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


Reply
 
Thread Tools Search this Thread
Old 22nd June 2011, 01:29   #106
Distinguished - BHPian
 
dhanushs's Avatar
 
Join Date: Oct 2009
Location: Bathery/BLR
Posts: 3,446
Thanked: 4,055 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by Poitive View Post
Hope we get to a useable conclusion of this thread (for various types of engine and fuel types).
Potive, be it carb, efi, TC'd, or be it petrol or diesel, or be it city or highway or hills: I would not recommend coasting in neutral. Please ALWAYS drive in gear, whatever be the case. Unless your clutch is shot.
dhanushs is offline   Reply With Quote
Old 22nd June 2011, 01:41   #107
BHPian
 
Poitive's Avatar
 
Join Date: Apr 2011
Location: 3rdRockFmTheSun
Posts: 850
Thanked: 280 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by dhanushs View Post
Potive, be it carb, efi, TC'd, or be it petrol or diesel, or be it city or highway or hills: I would not recommend coasting in neutral. Please ALWAYS drive in gear, whatever be the case. Unless your clutch is shot.
Thanks for the reply dhanushs.

The only time I really 'coast' in neutral is when I am about to approach a traffic signal and can see from a distance that it is red. It was more about how different engines may behave with regards to fuel consumption in various conditions.

Also not sure if one would save fuel while 'coasting' in neutral or in gear in the above case in various types of engines.
Poitive is offline   Reply With Quote
Old 22nd June 2011, 01:56   #108
Distinguished - BHPian
 
dhanushs's Avatar
 
Join Date: Oct 2009
Location: Bathery/BLR
Posts: 3,446
Thanked: 4,055 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by Poitive View Post
It was more about how different engines may behave with regards to fuel consumption in various conditions.

Also not sure if one would save fuel while 'coasting' in neutral or in gear in the above case in various types of engines.
While coasting in gear, otherwise known as overrun (When torque demand is zero, and torque is being supplied to the engine, not vice versa) fuel is cut off completely. This is the case in most ECM controlled vehicles, till a certain rpm, after which fuel supply is restored. Hence it DOES save fuel than coasting in neutral. There are certain exceptional cases whist overrun, fuel is supplied, like to prevent oxidization of cat, to control temp, and may be to clear dfp.

Now, in carb engines, idle amount of fuel is always supplied.

As for driving recommendation, I have mentioned in my previous post.

Last edited by dhanushs : 22nd June 2011 at 01:59. Reason: Changed 'ALL' to 'most' as I was unsure if ALL ECM's cut fuel.
dhanushs is offline   (2) Thanks Reply With Quote
Old 24th June 2011, 12:45   #109
Distinguished - BHPian
 
supremeBaleno's Avatar
 
Join Date: Dec 2005
Location: Weekdays@Chennai, Weekends@Kerala
Posts: 5,164
Thanked: 1,638 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by pranavt
On a flat road with no incline, it's very much possible. Speaking from experience.
I tried this the other day on my Baleno on a long & relatively empty road. Went up to 60kmph in 5th gear and took foot off the throttle - speed started reducing gradually to almost 20kmph, after which I had to shift down to 3rd to avoid knocking. Given this, I doubt your stock OHC would continue coasting in 5th gear, like the diesels do.

Quote:
Originally Posted by pranavt
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.
Had a query here. The last time I bumped off the rev-limiter many years ago, the car went limp suddenly (ie. it was not gradual deceleration). Since you used the same analogy for fuel-cutoff while coasting, should the car not go limp the moment fuel was cut-off on ECU recognising coasting? But since we don't see such behaviour while coasting, I have a feeling that minute amount of fuel still gets supplied and burnt, though maybe less than what is consumed at idling.
supremeBaleno is offline   Reply With Quote
Old 24th June 2011, 15:51   #110
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 supremeBaleno View Post
I tried this the other day on my Baleno on a long & relatively empty road. Went up to 60kmph in 5th gear and took foot off the throttle - speed started reducing gradually to almost 20kmph, after which I had to shift down to 3rd to avoid knocking. Given this, I doubt your stock OHC would continue coasting in 5th gear, like the diesels do.
Why wouldn't it? it would keep revving at idle RPM, and as long as the load isn't very high, it could keep doing it all day.

