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Old 19th June 2008, 19:27   #61
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Engine braking = loss of contro, skidding, front wheel lock

Now this is confusing.
Yes, if you use hard braking (on the verge of lockup) and then slot into a lower gear, you will definately lock up your wheels. There needs to be a balance between engine braking and regular braking. This becomes even more important when you drive on low traction road surfaces.

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Old 19th June 2008, 20:01   #62
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Post some clarifications

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Originally Posted by revvedup View Post
My 2 cents

1.Engine braking happens when your wheels are spinning faster than they can do in that particular gear so if one shifts at about 10 Km/h above the maximum speed in that gear it would lead to retardation in a safe manner while at the same timing insuring that there is not much damage to the
engine and tranny. Case in point is the swift(which does 100 in 2nd) so if you are say at 110 in 5th gear and shift to 2nd it would lead to engine braking.
I am not sure that it'll lead to engine braking, but it'll certainly take you to the verge of "engine breaking".

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Originally Posted by revvedup View Post

2. The purpose of heel and toe or blipping the throttle while braking+ downshifting is to ensure that that engine does not run out of air. Petrol engines in general change the amount of air by restricting their flow via the throttle instead of reducing its amount.

Not blipping the throttle would be like restricting the air supply of a person before he starts to run so what one actually does by blipping the throttle is to increase the pressure by increasing the air in the combustion chamber because each time the piston moves downward it creates an area of low pressure (think of it as air pump drawing air, and this is what increases in case of engine braking) until finally it can draw no more air finally resulting in the jerking sound accompanied by the halt. So by blipping the throttle you are actually reducing the effectiveness of engine braking (not completely required if you practice engine braking like how I have mentioned in the first point)

Engine braking is primarily used in racing/rally applications( this is not to say it cannot be used on the road) in order to prevent overheating of the braking system ( remember glowing red discs) because engines are typically liquid cooled whereas brakes even in F1 applications are air cooled so they are less prone to overheating than your brakes.
Dude.. toe-n-heel during racing isn't used to prevent overheating of the brakes and all that stuff that you have mentioned but to downshift as quickly as possible while keeping the car's balance neutral through a corner and having the right gear engaged while powering out of the exit of the corner.

The primary objective of toe-n-heel is to match the current engine revs to the expected revs of the next lower gear in the 'box making the change smoother, while conserving the clutch (as it is engaged for a very short time if a toe-n-heel maneuver is executed perfectly).

And Formula 1 car drivers do not use toe-n-heel, coz they don't have a manual clutch to start with. The throttle is blipped electronically when they go down the gearbox.

Watch this video, you will surely learn a thing or two about toe-n-heel. For those who want to know- the driver is Dori Kin- Drift King aka Tsuchiya Keiichi at the Tsukuba circuit for a qualifying time attack for the AE86 cup.


Last edited by doomsday : 19th June 2008 at 20:02. Reason: Added link
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Old 19th June 2008, 20:56   #63
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Quote:
Originally Posted by sathya_nars View Post
I used to try out different options different times, wanted to kick off discussion on it.

Consider we are going in top gear and in speed which is not comfortable for next lower gear - say, my car has 5 gears, running in 4th gear is comfortable (RPM below 1500) not more than 50kmph, I am speeding at 70kmph in 5th gear.

Now either due to known stops (signal/speed breaker) or sudden obstacle I need to reduce the speed of car. There are few ways.
  1. To stop within short time, I apply break and keep reducing gears quickly.
  2. If I start reducing at considerable distance from stop, then I first stop acceleration, reduce gears one by one (RPM may be between 1500 to 3000 at each gear) and finally the speed is reduced/fully stopped (briefly applying break).
Lets take the goal is to get good FE (since this situation happens freq in city rides) as well as not driving in higher RPMs to keep engine's condition good (am I right?).
The speed you mention, namely, 70 kmph in 5th gear, is really too low to bother with anything other than just lifting foot off the pedal and then braking if necessary and finally shifting to appropriate gear when you are ready to accelerate. This is best for fuel economy, i.e., slow down sufficiently in advance so that braking is minimal and then accelerate smoothly at the earliest opportunity, so that downshifting is minimal. I doubt if downshifting really helps in conserving fuel.

