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Old 17th June 2011, 19:21   #91
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Default Re: Does coasting save fuel?

I did this experiment today with ANHC AT:
I was going on a fairly open stretch (inner ring road in Bangalore).
When I ma applying constant throttle pressure to maintain 50-60kmph, the instant FE gauge shows around 10 kmpl
Now I take the feet off the accelerator, car starts coasting (in gear) and instant FE shows around 20 kmpl
Soon the vehicle is loosing momentum and I need to keep up with the traffic, so I accelerate, the instant FE slides back to 5kmpl

What I gain in coasting (and letting my car to lose momentum) I loose in accelerating back to the required speed. (true on a flat surface)

It all comes down to laws of physics. You need certain amount of power to keep propelling the car at certain speed. You may keep constant throttle or use the cycle of 'engine power-> kinetic energy -> more engine power'. End result should remain more or less the same.
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Old 17th June 2011, 19:35   #92
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Default Re: Does coasting save fuel?

The pulse method works. and the coasting in neutral too.

Ant taxi driver worth his salt knows this.

And i know for a fact that drivers who used to drive the bus lorry chassis for delivery over long distances use this method to save fuel. They sell the fuel they save. Try folowing one if u can catch one and you can see what they are doing because everything is in plain sight

And in my book, the taxi walas know best. Because no matter how much we theorize, they have practical experience, and nothing can beat that.
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Old 17th June 2011, 19:45   #93
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by Guna View Post
I did this experiment today with ANHC AT:
I was going on a fairly open stretch (inner ring road in Bangalore).
When I ma applying constant throttle pressure to maintain 50-60kmph, the instant FE gauge shows around 10 kmpl
Now I take the feet off the accelerator, car starts coasting (in gear) and instant FE shows around 20 kmpl
Soon the vehicle is loosing momentum and I need to keep up with the traffic, so I accelerate, the instant FE slides back to 5kmpl

What I gain in coasting (and letting my car to lose momentum) I loose in accelerating back to the required speed. (true on a flat surface)

It all comes down to laws of physics. You need certain amount of power to keep propelling the car at certain speed. You may keep constant throttle or use the cycle of 'engine power-> kinetic energy -> more engine power'. End result should remain more or less the same.

Well, if engine works more efficiently when giving out more power (and every engine does) and shuts down completely when coasting, then accelerate-coast-accelerate-coast will give better mileage for the same average speed as cruising at that average speed.

In your example let us say you are at 60 and to come down to 50 takes 200m (just an example - your actual numbers will differ) and then going back to 60 takes 50m (again just an example)

then if you do coast-accelerate cycle over 10km, you'll have travelled 2km accelerating and 8km coasting out of the total 10km.


If you just maintained a speed of 55, your fuel consumption would be 1ltr (at 10kmpl). While now your fuel consumption will be (2km/5kmpl + 8km/20kmpl) = 0.8ltr This is a 20% reduction in fuel consumption.

Of course my numbers (as well as probably your 5, 10 and 20 figures) are not accurate for real conditions, but the point is though it may seem like itll average out, it may not.
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Old 17th June 2011, 22:58   #94
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by Guna View Post
I did this experiment today with ANHC AT:

When I ma applying constant throttle pressure to maintain 50-60kmph, the instant FE gauge shows around 10 kmpl
Now I take the feet off the accelerator, car starts coasting (in gear) and instant FE shows around 20 kmpl
Tried this sometime back on ANHC. The instantaneous FE meter was shooting to 40 while coasting!

Very distracting.
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Old 18th June 2011, 10:43   #95
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Default Re: Does coasting save fuel?

I have been trying to experiment with this technique in city traffic in the last two days and I am sorry to report that it just does not work (in the city). Reasons being you just cannot anticipate what your fellow drivers will do, you think he will do one thing and he will end up doing the exact opposite. Another drawback of using this technique in city traffic is that by the end of your journey you are mentally exhausted by trying to anticipate what the idiot auto-wallah in front of you will do or which gap the bike in front of you will try to take. Like I said earlier, in India this technique can only be succesfully used on highways and not in our cities where we drive like we own the road.
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Old 18th June 2011, 11:03   #96
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dot View Post
Tried this sometime back on ANHC. The instantaneous FE meter was shooting to 40 while coasting!

