Using gravity to save money

[matt_w]

Hello everyone, thought I would pipe up (my first post - yay!)

Could the people that believe coasting is dangerous explain why they think it is - I am neither agreeing nor disagreeing; it is purely for scientific debate of course!

Let me explain why coasting _generally_ saves money. I'll try to be as simple as possible so will gloss over some facts about how this all works and use layman’s terms where possible (so please forgive me engine scientists).

When an engine is running, it requires energy to turn, the faster the engine is turning, the more energy is required over the same period of time. So for example an engine running at tick over (say 1000 rpm) uses less energy per second/minute/hour than one running at increased engine speed (say 2000 rpm). This energy to turn the engine ends up as heat (a waste for our purposes). This energy will be 'taken' from somewhere, whether that be the fuel or the kinetic energy (momentum) of the car.

The simplest example, flat road, approaching a junction where you have to stop:
The car has kinetic energy (momentum). This needs to be all used up to stop at the junction. In all cases, some will be used to overcome the normal things such as wind and tyre resistance.

The most fuel efficient way would be to switch off the engine at such a point where the car naturally comes to a stop without using the brakes at all. This is not practical and can be dangerous.

If you take the car out of gear or declutch, then all the kinetic energy can go towards the normal resistance, but some energy is now required from the fuel to turn the engine over. This would be generally less energy as the engine would be turning slower.

If you do not take the car out of gear or declutch, then some of the kinetic energy is being used to turn the engine as well as normal resistance (yes most modern cars cut the fuel supply above tick over when the you don't have your foot on the right hand pedal). If you were to come off the power at the same point as above then you would stop before the junction. (Ok actually the engine may just drag you to the line at tick over, but using extra energy to do so)

Therefore leaving the car in gear will be less efficient.

In either case if you are using the brakes, you are losing energy to heat and potentially not saving any energy (for useful purposes). You will find though that if you try to use all the energy without braking or turning the engine over faster though, you will generally annoy other road users, who are likely to overtake and brake in front of you anyway.

Planning ahead is the key and avoiding use of the brakes without annoying everyone else.

OK so what about downhill? Well it's basically the same except there is another source of energy to drive the car, potential energy (i.e. the car wanting to go down the hill on its own). Simply put, if you have to brake to keep the car at a sensible speed, it may as well be in gear, you'll save more money as you are not using fuel to turn the engine. If the car is at a constant speed or slowing a little then coasting could save you fuel.

If you have to top up the speed with some acceleration then it becomes a bit more grey and I cannot really explain it in layman’s terms without losing everyone or making myself go cross eyed, but basically I've ignored a whole bunch of things which all equate to useful energy being lost at each stage to un-useful energy or waste products. Please don't jump on me for using laymen terms, I know they are not technically correct in a number of areas

[brett53]

any of you considered you MAY be invalidating your insurance cover ?? - you know what they are like if they can find an excuse not to pay out

just a thought

[grahamhobbs]

I used to have an old Rover that had a big knurled knob on the dashboard that you could turn and it switched you automatically into freewheel whenever you weren't using the throttle. I always drove with it in this mode, I never thought of it as dangerous, it was a sedate and comfortable car.
However I wouldn't now dream of freewheeling using the clutch or slipping into neutral - but then I'm driving in London all the time.

[Minnesota]

In the Mid-west part of the USA, we call this "doing the Georgia Coast", I don't know why we call it that ? years ago, when traveling through the Rocky mountains on a Motorcycle, going down the mountain, I will shut off the engine and coast...many times you roll for several miles...one time I went 13 miles on one coast with the engine off. it's so amazing riding/coasting and it's whisper quiet. I'd never do this with other traffic around. In that 13 mile stretch, I probably came across 4 or 5 other vehicles (and there is absolutely no pedestrians, no homes, just open land, you need to watch out for wild animals mostly. most of minnesota is flat and it's not possible, so I haven't done this for years....and this is illegal in the USA too.

[paul123456]

Hello there ,

interesting .
I did a downhill course in Ireland once , using gravity , just using the brakes .
At the bottom of the hill it smelt like Tsjernobyl , the back brake pads were red hot and I
had to wait for 20 minutes for them to cool of.

Volkswagen now has a system that charges the battery when the car brakes .

What if Escher would have been an city architect , he could have redeveloped towns
that we would always travel downhill , that would be a massive contibution.

regards ,

Paul

[Big Al]

Well for my t'peneth worth I'm with the use less brigade. My exhaust started blowingon Aug 14th last year and I haven't saved the money to get a new one so I walk or use the bus. It's amazing how much petrol you save when you don't drive......

[brett53]

Volkswagen now has a system that charges the battery when the car brakes .

ah so they have finally caught up with mobility scooters then ?? - we have had "regenerative" breaking for years - actually on most modern scoots - that's all that DOES stop you + an electromagnetic break that cuts in when you let go the control ( some have an emergency / parking disk or drum break too ) - mind it works OK and will stop you pretty quick even on a steep hill - er just as well sometimes