Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 15th June 2011, 18:20   #76
Senior - BHPian
 
Join Date: Dec 2010
Location: Ghaziabad/Hyderabad/Mysore
Posts: 1,416
Thanked: 313 Times
Default Re: Does coasting save fuel?

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.

Thanks - so the real reason some fuel would be required occasionally is to prevent the catcon damage by oxidation
vina is offline   Reply With Quote
Old 15th June 2011, 18:20   #77
Senior - BHPian
 
pranavt's Avatar
 
Join Date: May 2008
Location: Mumbai
Posts: 1,628
Thanked: 272 Times
Default Re: Does coasting save 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?

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.
Regarding your infinite mileage query, it's actually a yes. As long as the engine is rotating, the alternator is generating electricity; and since vacuum is created because of a closed throttle, power brakes will also work fine. Edit: Guess I never considered long deceleration runs. However, if we are talking about short periods of engine braking, the post still holds true.

Devices like scangauge and/or onboard displays on fancy cars show an instantaneous response, so hypothetically, if those conditions were maintained throughout your whole trip, it would actually give you whatever wacky figure they are showing.

I have a seperate toggle switch wired directly to my fuel pump. During engine braking, I have turned off the fuel pump through that switch and the car has not done anything different. My first paragraph explains why. As long as I turn on the fuel pump again, everything stays normal and the engine won't stall. This point is to explain to you that fuel is not NEEDED when "coasting". However, the injectors are shut off automatically anyways, so I don't have to resort to this fuel pump business.


Quote:
Originally Posted by supremeBaleno View Post
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.
On a flat road with no incline, it's very much possible. Speaking from experience.


Quote:
Originally Posted by vikram_d View Post
Can you also find out if no fuel is injected, then what are the spark plugs doing at this point of time?
They are firing. But no fuel is injected, so no combustion, so no question of cylinder temperatures. Just to go further. When you are bumping the engine off the revlimiter, the ECU cuts the fuel for a specified amount of time so the revs don't cross the limit. You don't see engines exploding immediately. Same reason.

Last edited by pranavt : 15th June 2011 at 18:21.
pranavt is offline   Reply With Quote
Old 15th June 2011, 18:22   #78
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

Pranav, buddy. Read my last post on page number 5. I have accepted that we were all correct in our beliefs. It just so happens that the ECU is smarter than us. LOL. Peace bro.

Edit: Read it. . Always good to have a nice end to a constructive debate.

Last edited by vikram_d : 15th June 2011 at 18:26.
vikram_d is offline   (1) Thanks Reply With Quote
Old 15th June 2011, 18:23   #79
Senior - BHPian
 
pranavt's Avatar
 
Join Date: May 2008
Location: Mumbai
Posts: 1,628
Thanked: 272 Times
Default Re: Does coasting save fuel?

You posted when I was composing my post. Read my post, I edited it slightly.
pranavt is offline   Reply With Quote
Old 15th June 2011, 18:35   #80
Senior - BHPian
 
Join Date: Dec 2010
Location: Ghaziabad/Hyderabad/Mysore
Posts: 1,416
Thanked: 313 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
Pranav, buddy. Read my last post on page number 5. I have accepted that we were all correct in our beliefs. It just so happens that the ECU is smarter than us. LOL. Peace bro.

Edit: Read it. . Always good to have a nice end to a constructive debate.

One correction (important for those who plan to use this method to save fuel):

ECU will shut off fuel supply only as long as rpm is somewhat higher (200-400 depending on the car) than the idling rpm for the engine. If the rpm falls below (or was already below) the min. rpm - the ECU will maintain the min. rpm.

If somebody is trying to save fuel while slowing down - this is a good point to downshift. This will not effect going downhill at a constant high rpm.
vina is offline   (1) Thanks Reply With Quote
Old 15th June 2011, 18:53   #81
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
One correction (important for those who plan to use this method to save fuel):

ECU will shut off fuel supply only as long as rpm is somewhat higher (200-400 depending on the car) than the idling rpm for the engine. If the rpm falls below (or was already below) the min. rpm - the ECU will maintain the min. rpm.

If somebody is trying to save fuel while slowing down - this is a good point to downshift. This will not effect going downhill at a constant high rpm.
Minor correction to this point. The RPM range mentioned by should be 600 - 900 RPM above idling range. In the petrol swift the car idles at 800 PRM and the ECU resumes normal operation from deceleration from 1500RPM. In the Mondeo it starts normal duty at 1750 RPM where as it idles at 750 RPM. The SX4 also resumes normal duty at 1500 RPM and idles at 750 - 800 RPM. Am sure about these 3 vehicles because I own them and have experience with them.
vikram_d is offline   Reply With Quote
Old 17th June 2011, 00:29   #82
BHPian
 
Join Date: Feb 2009
Location: KL41-MH14
Posts: 79
Thanked: 18 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
True, the IAC cannot inject fuel. But when you are coasting in gear the engine is still kept running by using air from the IAC and very tiny amounts of fuel is still being sent in to the engine through the injectors. I had a wideband on my turbo swift and I know what happens. Have done umpteen number of tests on how the stock ECU behaves while coasting in gear.

