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Old 15th June 2011, 13:26   #61
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Default Re: Does coasting save fuel?

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Originally Posted by vikram_d View Post
So what you are saying is that fuel is completely cut off? If this is the case then what is happening with the spark plugs? Are they firing?

What I'm saying is fuel is either cut off or practically cut off. The video posted by @dot on this post (Does coasting save fuel?) shows it is totally cut off.

What I'm also saying is that even if it is not cut off - engine heating is not the reason to keep some fuel going.


I don't know about spark plugs, but I don't think they need to fire - they are controlled by ECU anyway so ECU can shut them down if needed or let them spark up - it wouldn't matter if there is too lean a mixture in the cylinder - it can not ignite.
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Old 15th June 2011, 13:43   #62
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Default Re: Does coasting save fuel?

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Originally Posted by srishiva;
Yesterday I was running at 50 km/h on my Vento. While coasting in gear it showed 80kmpl. It remained the same when I depressed the clutch also.
You have brought up an interesting point. When I TD-ed the Vento, the salesman told that Vento could go on for ever in 5th gear at 50kmph without throttle input, which I tried and confirmed. But I am sure this infinite trot cannot happen with zero-fuel - so the ECU is indeed supplying fuel in this mode, albeit bare minimum to sustain the speed. Same with the going-up-the-ramp-without-accelerator-input.

And I agree with vikram_d that fuel is not "completely" cut off - just that it is close to zero-fuel. A bare minimum is still being supplied even when coasting in gear.

Last edited by supremeBaleno : 15th June 2011 at 13:44.
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Old 15th June 2011, 13:53   #63
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
but the reason you give (cooling of the engine) doesn't make sense.
Hi Vina, not sure about the technicals, but then, I too have heard that sometimes, more fuel is injected to control engine temperature.

Quote:
Originally Posted by vikram_d View Post
So what you are saying is that fuel is completely cut off? If this is the case then what is happening with the spark plugs? Are they firing?
Yup!. fuel is completely cut off, but AFAIK, the ignition system remains HOT.
Quote:
Originally Posted by vikram_d View Post
Didn't understand this part.
It means that when fuel is required, the pump turns on and the engine keeps running.

Quote:
Originally Posted by supremeBaleno View Post
... the salesman told that Vento could go on for ever in 5th gear at 50kmph without throttle input, which I tried and confirmed.
Actually, this is not while coasting. In diesels with ECU and good low end torque, this is quite possible. Even when engine load tries to bring down the rpm, the ECU pumps fuel to try and maintain the idle rpm, that way, the car will maintain idle rpm and will move at speeds matched by gear ratio - even without any throttle input.

During coasting, the scenario is entirely different. The ECU cuts off fuel ONLY when the wheels drive the engine, and not vice versa. Also, AFAIK, even while coasting, below 1500-1300 rpm, ECU starts the fuel pump and injects a lean AFR.

Last edited by dhanushs : 15th June 2011 at 13:58.
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Old 15th June 2011, 14:02   #64
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Default Re: Does coasting save fuel?

@dhanushs, I know it is not coasting. Have experienced the same with a Punto_D. Even petrols do this, but at lower gears.
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Old 15th June 2011, 14:51   #65
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Default Re: Does coasting save fuel?

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Originally Posted by supremeBaleno View Post
You have brought up an interesting point. When I TD-ed the Vento, the salesman told that Vento could go on for ever in 5th gear at 50kmph without throttle input, which I tried and confirmed. But I am sure this infinite trot cannot happen with zero-fuel - so the ECU is indeed supplying fuel in this mode, albeit bare minimum to sustain the speed. Same with the going-up-the-ramp-without-accelerator-input.
Uh, 2 different things. Nobody is implying that fuel isn't supplied when idling. Any car can move without throttle input. If you have enough speed, the car can keep moving even in 5th gear without any throttle input. At that point, the engine is basically idling, so fuel is being injected. Since there is more load on an engine when it is in 5th gear as opposed to neutral, more fuel is injected even when the engine is revving at idle RPM.

