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


Reply
 
Thread Tools Search this Thread
Old 17th May 2006, 08:48   #46
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 22,367
Thanked: 22,583 Times
Default

Vivekiny2k,

If i had to guess i would say that its due to the fact that gears other than "D" on auto trannys provide engine breaking.

Maybe you should bring up autobox questions in the thread i had provided a link to in one of my earlier posts. A similar discussion was had there.

cya
R
Rehaan is offline   Reply With Quote
Old 17th May 2006, 13:08   #47
Senior - BHPian
 
Join Date: Dec 2004
Location: pune
Posts: 2,088
Thanked: 48 Times
Default

Quote:
Originally Posted by vivekiny2k
so it's useless for me (and @2fast4u) until i actually slam the brakes someday.
Gentleman, you need your brakes (and ABS) most at that very instant. So, it's not as useless as you think. No one crashes everyday

@Rehaan, a question for you! In this thread and earlier as well you have mentioned that taking your foot off accelerator will cut off fuel supply. Does it mean 0 fuel supply or fuel supply as required for only idling the engine? I feel it will be the later case, but then I can be wrong.
RX135 is offline   Reply With Quote
Old 17th May 2006, 13:25   #48
Distinguished - BHPian
 
supremeBaleno's Avatar
 
Join Date: Dec 2005
Location: Weekdays@Chennai, Weekends@Kerala
Posts: 5,164
Thanked: 1,636 Times
Default

Exactly what I also wanted to ask, @RX135.
supremeBaleno is offline   Reply With Quote
Old 17th May 2006, 22:56   #49
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 22,367
Thanked: 22,583 Times
Default

RX135 & SupremeBaleno,

As i have said -
Quote:
Originally Posted by Rehaan
When you are in neutral or have the clutch pressed, the car has to keep fuel going to the engine in order to keep it running. This uses fuel, naturally.
When you are in gear, and you take your foot off the accelerator, modern cars (which are fuel injected and pretty intelligent) sense the decelerating load on the engine and CUTS ALL FUEL SUPPLY TO THE ENGINE. This uses NO fuel. The only thing keeping the engine spinning is the rolling of the tires transfering power back up the drivetrain, keeping the engine spinning. And at the same time you are getting engine braking! Talk about a sweet deal.
When your car has enough momentum going to keep the engine turning when you take your foot off the accelerator (AND you are in gear without the clutch pressed), no fuel is injected into the engine, as there is no need for it to keep the engine spinning since its the force being transfered back from the wheels that is spinning the engine.
If the clutch was pressed or the car was in neutral, the fuel would be required to keep the engine spinning.

Hope thats clearer?
cya
R
Rehaan is offline   Reply With Quote
Old 17th May 2006, 23:45   #50
Senior - BHPian
 
Gordon's Avatar
 
Join Date: Feb 2004
Location: Mumbai
Posts: 2,420
Thanked: 162 Times
Default

Quote:
Originally Posted by Rehaan
no fuel is injected into the engine
Is this possible?!
If there is absolutely no fuel supplied => engine is stalled/off.
Gordon is offline   Reply With Quote
Old 17th May 2006, 23:45   #51
Senior - BHPian
 
Join Date: Dec 2004
Location: pune
Posts: 2,088
Thanked: 48 Times
Default

Quote:
Originally Posted by Rehaan
RX135 & SupremeBaleno,

As i have said -


When your car has enough momentum going to keep the engine turning when you take your foot off the accelerator (AND you are in gear without the clutch pressed), no fuel is injected into the engine, as there is no need for it to keep the engine spinning since its the force being transfered back from the wheels that is spinning the engine.
If the clutch was pressed or the car was in neutral, the fuel would be required to keep the engine spinning.

Hope thats clearer?
cya
R
Thanks Rehaan! I had understood about that wheel running the engine part and also the logic behind cutting off the fuel supply as you mentioned. I was just curious to know, what happens when NO fuel is supplied to the cylinders. Do the cylinders fire (since there is nothing to fire without fuel)? Does it change or should it change the exhaust note?