Very distracting.
In the Altis, doing the same thing would show 99.9 kpl on the MID. To answer the main question, coasting (in gear) definitely saves fuel. Not to the extent that the MID sometimes shows but a significant increase.
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Old 19th June 2011, 00:54   #97
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
I have been trying to experiment with this technique in city traffic in the last two days and I am sorry to report that it just does not work (in the city). Reasons being you just cannot anticipate what your fellow drivers will do, you think he will do one thing and he will end up doing the exact opposite. Another drawback of using this technique in city traffic is that by the end of your journey you are mentally exhausted by trying to anticipate what the idiot auto-wallah in front of you will do or which gap the bike in front of you will try to take. Like I said earlier, in India this technique can only be succesfully used on highways and not in our cities where we drive like we own the road.

trying it in our kind of city traffic will not only be ineffective (you mentioned the reasons) - it can be dangerous (again you have mentioned some of the reasons like mental exhaustion).
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Old 21st June 2011, 17:53   #98
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dot View Post
Tried this sometime back on ANHC. The instantaneous FE meter was shooting to 40 while coasting!

Very distracting.
Instant FE display is a hog wash. I have seen Altis showing 99.9 in such cases, which is probably it's idling FE. Average FE display option is more useful and will give a better indication. The only problem with average FE display is that you have to reset the trip meter every time you want to know the average FE for a particular trip. Btw, does any car provide a lifetime FE (averaged over the entire mileage) display option?
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Old 21st June 2011, 17:55   #99
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Default Re: Does coasting save fuel?

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Originally Posted by vasoo View Post
Btw, does any car provide a lifetime FE (averaged over the entire mileage) display option?
My Mondeo has it, but like I have mentioned in one of my previous posts, it's pretty easy to fool. I consider this hog wash too.
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Old 21st June 2011, 19:04   #100
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by Joe-Ker View Post
The pulse method works. and the coasting in neutral too.

Ant taxi driver worth his salt knows this.

And i know for a fact that drivers who used to drive the bus lorry chassis for delivery over long distances use this method to save fuel. They sell the fuel they save. Try folowing one if u can catch one and you can see what they are doing because everything is in plain sight

And in my book, the taxi walas know best. Because no matter how much we theorize, they have practical experience, and nothing can beat that.
Bus drivers and taxi walas are driving Diesel engines or petrol/cng with carbs. The discussion was focussed towards EFI since we are talking about injectors. There is no correlation between their vehicles and most of the private cars running on the road today.
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Old 21st June 2011, 20:42   #101
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Default Re: Does coasting save fuel?

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Originally Posted by pranavt View Post
Bus drivers and taxi walas are driving Diesel engines or petrol/cng with carbs. The discussion was focussed towards EFI since we are talking about injectors. There is no correlation between their vehicles and most of the private cars running on the road today.

Well cool down buddy, nothing in the thread title says "petrol only" - it is about whether coasting saves fuel. Why would you want to discriminate against us diesel-wallah guys?
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Old 21st June 2011, 21:21   #102
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
Well cool down buddy, nothing in the thread title says "petrol only" - it is about whether coasting saves fuel. Why would you want to discriminate against us diesel-wallah guys?
I think pranav was pointing out the difference between carbs and efi's, not petrols and diesels.
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Old 21st June 2011, 22:33   #103
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Default Re: Does coasting save fuel?

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Originally Posted by dhanushs View Post
I think pranav was pointing out the difference between carbs and efi's, not petrols and diesels.

Oh OK - read his post again and I'm a little confused - did he mean "diesel with carb" because diesels always work with injectors (glow plugs) even if they are not common rail, and I believe even indicab engines are common rail.

