does coasting save petrol

Wongsky

Wig wrote: »

No, all you are feeling/witnessing there is an automatic throttle to increase fuel/air supply to prevent stalling,

Rubbish - I'm talking at around 1200-1300rpm.

That's not to do with stalling, that's fuelling being introduced again. IACVs and ETMs and air valves do tend to be used to boost idle speed when moving, but no load (ie not in gear) which is probably contributory to the general vibe about coasting probably being less economical than no throttle in gear.

Wig wrote: »

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.

You're simply misunderstanding what I'm talking about.

Go drive your car, and where the roads are free and clear, when you can decelerate from normal road speeds in say 3rd, or even better 2nd, feel how that car responds when you're foot is off the throttle, and you slowing down revs dropping from around 2k, to just over 1k. My guess would be that at about 1200-1300rpms you'll feel a change in the engine response and the way the car moves - and I doubt it's an IACV or ETM strategy to prevent stalling at those engine speeds.

Wig wrote: »

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

Fuel pressure tends to be measured at the rail, the injectors will be under direct control of the ECU, and other sensors will also be telling software management about how much fueling, or not, the engine is using (ie downstream from the engine).

This idea that it's some myth, or really not that prevalent, is just reactionary - you're just compensating for not liking the idea - go google it - even some of the very earlier EFI systems cut fuel on overrun, given, nowadays, cars tend to be "tuned" for economy, by default, and certain emiission evaluations, and you've only got to log data for a few different engines to realise this is no myth.

Go search google, then come back and tell me there's going to be as many exceptions as inclusions. In fact, go google it then tell me your 2004 car doesn't cut fuel on overrun - 'cos I'm not buying your take on it - not for a New York minute.

Wongsky

GolfBravo wrote: »

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.

I'd agree with this - I wouldn't be obsessing too much about MPG readouts from various cars "computers". Some will be remarkably simplistic in how they measure it, and on an instantaneous basis, fuelling used at engine idle speeds, and on overrun, given likely road speed, is probably still going to be reported in a similar manner.

mattyprice4004

On a relatively modern diesel, idle uses around 0.1 - 0.2 gallons of fuel per hour (0.5 to 1 litre of fuel per hour). This varies by engine of course.

So for every minute you coast, you use between 8 and 16ml of fuel. Running on over-run a modern diesel will totally cut fuel use to zero, so is of course much better for economy... as long as you don't have to make up for it with more throttle use later.

Assuming a consumption of around 45mpg, every minute coasting will use enough fuel to cover 125 to 250 metres.

Note: This is all for diesel engines, based around my knowledge and experience with the Bosch VP30/VP37 fuel pumps (direct injection, Rover L series) and BMW M47 commonrail diesel engine. Both types of injection, although very different in operation, deliver similar economy and idle fuel consumption.

Wig

Wongsky wrote: »

Rubbish - I'm talking at around 1200-1300rpm..

So am I, That is roughly where the anti-stall throttle comes into play. On my cars the only apparent increase in fuel that I can witness is because any slower and the car would have stalled.

I'm not giving any "reactionary view". I have no view to take. All I am saying is that on threads like these you will always get people like you who come along and say,
"you will use more fuel if you coast out of gear because if you coast in gear the fuel gets cut off" Yet you provide no evidence that this is infact the case on all modern cars, no evidence on which cars it is true and which it is not true.

I don't think it is a myth, as I said earlier, I have no position on this, all I ask contributors to do is provide evidence. I am quite willing to look at google on the subject but having looked I can find no evidence....can you?

I don't like or dislike the idea, all I care about is driving to save fuel based on evidence, and in the absence of evidence based on my own experience.

tbourner

Wig wrote: »

I don't think it is a myth, as I said earlier, I have no position on this, all I ask contributors to do is provide evidence. I am quite willing to look at google on the subject but having looked I can find no evidence....can you?

Thing is most normal people know the basic physics of how cars work, and that when using engine braking the momentum of the vehicle means the engine can turn without needing fuel added, hell even wikipedia in all it's infallible glory has a section or 5 on the subject.
So most normal people assume the car manufacturers would make use of this, they don't need evidence. The problems come with the implementation, as engine temperatures, backpressures, air locks and so on can become a problem if you simply cut off the fuel all the way down to 800rpm. If THAT's what you want clarification on then you'll probably have to go away and look for it yourself, I just did a quick search on DFCO (Deceleration Fuel Cut-Off) and found this, so Toyota use it. I got that from a helpful Yaris forum. There was also a Scion forum where I found a link to this handy gadget, you could buy one of those if you really want some of your own 'evidence'.

