does coasting save petrol

tbourner

Wongsky wrote: »

Thing is, when driving, the most economical periods are going to be when you're travelling at a reasonable speed, your revs aren't too low, and your foot completely off the throttle. Next to that, will be when only the lightest use of the throttle is sufficient to maintain the desired speed.

So to get the best economy, is a combination of maximising those periods and using good observation, planning and anticipation to reduce periods of braking (ignoring new fangled regenerative braking for a second) and acceleration.

That's what I've been trying to get across.

It's also why I think cruise control isn't good for economy, maintaining a fixed throttle position is better, meaning you slow down up hills and speed up down the other side. There's a whole other argument though!

Dave_C_2

Interesting points being made here, and the answer to the OP isn't a simple yes or no. One of the problems is that it quickly stops becoming an academic discussion because at all times safety is paramount and cannot be ignored. On searching for answers on web sites like the AA, they all make the point that you should be in gear ready to accelerate away from trouble.
In gear without any throttle you get engine braking. A further complication is that as your speed decreases you should change down so that you can accelerate if needs be. This has the effect of making the engine braking more efficient, so instead of a free ride all the way to the bottom of an incline, you could be crawling to a stop half way down. Or you could use fuel to maintain your speed.:)

Not coming down on one side or the other, just saying that it's anything but simple.

Dave

Tobster86

The answer IS a very simple NO IT DOES NOT.

With Wig's assumption of 'having to accelerate sooner'; if your engine had that much internal drag, it would take a lot more fuel to sustain it at idle, so the model is flawed anyway. He's quoted made up numbers, but hasn't described an accurate and realistic situation where out of gear coasting saves fuel (because such a situation does not scientifically exist).

He is also describing the ability to govern fuelling to quantities less than required to sustain idle as a special feature because he read it in a manual for a Vauxhall.

Post 50 wrongly assumes that the accelerator is a fuel quanitity adjuster which it is not. Fuel quantity from an EFI system comes from where all the sensor readings are on the ECU's map, including accelerator position; and on mechanical multipoint injection systems, from the accelerator & govenor positions, plus a few other tweaking devices.


Here's a simple set of models.
Imagine an arbitrary car on a downhill slope wanting to sustain a specific speed. There are three possibilities:

-The vehicle would speed up either in neutral or in top gear. In this situation it is necessary to brake and/or select a lower gear to sustain the desired speed. In this situation, being in gear safely controls the speed and uses less fuel than coasting.

-The vehicle would speed up in neutral but slow down in top gear. In this situation, some accelerator would be required in gear but fuel quanitity would still be less than to sustain idle revs, and again provides more safe control.

-The vehicle would slow down in either neutral or top gear. In this case, being in neutral is illogical.

mkirkby

Tobster86: as someone who appears to have a working knowledge of how the old mechanical Bosch ve systems work, can you confirm that the one fitted to my xantias xud9 engine will cut fuel supply when no accelerator is used while moving fast enough in gear to keep the engine revs above idle?

Is the design actually clever enough to achieve this without an ecu? I'd just assumed it would keep a minimum amount of fuel going through to maintain idle but suppose if there was a feedback system to tell the pump what speed the engine was turning it could achieve the fueling you suggest it does

mikey72

Tobster86 wrote: »

...........In all cars you WILL USE LESS FUEL if you go down a long slope (or even approach a stop; the physics are no different) in gear than in neutral...............

I've got two on carbs, and one with mechanical diesel fuel injection that would disagree with you.
The carbs will have greater air flow at higher revs, even with the throttle closed, as the engine will be turning faster, and pulling more air through, so using more petrol.
The diesel will be on minimal throttle on the pump either way, and engine braking will decrease the speed faster in gear, so I'll need throttle sooner to keep moving on the flat, or have to lift off later in the first place.
That's simple physics.

Tobster86 wrote: »

...........-The vehicle would speed up in neutral but slow down in top gear. In this situation, some accelerator would be required in gear but fuel quantity would still be less than to sustain idle revs, and again provides more safe control....

That's inherently wrong.
The accelerator controls the airflow, either mechanically or electronically on a petrol engine.

If more acceleration is required in gear, that means more airflow is required and hence more petrol will be injected than in neutral at lowest airflow.

