Using gravity to save money

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Davy stephenson

Using gravity to save money

Post: #151964 Davy stephenson
Mon Apr 27, 2009 9:57 am

As the fuel prices grow there are many people who are turning to gravity for help, it is a subject which the government frown upon and say is highly illegal because of potential safety reasons, and one I do not condone, that is the passtime of free wheeling,

I know of people who have tried this and the results are quite intresting, on a 100 mile trip they could actually travel almost half that distance using gravity, saving themselves in the region of about 5 pounds for every 60 miles out of gear, but with the engine running to supply braking power.

A lot of this depends upon where you live, but I wonder how many have already thought of this ?

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Re: Using gravity to save money

Post: #151988 wulf
Mon Apr 27, 2009 1:00 pm

I'm not sure the braking power of the engine has any effect when you are out of gear although, as long as the engine is running, it is easy to slip into an appropriate gear if more control is needed. You can certainly go a lot further without having to open the throttle with the accelerator pedal when in neutral as opposed to when in gear, given even a relatively minor downhill gradient.

I do use this approach sometimes, given good visibility and low traffic density. Other hypermiling ideas which are even easier to apply are braking early and gently, accelerating slowly and not pushing the car too fast (having a car with a rev counter is handy in judging a sustainable level of energy consumption). What I don't have are the tools to gauge how effective this kind of driving is.

Wulf

ps. ... and, of course, looking for ways to reduce car usage will reduce your mileage costs even more, if you figure in the ground you cover by other means, like walking or cycling.
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Re: Using gravity to save money

Post: #152006 Davy stephenson
Mon Apr 27, 2009 3:37 pm

Hi Wulf

Where we live the roads are undulating there are some good runs of more than 4 miles or so, in a 50 miles trip either way you can free wheel 21 miles in one direction and 17.5 miles in the other, its when you go to fill up again one can see the difference in the cost, using your engine for braking is using an amount of fuel, idling is using very little, if you let the car idle it will go for hours before it runs out, that time free wheeling is saving you lots of fuel.

Once as you get used to a paticular journey you can even free wheel up and over a hill or rise in the road saving even more energy, momentum and gravity does the rest.

If you are taking every single weather climate into consideration a windy day can have a benefit one direction, and turning off the engine is really silly becauseyou losse the servo efect and can plow ever onwards.

If you have an old points version 1000cc fiesta like my wife with the perculator type gas conversion I made and fitted myself, you are then getting the equivalent to about 100 MPG, and with the wheeling thrown in a little more.

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Re: Using gravity to save money

Post: #152022 pumpy
Mon Apr 27, 2009 6:59 pm

Here in Norfolk, my car is in neutral wherever practicle. ( the county is not as flat as a lot of people suppose........ apart from that bit between Norwich & Gt.Yarmouth). I would never dream of 'free-wheeling' on busy or main roads, but on our local lanes i guess it can save you a few pennies & cut down on energy & emissions. I know that it is illegal to allow the car to run in neutral, but surely with common-sense applied, then what's the problem?
it's either one or the other, or neither of the two.

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Re: Using gravity to save money

Post: #152031 Annpan
Mon Apr 27, 2009 7:34 pm

Well, I do pray that no one is freewheeling when they meet me and my toddler on the bike heading the other way on one of these quiet roads.... sorry guys but somethings will never be worth that extra 2p saved per trip.
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Re: Using gravity to save money

Post: #152034 pumpy
Mon Apr 27, 2009 8:02 pm

Annpan wrote:Well, I do pray that no one is freewheeling when they meet me and my toddler on the bike heading the other way on one of these quiet roads.... sorry guys but somethings will never be worth that extra 2p saved per trip.

I totally agree, Annpan, but as i say ; with common-sense applied (i.e. only when it is safe to do so), then where's the problem?
it's either one or the other, or neither of the two.

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Re: Using gravity to save money

Post: #152037 dave45
Mon Apr 27, 2009 8:44 pm

What of the theory that fuel-injected cars use less fuel when in gear slowing down or cruising downhill than when idling in neutral? In other words the engine control system cuts off the fuel supply if it isn't doing useful work.

And doesn't Plod consider that you are not in full control of your car if you are coasting?

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Re: Using gravity to save money

Post: #152038 Annpan
Mon Apr 27, 2009 8:46 pm

pumpy wrote:
Annpan wrote:Well, I do pray that no one is freewheeling when they meet me and my toddler on the bike heading the other way on one of these quiet roads.... sorry guys but somethings will never be worth that extra 2p saved per trip.

I totally agree, Annpan, but as i say ; with common-sense applied (i.e. only when it is safe to do so), then where's the problem?


The problem is that you don't know when it is safe, you can never be 100% sure of safety of course, but to take extra risks is highly irresponsible.... which is why it is illegal... and does anyone think, for one instant, that they would be looking at the pedestrian or cyclist they have just hit with their 2 tonnes of metal, thinking "Oh well, at least I saved 14p on petrol today"

This for me, (as a cyclist, pedestrian, and never a driver) is a no brainer.
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Re: Using gravity to save money

Post: #152152 Davy stephenson
Wed Apr 29, 2009 6:58 am

dave45 wrote:What of the theory that fuel-injected cars use less fuel when in gear slowing down or cruising downhill than when idling in neutral? In other words the engine control system cuts off the fuel supply if it isn't doing useful work.