Quote:
Originally Posted by supremeBaleno View Post
Had a query here. The last time I bumped off the rev-limiter many years ago, the car went limp suddenly (ie. it was not gradual deceleration). Since you used the same analogy for fuel-cutoff while coasting, should the car not go limp the moment fuel was cut-off on ECU recognising coasting? But since we don't see such behaviour while coasting, I have a feeling that minute amount of fuel still gets supplied and burnt, though maybe less than what is consumed at idling.
The car does not go limp because the torque from the wheels is spinning the engine. When bumping off the redline, you feel the car go limp because it has been accelerating for a while and the power is cut off completely, giving you that feeling of hitting a wall. Enough proof, including citations to technical drawings and write-ups have been provided in the last few pages. There is enough reading material on the internet if you actually want to confirm the facts instead of just relying on your gut feeling.
pranavt is offline   Reply With Quote
Old 24th June 2011, 17:47   #111
Distinguished - BHPian
 
supremeBaleno's Avatar
 
Join Date: Dec 2005
Location: Weekdays@Chennai, Weekends@Kerala
Posts: 5,164
Thanked: 1,638 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by pranavt
Why wouldn't it? it would keep revving at idle RPM, and as long as the load isn't very high, it could keep doing it all day.
I doubt idle-rpm (750rpm in my case) in a petrol car would be enough to keep the car running in 5th gear. As I mentioned, the speed dropped to almost 20kmph and if I had not downshifted, the engine would have knocked and died due to being unable to support 5th gear at that rev/speed. Next time you get a chance to try this out, do post the results with your car.

Quote:
Originally Posted by pranavt
The car does not go limp because the torque from the wheels is spinning the engine. When bumping off the redline, you feel the car go limp because it has been accelerating for a while and the power is cut off completely, giving you that feeling of hitting a wall.
If the car has been acclerating for a while before fuel cutoff at redline, should not the torque from the wheels keep spinning the engine (like it does during coasting) instead of going limp ? Maybe the 2 scenarios are different & not comparable.

Quote:
Originally Posted by pranavt
Enough proof, including citations to technical drawings and write-ups have been provided in the last few pages. There is enough reading material on the internet if you actually want to confirm the facts instead of just relying on your gut feeling.
Thanks, will have a look - I had forgotten to subscribe to this thread, hence missed the action after my last post some days ago.
supremeBaleno is offline   (1) Thanks Reply With Quote
Old 24th June 2011, 19:23   #112
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 supremeBaleno View Post
I doubt idle-rpm (750rpm in my case) in a petrol car would be enough to keep the car running in 5th gear. As I mentioned, the speed dropped to almost 20kmph and if I had not downshifted, the engine would have knocked and died due to being unable to support 5th gear at that rev/speed. Next time you get a chance to try this out, do post the results with your car.
I have a diesel, so it may not be relevant here - but I have seen that if the car is already moving then if I take foot off the accelerator, the speed reduces slowly (in high gears VERY slowly) down to the point where idling rpm (roughly 800 on my Figo) us achieved, and then speed doesn't go down further. - Though car does make complaining noises as if it is about to stall, it doesn't really stall.

Quote:
Originally Posted by supremeBaleno View Post

If the car has been acclerating for a while before fuel cutoff at redline, should not the torque from the wheels keep spinning the engine (like it does during coasting) instead of going limp ? Maybe the 2 scenarios are different & not comparable.
Going limp means what? If it means your car stops immediately (or sees heavy engine braking) - you need to get your car checked or may be the ECU initiated braking to reduce your rpm after you hit the redline.

On the other hand if by "limp" you mean you experience loss of power - that is what is supposed to happen when fuel supply is cut. Even when I try coasting, there is no power, only the momentum of the car keeps it going.