On my swift while coasting in gear the AFR would go down to 17:1 and as soon as the car went below 1500 rpm, the ECU would resume its normal duty cycle and bring the AFR to about 14.5 - 15:1. Note these readings are without the throttle being applied. As soon as you step on the throttle the ECU would maintain a duty cycle of about 13 - 14:1. And I am talking about the stock ECU here and not the aftermarket one that I had on my car also. Oh and on idle with the stock ECU the AFR would 15.5 - 16:1.

Based on these readings I am very sure that at least in the Swift petrol coasting in gear is more economical than coasting in neutral and I am also pretty positive that this is true for most modern cars.


But according to what you are saying

Coasting will invariably at a higher engine rpm than idle rpm..Lets say 2000 avg. and idle rpm is 800-900rpm.

So while coasting in gear
in a minute it will use fuel at 17:1 2000 times

While coasting in neutral/ clutch pedal depresed
in a minute it will use fuel at 15.5 or 16:1 800 times.

Definitely the second case saves more fuel!!

Why has no one considered this aspect? So if i understand corectly, Fuel consumption is also a function of engine rpm. slightly Lower ratio at higher rpm still consumes more fuel than a slightly higher AF ratio at much lower rpm.

And i think i still hear the firing sound from a coasting engines...if fuel was fully cut off, would it not be very silent (cos of no detonation in the engine)

I am not considering the safety aspect here..just the FE

Last edited by Joe-Ker : 17th June 2011 at 00:43.
Joe-Ker is offline   Reply With Quote
Old 17th June 2011, 12:17   #83
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by Joe-Ker View Post
But according to what you are saying

Coasting will invariably at a higher engine rpm than idle rpm..Lets say 2000 avg. and idle rpm is 800-900rpm.

So while coasting in gear
in a minute it will use fuel at 17:1 2000 times

While coasting in neutral/ clutch pedal depresed
in a minute it will use fuel at 15.5 or 16:1 800 times.

Definitely the second case saves more fuel!!

Why has no one considered this aspect? So if i understand corectly, Fuel consumption is also a function of engine rpm. slightly Lower ratio at higher rpm still consumes more fuel than a slightly higher AF ratio at much lower rpm.

And i think i still hear the firing sound from a coasting engines...if fuel was fully cut off, would it not be very silent (cos of no detonation in the engine)

I am not considering the safety aspect here..just the FE
Good point. Never thought about it like this. But like we have already proved, that most modern vehicles shut off fuel completely while coasting in gear at high rpm and only inject very small amounts at periodic intervals to prevent cat con oxidation. Where as in idling the fuel is being continuosly injected to keep the engine running.

Having said that I firmly believe that the fuel saved by coasting in gear is a very small amount unless you happen to be going down my imaginary never ending downhill slope.

Last edited by vikram_d : 17th June 2011 at 12:19.
vikram_d is offline   Reply With Quote
Old 17th June 2011, 15:01   #84
Senior - BHPian
 
Join Date: Dec 2010
Location: Ghaziabad/Hyderabad/Mysore
Posts: 1,416
Thanked: 313 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post

Having said that I firmly believe that the fuel saved by coasting in gear is a very small amount unless you happen to be going down my imaginary never ending downhill slope.

Long ago I read an article on some people who routinely get double the usual FE from their vehicles. Coasting was one of the biggest parts of their tricks (the others are deliberately slow pickup, very active anticipation to avoid using brakes almost completely, never going faster than the sweet-spot for their vehicles)

It does save fuel - just search on the internet.
vina is offline   Reply With Quote
Old 17th June 2011, 15:09   #85
dot
Senior - BHPian
 
dot's Avatar
 
Join Date: Dec 2007
Location: Ban Chang
Posts: 1,681
Thanked: 714 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
Long ago I read an article on some people who routinely get double the usual FE from their vehicles. Coasting was one of the biggest parts of their tricks (the others are deliberately slow pickup, very active anticipation to avoid using brakes almost completely, never going faster than the sweet-spot for their vehicles)

It does save fuel - just search on the internet.
I was reading such articles too. It seems that these folks coast in neutral or sometimes even switch off engines!! I dont think we discussed the same thing in the last few pages.

http://www.metrompg.com/posts/pulse-and-glide.htm

OT: If you ask me, these types of fuel saving tricks are paranoid in nature.

Last edited by dot : 17th June 2011 at 15:22.
dot is offline   Reply With Quote
Old 17th June 2011, 16:54   #86
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
Long ago I read an article on some people who routinely get double the usual FE from their vehicles. Coasting was one of the biggest parts of their tricks (the others are deliberately slow pickup, very active anticipation to avoid using brakes almost completely, never going faster than the sweet-spot for their vehicles)

It does save fuel - just search on the internet.
I agree, but (sadly there is a but here..lol) I am talking about real world driving scenarios and not about scenarios where people go to extreme lengths the get high FE figures like the ones that you have mentioned.