Last edited by RX135 : 17th May 2006 at 23:47.
RX135 is offline   Reply With Quote
Old 18th May 2006, 00:10   #52
Senior - BHPian
 
vivekiny2k's Avatar
 
Join Date: Feb 2005
Location: cincinnati, jabalpur,chennai
Posts: 1,241
Thanked: 163 Times
Default

Quote:
Originally Posted by Rehaan
When your car has enough momentum going to keep the engine turning when you take your foot off the accelerator (AND you are in gear without the clutch pressed), no fuel is injected into the engine, as there is no need for it to keep the engine spinning since its the force being transfered back from the wheels that is spinning the engine.
R
I have my doubt on that. my car keeps rolling (as if in cruise control). There is slight deceleration but very less. it keeps running until i actually try to stop it.
I think it's same as putting it in 'D' from stanstill but not pressing the pedal. some amount of fuel is injected to keep the car just crawling.
vivekiny2k is offline   Reply With Quote
Old 26th January 2007, 11:28   #53
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Im not sure about the fuel cut-off part. I think the fuel supply is reduced but not cut off completely.

When you take your foot off the acc, the throttle valve closes and air is sucked into the cylinder only through the idling vent. So the engine isn't producing enuf torque/power for the car to accelelrate any further, this causes resistance (engine braking).

If fuel is cut off completely, there shouldn't be any exhaust note while decelerating?

Can some of the tuners clear this confusion?

Shan2nu
Shan2nu is offline   Reply With Quote
Old 26th January 2007, 14:14   #54
Team-BHP Support
 
theMAG's Avatar
 
Join Date: May 2006
Location: Long Island, NY
Posts: 7,033
Thanked: 1,470 Times
Default

Quote:
Originally Posted by netarchie View Post
Hi,
I tried searching the other threads before posting this query, but could not find what i was looking for.
here's my query..
While driving at a certian speed say above 60 - 70 Kms/ hour, when i need to slow down my car , if suppose i am approaching a red light/Speed breaker, i usually shift my gears to the neutral , directly from the 5th or 4th geat and break genlty before i come to a comple halt or the speed at which i need to take off once again? This way i dont have to use the clutch much, just once to shift to the neutral.
in short, i shift to the neutral from a higher seed/higher gear and let the car come to a speed at which i can engage 2nd or 3rd gear.
just to give an e.g. if iam driving in the expressway at say 100 km/hr and when i see the sign - Toll 1000 meters ahead - i simply shift to the neutral and break gently till I approach the Toll booth , rather than shifting to 4th,3rd ect.
I do this while driving downhill also.(shifting to the neutral) but i do not keep my foot over the brakes.

by doing this i feel i am saving fuel.
Please correct me if i am wrong in the follwoing the above said method.
Neutral is not recommended esp on downhills and highways. Being in gear assists with the engine braking reducing the wear on your brakes. But I slot into neutral too approaching a manageable stop situation like traffic signals etc. Speaking for myself, I like this practise.


EDIT: I realized after posting that this thread is almost a yr old. Feeling weird

Last edited by theMAG : 26th January 2007 at 14:16.
theMAG is offline   Reply With Quote
Old 28th January 2007, 13:12   #55
Newbie
 
Join Date: Jan 2007
Location: Boston
Posts: 21
Thanked: 4 Times
Default

So most of the responses in this thread have indicated that it is a terrible, dangerous thing to travel at speed in a manual-trans car while the stick is in neutral. I could not disagree more.

One of the reasons cited is "engine braking". While engine braking may have its benefits, one of them is not, primarily supplementing the car's wheel brakes. Essentially any modern vehicle can apply more force than is necessary to "lock" all four wheels with relatively little effort. What little force the engine can apply to them is negligible in comparison. Think you're saving your brakes? Think again. What's more expensive to replace, the brakes or the engine? Which would you rather wear out?

So why is engine braking used in racing? It isn't these days -- at least not on purpose. A long time ago when brake technology was much worse than it is now, it was of some benefit in saving the brakes to apply engine braking together with the main wheel brakes. These days brakes are so comparatively good that using the engine as a brake is not very helpful in the grand scheme of things. What race drivers ARE doing when downshifting prior to entering a corner is so that the engine will be ready to apply power on corner exit. It is both difficult and dangerous to downshift mid-corner while at the limit of traction, so they do it beforehand. That's the only reason.

That said, for very extended distances downhill, engine braking may be useful to prevent brake overheating, but most people aren't driving down mountain roads very often, and brake heat buildup is rarely a problem in normal vehicle use.


The other reason cited often is that it saves gas to engine brake. While I see the logic applied to this, I think that answer misses the point. ANY friction applied to the car to convert kinetic energy into heat is essentially reducing your gas mileage. Braking of any kind is by definition taking useful energy (forward momentum) and wasting it. On the one hand we have the engine using no fuel (above a certain RPM only) with the throttle closed while the engine friction decelerates the vehicle, or the engine using a very small quantity of fuel to stay idling while the vehicle gradually coasts down from speed. Since the coasting vehicle will travel further, the comparison isn't really a fair one.