I have observed that my Santro slows down pretty quickly from higher speeds (say 120+ kmph) even while in 5th gear, when I lift the foot off the pedal. My conjecture is that the tall-boy stance helps in providing more wind resistance, which, when coupled with the low weight of the Santro, effectively slows it down. Secondly the tall-boy stance also gives the driver maximum confidence while going to higher speeds, as you don't feel like you are really speeding even if the speedo shows 120 kmph, and the road view is great. Of course, the fuel economy is also bound to come down because of the higher wind resistance of a tall-boy car, if it is pushed too hard.

Last edited by rks : 19th June 2008 at 21:09.
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Old 20th June 2008, 03:01   #64
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Originally Posted by doomsday View Post
I am not sure that it'll lead to engine braking, but it'll certainly take you to the verge of "engine breaking".



Dude.. toe-n-heel during racing isn't used to prevent overheating of the brakes and all that stuff that you have mentioned but to downshift as quickly as possible while keeping the car's balance neutral through a corner and having the right gear engaged while powering out of the exit of the corner.

The primary objective of toe-n-heel is to match the current engine revs to the expected revs of the next lower gear in the 'box making the change smoother, while conserving the clutch (as it is engaged for a very short time if a toe-n-heel maneuver is executed perfectly).

And Formula 1 car drivers do not use toe-n-heel, coz they don't have a manual clutch to start with. The throttle is blipped electronically when they go down the gearbox.

Watch this video, you will surely learn a thing or two about toe-n-heel. For those who want to know- the driver is Dori Kin- Drift King aka Tsuchiya Keiichi at the Tsukuba circuit for a qualifying time attack for the AE86 cup.

No, it would not lead to engine breaking as you suggest. However at the same time if my procedure is incorrect please specify how you do it. Moreover could you throw more light on how engine braking helps to keep the car 's balance neutral through the corner.


Please re -read my post carefully, nowhere have I mentioned that heel -n-toe is used to prevent brake heating or that F1 drivers use it.

BTW, a big thanks for the video heel n toe from the Keiichi Tsuchiya himself .

Last edited by revvedup : 20th June 2008 at 03:04.
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Old 20th June 2008, 10:24   #65
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Quote:
Originally Posted by revvedup View Post
1.Engine braking happens when your wheels are spinning faster than they can do in that particular gear so if one shifts at about 10 Km/h above the maximum speed in that gear it would lead to retardation in a safe manner while at the same timing insuring that there is not much damage to the
engine and tranny. Case in point is the swift(which does 100 in 2nd) so if you are say at 110 in 5th gear and shift to 2nd it would lead to engine braking.
Engine braking happens whenever the engine RPM is less than the RPM required to rotate the wheels at that particular speed. Engine braking takes advantage of the fact that the engine RPM has a tendency to come down to idle RPM. Infact if you are doing 100 in 5th gear, pressing and releasing the clutch (without pressing the accelerator) will also provide engine braking. However the braking in 5th gear will not be effective as in 2nd gear. There is no need to redline while downshifting to provide engine braking.
Quote:
Originally Posted by revvedup View Post
2. The purpose of heel and toe or blipping the throttle while braking+ downshifting is to ensure that that engine does not run out of air. Petrol engines in general change the amount of air by restricting their flow via the throttle instead of reducing its amount.

