CDV Hydraulics Explained

[terraphantm]

Nope.

See the section titled "Incompressible flow through an orifice" in the link you gave.

Read that section a little more carefully. You forgot to take into account losses due to friction and turbulence; that's why that dimensionless coefficient is there.

I also refer you to the section titled "Permanent pressure drop for incompressible fluids"

[Dave_B]

Brain hurts. Will just pull the damn thing next time I'm under the car.

[wsmeyer]

I think the biggest difference is felt during downshifts. The clutch pedal can be released faster during a downshift without a CDV. With a CDV I feel like I have to leave my foot near the engagement point until it finally engages before fully removing my foot off the pedal.

Because fluids are incompressible, hydraulic systems cannot store potential energy.

Gases are compressible so pneumatic systems can store potential energy.

If there is movement in your system after you stop moving the pedal you have air in the lines = pneumatic system.

[danewilson77]

Because fluids are incompressible, hydraulic systems cannot store potential energy.

Gases are compressible so pneumatic systems can store potential energy.

If there is movement in your system after you stop moving the pedal you have air in the lines = pneumatic system.

Yep.....but pressure drop will still occur through an orifice with flow, correct?

[wsmeyer]

I'm absolutely in agreement with you, Derek. It's a scenario that generates repeatable results highlighting the difference between CDV and no CDV.

When I first got my car, I tried dropping two gears and stomping on it to see how quickly the car could get out of the way. Coming from a car where pedal position corresponded exactly to clutch disc position, I got hard on the gas as soon as my foot was off the clutch pedal, and the engine revs surged and settled like a failing clutch would, because, well... the clutch was slipping exactly the same way due to that restricted flow.


Actually, the above is a little simplified... the truth is that the clutch disc makes contact with the flywheel much sooner than the perceived "engagement point"... what we typically think of as clutch disc travel is actually more like "clutch disc travel" over the first 30% of clutch pedal travel followed by "progressively increasing clamping force" over the remaining 70% of clutch pedal travel; the engagement point we feel is somewhere in that 70% range where you hit a threshold of sufficient clamping force to hold the clutch disc and flywheel together without slippage.

That threshold amount of force rises slightly with increasing disparity between the rotational speeds of the clutch disc and flywheel (e.g., when revs are poorly matched), but for the most part it's in the same place. More accurately, the relatively-fixed amount of clamping force required to engage the clutch corresponds with a relatively-fixed position in the clutch pedal's travel. Or it should, if there's no flow restriction.

As quickly as you can get to that position, and thus that amount of clamping force, is how quickly you can get on the throttle after a shift. But the rate of application of that clamping force is what the CDV limits. So if your foot makes it to the point where that amount of clamping force is normally applied sooner than that force is actually applied, you'll get slippage until the CDV allows enough fluid through. In my experience, my foot could get there an entire second before the CDV caught up. That's enough time for some very annoying over-revving and slippage.

The movement of the diaphragm spring inside the pressure plate is a bit complex but hopefully you can see that there is a direct correlation between the clamping force and the linear travel of the throwout bearing. The clamping force is variable from 0-100% just as the linear movement of the throwout bearing is variable from 0-100% Whatever the correlation is it is fixed. if 20% linear movement of the throwout bearing yield a 50% reduction in clamping force, that will always be the same.

Now realize that the amount of linear travel of the throwout bearing is fixed to the volume of fluid that is pushed into the slave cylinder.

Since the CDV has no ability to store fluid, any fluid pushed out of the master cylinder, must go into the slave cylinder.

It doesn't matter which direction you are analyzing:

volume in = volume out

Therefore:

THE MOVEMENT OF THE THROWOUT BEARING RELATES LINEARLY TO THE MOVEMENT OF THE CLUTCH PEDAL

There is no exception to this, unless of course, you have compressible air in the lines.

[terraphantm]

Of course volume in = volume out. But that doesn't mean the CDV can't slowdown the whole system. And that's exactly what it does. Everything eventually goes where it needs to go, but it takes a little (and I do mean little -- the delay is probably small fraction of a second) longer. The clutch return spring is still there, and the fluid does move with the pedal. But there is enough resistance to prevent hard launches and make it difficult to perform shifts which require precise timing.

Think of drawing fluid through a syringe with a needle attached vs. one without a needle. If you want to draw fluid at the same rate through both, you'll need to exert a lot more force on the former (and there's generally a small empty space, meaning that you need a pretty large pressure gradient to move the fluid that quickly). Why is this? It's because the resistance through the tiny needle opening is quite small. You can think of the CDV in a similar manner -- the CDV is more restrictive than a direct connection between the lines. The diaphragm spring and clutch return spring exert the same force regardless of whether or not there's a CDV. As such it is reasonable to conclude that the increased resistance delays everything. As for why the difference is noticeable in lower gears than in higher gears, that's because you don't really need full clamping force to prevent the clutch from slipping in higher gears.


At the end of the day, all of the empirical evidence disagrees with your conclusion. Almost every single person who has done this mod noticed a difference, whether or not they bled the clutch (I've personally never bothered bleeding the clutch both times... mostly because I noticed my clutch line had blue fluid in it even though I never actually bled my ATE superblue through it). So either everyone is wrong and you're more well versed in hydraulics than every BMW owner, or your understanding of the mechanisms are flawed. Take your pick.

[wsmeyer]

I've personally never bothered bleeding the clutch both times

I'm not the least bit concerned about what other people think or want to believe and quite frankly, your statement above demonstrates that you really don't understand how hydraulic systems operate and negates all of you observations.

[wsmeyer]

Does there not have to be a pressure drop for the equation to hold true?

That equation would be used to calculate the movement of fluid from a high pressure area to a low pressure area.

If you had a hole in your line, P1 would be the pressure in the line and P2 would ~ 15psi (1atm) and the amount of fluid spraying out would depend on the size of the hole.

If your head gasket is leaking P1 would be the oil pressure ~ 60psi and P2 would be whatever the relief valve in the ET cap is ~ 20psi and again the flow of oil into the coolant would depend on the size of the hole.


Our clutch system is a bit different though. The pressure plate is providing the resistance to movement. You apply a force to the clutch pedal that increases the pressure in the hydraulic line. From Pascal's law: "increasing the pressure at any point in a confined fluid, there is an equal increase at every other point in the container" so according to that equation P1-P2 would always = 0 and there would never be any flow.

That doesn't mean the equation is incorrect, it's just not applicable to our system.

Think about this. If there is a bucket of water on the floor and I push it a distance with my foot obviously the water moved but could you calculate it with that equation? No.

[wsmeyer]

Read that section a little more carefully. You forgot to take into account losses due to friction and turbulence; that's why that dimensionless coefficient is there.

I also refer you to the section titled "Permanent pressure drop for incompressible fluids"

I didn't forget about anything. If this was an assignment in a Physics class we would have to account for a whole bunch of stuff that I neglected because I believe they are insignificant compared to the larger forces in our system:

compressibility of the fluid
amount of gasses in the fluid and it's effect on density
compressibility of the gasses in the fluid
friction losses through the CDV

There's probably more but if you tink they are significant you should start with friction as it would be the largest.

The orifice in the CDV is .060 inches in diameter and .0165 inches in length.

If we call the friction loss through the CDV 'f' and the friction loss through the rest of the system 'F' then f+F / F would give the error factor of neglecting it.

[Mtnman]

I really enjoy reading all of this, and enjoy the debate we can have on our site. Unlike other sites, we don't lob grenades from the back of the fight or make snide comments to others of differing opinions. Im glad this thread is here. Lets continue this discussion the same way it started. Facts, observations, and principle of physics. I look forward to more info Wsmeyer, terra, and dane. I have a hard time shifting 1-2 compared to other cars, and want to find out why before i spend the time to remove something bmw thought should be there.