For eg. You will gain some mileage advantage only if the downhill slope is a fairly long one and it is followed by a plain flat road. If the descent ends in a trough and you have to climb back uphill, then any advantage gained by coasting will be lost as soon as you step on the throttle to go uphill.
vikram_d is offline   Reply With Quote
Old 17th June 2011, 17:10   #87
BHPian
 
mtnrajdeep's Avatar
 
Join Date: Feb 2006
Location: Bangalore
Posts: 648
Thanked: 88 Times
Default Re: Does coasting save fuel?

I was checking the instantaneous mileage in the Ritz D. To my surprise, I found that on coasting, the instantaneous mileage used to shoot up and show in the range of 33-40 kmpl whereas when in gear, it never went up more than 12 kmpl.
mtnrajdeep is online now   Reply With Quote
Old 17th June 2011, 17:40   #88
Senior - BHPian
 
Join Date: Dec 2010
Location: Ghaziabad/Hyderabad/Mysore
Posts: 1,416
Thanked: 313 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
I agree, but (sadly there is a but here..lol) I am talking about real world driving scenarios and not about scenarios where people go to extreme lengths the get high FE figures like the ones that you have mentioned.

I was talking about guys in California who get it in regular driving conditions in regular (no special mods) cars. There was a feature in one of IEEE magazines or some such place. I have forgotten where and will need to dig it up.

Another technique they used that I forgot to mention was (I don't know what they call it) to accelerate up (say from 55 to 60) and then coast down (60 to 55) then accelerate up, then coast down .... They do it on flat stretches of long relatively straight highways.

I didn't figure how that would work till in this post http://www.team-bhp.com/forum/owning...ml#post2391681 (ARTICLE: How to get the maximum Fuel Efficiency) somebody posted a technical link showing that at a given rpm the engine is more efficient when it is generating more power. Since during cruising little power is needed, one way to use this fact is to accelerate - convert energy from fuel to KE at a high efficiency (higher than normal cruising) - store that KE in your vehicle (going from 50kmph to 60kmph increases the KE of the vehicle by almost 50% !) and then coast down to 50 while the vehicle consumes zero fuel.



By the way, the experts on engine control may pitch in to tell whether this is correct - but for diesels your car may already be coasting on its own without telling you when you are trying to cruise at constant speed. The pedal controls the fuel injection via the ECU and the ECU modulates the amount of fuel to be injected based on (its own interpretation of) what the driver is trying to do. ECU figures this out on the basis of activity on TPS sensor and its own algorithms.

So in theory at least ECU can by itself cut fuel in a few cycles when it deems more power to be unnecessary. It can already reduce the amount of fuel injected by reducing the time injectors are turned on (more complicated stuff like multiple injections etc. are also possible). But if the ECU programmers figure that best is to skip a stroke altogether then that can be done too.


In a car that is already implementing such techniques I don't think you'll save anything on top of ECU's own savings by deliberate coasting.
vina is offline   (1) Thanks Reply With Quote
Old 17th June 2011, 18:20   #89
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

Well, your point completely makes sense if you are doing this on a relatively flat highway in India. in city driving conditions I dont think it will be of any help at all. Moreover in other countries you have something called traffic sense and discipline. But you know that we in India seriously lack it.

Oh and the technique you are talking about is called Pulse and Glide. Dot has posted a link to it a couple of posts above.

Edit: Will try this technique on my next highway drive and post results.

Last edited by vikram_d : 17th June 2011 at 18:23.
vikram_d is offline   (1) Thanks Reply With Quote
Old 17th June 2011, 18:43   #90
Senior - BHPian
 
Join Date: Dec 2010
Location: Ghaziabad/Hyderabad/Mysore
Posts: 1,416
Thanked: 313 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
Well, your point completely makes sense if you are doing this on a relatively flat highway in India. in city driving conditions I dont think it will be of any help at all. Moreover in other countries you have something called traffic sense and discipline. But you know that we in India seriously lack it.

Oh and the technique you are talking about is called Pulse and Glide. Dot has posted a link to it a couple of posts above.

Edit: Will try this technique on my next highway drive and post results.

Last year my cousin was driving me around Bay Area and he was cursing city driving. I was thinking all the while "what is you problem - speedo shows over 30mph"

Highway driving was similar to what we get on DND flyover in Noida after midnight - no complaints except that the experience lasts barely a couple of minutes .

Point being - in their highway condition you can definitely coast as well as speed-and-coast easily. In their city conditions it will be possible many (if not most) times.

In our condition unfortunately it is one foot on the clutch and, if you are going up an incline, the other one on the pedal. There is no way you can save fuel.
vina is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
I save lives, and cars save me! Glory Glory Team-BHP Starfire Introduce yourself 5 6th August 2014 23:23
Coasting in an automatic car Mr_Bean Technical Stuff 1 18th December 2013 17:40
Does clutching save fuel or does more harm than good? skumar69 Technical Stuff 1 5th November 2008 17:55
ScanGuage II - Does it really help save fuel? adrena Technical Stuff 4 29th June 2007 09:17


All times are GMT +5.5. The time now is 18:41.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks