Coasting in neutral, good idea or bad?

DdraigGoch

I've wondered this too. My grandfather, back in the days when he had the ONLY car in the district!, used to switch the engine off at the top of a steep bank - nervewracking only when you know better, as children we thought it was hilarious, swooshing along silently!!

It makes no sense to me that if an engine is turning over, as it would be when it's stationary, but the wheels are turning and the friction of air and road are there, as opposed to the engine turning over and actively driving the wheels, with the same road and air friction .... that the engine is using LESS fuel actively driving than coasting!

Please can someone explain it in words of one syllable [two at most] - I am not the brightest bunny when it comes to mechanical stuff... LOL

Belnahua

When these conditions are met :-

The engine is up to operating temperature
The engine speed is higher than 1500rpm
The accelerator is not pressed

The fuel system cuts all the fuel to the engine (i.e. turns off the fuel injectors) and relies on momentum to turn it; when any of these conditions cease, the fuel is restored.

sarah_elton

Will probably get flamed for this, but my favourite driving game with myself used to be the multi-storey coasting game. On leaving a multi-storey car park, see how far down you can get without having to go into gear. Trick is to roll down each ramp quick enough to have enough momentum to get to the next.

Only once did I manage the full ten floors of my local without any slow people pulling out in front of me.

Of course this was back when I was young and stupid, and in my ancient Metro. Wouldn't engage in or encourage such behaviour now.

Belnahua

It was always that pale yellow Volvo 440 on the penultimate ramp that stopped you wasn't it? Not that I ever did it either......:rolleyes:

BillScarab

sarah_elton wrote: »

Will probably get flamed for this, but my favourite driving game with myself used to be the multi-storey coasting game. On leaving a multi-storey car park, see how far down you can get without having to go into gear. Trick is to roll down each ramp quick enough to have enough momentum to get to the next.

Only once did I manage the full ten floors of my local without any slow people pulling out in front of me.

Of course this was back when I was young and stupid, and in my ancient Metro. Wouldn't engage in or encourage such behaviour now.

I though I was the only person who did this! It's been a long time though and now I'm a grown up I wouldn't do it.

Inactive

LandyAndy wrote: »

:rotfl: :rotfl: :rotfl: :rotfl:

You tried to tell Inactive something.

:rotfl: :rotfl: :rotfl: :rotfl:

:rotfl: :rotfl: :rotfl:

Inactive

cajef wrote: »

Well tell that to a driving examiner, if you do it on the driving test it is a serious fault and instant fail.

Highway Code Rule 122.

Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because

Which supports my earlier comments..

Pew_Pew_Pew_Lasers!

Wig wrote: »

What do you base your opinion on?

I beg to differ.

It isn't opinion. It is fact for most vehicles.

I'd like you to tell me how you can be in full control of the vehicle when you have only the brakes to control vehicle speed, and not brakes + engine speed. Do tell me how descending a hill using only the brakes to control speed, and having to engage a gear by removing one hand from the steering wheel while negotiating a bend with the braked wheels unbalancing the car is somehow safer than leaving the vehicle in gear and keeping both hands on the wheel.

loofer

Belnahua wrote: »

When these conditions are met :-

The engine is up to operating temperature
The engine speed is higher than 1500rpm
The accelerator is not pressed

The fuel system cuts all the fuel to the engine (i.e. turns off the fuel injectors) and relies on momentum to turn it; when any of these conditions cease, the fuel is restored.

Just out of interest, why would it have to be over 1500rpm (or other Xrpm pre-set by the manufacturer)?

I would have thought the fuel would have cut off if engine speed is lower. Are you saying that if i'm going downhill at say 40mph, it is more fuel efficient for me to have it in 5th gear (circa 1750rpm) rather than 6th gear (circa 1250rpm) ...assuming it is safe for the speed to rise beyond 40mph anyway

tomstickland

Generali wrote: »

Surely if you're in neutral you only have road and air resistance. If you are in gear with no foot on the clutch then you have the resistance of the engine to contend with too. If you select a gear wisely you can reduce but not eliminate that.

Having said that, crashing your car probably isn't very money saving!

Yes, and the question is which out of two potentially good options is the best.
1-No fuel to engine, but engine provides resistance to the car rolling.
2-Engine burns fuel to idle, but car rolls further.

If you think about it in energy terms, then in one case the energy to keep the engine turning is taken from the car, so the car ends up travelling slowly. In the second case the energy comes from the petrol.
Frictional losses increase with speed, so the car that travels more quickly will throw away more energy in rolling resistance and drag. There are also engine inefficiencies at low rpm.

Hence my money is on no1 for most situations. The most efficient result would be come to halt just before needing to. In that case then all of the available kinetic energy at the start of the roll would have been used to drive the car and keep the engine turning.