Quote from lefty :

good luck with the studies.

LMAO.

Engine design engineers all over the world are throwing away their notes and copying and pasting scipy's text there so they can reinvent the internal combustion engine to the way that it actually works in scipy's mind.

How about that. I never knew that the exhaust valve, under no acceleration load, could just disregard the camshaft and stay 'slightly' open.

Have you been reading Caroll Smith again?

Engine braking is a real and useful driver aid. Of course, if all four wheels are being braked (by the discs/pads) as much as they can, then any engine braking will lock them up. But in real life you never ever brake like that - load transfer, surface irregularities, driver irregularities, dynamic tracks that are slightly different lap after lap, and the NEED to change down gears means that you always have something in reserve, generally at the back of the car. By changing down and letting the clutch engage you can either use it to improve car stability (try turning a car on the limit with it in neutral to see how hard that is!), or to aid braking. Some people rely on it more than others, and it doesn't always cause wear to dogs or clutches if done with skill (i.e. not by me).

When downshifting and heel/toeing the main problem isn't the nose DIVING, but the nose going UP. Many drivers, myself included, reduce brake pressure when touching the throttle, increasing braking distances and slowing lap times. The last thing ANY driver learns to do properly is brake (and standing starts). I don't think novice or pro drivers would ever make the nose dive MORE whilst blipping the throttle, because then it would be REALLY obvious how far from the braking limit you are.

In many ways you are not wrong. But not necessarily for the right reasons.

And you are right that doing something for a long time doesn't mean you are doing it right. But engine braking isn't as bad as you think.

P.S. I'm a graduated Mechanical Engineer now you mention it. I don't see what that has to do with this thread though, as no Engineering course on the planet will teach you about race car gear changing techniques and pitfalls.

P.P.S If engine braking can be used to slow the tyre speed in wet/icy conditions, how does that differ from dry conditions? If you can slow your tyre speed in the dry, surely you can do it in the wet/ice. You've just contradicted yourself mate

Quote from mrodgers :

How about that. I never knew that the exhaust valve, under no acceleration load, could just disregard the camshaft and stay 'slightly' open.

That bit is true - engine braking is primarily due to friction not compression. That is why you don't get more engine braking in a diesel (high compression) than a petrol (lower compression) or a turbocharged petrol (lower still). It's a common misconception that you've bought too. It has nothing to do with the exhaust valve staying slightly open at all - that's just silly.

Quote from tristancliffe :

When downshifting and heel/toeing the main problem isn't the nose DIVING, but the nose going UP. Many drivers, myself included, reduce brake pressure when touching the throttle, increasing braking distances and slowing lap times. The last thing ANY driver learns to do properly is brake (and standing starts). I don't think novice or pro drivers would ever make the nose dive MORE whilst blipping the throttle, because then it would be REALLY obvious how far from the braking limit you are.

that's what i ment, when you look at racers in T1 for example u can tell apart the ones that are braking properly cause the nose doesnt rise while blipping, but nosedives are there when a driver doesnt blip (i.e. uses the "engine braking") and he hasnt been using the brakes at 100 % before that.

Quote from tristancliffe :

P.P.S If engine braking can be used to slow the tyre speed in wet/icy conditions, how does that differ from dry conditions? If you can slow your tyre speed in the dry, surely you can do it in the wet/ice. You've just contradicted yourself mate

ofcourse it can, i never said it couldnt be used - but u missunderstood, in icy conditions when u touch the brakes just slightly they will lock almost right away, and if you just put it in 1st gear and drop the clutch they will slow down but you will have some steering control. in the dry conditions you are already using the brakes, that's why i said there's no reason to do it in the dry - cause brakes can be used to their limits

i also tought that thing with the intake valves not opening was silly but i've read and seen somewhere that camshafts can move out of position (to some extent that's how VTEC works).. but i guess that would be a bit idiotic.. apologize for that but i was concentrating too much on the fact that i was sure it wasnt the compression that is the cause of engine braking