Quote:
Originally Posted by supremeBaleno View Post
And I agree with vikram_d that fuel is not "completely" cut off - just that it is close to zero-fuel. A bare minimum is still being supplied even when coasting in gear.
You are joking right? I spent the last page explaining that it is completely cut off. Do you now want a video of the injectors shutting off during engine braking?
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Old 15th June 2011, 15:10   #66
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Default Re: Does coasting save fuel?

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Originally Posted by pranavt View Post
I spent the last page explaining that it is completely cut off.
pranavt, I have a question: As far as my tiny knowledgebase goes, ignition remains 'HOT'. Is this the same for all car's or, is it car specific?. I don't see the point.
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Old 15th June 2011, 15:14   #67
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Default Re: Does coasting save fuel?

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Originally Posted by drpullockaran View Post
For example: The GTX has peak torque at 4200 rpm so up shift at 25% above that rpm which works out to 5250 rpm. Though the GTX sings to 7000 rpm it is bound to damage the engine if revved to this level, though I have not heard of any GTX with a seized engine as yet. Down shift at peak torque values.

For fuel efficiency in all cars including the GTX up shift at 20% below peak torque rpm which for the GTX works out to 3360 rpm. Driving the GTX below 2000 rpm is counter productive since auto fuel cutoff in the GTX while decelerating only happens above 2000 rpm. Below 2000 rpm in the GTX fuel is continuously flowing even with your foot off the accelerator.
Pranav - does the highlighted / bold lines hold true in this thread?
the fuel is cut-off at and above a certain RPM when the foot is taken off the accelerator and the car in gear?

Drpullockaran also replied

Quote:
Thats the one part of the blue motion technology that the new passat is harping about which was there in the GTX 11 years ago. I do not know the reason why they do not implement it below 2000rpm since it is bound to save Fuel. They must have a very good reason to continue fuel flow below 2000rpm.
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Old 15th June 2011, 15:25   #68
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Default Re: Does coasting save fuel?

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Originally Posted by planet_rocker View Post
Pranav - does the highlighted / bold lines hold true in this thread?
the fuel is cut-off at and above a certain RPM when the foot is taken off the accelerator and the car in gear?
Hi, not sure about the GTX, but yes, I have read below 1500-1300rpm ECU starts the fuel supply. Only above it, is where there is complete fuel cutoff.

I believe it has to do something with throttle response in performance oriented cars.
Quote:
Originally Posted by dhanushs View Post
Also, AFAIK, even while coasting, below 1500-1300 rpm, ECU starts the fuel pump and injects a lean AFR.
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Old 15th June 2011, 15:30   #69
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Default Re: Does coasting save fuel?

Instantaneous consumption in my Punto D shows 50 kmpl (max value) whenever I am descending in higher gears 3, 4 & 5. Why coast at all? It's not even safe. I think I have seen 50kmpl while descending in second gear as well.

However, while going through horizontal surface, fuel is consumed even when you don't press the gas pedal. Noted that IC reduces in this case.

Last edited by clevermax : 15th June 2011 at 15:34.
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Old 15th June 2011, 15:51   #70
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Default Re: Does coasting save fuel?

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Originally Posted by pranavt View Post
You are joking right? I spent the last page explaining that it is completely cut off. Do you now want a video of the injectors shutting off during engine braking?
For the sake of having a good discussion, I have just one question.

Let's take a hypothetical scenario where there is a never ending downhill slope which is inclined enough to keep the vehicle moving in the fuel cut off RPM range. So by your logic while coasting down this imaginary slope, the vehicle should return infinite mileage. Do you think that will happen? Will you get infinite mileage?

The MID on different cars behave differently as we have seen in this very thread. Some will show '0' fuel consumption. Some will show the number the Max KMPL value as already pointed by Clevermax on the post above. I personally don't put too much faith in these MID's. Because these MID's can be fooled very very easily.