Not blipping the throttle would be like restricting the air supply of a person before he starts to run so what one actually does by blipping the throttle is to increase the pressure by increasing the air in the combustion chamber because each time the piston moves downward it creates an area of low pressure (think of it as air pump drawing air, and this is what increases in case of engine braking) until finally it can draw no more air finally resulting in the jerking sound accompanied by the halt. So by blipping the throttle you are actually reducing the effectiveness of engine braking (not completely required if you practice engine braking like how I have mentioned in the first point)
There are a number of incorrect facts here. If it was true we would not be able to idle our cars in neutral gear without pressing the accelerator since the lack of air supply would suffocate the engine. Engine braking is used not for stopping the car but for reducing the speed of the car. Also, in modern cars the fuel supply would be cut off when the ECU detects that the wheels are rotating faster than the engine.
Quote:
Originally Posted by revvedup View Post
3. Engine braking is primarily used in racing/rally applications( this is not to say it cannot be used on the road) in order to prevent overheating of the braking system ( remember glowing red discs) because engines are typically liquid cooled whereas brakes even in F1 applications are air cooled so they are less prone to overheating than your brakes.
In racing/rally downshifting is done not for braking but for swifter exits. In rallies in particular downshifting is generally used in combination with heel and toe technique and hand brakes to maintain the balance of the car and to provide the necessary drift.
Quote:
Originally Posted by revvedup View Post
4. From the above, one can conclude that engine braking although advantageous shouldn't be used excessively because it can lead to heating up of the clutch( reducing its bite so as to say) and engine ( especially if you also do your upshifts a little late).
The clutch will not heat up or wear down any faster if you rev match while downshifting.
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Old 20th June 2008, 11:03   #66
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No, it would not lead to engine breaking as you suggest.
Actually it is true. If you downshift into 2nd gear at 110kmph (when 2nd gear maxes out at 100), you're forcing the engine to revv beyond it's redline. By doin this, you're reducing engine life. Always make sure the engine is never revved beyond it's redline.

Quote:
Engine braking happens whenever the engine RPM is less than the RPM required to rotate the wheels at that particular speed.
Not really, you can have engine braking even when you revv match while you downshift (to match engine and transmission speeds). Engine braking happens bcoz there is not enough fuel/air being supplied to the engine for it to maintain or increase it's revvs.

Moreover, the pistons have to overcome the resistance provided during the compression stroke. This resistance is what really causes engine braking. Had there been no air inside the cylinders, the engine wouldn't have been able to slow down the car.

Thats why, in engines that use VCM (Variable Cylinder Management), they shut down some of the cylinders by sealing off the intake and exhaust valves, such that the piston inside those cylinders work in a vacuum, eliminating the resistance during the compression stroke. This makes it easier for the other (working) cylinders to function more effectively, which resuslts in a much more fuel efficient engine.

By letting the clutch off without matching the engine and transmission speeds, you tend to wear out the clutch much faster and this frictional resistance (between the flywheel and the clutch plate) is what contributes to the braking. Thats why you feel a greater resistance when you let go of the clutch without revv matching.

Shan2nu

Last edited by Shan2nu : 20th June 2008 at 11:10.
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Old 20th June 2008, 11:13   #67
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Quote:
Originally Posted by revvedup View Post

Moreover could you throw more light on how engine braking helps to keep the car 's balance neutral through the corner.
Quote:
Originally Posted by watashi75 View Post
In racing/rally downshifting is done not for braking but for swifter exits. In rallies in particular downshifting is generally used in combination with heel and toe technique and hand brakes to maintain the balance of the car and to provide the necessary drift.

The clutch will not heat up or wear down any faster if you rev match while downshifting.
This is a part of the explanation. Now imagine a racing scenario. A long straight (take the Tsukuba circuit back-straight for example, as everyone can refer to the video) followed by a long corner. Now you are doing big speeds just before your braking point. At the braking point if you simply stand on the brakes (without toe-n-heel) to slow the car down, the entire weight will be transferred to the front wheels. This is a dangerous situation, because at turn-in, the rear wheels will have much less grip and the car may go into oversteer at corner entry- which you may catch but anyway leads to loss of time on the track.
And remember, you haven't downshifted yet, so you are still in a gear too high to power out of the corner at optimum speeds, which again has two negatives. Obviously the exit is slow because of slower acceleration in the high gear, but more important the drive to the wheels (wheel rpm) may not be high enough to counteract the lateral forces which tend to throw the car sidewards (thats why it is said that one should never lift off mid-corner)

Now go round the same corner using toe-n-heel. You come to your braking point and brake while downshifting at the same time. The weight doesn't transfer abruptly to the front wheels, ensuring both ends of the car have adequate grip. Since you have downshifted at the same time, you also have the optimum gear handy to exit out of the corner- leading to a stable and fast exit.

As said earlier, the clutch will not heat up or wear out during downshifting, but it'll be quite the opposite. Since you are using the clutch for much lesser times and the transitions between gears are smoother- the clutch life is prolonged. Probably the only downside is the fact that you are using a little extra fuel.

