does coasting save petrol

Tobster86

mkirkby wrote: »

Down Sutton bank.....do you think "0800 no win no fee ltd" would take my case :mad:

I inadvertently experienced this test there, it was the location that the old Lucas injector pump on my first 306 decided it didn't like low quality biodiesel!

mkirkby

Tobster86 wrote: »

it was the location that the old Lucas injector pump on my first 306 decided it didn't like low quality biodiesel!

I was under the impression they didn't much like diesel anyway....

mkirkby

Tobster86 wrote: »

In a safe and convenient downhill location, or flat location with sufficient momentum, engage a gear that gives adequate feel of engine braking. Take your foot off the accelerator and perceive the effect of engine braking.

Well I'm convinced. Old 1998 Xantia with totally mechanical Bosch VE diesel pump and there is no difference in engine braking between in gear downhill whether engine off or on.

In addition, accelerating on the level in 3rd gear then allowing the car to slow in gear shows a linear slow down until just above the idle point at which point there is a discernible change in engine note and in the rate of change of deceleration as the fueling suddenly cuts in. In any gear if below the idle speed the car will accelerate by itself until the *exact* idle speed is achieved so there is certainly some feedback there.

In fact the pump will actually add extra fuel to maintain idle speed if needed. It will go up a one in six hill in 1st gear, maintaining the correct idle speed all the way.

Very clever piece of kit considering it's all without computers.

Tobster86

mkirkby wrote: »

Well I'm convinced. Old 1998 :rotfl:Xantia with totally mechanical Bosch VE diesel pump and there is no difference in engine braking between in gear downhill whether engine off or on.

Just done the same test in my Peugeot 306 with the same engine & pump (and fuel type by the sound of it ) and reached the same conclusion. You can't even tell that the engines off other than through lack of throttle response and dashboard lights.

Will try the BMW 323i next.

Notmyrealname

Can't believe that someone who thinks they're technically competent enough to tell someone who is that they don't know what they're on about can think there'd be a difference with a diesel or fuel injected engine using engine braking with the ignition in the on or off position.

You will find the same conclusion on every single car manufactured from 1993 onwards and every single diesel made in the last half century no matter whether it is a car or a lorry or a combine harvester.

Tobster86

Notmyrealname, I think you've got the wrong end of the stick, although I can't fully make sense of your first paragraph.

I fully expect the same result from all vehicles as you've described; I only came up with the test idea as a means of proving it since the posters I was arguing my point against wouldn't accept a technical description, only real world observations.

I would be very interested to see the effect with a carb aswell though.

Wig

I don't think the test described would show any difference on any car or motorcycle from 1908 onwards....

But I await anyone who can say they found a discernable difference on an old car. I have an old car, but its not on the road at the mo, so I can't check.

We need a few 'controls' on the test to set the benchmark.

Tobster86

Feel free to propose some controls.


Tested the BMW 323i (2.5l 6cyl petrol, multipoint direct injection) this evening, the end result was the same but with an interesting observation that could give some credit to Wigs' earlier 'feature' argument; when you take your foot off, there is a slight delay of about half a second before the full extent of engine braking suddenly becomes apparent. In other words, the ECU makes a positive, delayed decision to cut the fuel to what appears to be nothing.

Half a second is a hell of a lot of time in terms of computing. The weighing machines I work with, that use 22Mhz Zilog derived processor does about 50 'ticks' in that time (full processing of a single raw sample). This looks more like a programmed time delay.

So I can think of two possibilities:
-It really is a specific 'feature'.
-There's a technical reason for sustaining some fueling at that moment. At least one manufacturer claims that it's a special feature.

I'm actually now more interested to see results from very low spec modern cars - there might be something that's just awful enough to discredit my argument!

So, anyone out there with a Kia Picanto...

mikey72

I've a vauxhall.
On overrun, it does cut fuel eventually, but then engine braking slows the car dramatically on a petrol engine, but after a period of several seconds. It then kicks in again at 1200 rpm or so, and the car will pull on idle, and refuel on load. So cutting fuel isn't for long.
Personally, I find it better to coast, it will run over twice as far, so I can drop into neutral a lot earlier, and offest the cut out.

I haven't run the diesel van, but from the spec, it probably produces less than 10hp on idle, (about the same as a moped or two) with no load. It'll use about a litre of diesel every hour. So, overrun in gear wouldn't really be any different than coasting out of gear, to the fuel use, so we're back to the type of governor. Either max, min, or constant speed. I know mine isn't constant speed, it'll keep min revs, even on load up a hill, so it won't cut the fuel on overrun.
Then again, I use the creep, and diesel engine braking is minimal compared to petrol, I rarely drop the van out of gear, as it's the same response as in gear.

EazyDuz

I agree with above, my car will cut fuel with no throttle but the engine braking is ridiculous.
Case and point, there is a slight downhill road which goes straight through town about 1.8 miles.
I used to go down that road at 1am when coming back from friends, when i was at the top of that slight hill, i coasted the car. It would literally roll all the way to the bottom, 1.8 miles, using the idle RPM fuel consumption (hardly anything whatsoever).
If i did the same thing but in gear, the engine breaking would be so bad i'd be constantly adding throttle on the way down the hill, therefore using much more fuel.