Engine braking down hill

899cc · Joined: 19 Mar 2006 Posts: 185 Location: Berkshire

I got into a discussion/argument when people were saying that they all coast down a certain hill to save fuel. I tried to explain that it actually uses more fuel (and you have less control etc), but even after an explaination they were looking at me as if I was stupid. In the end I just gave up because it wasn't worth arguing.

On this particular road, it is 30mph, and quite steep. I said I use 2nd gear, and don't touch the pedals, it actually keeps it at exactly 30mph almost all of the time! One of the things somebody said is that surely that wears the engine more, and I was wondering how true this is.

My engine must be doing about 4000rpm (no rev counter) in 2nd at 30mph. If my engine was cold I would use 3rd because that is quite high, although I wouldn't worry with a warm engine. When going down hill though, is the engine wear significantly reduced becuase the engine is doing no work? I would expect only a very small reduction in wear (if any) because you've still got friction, which is probably the biggest cause for wear.

What would you say if you had an associate with these arguments?

ScoobyChris · Joined: 01 May 2006 Posts: 1033 Location: Andover, Hants

I don't think there's any significant engine wear and it's no different to driving along a road at 4k rpm. The engine is turning over in both instances, with the only difference being that instead of the engine driving the wheels, the wheels are driving the engine.

Chris

899cc · Joined: 19 Mar 2006 Posts: 185 Location: Berkshire

7db · Joined: 06 Apr 2006 Posts: 1691 Location: London

Of course if you want to save fuel, just turn the whole thing off and roll down the hill in neutral.

But then I suspect you don't want to save fuel -- you want to drive safely first and foremost and then save fuel.

Which gear will you need next to go fast again? Use that one, or one a bit higher. Use the brakes to slow.

ScoobyChris · Joined: 01 May 2006 Posts: 1033 Location: Andover, Hants

899cc · Joined: 19 Mar 2006 Posts: 185 Location: Berkshire

michael769 · Joined: 17 Sep 2007 Posts: 166 Location: Livingston

In modern electronic fuel injected cars the fuel supply to the engine is completely shut off on overrun (when the engine is not being used to driver the car, for example going down a steep hill), so there is no advantage to coasting. Even shutting it off completely would not save any fuel and ironically will actually consume more as the act of restarting the engine requires some fuel to be consumed.

EDIT: Thinking about it. When coasting the engine revs will drop, won't you actually consume more fuel when you bring up the clutch, as you need to bring the revs back up to whatever is required for the gear? It may be that coasting is actually using more fuel - unless of course the driver does not care about how rough he is being with the car.

As far as the rev level is concerned virtually all of the wear to an engine occurs when it is cold. Once the oil is up to temperature it makes little difference if the engine is at 1,500rpm or at 5,500.

Red Herring · Joined: 20 Oct 2007 Posts: 494

If we want to take this argument to the ultimate conclusion over fuel I would say you actually use more fuel when coasting because with the drive disconnected the engine needs to burn fuel to keep running, where with the clutch engaged gravity will do it for you. If you think 2nd is a little high in the rev range why not compromise and use third, with just the faintest and smoothest of rub on the brakes as well?

7db · Joined: 06 Apr 2006 Posts: 1691 Location: London

Horse · Joined: 05 Feb 2007 Posts: 495 Location: Darkest Berkshoire

Mr Cholmondeley-Warner · Joined: 22 Jan 2007 Posts: 507 Location: Swindon, Wilts

I've experimented with this in a past life, and actually up to a point the argument about "modern engines shut off the fuel bla bla bla" is correct. However, as the effect of gravity is reduced by having to drive an empty engine, the car gradually slows down. The effect on overall trip MPG ends up being worse, because eventually you have to speed the car back up again. There's a nice hill on the M4 coming down to J15 (Swindon East) where I have carried out lenghty trials of this process.
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Nick
North Wilts RoADAR

Horse · Joined: 05 Feb 2007 Posts: 495 Location: Darkest Berkshoire

899cc · Joined: 19 Mar 2006 Posts: 185 Location: Berkshire

martine · Joined: 17 Oct 2005 Posts: 1267 Location: Bristol, UK

On BBC breakfast TV this morning they had a discussion about eco-driving and someone emailed in 'I live in Wales with lots of hills and always coast downhill thereby saving loads of fuel' Rolling Eyes

It wasn't picked up by anyone in the studio...almost felt like emailing in in my best 'adenoid' text: "actually I think you'll find..." etc.
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Martin - Bristol IAM Group Senior Observer and Secretary.