Now I'm not going to spend all day searching for DFCO tech documents of various manufacturers, building a table of data cross-referencing model years with DFCO implementation for you, because I believe it to be true based purely on the proven physics of the situation.

So I'll just end on the Toyota technology, and say that my old Supra of 1993 design didn't have this technology. Somebody told me once what the duty cycle was on overrun and it wasn't zero.

Wongsky

Wig wrote: »

So am I, That is roughly where the anti-stall throttle comes into play. On my cars the only apparent increase in fuel that I can witness is because any slower and the car would have stalled.

Your car would have stalled around 1200-1300rpms when in gear and moving on the road?

Wig wrote: »

I'm not giving any "reactionary view". I have no view to take. All I am saying is that on threads like these you will always get people like you who come along and say,
"you will use more fuel if you coast out of gear because if you coast in gear the fuel gets cut off"

And as a generalism, that holds true.

Thing is, the converse applies - in threads like these, you always get people who go against common knowledge, and it appears because the topic goes against their ingrained methods of driving, that they have some investment in.

You get much the same in threads that talk about people changing down through gears when slowing down, or braking, then changing to the one target gear. And the main issue, always seems to be that people will argue because more current thinking makes their methods of driving which they've done for years and years, suddenly look a bit unnecessary, but they're unwilling to change their habit formed behaviour.

Wig wrote: »

Yet you provide no evidence that this is infact the case on all modern cars, no evidence on which cars it is true and which it is not true.

Why would you imagine there would be modern cars where it isn't true?

If you're in any doubt, write / email Fiat (your car is a Fiat, right?) and ask them? Or buy a cheap ELM dongle, and log data, and see what sort of metrics are available - if nothing else, you should have record of things like roadspeed, throttle opening, and what sort of lean / rich condition your engine was running.

The reason why I suggested you look on google, being simply you'll find countless people, discussing countless different makes and models. Why would you think your car will be magically different?

I'll give you a couple of examples - neither of which are particularly very recent. There's a year 2000 Ford Focus, 1.6 petrol in the household. As well as the normal things you can expect from an EFI engine, one other thing it details in the manual, being what it does as a strategy should the coolant temperature get quite high. It runs on alternating 2 cylinders, and blows air (ie cuts injection) to the others, in order to try and keep the temperature down (I wonder if Rover had done something similar with the K series, maybe their issues would have been lessened...). And a few years back when the 300C came out with a Hemi engine, they made a big song and dance about what it will do at light throttle when crusing (run on 4 cylinders).

Now don't you think if different manufacturers go to such efforts, and clearly have BIG interests in making their cars look fuel efficient, that any semi modern EFI petrol engine car will behave loosely the same, in looesely the same conditions?

Why would you imagine there would be exceptions - it's almost free gains in terms of fuel efficiency figures.

Wig wrote: »

I don't think it is a myth, as I said earlier, I have no position on this, all I ask contributors to do is provide evidence. I am quite willing to look at google on the subject but having looked I can find no evidence....can you?

Can you not see countless different links, often with logged data, of people discussing this on various makes and models?

Why would you imagine that Fiat have magically done something completely bucking the trend on you car / model?

Wig wrote: »

I don't like or dislike the idea, all I care about is driving to save fuel based on evidence, and in the absence of evidence based on my own experience.

Well as a generalism, petrol EFI cars (and probably diesels, too) cut fuelling on overrun, above a certain lower threshold in revs - and have done so for a LONG time (decades).

And also, as a generalism, it is more fuel efficient to appreciate that, and bear it in mind when driving. Sure, there may be some odd, contrived exceptions - that I'll keep an open mind about, in terms of them being very specific alteratives for very specific situations - but outside of that, I think the generalism holds true - petrol EFI cars from modern times cut fuel on overrun, and in general, it's more fuel efficient to leverage that rather than coasting out of gear.

If you think that's controversial, but you really DO want to learn more, rather than just arguing against that, then do more research, rather than simply disputing what most people (reasonably correctly) spout as common knowledge.