Quote:
Originally Posted by supremeBaleno View Post

Thanks, will have a look - I had forgotten to subscribe to this thread, hence missed the action after my last post some days ago.

vina is offline   Reply With Quote
Old 24th June 2011, 20:31   #113
Distinguished - BHPian
 
supremeBaleno's Avatar
 
Join Date: Dec 2005
Location: Weekdays@Chennai, Weekends@Kerala
Posts: 5,164
Thanked: 1,638 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina
I have a diesel, so it may not be relevant here - but I have seen that if the car is already moving then if I take foot off the accelerator, the speed reduces slowly (in high gears VERY slowly) down to the point where idling rpm (roughly 800 on my Figo) us achieved, and then speed doesn't go down further. - Though car does make complaining noises as if it is about to stall, it doesn't really stall.
The 2 diesels I drove (Punto & Vento) will keep running till they run out of fuel without any A-input. And also no complaining noises about stalling, that you see on the Figo. With the Vento, the speed stays at 50kmph and on the Punto IIRC it was 40kmph - does not go below this.

Quote:
Originally Posted by vina
Going limp means what? If it means your car stops immediately (or sees heavy engine braking) - you need to get your car checked or may be the ECU initiated braking to reduce your rpm after you hit the redline.
It's almost like an ultra-quick deceleration - very contrasting compared to the quick acceleration achieved when one redlines a gear. I believe this is normal behaviour - maybe ECU induced.
supremeBaleno is offline   Reply With Quote
Old 25th June 2011, 13:29   #114
Senior - BHPian
 
vikram_d's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 2,380
Thanked: 936 Times
Default Re: Does coasting save fuel?

I think the jerking caused at redline is more because of the sudden reduction of ignition timing as well as sudden fuel cut off.

Where as while you are coasting the timing gets reduced but not so drastically.
vikram_d is offline   Reply With Quote
Old 25th June 2011, 14:18   #115
Senior - BHPian
 
Rahulkool's Avatar
 
Join Date: Oct 2007
Location: Chennai
Posts: 1,265
Thanked: 568 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by supremeBaleno View Post
I doubt idle-rpm (750rpm in my case) in a petrol car would be enough to keep the car running in 5th gear. As I mentioned, the speed dropped to almost 20kmph and if I had not downshifted, the engine would have knocked and died due to being unable to support 5th gear at that rev/speed. Next time you get a chance to try this out, do post the results with your car.

If the car has been acclerating for a while before fuel cutoff at redline, should not the torque from the wheels keep spinning the engine (like it does during coasting) instead of going limp ? Maybe the 2 scenarios are different & not comparable.

Thanks, will have a look - I had forgotten to subscribe to this thread, hence missed the action after my last post some days ago.
I can go till 5th gear in my palio without touching the accelerator , this is with AC on, havent tried it without AC.
Rahulkool is offline   Reply With Quote
Old 25th June 2011, 19:16   #116
Distinguished - BHPian
 
supremeBaleno's Avatar
 
Join Date: Dec 2005
Location: Weekdays@Chennai, Weekends@Kerala
Posts: 5,164
Thanked: 1,638 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by pranavt
Why wouldn't it? it would keep revving at idle RPM, and as long as the load isn't very high, it could keep doing it all day.
My experiment yesterday seems to prove you both right & wrong.

Right because, I tried to coast in 4th gear yesterday after hitting 50kmph and the speed slowly dipped to 20kmph. I thought it would knock and die, but decided to check how low it would go. It retained the speed of 20kmph in 4th gear and kept ambling, like you said.

Wrong because, it was not at idle rpm. When it was ambling at 20kmph without A-input, the rpm was like 400rpm (idle is 750rpm).

I did not get a long enough stretch to try 5th gear, but after this experiment, I am guessing the car will still amble along at ~20kmph in 5th gear without A-input at same 400rpm or so.

