primerGrey

iTrader: (13)

I have a theory as to the purpose of the restriction pills in the pressure source lines for the pre-control and waste-gate control solenoids, that I haven't read already described, so I thought I'd share it. Just to state explicitly - I don't know if this is correct.

The pill (like any throttle) creates a pressure drop across it that is proportional to the flow rate through it. In a static (zero or very low flow) the pressure on both sides of the pill is the same. At high flow velocities, the pressure on the upstream side will be greater than the pressure on the downstream side.

So the pill acts like a damper - its effect is proportional to flow velocity - a value controlled by the PFC duty cycle.

The location of the pills, on only the pressure source side of the actuator, is worth noting. They slow the speed that boost can be lowered, but do nothing to slow how quickly boost can rise. Control system designers usually avoid this kind of asymmetry non-linearity, since it makes the overall system much more difficult to analyze. So it seems unlikely they were meant to be an active part of the control system.

Sampled controls systems have a limitation called the "nyquist frequency". It is the maximum frequency that the system is able to effectively control. With a perfect linear system, this frequency is half the sample rate. In practice, control engineers typically want to see a sample rate of 10 times the maximum open loop frequency.

I think the pills are there to damp out any high frequency dynamics (pressure waves, hose wall dynamics, whatever) between the actuator (precontrol or waste-gate) and the turbo. The turbo itself is driven by a pulsed exhaust of widely varying frequency (dependent on RPM), so could excite a broad range of resonances in the loop from turbine->compressor->boost hose->diaphragm actuator->waste gate->turbine. If this system does have high frequency dynamics that are beyond the ability of the solenoid control system to respond to, and these effects were at a resonant frequency of the system, boost would oscillate and there is nothing the active control system would be able to do about it. And I think the stock (and PFC) boost control systems have a relatively low sample rate (perhaps because those computers are controlling the entire engine as well as boost), and I'm going to guess that aftermarket dedicated electronic boost controllers have higher sample rates. The pills are placed exactly where you would want them to be if their job is to damp out any of these uncontrollable high frequencies. They act like low-pass filters in front of the digital control system.

If this is true, than pill diameter should be adjusted not based upon the amount of boost desired, but upon the sample rate of the active controller being used. One would think that electronic boost controllers are thus more likely to be able to do without pills (which I think is the general consensus, and my experience). And pure analog controllers like manual boost controllers of course have infinite sample rate, so are the most likely to work well without pills.

And making changes to PFC or stock boost controllers like adding high speed 3 way solenoids that speed solenoid actuation but do nothing to change the underlying system sample rate, will not dramatically improve the ability of the system to run without pills (which has been my experience).

arghx

iTrader: (3)

That's certainly a more detailed of an explanation than my picture of a bottle of barbecue sauce with an orifice in it

I wish electric wastegates were an easy to install aftermarket thing. They are much easier to control from my experience.