Quote from scipy :

i dont think so.. when the engine is coasting there is almost no compression beeing made, cylinders are running almost on vacuum with intake vales beeing steady and exhaust valves opening slightly. what you are using to slow down is: let's say you're on 6000 rpm in 6th gear, start braking and downshift without a throttle blip, the engine has already started to slow down - it had some kinetic energy on 6000 rpm, and that energy is going down, also taking into consideration the rotating mass of the flywheel and the clutch assembly.. when the clutch reingages it will spin that mass once again, and the thing that opposes you is friction, not compression..

I see what you are saying, that is called "inertia" not "friction" - you will get the same effect if the system was perfectly frictionless because you still need energy to spin-up the components of the system. BTW, you do realise that friction is a constant force and not proportional to speed?

But, that doesn't explain why you get more engine braking at 6000rpm compared to 2000rpm (forget changing gear, just lift off at different rpms).

Quote from sgb27 :

But, that doesn't explain why you get more engine braking at 6000rpm compared to 2000rpm (forget changing gear, just lift off at different rpms).

I think you pump air 3x as fast through the whole engine at 6000rpm than 2000rpm, and that air flowing through the whole thing creates some drag that takes energy away from pistons. I guess 3x as fast translates to 9x as much drag...

Also, the simple act of compressing air then expanding it back costs energy. Compressing heats up the air, raising its temperature above the outside. Since the air is hotter, some heat (energy) is lost to the outside and can't be reclaimed while expanding. You do that more cycles per minute, you lose more energy per minute= higher engine braking.

But that's the thing - compressing the air doesn't cause engine braking - most of the energy goes back into the engine on the expansion stroke.

Do you REALLY get more engine braking at higher engine speeds? Or is it just that at 6000rpm there is more noise and speed so you think you are slowing quickly, but at 2000 the idle control is starting to become noticable and effectively opens the throttle.

People think that compressing the air in an engine must slow it down a lot, but don't for get that another cylinder is expanding, taking energy from the air at the same rate. Net energy 'used' is very little. Even on a single cylinder, the compression will be balanced by the expansion, just not at quite the same time.

Yes it does use energy, but at road speeds the engine friction is way more!

Quote from tristancliffe :

But that's the thing - compressing the air doesn't cause engine braking - most of the energy goes back into the engine on the expansion stroke.

Do you REALLY get more engine braking at higher engine speeds? Or is it just that at 6000rpm there is more noise and speed so you think you are slowing quickly, but at 2000 the idle control is starting to become noticable and effectively opens the throttle.

People think that compressing the air in an engine must slow it down a lot, but don't for get that another cylinder is expanding, taking energy from the air at the same rate. Net energy 'used' is very little. Even on a single cylinder, the compression will be balanced by the expansion, just not at quite the same time.

Yes it does use energy, but at road speeds the engine friction is way more!

Do you really NOT get more engine braking at higher engine speeds? I do. My motorbike redlines at some 20km/h in 1st gear, where the air drag is still negligible. The braking I get out of a closed throttle is much greater at 20 km/h than at 10km/h. And the engine braking reduces to 0 when speed corresponds to idle rpm, as it should. Oh, and it uses a simple carburetor so no such thing as idle control.

The force used to compress can't be balanced by expansion from another cylinder. Not because the time is out of phase, but the force used to compress will be different from the force from expansion. Without some proper data concerning the heat conduction, I don't think you can conclude if the energy loss through heat is way less or more than through other friction. You only know it is present.

Have you measured the engine friction at different speeds? Whilst it's not going to be constant, it will be more than the losses through compression until very very slow engine speeds.

Your bike will have idle control, otherwise it would stall and not idle. At zero throttle, there are idle bypasses and jets (and drillings) that ensure the right amount of fuel/air is metered to hold a steadyish idle. That is idle control. All engines will have it, from lawnmovers to F1 cars. That means that at lower revs the rate of decelleration (in terms of revs) will fall as the fuel on a closed throttle starts to balance what the engine needs to idle.

Perhaps do the test without the engine running, so that there is no fuelling, and no ignition?

Engine braking reduces to zero because you have some throttle (i.e. the fuelling is correct for that load condition to maintain engine speed). Just like at full throttle, max speed there is no engine braking because the fuelling matches the load requirements.

Yes there will be a difference in energy used to compress air versus energy gained by expanding air. If you ignore heat, which just makes everything harder to examine, you will see that friction in bearings, rings etc will have more of an effect than compressing some air (and it won't be much air, because the throttle will be closed and intake pressures will be negative gauge).

argh, yes inertia.. i was thinking that and writing friction, friction is pretty constant (heat and tollerances taken into consideration).

You can use engine braking WITHOUT the engine even being on. You just have to be travelling at speed before you turn off the engine. OR just get a tow

Quote from tristancliffe :

Your bike will have idle control, otherwise it would stall and not idle. At zero throttle, there are idle bypasses and jets (and drillings) that ensure the right amount of fuel/air is metered to hold a steadyish idle. That is idle control. All engines will have it, from lawnmovers to F1 cars. That means that at lower revs the rate of decelleration (in terms of revs) will fall as the fuel on a closed throttle starts to balance what the engine needs to idle.

Perhaps do the test without the engine running, so that there is no fuelling, and no ignition?

Engine braking reduces to zero because you have some throttle (i.e. the fuelling is correct for that load condition to maintain engine speed). Just like at full throttle, max speed there is no engine braking because the fuelling matches the load requirements.

Yes there will be a difference in energy used to compress air versus energy gained by expanding air. If you ignore heat, which just makes everything harder to examine, you will see that friction in bearings, rings etc will have more of an effect than compressing some air (and it won't be much air, because the throttle will be closed and intake pressures will be negative gauge).

Ok I thought your "idle control" meant some computer controlled fuel injection as engine speed drops, that is probably present on all modern sedans. But old-style idle doesn't need control, really. Idle is just a set % of throttle that remains open no matter what.

My bike has this switch that cuts ignition. Once cut, the engine braking is generally stronger, and only drops to 0 right at standstill. But, like said, the engine braking is still higher at higher rpm even without burning fuel.

Here's a test: go to 1st gear and slide down a long slope, throttle off. Does the car settle down to a steady speed? If the car settles to a speed, then you have higher engine braking at higher rpm which brakes you more when you're fast, and brakes you less when you're slow. Autos don't work as their engine braking doesn't go through the torque converter, but my bike does that.

If you'd like to argue heat is not important by ignoring heat, then we may as well ignore friction and argue friction isn't important. Besides, trying to pump all the air from intake to exhaust through a tightly closed throttle is probably a very big drag source. That I can experiment by switching off ignition and compare open/closed throttle.

With the throttle closed you want the throttle closed, not a couple of % open. Carbs have idle bypasses for that. If you can see light around the butterflies with the throttle closed then your carbs are knackered

My bike had the same ignition cut. I never tried using it whilst riding though

I was ignoring heat because on a closed throttle the heat processes aren't important. Besides which we are dicussing whether the air pump nature of an engine provides the braking or the engine friction. My sources are: Caroll Smith, Guy Croft, Keith Duckworth (though I couldn't tell you where I read Keith's prose on the matter, because I've long forgotten which book), and my research lecturer at Uni. None of this has been tested by me.

Quote from tristancliffe :

That bit is true - engine braking is primarily due to friction not compression. That is why you don't get more engine braking in a diesel (high compression) than a petrol (lower compression) or a turbocharged petrol (lower still). It's a common misconception that you've bought too. It has nothing to do with the exhaust valve staying slightly open at all - that's just silly.

Tristan, i have some anacdotal(sp?) evidence to support your theory, i too thought compression would be a mayor part of the equation but i dont think so, i had my car fuel cut on me due to overboost (boost soloniod failure) at full chat, that gave me MASSIVE engine breaking for a moment (1-2 secs?) much more than normal engine braking, i can only attribute this to there still being intake pressure makeing full compression and once that had gone the car slowed normally. (i was throwed forward so hard i hurt my neck and luckly i was going in a straight line, i would of spun in a turn)

I now also believe that the compression isnt a mayor factor in the resistance of the engine to turning.

Simon

Quote from tristancliffe :

With the throttle closed you want the throttle closed, not a couple of % open. Carbs have idle bypasses for that. If you can see light around the butterflies with the throttle closed then your carbs are knackered

Hmm...not, but probably we're both over generalizing. My bike has a screw that blocks the throttle low limit, used for adjusting idle rpm, so the throttle is always open at least a little. But be it bypass or non-closed throttle, idle means allowing very restricted air intake, which in the case of engine braking should be exactly the same thing.

Quote :

My bike had the same ignition cut. I never tried using it whilst riding though

I was ignoring heat because on a closed throttle the heat processes aren't important. Besides which we are dicussing whether the air pump nature of an engine provides the braking or the engine friction. My sources are: Caroll Smith, Guy Croft, Keith Duckworth (though I couldn't tell you where I read Keith's prose on the matter, because I've long forgotten which book), and my research lecturer at Uni. None of this has been tested by me.

Your logic is circular. Heat processes aren't important so heat is ignored. Heat is ignored so compression loss is negligible. Compression loss is negligible so heat processes aren't important... It just doesn't say anything. Otherwise there's no good reason to ignore heat processes.

Can you quote anything that says pumping air through a tight opening offers negligible engine braking?

sure, you can rev a UF 1000 to 20000 RPM. Once.

Scipy, i don't like your attitude.

Quote from MikeB :

Scipy, i don't like your attitude.

That made me laugh so much, the thread had totally gone dead and you just say you don't like his attitude? hahahaha

Quote from MikeB :

Scipy, i don't like your attitude.

I like it. +1

The truth can be told. That's what I always say.

You want the truth? You can't handle the truth!



... nevertheless there's one thing I still believe: in LFS, you can brake on the verge of locking wheels and still decrease your braking distance with heavy engine braking, which - obviously and for the reasons scipy stated - makes NO sense in RL, but it appears (yeah, APPEARS to me, still) that it works in LFS.

maybe because in LFS engine braking is programmed in the aerodynamics.

Bandit, it DOES work in LFS. I don't care if it works in r/l the fact is, the WR times, are used by changing into the gear for the upcoming corner, pretty much instantly. Going into the red of the powerband. It does work with any car, people seem to think, going flat out in FXO and jamming it into 1st has something to do with this conversation, well it doesn't.

my attitude is totally pwnage (imo - and mo is the only one that mathers to me). i should just try to do what i've been doing since i bought S2.. just stay away from forums. someone said it perfectly: the more posts u have, the worse ur driving is.

i've been applying the same policy in real life, when i come across an ignorant person with the "i am right because i've been doing it so for xy years", i just have no reason to share any knowlege with them, and knowlege is power and i like power. i also like the looks on their faces after getting beaten by their vehicle of choice. i absolutley LOVE the wide variety of excuses that follow that look.

p.s. i dont know everything, and i'm inteligent enough to recognize that. why other people can't even consider the posibilty of beeing wrong is beyond me, and there it will stay cause i don't want a bleading ulsar in my stomach from dealing with those sorts of people.

kiss to tristan, i'm outta here.

+1 for scipy being a dick...
~Bryan~

Quote from dropin_biking :

+1 for scipy being a dick...
~Bryan~

+1

-1 for the drift tho

I'd quite happily race him in any car. He doesn't seem that quick really.