Again, I would say that it is not particularly inefficient, and definitely not unsafe to coast in neutral at speed.

-Chris


P.S. How's that for a first post?

What do I have to do to get a moderator to waive the 50-posts rule?
Multiades is offline   (1) Thanks Reply With Quote
Old 28th January 2007, 15:33   #56
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Race cars never use engine braking as such. They need to increase the revvs as they brake n downshift, in order to match engine speed with gear speed to attain a smooth and efficient downshift.

On a FWD you maynot feel much but if you've tried downshifting quickly without revvmatching on a RWD, you'l realise that the engine resistance causes the rear wheels to reduce their speed which can unsettle the car (similar to using the hand brakes).

This happened to me when i was driving the Innova. The car twitched for a sec and had i not pressed the clutch again, the car would have done a 180 spin right there.

It's not about saving the brakes or the engine, it's about being in control.

I've made it a habbit to revv match whenever i downshift and i never use neutral unless the car is about to halt.

Shan2nu

Last edited by Shan2nu : 28th January 2007 at 15:41.
Shan2nu is offline   Reply With Quote
Old 29th January 2007, 00:01   #57
Newbie
 
Join Date: Jan 2007
Location: Boston
Posts: 21
Thanked: 4 Times
Default

Quote:
Originally Posted by Shan2nu View Post
Race cars never use engine braking as such. They need to increase the revvs as they brake n downshift, in order to match engine speed with gear speed to attain a smooth and efficient downshift.
Exactly, which is how all downshifts should be performed. I double-clutch and heel-toe religiously for practice, and I've used the techniques on a racetrack as well (see sig pic).

Quote:
Originally Posted by Shan2nu View Post
On a FWD you maynot feel much but if you've tried downshifting quickly without revvmatching on a RWD, you'l realise that the engine resistance causes the rear wheels to reduce their speed which can unsettle the car (similar to using the hand brakes).

This happened to me when i was driving the Innova. The car twitched for a sec and had i not pressed the clutch again, the car would have done a 180 spin right there.
In any car, RWD, FWD, or AWD, "lift throttle" can cause oversteer. It's very much more pronounced in a front-heavy RWD car with a large engine, like some of those silly cars we have here in the USA (Camaro, Mustang, Firebird, GTO, you name it). Both engine braking on the rear wheels and weight transfer make it easier to run into this problem, but it would seem you've given me supplemental material for my next point...

Quote:
Originally Posted by Shan2nu View Post
It's not about saving the brakes or the engine, it's about being in control.

I've made it a habbit to revv match whenever i downshift and i never use neutral unless the car is about to halt.
You make a good point that I meant to state originally. Everybody would like to think they're using good technique, but I would bet that 90%+ of people who are told to engine brake will do it without rev-matching and run into the problem you described before. Never mind the confusion involved in a panic-stop, if someone somehow develops the wrong reflex for stuffing the car in gear and popping the clutch (thinking it helps them) they will waste valuable braking time and risk loss of control if the car skids.

If they're driving a RWD car, even if they have been engine-braking for a while and aren't abruptly downshifting when the panic-stop comes, the driver has still inadvertantly increased the rear brake bias. The braking effect will still be mild next to the wheel brakes, but could very easily be enough to lock the rear wheels and cause sudden, unexpected oversteer.

-Chris
Multiades is offline   Reply With Quote
Old 29th January 2007, 08:43   #58
Team-BHP Support
 
Vid6639's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 14,468
Thanked: 19,375 Times
Default

Chris, wow that was a lot of info.
There is one point I would like to point out. As you said most of the modern cars have very powerful brakes, in which case engine braking is negligible.

But in India there are quite a few cars which don't. We have cars with all round drums and no brake boosters.

I had a car which was very popular here called the Maruti/Suzuki 800 which was difficult to stop without downshifting during any sort of braking even normal braking if you put your foot on clutch or if you left it in gear. You had to downshift if you wanted to stop.

If you brake and the vehicle is in neutral you dont have full control of your car. In india any sort of hard braking may very well lead to your car getting rear ended as vehicles tailgate. In those situations it may be necessary to hit the brakes glance in the mirror and judge your braking so that you stop as close to the vehicle in front as possible. You may need to accelerate after hitting the brake to prevent a rear end collision.

US road conditions are ideal and for the track what you say is true. In india the roads are like the track and we need to put the power down asap just to avoid these situations.