A small example to illustrate the above point. My Mondeo has a L/100kms counter. I can very easily fool this counter into coming as low as 5.5L/100kms. Which is something like 18kmpl. Not even in my wildest dreams will the Mondeo do 18kmpl that too in the city. It can only do that if I have a death wish and try coasting it with the engine switched off. All I have to do to fool the counter is to tank up gas all the way till the top of the fuel filler cap. Fooling an ECU or an MID is just as simple as that. To get proper mileage figures I stick to the auto cut off of the fuel dispenser gun. It instantly jumps up to 8.8 - 9.1/100kms in one tankful of gas.

Last edited by vikram_d : 15th June 2011 at 16:02.
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Old 15th June 2011, 15:55   #71
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by pranavt
Any car can move without throttle input. If you have enough speed, the car can keep moving even in 5th gear without any throttle input.
The any-car part is not true - specific cars yes, especially diesel. Try the underlined part of your post above on your OHC - I doubt it would keep moving in 5th gear without A-input.

Quote:
Originally Posted by pranavt
You are joking right? I spent the last page explaining that it is completely cut off. Do you now want a video of the injectors shutting off during engine braking?
I am not. Call me a doubting thomas, but yes, if you put up a video of the injectors shutting off, I will believe.
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Old 15th June 2011, 16:54   #72
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
....
Quote:
Originally Posted by supremeBaleno View Post
....
Quote:
Originally Posted by vikram_d View Post
...
Guys, I just called my friend who is working in Bosch, in injection. As per him, NO FUEL is injected in diesel vehicles using Bosch ECU's during overrun.

There may be special cases where the ECU might inject fuel like in dpf regeneration, but in ordinary conditions - NO. NO FUEL is injected.

Give me some time, I will dig deeper into the topic and update.
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Old 15th June 2011, 17:32   #73
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dhanushs View Post
Guys, I just called my friend who is working in Bosch, in injection. As per him, NO FUEL is injected in diesel vehicles using Bosch ECU's during overrun.

There may be special cases where the ECU might inject fuel like in dpf regeneration, but in ordinary conditions - NO. NO FUEL is injected.

Give me some time, I will dig deeper into the topic and update.
Can you also find out if no fuel is injected, then what are the spark plugs doing at this point of time?
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Old 15th June 2011, 17:34   #74
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dhanushs View Post
Hi Vina, not sure about the technicals, but then, I too have heard that sometimes, more fuel is injected to control engine temperature.

you are right - when the engine needs to generate lots of power, to cool down the exhaust more fuel than needed may be injected. This works because heat requires to evaporate the fuel will reduce temperatures.

However while costing - no fuel at all need be injected. Merely by compression no petrol engine can generate enough temperature to cause any damage (otherwise every single petrol engine would knock). Even in diesels temperature with fuel is far higher than temperature without fuel.


While coasting fuel may or may not be injected (and from what I have read in a lot of techy places it is not in modern cars - as long as the rpm remains somewhat above idling) - but cooling is not the reason to inject fuel.


Quote:
Originally Posted by vikram_d View Post
For the sake of having a good discussion, I have just one question.

Let's take a hypothetical scenario where there is a never ending downhill slope which is inclined enough to keep the vehicle moving in the fuel cut off RPM range. So by your logic while coasting down this imaginary slope, the vehicle should return infinite mileage. Do you think that will happen? Will you get infinite mileage?


And my friend why will you not get infinite mileage (on such a stretch if it is infinitely long) in case the energy gained by going downhill is enough to run all your auxiliary systems (electricals including lights, music etc., AC if you have it on ...)?

Last edited by vina : 15th June 2011 at 18:01.
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Old 15th June 2011, 18:11   #75
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Default Re: Does coasting save fuel?

Finally found some good data for this argument/debate/doubt.

WE have all been partially right in all that we have said. The ECU does everything that we have all said. It does cut off fuel completely, but it will pulse injectors occasionally if the deceleration is a long one. Attached some good reading.

And going by this write up we will not get infinite mileage.

Edit: And we also need to figure out which vehicles use the Fuel Tau Cut mode. Wonder where to look. LOL.
Attached Thumbnails
Does coasting save fuel?-untitled.png  


Last edited by vikram_d : 15th June 2011 at 18:16.
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