The same principle applies to a diesel, but controls the diesel injected instead.
Either way more energy = more throttle = more fuel = more air, to keep the mixture correct.

And if the vehicle would slow down in gear, more energy is required to move the entire car, as opposed to simply keeping the engine idling

mkirkby wrote: »

Tobster86: as someone who appears to have a working knowledge of how the old mechanical Bosch ve systems work, can you confirm that the one fitted to my xantias xud9 engine will cut fuel supply when no accelerator is used while moving fast enough in gear to keep the engine revs above idle?

Is the design actually clever enough to achieve this without an ecu? I'd just assumed it would keep a minimum amount of fuel going through to maintain idle but suppose if there was a feedback system to tell the pump what speed the engine was turning it could achieve the fueling you suggest it does

No.
It fuels regardless.

Wig

Tobster86 wrote: »

The answer IS a very simple NO IT DOES NOT.

With Wig's assumption of 'having to accelerate sooner'; if your engine had that much internal drag, it would take a lot more fuel to sustain it at idle, so the model is flawed anyway. He's quoted made up numbers, but hasn't described an accurate and realistic situation where out of gear coasting saves fuel (because such a situation does not scientifically exist).

According to you -with no experimental data to back up your assertion- i.e. you are only guessing and estimating, just like you say I am doing, the difference is I admit in my posts that they are guesses and estimations...because that's all we have got in this argument, which pops its head up in lots of threads from time to time, we have people like you who say without any evidence that your version is the correct version. And I say, "don't believe people like you because you have no evidence".

He is also describing the ability to govern fuelling to quantities less than required to sustain idle as a special feature because he read it in a manual for a Vauxhall.

And? You know more than Vauxhall?

Post 50 wrongly assumes that the accelerator is a fuel quanitity adjuster which it is not.

Where does it say that?

Here's a simple set of models.
Imagine an arbitrary car on a downhill slope wanting to sustain a specific speed. There are three possibilities:

-The vehicle would speed up either in neutral or in top gear. In this situation it is necessary to brake and/or select a lower gear to sustain the desired speed. In this situation, being in gear safely controls the speed and uses less fuel than coasting.

-Because you say so, so it must be true, right......but you ignore what happens when the incline finishes what distance each car would reach before having to apply throttle.

-The vehicle would speed up in neutral but slow down in top gear. In this situation, some accelerator would be required in gear but fuel quanitity would still be less than to sustain idle revs, and again provides more safe control.

- Because you say so, so it must be true.....right

-The vehicle would slow down in either neutral or top gear. In this case, being in neutral is illogical.

Because you say so, so it must be true, riiiight. except that in neutral the rate of deceleration would be less.....and there would be no need to maintain a specific speed, but more likely a desire to be above a specific speed, so the car in gear (having reached the top of the slope at a speed greater than the lowest desired speed) would decelerate to the lower speed for the application of throttle before the car driven out of gear.

Tobster86

mkirkby wrote: »

Tobster86: as someone who appears to have a working knowledge of how the old mechanical Bosch ve systems work, can you confirm that the one fitted to my xantias xud9 engine will cut fuel supply when no accelerator is used while moving fast enough in gear to keep the engine revs above idle?

Is the design actually clever enough to achieve this without an ecu? I'd just assumed it would keep a minimum amount of fuel going through to maintain idle but suppose if there was a feedback system to tell the pump what speed the engine was turning it could achieve the fueling you suggest it does

Yes it does, it's nothing greatly clever just very finely engineered & machined.

There are a LOT of parts in the pump so I won't go into full detail, but basically the bottom half of the pump contains a plunger. Diesel (/vegetable oil!) in the plunger can go one of two ways; through a valve to the injector pipe, or though a pressure release hole. A control sleeve surrounds the plunger, allowing the pressure release hole to be exposed at a certain point in each injector cycle. It's position determines the fuel quantity. The control sleeve connects to the top half of the pump which contains the centrifugal governor, idle spring and accelerator lever; all of which determine it's position on the plunger. At high revs, the centrifugal governor extends, moving the control sleeve forwards on the plunger and exposing the pressure release hole for more of the cycle. This, combined with no load from the accelerator lever exposes it further than when it's at idle. At idle, the idle spring (plus optional fast idle lever connection) maintains a minimum fuel quantity regardless of accelerator position.

mikey72 wrote: »

I've got two on carbs, and one with mechanical diesel fuel injection that would disagree with you.
The carbs will have greater air flow at higher revs, even with the throttle closed, as the engine will be turning faster, and pulling more air through, so using more petrol.

