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


Reply
 
Thread Tools Search this Thread
Old 8th October 2013, 21:41   #46
Distinguished - BHPian
 
Join Date: Oct 2011
Location: India
Posts: 9,364
Thanked: 13,330 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by ahmad.007 View Post
I mean to say regardless of the ratio, the engine will still put a load on the spinning of the wheels, no matter however small it may become.
I agree but when the transmission in on 4th or 5th gear being the overdrive gear the engine output is lower than the transmission output so what I want to say is that the transmission is connected to the wheels and in turn the output is high so the engine won't decelerate quick and won't need fuel to be alive but use the inertia that is produced by the transmission output. May be a short bursts are given to maintain the temperature but the supply will be lower as compared when shifting to Neutral gear.

Anurag.
a4anurag is online now   (1) Thanks Reply With Quote
Old 8th October 2013, 22:38   #47
BHPian
 
ahmad.007's Avatar
 
Join Date: Jun 2013
Location: Lucknow
Posts: 216
Thanked: 66 Times
Default

Quote:
Originally Posted by a4anurag View Post

I agree but when the transmission in on 4th or 5th gear being the overdrive gear the engine output is lower than the transmission output so what I want to say is that the transmission is connected to the wheels and in turn the output is high so the engine won't decelerate quick and won't need fuel to be alive but use the inertia that is produced by the transmission output. May be a short bursts are given to maintain the temperature but the supply will be lower as compared when shifting to Neutral gear.

Anurag.
Suppose I am doing 70 kmph. Engine is at 2000 rpm, 5th gear.
Accelerator is not being touched, but there is some fuel being fed. Say 0.5 units per revolution (my assumption may be wrong). So, at 2000 rpm, this comes to 0.5*2000=1000 units per sec (with engine breaking, also slowing down the vehicle much faster, i.e., covering lesser distance in same time, so more engine runtime).
Now I shift to neutral, rpm falls to 800. Now, for sustainence of the engine, the fuel needed is a little bit more (but generally at idle the mixture is very lean) say 1 unit per revolution (It may be more than that, I have not much knowledge, but I am just guessing). So the consumption comes out to be 800*1= 800 units per sec. And the vehicle is moving freely, which means I will deaccelerate slightly lesser, therefore reaching a certain distance faster and lesser engine runtime.
So which method uses lesser fuel? I need some authentic technical data for getting through this.
This is what runs through my mind sometimes. Feel free to bash me. :lol
ahmad.007 is offline   Reply With Quote
Old 8th October 2013, 23:41   #48
Distinguished - BHPian
 
Join Date: Oct 2011
Location: India
Posts: 9,364
Thanked: 13,330 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by ahmad.007 View Post
So which method uses lesser fuel? I need some authentic technical data for getting through this.
This is what runs through my mind sometimes. Feel free to bash me. :lol
Check this as I just read it in Wikipedia and the part I have mentioned in my earlier posts.

Quote:
Coasting or gliding

When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required
Source: http://en.wikipedia.org/wiki/Energy-efficient_driving

Check these two links also:

http://www.agcoauto.com/content/news/p2_articleid/215

http://www.thenakedscientists.com/fo...?topic=42659.0


In the second link when I read through and in between the posts I forgot why did I open that link, only after going to the second page I remembered the reason. In those two pages there is sight off-topic discussion too and the Senior members are also discussing OT. Point that I have to make here is the Quality of Team-BHP, its Mods and we members who stick to the topic and follow rules.

Anurag.
a4anurag is online now   Reply With Quote
Old 9th October 2013, 00:19   #49
BHPian
 
ahmad.007's Avatar
 
Join Date: Jun 2013
Location: Lucknow
Posts: 216
Thanked: 66 Times
Default

Quote:
Originally Posted by a4anurag View Post

Check this as I just read it in Wikipedia and the part I have mentioned in my earlier posts.

Source: http://en.wikipedia.org/wiki/Energy-efficient_driving

Look what I found here-

Quote:

Burn and coast

Burn and coast is also known as pulse and glide. This method consists of rapid acceleration to a given speed (the "burn" or "pulse"), followed by a period of coasting down to a lower speed, at which point the burn-coast sequence is repeated. [26] Coasting is most efficient when the engine is not running, although some gains can be realized with the engine on (to maintain power to brakes, steering and ancillaries) and the vehicle in neutral, or even with the vehicle remaining in gear [citation needed] . Most modern petrol vehicles cut off the fuel supply completely when coasting (over-running) in gear, although the moving engine adds considerable frictional drag and speed is lost more quickly than with the engine declutched from the drivetrain.

