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 ...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 ...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 ...
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 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 ...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 ... 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
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