PS: the info provided by you and Clint in this short span of time is invaluable for many.

Cheers

Last edited by Vid6639 : 29th January 2007 at 08:45.
Vid6639 is offline   Reply With Quote
Old 29th January 2007, 10:04   #59
Newbie
 
Join Date: Jan 2007
Location: Boston
Posts: 21
Thanked: 4 Times
Default

Quote:
Originally Posted by vid6639 View Post
Chris, wow that was a lot of info.
There is one point I would like to point out. As you said most of the modern cars have very powerful brakes, in which case engine braking is negligible.

But in India there are quite a few cars which don't. We have cars with all round drums and no brake boosters.

I had a car which was very popular here called the Maruti/Suzuki 800 which was difficult to stop without downshifting during any sort of braking even normal braking if you put your foot on clutch or if you left it in gear. You had to downshift if you wanted to stop.
Very interesting - I'd considered that your cars may be different but I'm a little surprised to hear about 4-corner drums. I would expect that the cars there are very light, too (800-1200cc engines sound common), so the brakes may not be as comparatively bad as you think, just that they require more effort. The lack of a brake booster is probably a bigger part of the problem (and race cars don't have 'em either ).

There are some 10-20 ton public transportation buses here that have front drums. Drum brakes shed heat poorly and are of generally cheap construction, but shouldn't be so bad at single-stop braking if they are sized properly. Most big trucks use drums.

Quote:
Originally Posted by vid6639 View Post
If you brake and the vehicle is in neutral you dont have full control of your car. In india any sort of hard braking may very well lead to your car getting rear ended as vehicles tailgate. In those situations it may be necessary to hit the brakes glance in the mirror and judge your braking so that you stop as close to the vehicle in front as possible. You may need to accelerate after hitting the brake to prevent a rear end collision.

US road conditions are ideal and for the track what you say is true. In india the roads are like the track and we need to put the power down asap just to avoid these situations.
While I've never driven in India, my co-worker was there recently and told me a little about it. US driving conditions (roads, drivers) are far from ideal, but it sounds as though it is more congested there.

Was that in-favor of gradual braking to sudden braking? If so, no arguments there, but a panic stop is a panic stop, and engine braking probably doesn't help much in that circumstance.


Let me give an example from where I live... There is a highway near my house that leads into the city. The speed limit is about 55mph (~100kph) and I'm usually going a little bit faster than that. On this highway there is a hill just under 1mi (1.6km) long with an elevation decrease of about 300ft (~100m). Starting at the top of the hill, I pop the car into neutral and coast down. Over this distance and despite the steep hill, the car will lose speed due to tire, bearing, and air friction with no help from engine braking. If I were to just lift off the throttle it would be going very slowly indeed by the bottom of the hill. Coasting in this case probably saves a little bit of fuel and gives the engine a brief rest without harming anything.

Quote:
Originally Posted by vid6639 View Post
PS: the info provided by you and Clint in this short span of time is invaluable for many.

Cheers
Thanks very much. We've been doing the car thing for a while now, and are no strangers to enthusiast forums. Always a pleasure to share whatever experience we do have.
Multiades is offline   (1) Thanks Reply With Quote
Old 29th January 2007, 12:15   #60
Senior - BHPian
 
iraghava's Avatar
 
Join Date: Oct 2005
Location: Bhaiyyaland
Posts: 8,028
Thanked: 153 Times
Default

Welcome to the forum Chris, that was quite a first post!! I believe now that you've given us some very good technical knowledge perhaps you won't mind posting an introduction of youself!

Quote:
Originally Posted by Multiades View Post
P.S. How's that for a first post?
Bloody good!!

Quote:
Originally Posted by Multiades View Post
What do I have to do to get a moderator to waive the 50-posts rule?
Ummmm... sorry but no bypasses here, spend sometime on the forum & get your post count to 50+ & the magic rule is waived off!!
iraghava 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
Palio 1.2 nv - Pressing the clutch pedal while starting the car kapilsharma80 Technical Stuff 24 18th February 2013 22:12
Xing: High rev when pressing the clutch during gear change raghu Technical Stuff 11 11th October 2009 07:06
cheen-cheen-cheen noise when pressing accelerator vebmetal Technical Stuff 15 27th August 2008 13:33
Which is the right Sequence Clutch-Brake Or Brake-Clutch ankura12 Technical Stuff 2 9th July 2007 10:02


All times are GMT +5.5. The time now is 14:10.

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