Hope this explains it.
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Old 20th June 2008, 14:47   #68
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If you ask me, its a matter of faith. My experience is like this. I am driving with a car about three car lengths in front. We are moving at 50 maybe a little more. The car in front slows down, not drastically, but so that in about three seconds the gap will become just a car length. As soon as I see the car slowing down, (more for practising than anything else) I take my foot off the A pedal and downshift a gear to 3rd, but don't touch the A pedal again. The moment that I press the clutch, the car surges forward, and I have an impulse to brake to avoid what may be a collission. But the moment I release the clutch, the car slows down quickly enough to keep safe distance.

So, to me its just a matter of faith and knowledge of the extent of engine braking. That can surely be achieved with practice.
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Old 1st July 2008, 12:18   #69
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Default Better FE using engine braking?

A couple of posts on this thread suggest that engine braking increases FE. I'm not exactly sure of how this happens. Consider the following scenario when i dont use engine braking:
1. I see a signal coming up ahead is red at a distance of say 200m. I have sufficient speed/momentum to carry me to the last car in the sgnal very easily (Assume i'm in 4th).
2. I shift the car to neutral/hold the clutch, so that the engine is now idling at 900 RPM.
3. I use my brakes to slow the car and come to a stand still at the appropriate place.

Using engine braking, i do the following:
1. I see the signal coming up...
2. I downshift to 3rd, 2nd to slow down and then eventually brake to stop.
At this time my engine is revving at over 1000 rpm throughout.

Wont the engine consume lesser fuel if its running at lower RPMs than it will be if its running at a higher RPM for the same period of time?
(I understand that the energy efficiencies here will be different. In case of engine braking, the engine's power is being used to stop the car while in the other case, its being wasted by idling.)
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Old 1st July 2008, 13:51   #70
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Quote:
A couple of posts on this thread suggest that engine braking increases FE. I'm not exactly sure of how this happens. Consider the following scenario when i dont use engine braking:
1. I see a signal coming up ahead is red at a distance of say 200m. I have sufficient speed/momentum to carry me to the last car in the sgnal very easily (Assume i'm in 4th).
2. I shift the car to neutral/hold the clutch, so that the engine is now idling at 900 RPM.
3. I use my brakes to slow the car and come to a stand still at the appropriate place.

Using engine braking, i do the following:
1. I see the signal coming up...
2. I downshift to 3rd, 2nd to slow down and then eventually brake to stop.
At this time my engine is revving at over 1000 rpm throughout.

Wont the engine consume lesser fuel if its running at lower RPMs than it will be if its running at a higher RPM for the same period of time?
(I understand that the energy efficiencies here will be different. In case of engine braking, the engine's power is being used to stop the car while in the other case, its being wasted by idling.)
What you say is true but most performance drivers don't use engine braking with FE in mind. When i use engine braking, i also perform "heel n toe". This is the worst thing you can do wrt FE, since you have to constantly revv the engine everytime you downshift.

But my priorities are diff. My idea is to stay in a gear, which will give me maximum acc once i've done with my braking, while keeping the downshifts as smooth as possible.

Even when i'm not driving for performance, i shift to a gear that maynot give me the best acc but atleast better than the gear i was in when i started braking. This method is a bit more FE.

Shan2nu

Last edited by Shan2nu : 1st July 2008 at 13:53.
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Old 1st July 2008, 14:11   #71
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For Heel and Toe and why we use it.

TurnFast! • Heel-Toe Downshift

The article assumes that you in a left-hand drive car. Please adapt it to RHD.

Thanks,
Raaj K S
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Old 1st July 2008, 14:29   #72
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The article assumes that you in a left-hand drive car. Please adapt it to RHD.
That doesn't affect the footwork or the gear pattern. Only diff is the gearstick on your right hand side in a LHD.

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Old 1st July 2008, 15:07   #73
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You are right..i was indicating on the hand movement only! ;-)
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Old 1st July 2008, 15:26   #74
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You are right..i was indicating on the hand movement only! ;-)
Hehe, i think when you sit in a RHD, you wouldn't expect the gear stick to be where the driver's door is.

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Old 13th July 2008, 09:48   #75
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There are definitely some misunderstandings and incorrect assumptions in the last few pages of this thread. Let me chip in my 2 cents on these issues.

