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Old 22nd June 2013, 18:40   #196
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Default Re: Does coasting save fuel?

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Originally Posted by saket77 View Post
IPardon me for still questioning & arguing but my thinking is that very small quantity of fuel might be used. And the ECU may register this 'very small quantity' as Zero.
The fuel flow that i have seen in the tdci is about 2-3 counts per second(idling, without AC). When coasting in the 5th gear, the rpm dropped from 2k to 1k, and until that the count remained the same. So i would doubt if a small quantity is injected, otherwise it would have shown up within some distance.

Regarding the acceleration downhill and DTE, i will check it and report later.

PS1:I drove a Ritz diesel this afternoon, and it's instantaneous FE shows 30 kmpl when coasting down a slope.

PS2: Figo/Fiesta tdci owners can check the fuel flow using the self-diagnostic mode. The parameter to watch is 'FL'.

http://www.fordwiki.co.uk/index.php/...iagnostic_Mode
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Old 22nd June 2013, 19:40   #197
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Default Re: Does coasting save fuel?

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Originally Posted by saket77 View Post
...no fuel & no air intake while coasting, there will be literally Zero emission by the car. Isn't it?
That is the whole purpose of ECU programming - achieving zero emissions. I'm no expert at how ECU codes are written, but I hope someone who does write (or work with) them can enlighten us further.
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Old 22nd June 2013, 20:36   #198
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... The judder would start immediately the engine is turned off ...
No it wouldn't, not in 4th gear, and not as long as momentum is sufficient to overcome compression. The juddering is when it is reaching the 'just about able to' - right after that the wheels will lock (unable to overcome cylinder compression) unless clutch is depressed. Sparking has no role in this if there is no fuel.

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Originally Posted by SS-Traveller View Post
It would completely depend on the ECU's software, whether you get zero fuel or a micro-quantity of fuel past the injectors when the engine is overrunning. AFAIK, older cars (pre-BS-III, pre-2005) did not have a complete fuel cut-off program written into the ECU software. ...
No, it is not required to be 'cut off' actively by software (e.g. when there is a post-crash instruction from Airbag system). The software is the same. The reference data in the Map corresponding to the extant conditions (0 A pedal, wheel speed > corresponding crankshaft speed for that point) is zero fuel.

Last edited by DerAlte : 22nd June 2013 at 20:53.
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Old 22nd June 2013, 22:50   #199
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Default Re: Does coasting save fuel?

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Originally Posted by DerAlte View Post
No it wouldn't, not in 4th gear, and not as long as momentum is sufficient to overcome compression. The juddering is when it is reaching the 'just about able to' - right after that the wheels will lock (unable to overcome cylinder compression) unless clutch is depressed. Sparking has no role in this if there is no fuel.
Sir, then it would mean that the engine will sound the same when the car is coming down a steep slope/ hill (with engine braking being used) in both engine start and engine off position? However, its a dangerous experiment to undertake, hence I would strongly discourage it, but just for the sake of asking.

Also, would like to ask ramzsys about the Ritz that displayed 30KMPL while descending in coasting mode, shouldn't the FE should be something like infinity or no/ blank data as no fuel was being used?
On the contrary, a fuel consumption rate which would return 30 KMPL while coasting is much realistic, isn't it?

@ SS-Traveller: Sir, I think you misunderstood my context of 'emission'. By emission, I meant any exhaust gas coming out of the tailpipe and not only the harmful gases. The objective of the modern MPFI systems & Cat-cons in my opinion is to reduce the harmful components of the emission and not to eliminate the emissions itself. Hope I made my point clear.

To all my FMs, I know, I am asking too many questions, but I request you all to take this as a constructive discussion.
Thanks for the understanding.

Regards,
Saket
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Old 22nd June 2013, 23:48   #200
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Default Re: Does coasting save fuel?