Causes of pulse-and-glide energy saving

Much of the time, automobile engines operate at only a fraction of their maximal efficiency, resulting in lower fuel economy (or what is the same thing, higher specific fuel consumption (SFC)). [27] Charts that show the SFC for every feasible combination of torque (or Brake Mean Effective Pressure) and RPM are called Brake specific fuel consumption maps. Using such a map, one can find the efficiency of the engine at various rpms, torques, etc.

During the pulse (acceleration) phase of pulse and glide, the efficiency is near maximal due to the high torque and much of this energy is stored as kinetic energy of the moving vehicle. This efficiently-obtained kinetic energy is then used in the glide phase to overcome rolling resistance and aerodynamic drag. In other words, going between periods of very efficient acceleration and gliding gives an overall efficiency that is usually significantly higher than just cruising at a constant speed. Computer calculations have predicted that in rare cases (at low speeds where the torque required for cruising at steady speed is low) it's possible to double (or even triple) fuel economy.

Very interesting. But its wikipedia, I am not too sure of the authenticity of the content.
ahmad.007 is offline   Reply With Quote
Old 9th October 2013, 01:36   #50
Distinguished - BHPian
 
Join Date: Oct 2012
Location: Delhi
Posts: 3,062
Thanked: 6,203 Times
Default

Quote:
Originally Posted by ahmad.007 View Post

Look what I found here-

Very interesting. But its wikipedia, I am not too sure of the authenticity of the content.
I'd say it is true. Its very simple to prove mathematically, or for real on a test track. Im not sure what value it has in real life. Apart from the constant accelerations and decelerations which are not very comfortable, cars operate in real traffic and you need to adjust speed to other cars, road conditions etc.

Jeroen
Jeroen is offline   (1) Thanks Reply With Quote
Old 9th October 2013, 11:25   #51
Senior - BHPian
 
DerAlte's Avatar
 
Join Date: Mar 2007
Location: Bangalore
Posts: 8,076
Thanked: 2,876 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by ahmad.007 View Post
Suppose I am doing 70 kmph. Engine is at 2000 rpm, 5th gear. ...
Sir, please first read and understand how Fuel Injection systems today work. The problem is in the model that you use to understand a concept or phenomenon. If that model is wrong, you would be going further and further away from whatever anyone gives you as inputs.

Your assumption here is that fuel uptake is a 'happening' due to crankshaft rotation (and consequent suction stroke pulling in fuel+air mix), but that is with carburettors where your logic might work. With electronically controlled injection systems that is a completely wrong mathematical model to work with. In these systems, the intelligence (ECU) doesn't allow any fuel to be injected when there is no torque demand (no load on engine from transmission). Due to emission considerations, the ECU does periodic low quantity injection every few strokes.

Quote:
Originally Posted by ahmad.007 View Post
Look what I found here ...
Seriously? Sure, I have done such funny experimentation on vehicle simulators - but I have kept the results at an arms length or derived vicarious amusement out of the same. Unfortunately driving in the real world with real engines is a different ball game - doesn't always follow regular mathematical algorithms. Try following an autorickshaw, I am sure you will understand the stochastic randomness that governs driving.
DerAlte is offline   Reply With Quote
Old 10th October 2013, 15:03   #52
BHPian
 
ahmad.007's Avatar
 
Join Date: Jun 2013
Location: Lucknow
Posts: 216
Thanked: 66 Times
Default

Well today I tested it, and found out that there is actually no fuel going to the engine when it is deaccelerating in gear. So turns out it is better to retain a gear rather than turning to neutral.
ThankYou DeAlte sir and others for providing helpful and authentic information.
ahmad.007 is offline   Reply With Quote
Old 10th October 2013, 15:19   #53
BHPian
 
::CMS::'s Avatar
 
Join Date: Sep 2013
Location: Bangalore
Posts: 571
Thanked: 462 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by ahmad.007 View Post
today I tested it, and found out that there is actually no fuel going to the engine when it is deaccelerating in gear
How did you test the fuel consumption during gliding in a pulse and glide driving method?
::CMS:: is offline   Reply With Quote
Old 10th October 2013, 15:23   #54
Distinguished - BHPian
 
saket77's Avatar
 
Join Date: Dec 2012
Location: Ranchi
Posts: 3,189
Thanked: 4,214 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by ahmad.007 View Post
Well today I tested it, and found out that there is actually no fuel going to the engine when it is deaccelerating in gear. So turns out it is better to retain a gear rather than turning to neutral.
ThankYou DeAlte sir and others for providing helpful and authentic information.
Would be interested to know how did you test it? Did you attach some diagnostic devices?
saket77 is offline   Reply With Quote
Old 10th October 2013, 16:28   #55
BHPian
 