Having been a motor tech for 25 years I can honestly say that a fuel injected car uses the same amount of fuel as a non injected car, as long as the engine is running its using fuel if its in gear or not,, when you are in gear the engine is slowing you down and you will not be able to free wheel as far, making you use more fuel,

Did you see the program on TV some tme ago where a guy in Rotherham I think it was, free wheeled with the engine turned off, I can tell you this is very dangerous because the steering lock might just come on resulting in no steering, whoooooopps.


And doesn't Plod consider that you are not in full control of your car if you are coasting?


That's right they frown upon it, However, an automatic car is not in gear when you take your foot off of the throttle, so you are in effect free wheeling,

I don't want to turn this thread into a political one, but I would like to say a responsible free wheeler is much safer than a boy racer in gear,

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Re: Using gravity to save money

Post: #152156 Greenbeast
Wed Apr 29, 2009 7:31 am

i'm a bit miffed, if the engine is on and therefore you have power steering, and you have your foot over the brake, where is the problem?

You can still steer and brake?

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Re: Using gravity to save money

Post: #152158 Davy stephenson
Wed Apr 29, 2009 7:44 am

Greenbeast wrote:i'm a bit miffed, if the engine is on and therefore you have power steering, and you have your foot over the brake, where is the problem?

You can still steer and brake?


There is no problem mechanically, as you say as long as the engine is running you have the power for steering and for braking hard, the problems arises due to legality, its not condoned by the government and most motoring associations.

When you are towing a second car without brakes it is just as dangerous as running without the gearbox engaged, if your using a rope its not as safe as using a fixed pole, which then pushes the towing car from behind, there are so many different angles you can come from that are not safe,

its all down to commom scense and confidence, experience et etc, I'm not condoning the passtime, you must do or not at your own leasure.

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Re: Using gravity to save money

Post: #152209 dave45
Wed Apr 29, 2009 7:03 pm

it is not so clear

http://www.honestjohn.co.uk/forum/post/ ... tm?t=68659

"When rolling downhill you use no fuel (in a diesel, not sure re petrol)), everything that comes off the engine works (servos, power steering, etc) and by placing your foot back on the accelerator you have instant response.... "

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Re: Using gravity to save money

Post: #152219 pumpy
Wed Apr 29, 2009 8:00 pm

wish i hadn't bothered getting involved with this one. i know what i mean & i know my local roads, i also know what is safe & what isn't. i also know the law. i also know when common-sense prevails.
it's either one or the other, or neither of the two.

Davy stephenson

Re: Using gravity to save money

Post: #152220 Davy stephenson
Wed Apr 29, 2009 8:03 pm

A modern diesel engine has all the same attributes as the modern fuel injected petrol engine direct injection and a pressurised fuel or common rail system in cluding cat and electronics, the fuel is the only differnce nowadays.

Most on board computers are showing mean MPG and will show diferent ratios on the same trip, which is based upon throtle position sensors or TPS and engine speed, MAP sensors, versus gear train measurements, as long as the engine is running it is using fuel or the engine would not be running, only then is it not using any fuel.

If the engine is braking with no throtle being used the car is slowing down under gearbox and engine drag, it is true that there is instant responce as the throtle is depressed again, and

depending what gear you are using depends upon how much torque is needed to bring the car back to optimum efficiency, thus wasting fuel, the more torque that is require the more fuel will be used and modern especially diesel engines have so much torque that starting again after longer periods of slow down without changing gear is easier to acheive.

The best fuel economies are to be found on many of the higher class German deisel engines which run with very high pressure systems, which are all obtained at around the 56 miles per hour, go much faster than this and the modern engines with around 150 bhp are not much better than older less powerful engines, this is why the fuel prices for diesels is much higher than petrol,

The more power an engine has the more fuel will be used on take off, if you free wheel you will save fuel, I find it gives me a 5 pound saving over every 60 miles out of gear, again many frown upon this scenario, but it does work.

I started this thread, I don't know what else to say, so this is my last post on this subject

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Re: Using gravity to save money

Post: #152229 dave45
Wed Apr 29, 2009 8:50 pm

The way I look at it is: either
a) the engine is turning the wheels (burning fuel) or
b) the wheels are turning the engine (no fuel necessary) or
c) you are freewheeling (tickover fuel is needed)

under b or c you could be slowing, cruising or accelerating depending on gradient.

I have a diesel and on a boring journey I kept resetting the trip computer and trying different constant speeds to see the effect on MPG. I managed to get it off the scale (99.9 mpg) in 6th gear down a long gentle gradient of several miles on the M6 . If I was freewheeling I'd have to keep boosting the speed as it slowed. But would I have improved on the MPG?

Some testing required. There must be a definitive answer.


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