Quote:
Originally Posted by Rahulkool
I can go till 5th gear in my palio without touching the accelerator
Rahul, I did not understand. Do you mean that you can go from 1st to 5th without using the accelerator ? That's another new experiment for me to try out.
supremeBaleno is offline   Reply With Quote
Old 27th June 2011, 11:32   #117
Distinguished - BHPian
 
dhanushs's Avatar
 
Join Date: Oct 2009
Location: Bathery/BLR
Posts: 3,446
Thanked: 4,055 Times
Default Re: Does coasting save fuel?

Quote:
Originally Posted by supremeBaleno View Post
I did not get a long enough stretch to try 5th gear, but after this experiment, I am guessing the car will still amble along at ~20kmph in 5th gear without A-input at same 400rpm or so.
No, it would amble along at idle rpm. Another way to try this is to try and accelerate the car using idle rpm. ie, using only clutch. It would take a lot of time and patience, but still, you will be able to reach idle rpm in 5th gear.

The jerks felt while hitting the revv limiter is the loss of acceleration. You can feel the same by just taking your foot off the pedal near to the redline. While coasting, the car experiences little change in acceleration.
dhanushs is offline   Reply With Quote
Old 27th June 2011, 12:26   #118
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 dhanushs View Post
No, it would amble along at idle rpm. Another way to try this is to try and accelerate the car using idle rpm. ie, using only clutch. It would take a lot of time and patience, but still, you will be able to reach idle rpm in 5th gear.

The jerks felt while hitting the revv limiter is the loss of acceleration. You can feel the same by just taking your foot off the pedal near to the redline. While coasting, the car experiences little change in acceleration.
@supreme reported that in 4th his car coasts at 400rpm


May be the idle rpm when the car is moving with clutch engaged is sligthly lower? I mean the car acts as an "extra flywheel" even when it is going at a constant speed.


In my Figo, as long as clutch is engaged, the car resists going below idle rpm (e.g. while engine braking + brakes I can see that once the rpm is near idle, the braking is less than with brakes alone - engine is not only not braking, it is trying to make car move)
vina is offline   Reply With Quote
Old 27th June 2011, 12:50   #119
Senior - BHPian
 
Fordmanchau's Avatar
 
Join Date: Jul 2009
Location: Hyderabad
Posts: 1,651
Thanked: 291 Times
Default Re: Does coasting save fuel?

Guys, thanks to discussion here, I have been practising COASTING to save fuel.
As mentioned here,I am letting go of the A-pedal at any incline, minimised the use of brakes by slowing down ahead of traffic signal,etc. This way I managed to bump up my city mileage from a poor 7.8kmpl to a decent 10.3 kmpl. Although, all of this would not be because of new driving practices but still a fair amount of credit goes to the new technique.

Other thing, I am doing is when I see a decent open stretch, straight or incline, I take the foot off the pedal and move to higher gears. Like , if I am on 2nd or 3rd, I lift my foot of the A-pedal change to 3rd, 4th or 5th gear, if there's no traffic ahead.

I have observed that this helps to maintain the momentum for a little long.

This weekend I drove to Srisailam and had enough of straight and incline stretches with no traffic. I used to accelerate till 80kmph at 4th gear, lift off the foot from A-pedal and then move to 5th.
The rpm slowly dropped to 2k at 5th to 1.5k till 60kmph and that is when I used to pedal again to maintain good momentum.
I managed a decent 16.7kmpl of pure highway mileage. 450kms from 27litres of fuel. and overall mileage of 14.75kmpl from 34.5 litres of fuel.

I have seen these figures before, but these practices helped me complete the trip without refuelling.
Fordmanchau is offline   Reply With Quote
Old 27th June 2011, 13:06   #120
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 Fordmanchau View Post
.... This way I managed to bump up my city mileage from a poor 7.8kmpl to a decent 10.3 kmpl.

...
I managed a decent 16.7kmpl of pure highway mileage. 450kms from 27litres of fuel. and overall mileage of 14.75kmpl from 34.5 litres of fuel.

...
Are these numbers for petrol vehicle? In that case pretty good I would say.
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 10:18.

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