Stock Boost Control - Closed loop PWM or just Boost On / Off

[cbatters]

I would like to open a thread to collect information about the stock boost control system as a pre-cursor to low cost electronic boost control.

Of particular interest is whether anyone has already logged the boost solenoid control line to determine if the ECU is actually doing closed loop control (using PWM) or just using the solenoid to enable / disable boost. (with actual boost level controlled by flow restriction)

If no data is available, my first project will be to collect/post the solenoid control signal data.



Clnit

[RobBeck]

I have been interested in this phenomenom for quite some time, although I am not skilled nor motivated enough to pursue... I am still highly interested.

With that,

SUBSCRIBED.

Rob

[cbatters]

Originally Posted by RobBeck

I have been interested in this phenomenom for quite some time, although I am not skilled nor motivated enough to pursue... I am still highly interested.

With that,

SUBSCRIBED.

Rob

.. and I finally have the onboad instrumentation necessary to collect this information.

Ready to log boost control on the way to work in the AM - will post early findings.


Clint

[cbatters]

OVERVIEW

1. Stock boost soenoid is powered by the MFI circuit which provides +12V to black lead of the solenoid as soon as the ignition is turned on

2. Boost solenoid is energized when the ECU pulls the white lead of the solenoid to ground.

3. Boost control signal is a constant 16 HZ with varying duty cycle

4. Duty cycle appears to be a function of RPM regardless of load / boost

RPM DUTY CYCLE COMMNENT
2000 100% Constantly energized
3000 70%
4000 60%
5000 55%
6000 50%

OBSERVATIONS
Stock boost control system appears to be little more than a fancy air bleed valve with the added option to turn down boost to wastgate pressure. Further, because it does not appear to use closed loop control, the condition of the solenoid and any restrictions in the amount of air it releases at 50% duty cycle could have a negative impact on boost.

This is consistant with an earlier observation that removing the flow restriction in the boost solenoid nipple increased max boost at lower RPM but did seem to increase boost at higher RPM.

The good news is that it appears that the stock solenoid may be suitable for closed loop control of boost using PWM. (Still trying to figure out why sompeone has not done this in the past. Seems much more interesting than an MBC.)

NEXT STEPS
I need to make some changes so I can log duty cycle instead of raw boost control signal. I am also going to add a pressure transducer to the boost control line so I can log boost control line pressure and see how it correlates to actual boost.


Clint

[RobBeck]

What is your thoughts on the results of changing the actuator springs... do you think that would cause a boost increase proportional to the new spring minus 6psi?

Rob

[cbatters]

Originally Posted by RobBeck

What is your thoughts on the results of changing the actuator springs... do you think that would cause a boost increase proportional to the new spring minus 6psi?

Rob

Had never really thought about it...

Currently headed in the direction of a low cost EBC add-on (<$100) for the logger expansion I have been working on for the last couple months. Coded the PWM control module today. Working watchdog timer / failsafe operation tonight (critical for any closed loop control.)


Clint

[cbatters]

Searching for any info that may have been posted earlier about MBC versus EBC in terms of boost control/spiking.

Haven't found any hard data about the quality of boost regulation - perhaps because few people have ever hooked up boost logging.

If you have any data / links please advise.



Clint

[SHiVeR-91VR4]

I did some research and found out quite a bit about how the MBC's and EBC's work. There are several different types of solenoids people used and regulator valves to get their spikes out. But most of everyone seemed to go towards the MBC because it was soo cheap and can regulate boost quite well if designed properly.

cbatters, u ever take an advanced controls class, or done any work on them? There is a way to make transfer functions that would regulate boost exactly how we wanted. You can write a set of functions to control boost to a preset level without spiking (underdamped response) and to rise to that level as quick as you wanted. These can then be easily implemented in digital logic.

[cbatters]

Originally Posted by SHiVeR-91VR4

I did some research and found out quite a bit about how the MBC's and EBC's work. There are several different types of solenoids people used and regulator valves to get their spikes out. But most of everyone seemed to go towards the MBC because it was soo cheap and can regulate boost quite well if designed properly.

cbatters, u ever take an advanced controls class, or done any work on them? There is a way to make transfer functions that would regulate boost exactly how we wanted. You can write a set of functions to control boost to a preset level without spiking (underdamped response) and to rise to that level as quick as you wanted. These can then be easily implemented in digital logic.

10+ years in process instrumentation / embedded control systems. Which is why I am amazed that people report similar success rates with MBC and EBC. A good EBC should be able to run circles around the MBC - executing PID or non-linear control to produce the desired control response. (under damped, over damped or critically damped response)

http://hyperphysics.phy-astr.gsu.edu/hbase/oscda2.html
http://users.erols.com/jyavins/servo.html



Clint

[cbatters]

A little more food for thought to keep the thread alive.

MBC-Bleeder Valve
When I first started reading up on increasing boost this is what I thought everyone was using but I was very confused abut the differences in how people were hooking them up. As it turns out, some people were installing MBC-Bleeder valves and others were installing MBC-Spring Type valves. (I have to believe that at least a few engines suffered from this confusion!)

MBC-Bleeder Valve can be used by itself or in combination with the existing boost control solenoid. The device works by bleeding off some of the air volume thus lowering the pressure in the control lines going to the wastgate actuators.

This was the first clue that the stock ECU was not really doing closed loop control of boost. If it was, it would have corrected for the loss of pressure in the control line by reducing the duty cycle on the solenoid.

If properly adjusted, a Bleeder Type MBC should produce an overdamped or critcally damped control response because a rapid rise in boost pressure will produce an immediate and proportional rise in control line pressure.

This type of MBC will provide a slow spool up unless the stock EBC is left connected.

MBC-Spring Type
This seems to be what most people are referring to now when they talk about MBC.

This type of MBC has a valve and a spring to control the pressure in the wastgate control line. When manifold pressure gets above some threshold pressure (controlled by adjusting a screw/spring) the valve begins to open pressurizing the control lines to open the wastgates, lowering boost pressure. There is also a facility to bleed off the pressure in the wastegate control lines otherwise they would remain pressurized/open.

I understand how this could work well steady state but I expect it would provide an underdamped control response (leading to boost spikes) during rapid boost transients.

Control line air volume might be very critical for proper operation becaause it introduces a delay in the control reponse time. Clever design of MBC valve could provide a non-linear flow response that would allow a proportional volume of air to flow in advance of the valve fully opening.

I would expect to see large differences in quality of boost control based on subtle differences in valve design / quality control.

It would be interesting to collect some boost / control line p[ressure logs from different MBCs to see which ones really control the best.


Good vendor writeup on MBC control. http://www.boostcontroller.com/display_page.php?i=19

(Although they do not offer any hard data on the quality of their control versuus bleeder valve MBC or EBC.)



Clint