does coasting save petrol

mkirkby

Nearly_Old wrote: »

On boring long journeys when a decent hill comes along and it isn't going to inconvience anybody else I still try and find the "speed" that the car will maintain and get 999.99mpg.

I found that to on a modern diesel I drove a while back. But I think you will find that the 999.9 is not actually 999.9; more like infinity because it's not using any fuel at all. Instant mpg = distance/fuel used and it's probably taking samples every few seconds. If you're not using any fuel the value would be dist/0 which is infinite so the programmer of the ecu decided it would be best to display the biggest number the display could handle.

We should be lucky that the programmer caught that and didn't actually do the division and crash the computer...

Tobster86

We check every division with the divisor being >0, and return the maximum for that data size if not; because otherwise it can send small embedded computers well and truly belly up! In my industry (food processing) this usually results in a sausage roll and pastry pile-up.

Wongsky

00ec25 wrote: »

OK , I have an old car , no idea if it's fuel injected (1996 Ford Mondeo 1.8) but the engine certainly does not cut out when I do the 1/4 mile coast home - I can't swtich off the engine as there are 2 bends and the loss of power steering would be "inconvenient" in that case, so I'm out of gear with engine running at idle to keep the steering going

A 96 Ford Mondeo - assuming it's petrol, would be fuel injected.

And on the overrun - ie foot off the throttle, in gear with the engine doing, say, 1300 revs or more, almost certainly, the ECU will cut fuelling to the / by the injector(s). If you coast (ie put it in neutral and let it idle) the ECU will be fuelling at least at engine idle rates - probably a little higher, as they tend to boost idle when moving.

When revs get below a certain point - probably somewhere a little over 1000rpm, probably around the 1200-1400rpm point, the ECU will possibly start adding a little fuel, on overrun, just to prevent stalling - but this can vary quite a bit, depending on FI systems.

Wongsky

Dave_C wrote: »

Going down a very long hill. In gear with no throttle you get engine braking and only travel at say 20mph. Comparing this with an engine at tickover is fair and engine braking uses less fuel

However on some long hills it is possible to coast at a much higher speed, say 40 mph. To maintain this with the engine engaged requires a fair bit of throttle - more than just tickover. So coasting uses less fuel to maintain the higher speed.

At some point in the middle is the break-even point.

What about automatic cars in drive - where there is no engine braking?

Dave

What about them?

They will still cut fuel on overrun, over a certain lower threshold of revs.

And it would be wrong to say they have no engine braking - they just don't have anything like the same degree as manual cars.

Wig

Despite many threads on this subject, no-one has ever produced any evidence that many/most or which models of fuel injected cars have overun as a standard feature.

I have a Punto 2004 with a mpg display and I cannot see any difference between coasting in-gear or out of gear, i.e. I cannot see any evidence that coasting in-gear shuts off the fuel.

So I would advise you take "a huge pinch of salt" with any poster who tells you that coasting in-gear will shut off your fuel supply, and thus will save you money in the long-run......apart from the fact that coasting out of gear allows you to go faster for longer before having to apply any throttle, there is simply no evidence out there to say that many, most or all cars with ECUs will shut off the fuel, and no list of car user manuals that say that this is a feature of the cars design.
*And then you go into the territory of asking the question....
Of those cars that defintately do have this overun feature - how many seconds does it take to kick in? Because when you dip the clutch and lift the throttle the fuel reduction is immediate, and so your savings are immediate. The same is true of coasting in-gear, the fuel supply is reduced in the same way immediately but how many more seconds until the car shuts off the supply completely? And at what speed/RPM does it discontinue?

However. I do have my own experience to answer your OP question. When my car is driven casually but always in gear it returns an avg mpg of 43mpg. When my car is driven to coast out of gear at every opportunity it returns 53 - 54 mpg.

Wongsky

Wig wrote: »

Despite many threads on this subject, no-one has ever produced any evidence that many/most or which models of fuel injected cars have overun as a standard feature.

I have a Punto 2004 with a mpg display and I cannot see any difference between coasting in-gear or out of gear, i.e. I cannot see any evidence that coasting in-gear shuts off the fuel.