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Originally Posted by saket77 View Post
Ritz that displayed 30KMPL while descending in coasting mode, shouldn't the FE should be something like infinity or no/ blank data as no fuel was being used?
On the contrary, a fuel consumption rate which would return 30 KMPL while coasting is much realistic, isn't it?
It may be a upper limitation in the calculated FE. For example, in the youtube link posted by Jereon, you can see that coasting returns an instantaneous FE of 999.9 mpg.
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Old 22nd June 2013, 23:52   #201
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Originally Posted by Guna View Post
... In that case the engine sound should change and when the throttle is given it should come to life (again sudden change in engine note). ...
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Originally Posted by saket77 View Post
... the engine will sound the same when the car is coming down a steep slope/ hill (with engine braking being used) in both engine start and engine off position? ...
1. You can't / don't hear the engine usually at all in a car

2. You hear the exhaust *after* the muffler. The muffler, much like a capacitor, removes all the peaks and transients. Effectively you hear the resonant exhaust pressure reduction, not the engine. This sound will be the same in coasting and fuelled operation

3. Only in engines with a free flow exhaust and no muffler one gets to hear something that can be related to the engine, and the engine sounds are different for coasting and fuelled operation

4. In older (carburetted) engines one could hear a bit of backfire in the exhaust manifold from unburnt fuel in the exhaust while coasting. With fuel injection, especially with a lambda sensor, one no longer hears that, because unlike idling fuel quantity that goes through carbs, fuel injection doesn't allow any fuel through

5. If you were somehow able to hear the engine (mic on engine block?), at any RPM fuelled operation sound will be slightly louder with a low timbre (due to the cylinder charge explosions), and the coasting (unfuelled) operation sound without it
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Old 23rd June 2013, 08:15   #202
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Default Re: Does coasting save fuel?

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@Jeroen: I am in agreement of the fact that coasting in gear would use lesser fuel, but not so convinced that its literally ZERO fuel, of course before reaching the idling rpm while coasting.
That is correct, it is not zero. That's what I meant with my statement "safe the occasional burst of fuel". The ECU does inject fuel even whilst coasting, but intermittently. Not sure exact, I think it varies depending on many factors, but it could be spaced out over several seconds.

It's a small amount so it is very likely it gets registered as "zero fuel usage" on the display

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Old 23rd June 2013, 11:38   #203
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Default Re: Does coasting save fuel?

Maybe stupid, but I'm just wondering wildly whether this air can be utilized in any way, which gets compressed in a stroke and expands wastefully in the next stroke, while coasting in gear.
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Old 23rd June 2013, 14:04   #204
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Default Re: Does coasting save fuel?

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Maybe stupid, but I'm just wondering wildly whether this air can be utilized in any way, which gets compressed in a stroke and expands wastefully in the next stroke, while coasting in gear.
Yes, you can utilise it. For engine braking.
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Old 23rd June 2013, 20:33   #205
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Default Re: Does coasting save fuel?

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Yes, you can utilise it. For engine braking.
And after that? Just allow it to expand freely in the supposed combustion stroke next? I was thinking more of any studies in the direction of converting that stored energy with some additional plumbing. It is a wild, incomplete and may be a stupid thought.
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Old 23rd June 2013, 23:51   #206
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Default Re: Does coasting save fuel?

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Originally Posted by Jeroen View Post
... "safe the occasional burst of fuel". The ECU does inject fuel even whilst coasting, but intermittently. Not sure exact, I think it varies depending on many factors, but it could be spaced out over several seconds. ...
If we keep in mind the following during coasting:
* Energy to keep the crankshaft rotating (overcoming compression) comes from the vehicle's momentum, so no further energy needs to be supplied
* The cylinder temperature is maintained high enough that even in cold weather glow plug need not be used to restart engine
* At any speed above the RPM that reliable start normally occurs, injecting diesel would result in combustion (cyl / air temp at the end of compression stroke can support ignition)
* Stall speed (severe juddering then engine stall) is usually a bit less than idling RPM. so as long as the crankshaft doesn't reach this speed, injecting fuel would bring it to stable operation again

Why would the ECU give a burst of fuel every, say, 50-75 revolutions @ 1500 rpm? And what factors would those bursts depend on?
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Old 24th June 2013, 00:10   #207
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Default Re: Does coasting save fuel?