Also Logan's diesel engine is definitely at least as modern as many of the other diesels around - why would Logan drivers' techniques not work?
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Old 21st June 2011, 22:59   #104
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
Well cool down buddy, nothing in the thread title says "petrol only" - it is about whether coasting saves fuel. Why would you want to discriminate against us diesel-wallah guys?
Since the last few pages were about EFI specifically, I posted that. In a carbed engine, coasting the car in neutral would probably save more fuel than engine braking because there are no injectors to shut off. As for the point about diesel, my bad. I completely forgot that some of you were referring to diesels and I just assumed everyone was talking about a petrol engine. My other regular forum is specific to my car's engine-make, so a lot of details are taken for granted which has made it a habit.
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Old 22nd June 2011, 01:14   #105
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Default Re: Does coasting save fuel?

Am trying to get to how this thread progressed and then asking for a summing up below.

Quote:
Originally Posted by Poitive View Post
It has been said a few times on this thread that the fuel supply is totally cut off when one is coasting downhill.

I read about engines a long time back and with respect to 2 stroke mototbikes. They used to need a 2T oil to keep the the cylinder/piston rings lubricated. Am not sure of this but, in 4stroke petrol engines, isn't the lubrication from the fuel itself? (In Diesel engines, I presume some of the engine oil is used for it).

If there is no fuel supply to the chamber (cylinder/piston rings), how does it get lubricated?
================


Vikram's explanation above takes care of the lubrication aspect as well.

Could some of the experts on the forum clarify if the fuel is indeed totally cut off?
The above is where a lot of debate started.

Quote:
Originally Posted by Poitive View Post
Thanks mate!
The whole thing is kind of complex for me unless I totally focus on it.
===================

Why I am asking is if it would hurt any car (Diesel, Petrol, Crdi, Mpfi, Carburetor based) if one was coasting, with regards to possible lack of lubrication.
This is where it got a bit too techie for some (or maybe many) of us.

Quote:
Originally Posted by vikram_d View Post
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.
http://www.team-bhp.com/forum/techni...ml#post2392914 (Does coasting save fuel?)

The above post is where a lot of issues were largely put to rest, technically.

Quote:
Originally Posted by Joe-Ker View Post
The pulse method works. and the coasting in neutral too.

Ant taxi driver worth his salt knows this.
Quote:
Originally Posted by pranavt View Post
Bus drivers and taxi walas are driving Diesel engines or petrol/cng with carbs. The discussion was focussed towards EFI since we are talking about injectors. There is no correlation between their vehicles and most of the private cars running on the road today.
Quote:
Originally Posted by vina View Post
Well cool down buddy, nothing in the thread title says "petrol only" - it is about whether coasting saves fuel. Why would you want to discriminate against us diesel-wallah guys?
Quote:
Originally Posted by dhanushs View Post
I think pranav was pointing out the difference between carbs and efi's, not petrols and diesels.
Quote:
Originally Posted by vina View Post
Oh OK - read his post again and I'm a little confused - did he mean "diesel with carb" because diesels always work with injectors (glow plugs) even if they are not common rail, and I believe even indicab engines are common rail.

Also Logan's diesel engine is definitely at least as modern as many of the other diesels around - why would Logan drivers' techniques not work?
Quote:
Originally Posted by pranavt View Post
Since the last few pages were about EFI specifically, I posted that. In a carbed engine, coasting the car in neutral would probably save more fuel than engine braking because there are no injectors to shut off.
In the above posts we got talking about different engines again.
===========================

Now, as seen in the first couple of posts, I was trying to get some focus on the different engines we could be considering. I largely stepped out of the discussion, as it was too techie for me.

Now can any of the Auto Techies (automobile engineers) please sum up what has been discussed in the thread for laymen like me. May I even suggest a format which may be useful to many.
  1. Background
  2. Definition/Understanding of the term Coasting
  3. Engine type : (Carburetor based Petrol, CRDI Diesel, etc etc)

    Driving Recommendations for
    • City
    • Highways
    • Hills
This could be repeated for the various types of engines/fuels.

Thanks mates. Hope we get to a useable conclusion of this thread (for various types of engine and fuel types).
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