Wig

mattyprice4004 wrote: »

On a relatively modern diesel, idle uses around 0.1 - 0.2 gallons of fuel per hour (0.5 to 1 litre of fuel per hour). This varies by engine of course.

So for every minute you coast, you use between 8 and 16ml of fuel. Running on over-run a modern diesel will totally cut fuel use to zero, so is of course much better for economy... as long as you don't have to make up for it with more throttle use later.

Assuming a consumption of around 45mpg, every minute coasting will use enough fuel to cover 125 to 250 metres.

I agree with those figures approximately, my own estimate is that idle uses 11ml per minute on my punto. which equates to 117 metres at 45 mpg

However, no throttle in-gear would require throttle to be applied much earlier than no throttle out-of-gear, so much so, that you can coast out of gear for half a mile or more longer than you could have done had you been in gear. Assuming in gear allows travel of 200metres and out of gear allows travel of 1000metres

Assuming your car has overrun = 200metre free travel + 800m at 45mpg
800m = 75ml of fuel
1000m = 75ml of fuel (includes 200m free travel)

Out of gear = 1000m at Avg 40 mph those 1000m took you 56 seconds
56 seconds at idle is approximately 11ml of fuel

So you can compare now,
in gear the distance cost you 75ml*
out of gear the distance cost you 11ml of fuel
*This assumes your car is fitted with overrun.

Of course assessing how much fuel was used over the 800m with a bit of throttle is really just guesswork and the figures assume an avg of 45 mpg for those 800m when it could have been less, considering it could have been a downhill section of the road. In the end, it is all just guesswork when written on paper, and there are so many variables like at what speed you reapply throttle, and even given that your chosen speed for reapplying throttle may be 40mph, you may be approaching a roundabout at 40mph and choose not to re-apply the throttle............ which is why it is important to stress that no-one can categorically say which method will save you the most money (if we assume that the car has overrun).

But if someone was to drive 1000 - 5,000 miles on each method maybe we could start to get some data on the subject. I have some data on the out of gear method, but it was spoiled by the last 250 miles being driven on a motorway journey which did not match the first 750 miles - I got 47mpg over the 980miles.

mikey72

mattyprice4004 wrote: »

...........Assuming a consumption of around 45mpg, every minute coasting will use enough fuel to cover 125 to 250 metres. ............

Level or uphill coasting every time, downhill, stayng in gear works for me normally.

I coast a mile or so to a fairly busy junction reguarly, if I stay in gear I find the engine braking means I have to accelerate again, whereas coasting out of gear gives me well over the 250m difference.
It's a junction where I need to slow from 60, to a stop, slightly downhill, so out of gear is just enough momentum to kep it going. In gear, the engine braking means it still needs power on about halfway.

tbourner

Ah Wig is talking about distance traveled on a certain amount of fuel, whereas everyone else is talking about fuel used over a certain distance.

So yes if you have a big empty road and you want to roll as far as possible you should just drop it out of gear and roll with the engine idling, cos you'll roll at that higher speed for much longer, eventually slowing down due to friction. Whereas if you leave it in gear you'll slow down really quickly with engine braking, and have to use throttle again.
But in the situation where you're driving along and a light ahead goes red, you have a set distance to stop in and you want to use as little fuel as possible, then you're better off leaving it in gear and engine braking to the lights - you may or may not stop way before the lights, the best option really would be to slow down to an optimum speed of around 10mph and then drive on enough throttle to maintain that speed until the lights go green - hopefully timing it just right. Incidentally that would also be the best option in the first situation, engine brake down to a sensible speed where the engine is at optimum (maybe 1600rpm in 4th gear?) then cruise for as far as possible.

Wig

tbourner wrote: »

But in the situation where you're driving along and a light ahead goes red, you have a set distance to stop in and you want to use as little fuel as possible, then you're better off leaving it in gear and engine braking to the lights

If you know that your car will coast in-gear no problem over the distance to the lights, then I would agree, you are better leaving it in gear, because even though you don't know for sure that your car has the overrun feature you have nothing to lose by coasting in-gear and possibly something to gain by doing so.

Incidentally that would also be the best option in the first situation, engine brake down to a sensible speed where the engine is at optimum (maybe 1600rpm in 4th gear?) then cruise for as far as possible.

This would need to be accurately measured for fuel consumption over the different methods before saying which method is the best method. Until that has been done it is down to driver belief and driver preference.