HillMan's Avatar
 
Join Date: Jul 2013
Location: Bangalore
Posts: 698
Thanked: 401 Times
Default

Quote:
Originally Posted by SS-Traveller View Post
You need to take a closer look at Mahindra's MicroHybrid system, and you'll realize that (a) use of handbrake is not required to turn off the engine; (b) the engine cuts off 2-3 seconds (not 1 minute) after coming to a rolling stop with clutch pedal disengaged and shift in neutral.
I tried the micro hybrid option on a cousins W8. The switch off is not as quick as 2-3 second. It wasn't a detailed analysis. May be I wasn't doing it right for the system to kick off.

That brings me to a question. What happens to turbo cooling which the manufacturers recommend the engine to run for a while before turning off the engine.
HillMan is offline   Reply With Quote
Old 10th October 2013, 17:33   #56
BHPian
 
ahmad.007's Avatar
 
Join Date: Jun 2013
Location: Lucknow
Posts: 216
Thanked: 66 Times
Default

Quote:
Originally Posted by saket77 View Post

Would be interested to know how did you test it? Did you attach some diagnostic devices?
No, I am not that deep into technical stuff.
What I did was I took my Beat D to 50 kmph in 2nd gear and let go off the accelerator, the car started deaccelerating at a constant rate, then I turned the ignition to 'off' and there was absolutely no percievable difference in deacceleration or any sound coming from the engine. I turned it back on and both the speedo and tacho climbed to their respective positions without any jerk or anything, and the process was seamless.
I tried this multiple times in different speeds and rpms, and I only found jerks when the the ignition was turned on near idling rpms (>1000).
This I feel is sufficient for the conclusion I made earlier.
ahmad.007 is offline   Reply With Quote
Old 10th October 2013, 18:23   #57
Distinguished - BHPian
 
Join Date: Oct 2012
Location: Delhi
Posts: 3,062
Thanked: 6,203 Times
Default

Quote:
Originally Posted by ahmad.007 View Post

No, I am not that deep into technical stuff.
What I did was I took my Beat D to 50 kmph in 2nd gear and let go off the accelerator, the car started deaccelerating at a constant rate, then I turned the ignition to 'off' and there was absolutely no percievable difference in deacceleration or any sound coming from the engine. I turned it back on and both the speedo and tacho climbed to their respective positions without any jerk or anything, and the process was seamless.
I tried this multiple times in different speeds and rpms, and I only found jerks when the the ignition was turned on near idling rpms (>1000).
This I feel is sufficient for the conclusion I made earlier.
There is another thread on the forum where this is discussed in great detail. Injection based engines typically use more fuel idling then when leaving it in gear slowing down. the ECU essentially shuts down the fuel, save for the occasional little burst. If you have a multi information display that shows average fuel consumption and you would do the above test again you can observe that effect. There are a whole bunch of YouTube video out there of car owners testing this and coming to the same conclussion.
Jeroen

Quote:
Originally Posted by HillMan View Post

I tried the micro hybrid option on a cousins W8. The switch off is not as quick as 2-3 second. It wasn't a detailed analysis. May be I wasn't doing it right for the system to kick off.

That brings me to a question. What happens to turbo cooling which the manufacturers recommend the engine to run for a while before turning off the engine.
Nothing happens to the Turbo. The whole turbo cooling thing is, especially on these modern cars, is largely a myth. On a turbo car which has this start stop mechanism, obviously this has been taken into account and you wont find the recommendation to leave the engine idling for a while in the manual.

Jeroen

Last edited by aah78 : 10th October 2013 at 20:31. Reason: Posts merged. Please use QUOTE+ or "" (MULTI-QUOTE) when responding to multiple posts. Thanks!
Jeroen is offline   Reply With Quote
Old 10th October 2013, 19:36   #58
Newbie
 
Join Date: Mar 2013
Location: Coventry, UK
Posts: 5
Thanked: 3 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by Aroy View Post
The maximum FE is at the peak torque. As you move away from the peak torque (both up and down) the fuel consumption increases impacting FE.

Lower FE means that some fuel is left unburnt, so at very high or very low RPM where the torque is much below the peak torque, fuel will not be fully used up, hence some fuel will remain unburnt.

