mpg.

The distance the car will freewheel is distance - less decelleration due to wind resistance.

There's also he rolling resistance in there. The wind resistance goes up (roughly) with the square of the speed, and rolling resistance is (pretty much) constant. If the only resistive force was wind resistance, you'd be able to push a stationary car with your little finger.

If you are an a car in top gear instead of out of gear then you will be using more energy to keep that engine spinning at 1000rpm than if you are simply idling because you are not only ticking the engine over you are also rotating the driveshafts and clutch assemblies.

No. If the car is moving and the wheels are turning, those parts are turning too - regardless of whether it's in gear or out of gear.

This could easily be proven with a quiet road with a gentle slope followed by a flat section.

Choose a start point and do two test rolls.
One with the engine in neutral passing a start point while rolling at a set speed before releasing the brake and freewheeling down the hill.
Another with the vehicle in top gear and passing the same start point at the same initial speed and then releasing the brake.

If the car does not travel as far in gear then it proves that energy needs to be applied how much depends on how far you then have to drive the car to reach the point where the car rolled to in neutral.

It wouldn't really prove what's under discussion though. (which is whether you'd use more or less fuel). You'd have to start your test at a particular speed and end it at a particular speed that is ABOVE the speed at which the engine gets close to idle speed in whatever gear you conducted the test. This is because as it gets towards idle, the over-run fuel cut-off would cease working and fuel would once again be provided to the engine (which would then try to propel the car (albeit labouring because it was more or less on tickover).

It's absolutely true that the car in-gear will slow down quicker than the one out of gear (especially if the gear is low and the speed at which you conduct the test is high). The question is whether you'd use more fuel in one case or the other. In the one case, the car slows faster due to engine braking, but is using zero fuel while it does so. In the other case, the car takes longer to come to a stop, but uses more fuel in doing so, because the engine is idling. I'm sure there's a "break-even" point where one is more economical than the other, but I imagine it varies greatly with the conditions and from car to car.

 

[kangersroo]

I think you will find the highway code says free-wheeling is not a method allowed.

"
If you coast during a driving test it is a 'major fault' and an automatic fail.. so that says it all "