Hyper-miling - coast or idle down hills?

[youngsyr]

Assuming you are at the top of a hill with a clear run down it, no speed limits and a hill on the other side that you need to go up that is also clear. Your car is a fuel-injected, petrol-engined, RWD roadster that loses 17% of engine output in transmitting the power to the ground.

Is it better to:

a) coast down the hill in neutral/clutch in and use 100% of the energy from the hill plus whatever amount of fuel it takes to keep the engine idling; or

b)leave the car in gear and coast down the hill on a closed throttle, thereby shutting down the fuel injectors, but losing 17% of the potential energy from the hill via the transmission?

[Rawwr]

c) don't drive like a spackmong.

[LuS1fer]

It is illegal to coast as you're not in control of the car. Wheels should always have drive to them. As to the point of your post, nobody cares.

[Arese]

Rawwr said:

c) don't drive like a spackmong.

[kambites]

You want to be in gear with the clutch up and the revs above the idle fuel cut-off point (typically around 2000rpm in modern petrol engines I think?). Assuming the hill is steep enough to maintain speed, that is.

Or just switch the engine off and bump start it at the bottom.


ETA: Oops. just re-read the question properly. The answer must depend on the car and the hills, I'd imagine. If you're allowed to turn off the engine, that's probably still the best approach.

Edited by kambites on Friday 29th October 10:43

[Jonny671]

doogz said:

Nail it down the hill. As fast as you can

If there is no speed camera that could get you I suppose giving it abit more beans rather than just coasting or no loud one, it would give you better momentum up the other side of the hill.

[Rawwr]

d) stop at the top of the 1st hill, smash it off the rev-limiter and side-step the clutch. See if you can have the rear wheels still smoking by the time you get to the top of the 2nd hill.

[youngsyr]

I should probably make it clear that this is purely a theoretical/technical question, I'm not suggesting that anyone is actually that bothered about saving 0.00001p by using the "best" method of getting down a hill!

As for the legalities, who says this is in the UK (not to mention that 99.9999% of UK drivers break the law every time they drive in one way or another)? Anywaym it's a theoretical question, legalities don't come into it.

[HellDiver]

Answer is B. Fuel injected engines in that scanrio use no fuel at all. If you pop it in to neutral, then the engine needs to use fuel to stay idling.

[Rawwr]

e) park up the roadster and stop for a delicious picnic with a loved one, enjoying fine August sunshine and artisan sausage rolls.

[kambites]

HellDiver said:

Answer is B. Fuel injected engines in that scanrio use no fuel at all. If you pop it in to neutral, then the engine needs to use fuel to stay idling.

That's the mistake I made when I first read the question. You'll use less fuel going down, but more fuel going up the hill the other side because the engine braking will mean you're going slower at the bottom.

[youngsyr]

Jonny671 said:

doogz said:

Nail it down the hill. As fast as you can

If there is no speed camera that could get you I suppose giving it abit more beans rather than just coasting or no loud one, it would give you better momentum up the other side of the hill.

Whether there's a hill on the other side or not is irrelevant I think, all that matters is that you can use all of the energy picked up from the hill one way or another.

Whether that's switching off the engine at the bottom of the hill and coasting to a stop 10 miles down the (flat) road or getting you 1/4 of the way up the next hill before you need to open the throttle, it's all the same - you're simply trying to maximise the energy gained from the hill and reduce the amount of petrol used over the journey?

[Mr Gear]

kambites said:

You want to be in gear with the clutch up and the revs above the idle fuel cut-off point (typically around 2000rpm in modern petrol engines I think?). Assuming the hill is steep enough to maintain speed, that is.

Or just switch the engine off and bump start it at the bottom.

This. If you don't need to maintain a constant speed... i.e. you can go as fast as you want. If you are trying to stop the car from running away from you because the hill is so steep, then you want to have the car in gear, clutch engaged, with your foot off the accelerator.

[Rawwr]

f) mount your car on casters and enjoy the thrilling ride. Use the time on the downhill trip to notify the emergency services.

[youngsyr]

kambites said:

HellDiver said:

Answer is B. Fuel injected engines in that scanrio use no fuel at all. If you pop it in to neutral, then the engine needs to use fuel to stay idling.

That's the mistake I made when I first read the question. You'll use less fuel going down, but more fuel going up the hill the other side because the engine braking will mean you're going slower at the bottom.

Ok, forget the hill on the other side, it's a flat road as far as the eye can see.

Does that change the answer?

[kambites]

It will depend on the length and steepness of the hill. I think you want to engine brake until you get to a certain point, then coast to pick up speed so you are approaching terminal velocity when you reach the bottom of the hill.

[TTwiggy]

LuS1fer said:

It is illegal to coast as you're not in control of the car. Wheels should always have drive to them.

Really? So every time you change gear you break the law? How do the police check up on this then?

[youngsyr]

Mr Gear said:

kambites said:

You want to be in gear with the clutch up and the revs above the idle fuel cut-off point (typically around 2000rpm in modern petrol engines I think?). Assuming the hill is steep enough to maintain speed, that is.

Or just switch the engine off and bump start it at the bottom.

This. If you don't need to maintain a constant speed... i.e. you can go as fast as you want. If you are trying to stop the car from running away from you because the hill is so steep, then you want to have the car in gear, clutch engaged, with your foot off the accelerator.

So, the amount of fuel used on idle is less than the amount that would be gained from the additional 17% of gravitational energy gained by removing transmission losses with the clutch down?

[minimatt1967]

[quote=Rawwr enjoying fine August sunshine and artisan sausage rolls.
[/quote]

Is that a euphemism?

[Mr Gear]

youngsyr said:

kambites said:

HellDiver said:

Answer is B. Fuel injected engines in that scanrio use no fuel at all. If you pop it in to neutral, then the engine needs to use fuel to stay idling.

That's the mistake I made when I first read the question. You'll use less fuel going down, but more fuel going up the hill the other side because the engine braking will mean you're going slower at the bottom.

Ok, forget the hill on the other side, it's a flat road as far as the eye can see.

Does that change the answer?

Depends on if you are trying to maintain a set speed (say 30mph) or if you can use the hill to gain as much momentum as possible.