That said you can design engines with pretty flat torque over a wide RPM range or with torque peaking at higher RPM to produce more power at high speeds. Most of the heavy trucks have pretty narrow torque bands to increase power. The down side is that you need a large number of gears (at times 12 or more) to get a wide range of road speeds. In contrast earth moving machinery engines are designed with flatter torque to accommodate a wide range of engine speeds for powering accessories and implements.
Actually, I think that should be re-phrased. At peak torque (high load), you will have minimum fuel consumption i.e. Brake Specific Fuel Consumption (BSFC). But that does not translate to maximum FE for the car. The BSFC/fuel consumption is minimum since it is relative to the power produced at peak torque, which means that fuel consumed is less 'per kW' power, since the engine is efficiently producing high power (i.e. the engine efficiency is maximum in this region).

Now, this high power makes the vehicle go faster,and gives it good acceleration to surge ahead, but if you see the overall fuel economy of the vehicle (i.e. the vehicle efficiency), that will not be the best FE you can get from the vehicle (remember:aerodynamic losses also increase at higher speeds,gear ratios, etc also decides where/at what speed you can get the best FE in your car).

In short, it would be fair to assume that max FE will normally be at some point lower than at the peak torque engine speed, though this point will be different for every car.
auto.enthusiast is offline   Reply With Quote
Old 10th October 2013, 23:46   #59
BHPian
 
HillMan's Avatar
 
Join Date: Jul 2013
Location: Bangalore
Posts: 698
Thanked: 401 Times
Default Re: Do High RPMs result in unburnt fuel?

Quote:
Originally Posted by Jeroen View Post
Nothing happens to the Turbo. The whole turbo cooling thing is, especially on these modern cars, is largely a myth. On a turbo car which has this start stop mechanism, obviously this has been taken into account and you wont find the recommendation to leave the engine idling for a while in the manual.
The manual only suggests to turn off the engine in idling. Usually you are not in turbo while approaching your destination and parking the car, so you have already given turbo time to cool down. In this case Idling Rule may not be effective. No harm idling a little while after a enthusiastic or hard drive while you are occupied putting your Shades back into its pouch, disconnecting the phone from charger etc.

Micro-hybrid technology might be intelligent enough and the engine and electricals designed to handle the frequent start stop load for city driving.

Here is a good read on this topic; http://www.team-bhp.com/tech-stuff/i...o-charged-cars

Last edited by HillMan : 10th October 2013 at 23:49.
HillMan is offline   Reply With Quote
Old 11th October 2013, 00:09   #60
Distinguished - BHPian
 
Join Date: Oct 2011
Location: India
Posts: 9,364
Thanked: 13,330 Times
Default

Quote:
Originally Posted by ::CMS:: View Post
How did you test the fuel consumption during gliding in a pulse and glide driving method?
Quote:
Originally Posted by saket77 View Post
Would be interested to know how did you test it? Did you attach some diagnostic devices?
Quote:
Originally Posted by ahmad.007 View Post
No, I am not that deep into technical stuff.
Just tried checking for fuel supply when in gear and when in neutral add discussed in our earlier parts. Used my ELM 327 OBD II reader.

Kept the car constant at 80 kmph:
Name:  IMG20131010WA0010.jpg
Views: 245
Size:  91.2 KB

Kept the car constant at 100 kmph
Name:  IMG20131010WA0011.jpg
Views: 251
Size:  91.4 KB

At 80 kmph with my leg OFF the accelerator pedal
Name:  IMG20131010WA0012.jpg
Views: 244
Size:  90.4 KB

At 100 kmph with my leg OFF the accelerator pedal
Name:  IMG20131010WA0013.jpg
Views: 262
Size:  90.6 KB

In neutral and idling. Check the fuel flow per minute:
Name:  IMG20131010WA0014.jpg
Views: 277
Size:  90.7 KB

When in gear at off the accelerator pedal the dials read '0 cc/min' be on idle the dials give a reading of '10 cc/min'

Anurag.
Attached Images
 

Last edited by a4anurag : 11th October 2013 at 00:14.
a4anurag is online now   (2) Thanks Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Race Result - A1GP Zuhai - INDIA WINS!! MustangMan Int'l Motorsport 35 20th December 2007 00:38
Tyre Comfort - unexpected result from calculator comfortfreak Technical Stuff 10 29th October 2007 22:50
Fiat Spa Financial result 2007 - Growing strong Firebird The International Automotive Scene 1 29th October 2007 11:46
Which box do u recommend for this woofer ? Result will be compared ! low_bass_makker In-Car Entertainment 10 17th April 2006 10:13


All times are GMT +5.5. The time now is 03:19.

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