So I would advise you take "a huge pinch of salt" with any poster who tells you that coasting in-gear will shut off your fuel supply, and thus will save you money in the long-run......apart from the fact that coasting out of gear allows you to go faster for longer before having to apply any throttle, there is simply no evidence out there to say that many, most or all cars with ECUs will shut off the fuel, and no list of car user manuals that say that this is a feature of the cars design.
*And then you go into the territory of asking the question....
Of those cars that defintately do have this overun feature - how many seconds does it take to kick in? Because when you dip the clutch and lift the throttle the fuel reduction is immediate, and so your savings are immediate. The same is true of coasting in-gear, the fuel supply is reduced in the same way immediately but how many more seconds until the car shuts off the supply completely? And at what speed/RPM does it discontinue?

However. I do have my own experience to answer your OP question. When my car is driven casually but always in gear it returns an avg mpg of 43mpg. When my car is driven to coast out of gear at every opportunity it returns 53 - 54 mpg.

That's just reactionary nonsense borne out of ignorance, with many cars you can actually feel the fuelling kick in again when the revs get low enough and the gear being used is low enough for it to be clearly apparent.

And if that wasn't sufficiently convincing, you can connect OBD devices and log live data whilst driving. I can assure you it's not an urban legend. And many car models share the same, or very similar FI systems.

Tobster86

Wig, I'm sorry but you clearly just do not know what you are talking about.

There is no 'overrun' feature and no 'shutting off of fuel'.

Injection systems respond to demand for load. The second your foot is off the accelerator, there is no demand and fuelling is cut to close to if not exactly nothing. It doesn't matter how high it's revving, there is no demand.

When you apply the clutch, the revs drop and demand comes from the requirement to sustain idle revs.

There may be an exception to some degree for old carb systems, but all proportionally governed mechanical diesel injection or electronic multipoint injection system works as I've described.

I'm betting your Punto has single point injection and doesn't have any kind of fuel flow sensor, so makes a best guess at mpg from perhaps throttle position and rpm. Infact, single point systems probably do still fuel a bit on no load.

Wig

"overrun", is Vauxhall's name for the fuel supply cutting off when lifting off the throttle and remaining in gear, if it's used by Vauxhall it may be used by other manufacturers for the same feature.

Quote from a Vauxhall user manual:
"Overrun*
The fuel supply is automatically shut off during overrun, e.g. when the vehicle is being driven down long gradients or when braking. To enable the overrun cut-off to take effect, do not accelerate during overrun and do not de-clutch. To prevent damage to the catalytic converter, overrun is temporarily de-activated when the catalytic converter temperature is high
* = Equipment options not available in all vehicles, all models, all countries"

If as you say, there is fuelling.... "close to if not exactly nothing" then the fuel has by definition been cut off so let's start by not splitting hairs please.

Now, given that this is a known feature of *some* fuel injected models. The question remains how common is the feature? What evidence do you have that this is a very common feature? What evidence do you have that this is how all/most or many modern cars work?

My Punto is multi-point injection. I don't know if it has a fuel flow sensor or if it estimates, but it should be able to determine if it is in overrun or not in either case.

Wig

Wongsky wrote: »

That's just reactionary nonsense borne out of ignorance, with many cars you can actually feel the fuelling kick in again when the revs get low enough and the gear being used is low enough for it to be clearly apparent..

No, all you are feeling/witnessing there is an automatic throttle to increase fuel/air supply to prevent stalling, this does not prove that before the automatic throttle intervened there was no fuel reaching the engine, it just proves that there was not enough fuel reaching it to overcome the load at low speed. i.e. there could easily have been an "idling" (and completely unecessary) amount of fuel supply prior to the intervention.

And if that wasn't sufficiently convincing, you can connect OBD devices and log live data whilst driving.

You can, but only if you have the data to collect, i.e. a fuel flow sensor.

GolfBravo

I think there are way too many variables there to claim that overrun will save fuel over coasting every single time - I'm sure there are instances where you can actually build up enough momentum and speed going downhill to actually save fuel (going faster and further than in gear).

But I guess "on average" it would be better for fuel economy to leave the car in gear.

It is also probably better for the DMF to just leave the car in gear.

And don't trust the trip computer too much, it isn't a scientific measuring device - the "current MPG" figure is usually calculated from airflow past the mass airflow meter, manifold vacuum and engine rpm. Car manufacturers use different algorithms to calculate the MPG, and their accuracy varies between manufacturers.