Engine braking

[Joenavy85]

Originally Posted by Christ

So your LED blinks about 1000 times a min?

i have a small capacitor (i forget the rating) that keeps the LED from flashing, and then a diode on the + side to prevent the capacitor from providing any current to the injector

[Christ]

Ok, beyond my electrical knowledge. Thanks for playing! LOL.

[JMac]

So you're saying over-revving the engine by downshifting isn't actually straining the clutch, it's only like if you stay on the gas while upshifting?

Justjohn, maybe you'd like the way my brain figures this out. Basically, when downshifting to engine brake, there is almost no wear on the clutch because the momentum of the car (lots of mass / energy) is being used to push the engine (much less mass) up to speed. Conversely, when using the engine to bring the entire automobile up to speed i.e. accelerating, the engine (very little mass) must accelerate the car (lots of mass) and so the clutch must be used and slipped in order to get the car moving without killing the engine.

Basically, it's a lot easier on the clutch to downshift (like while doing engine braking) than it is to upshift for acceleration from zero. I don't think one would need to worry about clutch wear if he is downshifting to engine brake.

I sure hope that makes some sense. I'm no physicist, that's for sure.

JMac

[jedi_sol]

Originally Posted by justjohn

So you're saying over-revving the engine by downshifting isn't actually straining the clutch, it's only like if you stay on the gas while upshifting?

Its not "over-revving" the engine, its more like giving the throttle a little "blip" while you are downshifting to match the speed of the engine with the speed of the transmission (hence the term "rev-matching"). When the engine/transmission are perfectly matched at the same speed, there is no wear on the clutch at all.

It's mainly a technique used in race driving (heel toe shifting) to keep the car's weight distribution balanced while braking into a corner ...but that's another topic all together. But it's also a useful application in the real word in that it saves wear on the clutch.

You want to try to blip the throttle to put the revs where they "should" be in the lower gear. It takes practice to get it just right. You know youve got it right is when you downshift, there is no jerking/shifting of the car.

Skip to 1:47
http://youtu.be/W7bb2OKFKgc

[jtbo]

I believe term is rev matching, during times without syncromesh (sorry if word is wrong, non english speaker here) boxes it was required to get gear changed (also there was need to double clutch to get box to same speed if I'm not mistaken).

In racing rev matching is needed for smoothness, so you don't jerk balance of car, in normal driving least wear to parts comes when you rev match, idea is to put engine to revs that it will have for next gear, then you can quickly just dip the clutch or even shift without the clutch.

When aiming for low fuel consumption, I believe smoothness is equally important as it is with racing, any strong jolt is wasted energy and adding wear and tear needlessly.

I think that when you can drive smoothly without panic when you don't have clutch or brakes, then you are in a zone of optimal MPG?

[justjohn]

Thanks for all the good replies. I'm definitely learning.


I think there must be some fundamental flaw in my understanding of the system though, because rev-matching seems counter-intuitive in terms of wear to me. Oh wait, I think I may have just made a breakthrough. Hang on and tell me if this is right.

"It's not using the clutch when RPMs are high that's hard on it, it's when there is a large difference between where the engine speed is and where it 'should' be for whatever gear you're going to."

If that's right then I've got it. Cause when I learned that keeping on the gas during upshifting was bad for it, I originally assumed the logical link was simply "high RPM = bad for the clutch." Assuming that's wrong, then that's where most of my confusion was coming from.

I always thought rev-matching was just for clutchless shifting, which I have wanted to learn for a while as well, but I've been stuck with an automatic.

[Christ]

That's pretty much it, exxcept that keeping on the throttle while up shifting increases the rpm while the goal of upshifting is to lower rpm. End result is lots of heat, while the clutch disc acts like a brake, using the vehicles mass, thus resistance to acceleration, to slow the engine to a 1:1 ratio with the input shaft of the transmission.

There are times when this action is beneficial, but we won't discuss those now.

As another thought - it's actually much safer to use high rpms to slow down than to rev high under load. Slowing places less stress on the engines internals than accelerating.

Consisting takes advantage of otherwise wasted energy, frictional losses and pumping losses, gear and drive train losses, to slow the vehicle. Done properly, it also greatly d saves on brakes. An added bonus is that by teaching yourself to drive proactively enough to downshift and use the brakes only when absolutely necessary, you also will be a safer, more economical driver.

[jtbo]

Originally Posted by justjohn

Thanks for all the good replies. I'm definitely learning.


I think there must be some fundamental flaw in my understanding of the system though, because rev-matching seems counter-intuitive in terms of wear to me. Oh wait, I think I may have just made a breakthrough. Hang on and tell me if this is right.

"It's not using the clutch when RPMs are high that's hard on it, it's when there is a large difference between where the engine speed is and where it 'should' be for whatever gear you're going to."

If that's right then I've got it. Cause when I learned that keeping on the gas during upshifting was bad for it, I originally assumed the logical link was simply "high RPM = bad for the clutch." Assuming that's wrong, then that's where most of my confusion was coming from.

I always thought rev-matching was just for clutchless shifting, which I have wanted to learn for a while as well, but I've been stuck with an automatic.

Remember that there are box input shaft rpm and engine rpm, it does not matter if engine rpm is high or low, it matter what speed difference is between two. Well when speaking of wear, also clutch slipping at high speed wears more than at slow speed, but that also is more like difference in those two rpm than exact engine speed.

You can have engine at low rpm and box input shaft at high rpm and again you get more wear than would be optimal.

Gearbox input shaft speed depends from which gear is in.

That is how I understand it, but also gearbox internals have bit mystery to me.

edit: One thing I remembered, some engines do burn oil when engine braking at high rpm, so that is good to remember, frequent oil checks are not bad thing if engine braking near redline.

[justjohn]

Cool.

So that would mean it's "safe" to use more than just mild engine braking, but only if you know how to rev-match. Correct?

And that would mean the ideal situation would be coasting down when possible, rev-matched engine braking when needed, and brakes rarely. Yes?

[jtbo]

Originally Posted by justjohn

Cool.

So that would mean it's "safe" to use more than just mild engine braking, but only if you know how to rev-match. Correct?

And that would mean the ideal situation would be coasting down when possible, rev-matched engine braking when needed, and brakes rarely. Yes?

Well, I don't think you can really cause damage if engine braking sensible manner, cars are so fool proof these days and have been at least 15 years, that it would require quite loony driver to get any damage caused.

However when maximizing life of components, then yes, rev matched shifts and moderate rpm engine braking, I would avoid high rpm engine braking for several reasons, noise, control of vehicle, possible oil consumption increase and such.

Also clutch wears only when slipping and it is not slipping unless one operates the clutch, or then clutch is weared out or is too small.