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Originally Posted by DerAlte View Post

...., so no further energy needs to be supplied

....


Why would the ECU give a burst of fuel every, say, 50-75 revolutions @ 1500 rpm? And what factors would those bursts depend on?
I'm not sure how long it would take but my rpm goes down below 1500 on its own; I don't think there is any fuel injection going on.

Other than that, once at or below idling, fuel injection does begin, to prevent stalling.
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Old 24th June 2013, 01:23   #208
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Default Re: Does coasting save fuel?

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Why would the ECU give a burst of fuel every, say, 50-75 revolutions @ 1500 rpm? And what factors would those bursts depend on?
I don't know if this post could be of help on the topic.

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Originally Posted by DerAlte View Post
Stall speed (severe juddering then engine stall) is usually a bit less than idling RPM. so as long as the crankshaft doesn't reach this speed, injecting fuel would bring it to stable operation again
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Originally Posted by vina View Post
Other than that, once at or below idling, fuel injection does begin, to prevent stalling.
Reading this and this (Does coasting save fuel?), and from my driving observation, I was thinking that the stall speed calculated by the ECU, while in gear, is more than the idle RPM.

IIRC, the ECU on my Micra takes over and pushes the car (sometimes it launches forward as if I had stepped on the gas when I had not, which require some braking effort to keep the deceleration constant) way before the needle hits idle RPM.
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Old 24th June 2013, 11:05   #209
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Default Re: Does coasting save fuel?

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I don't know if this post could be of help on the topic. ...
Thanks. It explains partially, since it only talks of catalyzer safety. How 'long' is 'long deceleration'?

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Originally Posted by thoma View Post
In the context of the engine, 'RPM' need not be continuous revolutions (a bit abstract, but bear with me). Everything from starting to stall control works on crankshaft angular speed (which can be simplified to RPM equivalent) and angular acceleration. It is possible for the crankshaft angular speed to suddenly increase (detection of firing) and decrease (stalling) within one revolution

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... IIRC, the ECU on my Micra takes over ...
'Natural' phenomenon of lugging / juddering. ECU can't do anything till you do something to come out of it. ECU can't do anything since it is not aware of conditions in the physical world. In autoboxes the clutch will be activated and injection resumed to preclude stall. In manuals the driver has to either declutch or accelerate, otherwise it will stall.

Last edited by DerAlte : 24th June 2013 at 11:06.
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Old 24th June 2013, 11:21   #210
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Default Re: Does coasting save fuel?

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Originally Posted by thoma View Post
And after that? Just allow it to expand freely in the supposed combustion stroke next? I was thinking more of any studies in the direction of converting that stored energy with some additional plumbing. It is a wild, incomplete and may be a stupid thought.
If there is additional plumbing, then imagine the supposed compression stroke- no fuel and air to expand. Therefore, with the valves closed, it would result in a vacuum. I suppose, depending on the flywheel, transmission, and speed, it might be possible to create this vacuum without jamming the piston. BUT what happens next during the exhaust stroke? Either the piston would fly up, or there would be a horrible back flow from the exhaust pipe into the chamber. Probably a bit of both. The juddering and vibration this would result in, not to mention the turbulent flow, would probably make for a horrid driving experience. And I think it is probable that it WOULD jam the piston. An engine redesign capable of dealing with this phenomenon would probably gain some efficiency, but it wouldn't be worth the effort doing this for meager gains in coasting efficiency.

Now, with a 12 cylinder engine, it might not be so bad...

What the compression achieved, is doing during the combustion stroke is pushing the piston down- meaning it is enabling another intake stroke in one cylinder, compression in a second, and exhaust in the last. This is assuming a four cylinder engine without fancy timing.
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