Quote:
Originally Posted by revvedup View Post
...1.Engine braking happens when your wheels are spinning faster than they can do in that particular gear so if one shifts at about 10 Km/h above the maximum speed in that gear it would lead to retardation in a safe manner...
As the others have said, this is (totally) incorrect and not good for your engine either.
"Engine braking" is simply the additional retardation provided by the friction inside the engine when it is revolving (and not being fed any air/fuel). This can happen at 2000rpm or 8000rpm - as long as the engine isnt getting enough air/fuel to spin on its own at the current RPM.

Quote:
Originally Posted by revvedup View Post
...2. The purpose of heel and toe or blipping the throttle while braking+ downshifting is to ensure that that engine does not run out of air. Petrol engines in general change the amount of air by restricting their flow via the throttle instead of reducing its amount...
The purpose of heel-toe is quite simple - revv-match (heel on accelerator) while braking (toe on brake) at the same time!

Quote:
Originally Posted by Shan2nu View Post
...
Moreover, the pistons have to overcome the resistance provided during the compression stroke. This resistance is what really causes engine braking. Had there been no air inside the cylinders, the engine wouldn't have been able to slow down the car.
As per my understanding this is not really correct Shan.

The MAIN reason for engine braking is FRICTION (not the compression of air).

Think about this - in a 4cyl engine one piston is on the compression stroke which is definitely causing resistance since it is squishing the volume of air to about 1/10th its volume (eg. 10:1 compression ratio), BUT at the same time another cylinder is on the power stroke (but there is no detonation because the drivers foot is not on the gas during engine braking), so the air in that cylinder is expanding to about 10times its squished volume - which is providing some force pushing down on the piston - and hence cancelling out the compression stroke resistance!

End result? In theory the compression/expansion cancel each other out, so you are left with only friction!


Quote:
Originally Posted by Shan2nu View Post
Thats why, in engines that use VCM (Variable Cylinder Management), they shut down some of the cylinders by sealing off the intake and exhaust valves, such that the piston inside those cylinders work in a vacuum, eliminating the resistance during the compression stroke. This makes it easier for the other (working) cylinders to function more effectively, which resuslts in a much more fuel efficient engine....
Once again, even if those cylinders were working in vaccuum, (it would be the other way around) the power stroke would be causing resistance and the compression stroke would be aided by the original vaccum.

However, think about this - what if the exhaust valve opened after the compression stroke and then closed leaving a lower pressuse in the cylinder just in time for the (unfueled) power stroke - that would be a win-win for retardation! There would be resistance on the pistons way up (squishing air) AND on the way down (sucking on low pressure), and that is the principle on which jake-brakes (also known as compression-release brakes) that are found of heavy vehichles work!


Quote:
Originally Posted by prabuddhadg View Post
...The moment that I press the clutch, the car surges forward...
In that case your car is already engine braking, and pressing the clutch is disconnecting the retardation provided by the engine!

Quote:
Originally Posted by paranoidAndroid View Post
... Consider the following scenario when i dont use engine braking:
1. I see a signal coming up ahead is red at a distance of say 200m. I have sufficient speed/momentum to carry me to the last car in the sgnal very easily (Assume i'm in 4th).
2. I shift the car to neutral/hold the clutch, so that the engine is now idling at 900 RPM.
3. I use my brakes to slow the car and come to a stand still at the appropriate place.

Using engine braking, i do the following:
1. I see the signal coming up...
2. I downshift to 3rd, 2nd to slow down and then eventually brake to stop.
At this time my engine is revving at over 1000 rpm throughout.

Wont the engine consume lesser fuel if its running at lower RPMs than it will be if its running at a higher RPM for the same period of time?...
Engine braking saves fuel.
If you car was engine braking all the time you would use ZERO fuel.

Please see my explanation in this post : http://www.team-bhp.com/forum/techni...tml#post226145 (Shifting to Neutral or Pressing the Clutch when Braking - Is this right?)


Also, this thread seems to be discussing the same issues that were discussed in-depth in the link above, so we should try and wrap it up here.

cya
R

Last edited by Rehaan : 13th July 2008 at 09:51.
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