Hence my earlier post

There may be an exception to some degree for old carb systems

I don't know a great deal about carbs, but I don't believe what you say about greater air flow; if I was designing a carb I'd have a strong enough butterfly valve spring to ensure that there was no air flow at high revs with no throttle; for the purpose of not wasting fuel and improving engine braking. Incidentally; what percentage of vehicles on the road still have carbs?
As for your diesel, read my explanation above. I've completely stripped and rebuilt several Bosch VE/VP injector pumps and interchanged parts between them. There is no 'minimum fuel' setting. There is a minimum accelerator lever position, minimum and maximum governor positions, full load quantity adjuster, and what is referred to as the 'anti-stall' setting for the idle spring (overcome by the governor at high revs, it's only a small spring). On pumps for turbocharged engines there's also a boost compensator adjuster which increases fueling in response to turbo boost pressure.

Wig wrote: »

According to you -with no experimental data to back up your assertion- i.e. you are only guessing and estimating, just like you say I am doing, the difference is I admit in my posts that they are guesses and estimations...because that's all we have got in this argument, which pops its head up in lots of threads from time to time, we have people like you who say without any evidence that your version is the correct version. And I say, "don't believe people like you because you have no evidence".

No I'm not; I understand how the systems work scientifically and have fully & reasonably explained how they work and acknowledged possible exceptions. I haven't presented hypotheses I've presented facts.

You've:
-Quoted a bit of sales jargon from a Vauxhall manual.
-Made up numbers.
-Not actually given any definitive example scenarios that flawlessly prove what you've saying.

You and Mikey72 are also misunderstanding a vital point I'm making; sustaining a driving speed. People drive to a speed governed by immediate conditions and adjust the vehicle controls to sustain that speed, including compensating for incline. I'm not suggesting sticking the car in a low gear and driving artificially slower than you would normally; I'm suggesting keeping the car in gear and maintaining your normal speed, whether through applying some accelerator or progressive braking as required; and stating the FACT that this method uses less fuel that coasting out of gear.

Credit to Mikey72 where it's due; he has demonstrated some level of experience and knowledge that qualifies what he's saying. I happen to believe it's incorrect, but gave it significantly more thought because of this. Wig on the other hand has demonstrated no such knowledge or experience, and I think I'm giving his comments far more consideration than they deserve.

Tobster86

By the way, have a photo from a few years ago showing a decapitated Bosch VE fitted to my old Audi 80. I had failed to replace the idle spring and had problems with it cutting out at the time.



You can see governor assembly (toothed mechanism bottom left, with piston and governor weights emanating from it, one of the weights has shifted outwards), and my questioning why there was a lack of a spring (and also if my blanking of a timing adjustment mechanism was valid); that spring being the idle spring. The top right lever is the full load quantity adjuster, the adjacent assembly connects to the accelerator lever. There is no fast idle on this pump.

This is how you learn stuff. Not by reading Vauxhall user manuals.

tbourner

Talking of Vauxhall user manuals:
http://www.autoexpress.co.uk/car-news/consumer-news/35408/watchdog-daytime-running-lights-rip

mikey72

Tobster86 wrote: »

........... At high revs, the centrifugal governor extends, moving the control sleeve forwards on the plunger and exposing the pressure release hole for more of the cycle. .......................

The governer simply stops the engine over reving by limiting fuel, it's got nothing at all to do with anything discuissed here.
We're not talking about the governor limiting high revs, we're talking about keeping it in gear with no throttle.

Tobster86 wrote: »

I don't know a great deal about carbs, but I don't believe what you say about greater air flow; if I was designing a carb I'd have a strong enough butterfly valve spring to ensure that there was no air flow at high revs with no throttle; for the purpose of not wasting fuel and improving engine braking. Incidentally; what percentage of vehicles on the road still have carbs?..............

It's physics, so what you believe won't really affect it. You need to actually understand how they work.

Tobster86 wrote: »

No I'm not; I understand how the systems work scientifically and have fully & reasonably explained how they work......................

But you've completely misunderstood what the governor does, so your conclusion is wrong I'm afraid.