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Boost solenoids:

 Way more than anyone but a geek would need to know about wastegate control solenoid  valves:


SEVERE DAMAGE CAN OCCUR IF YOU GET THIS WRONG !!! CIRCUIT SE7EN WILL ACCEPT NO RESPONSIBILITY IN ANY WAY.

Here is another useful link : http://en.wikipedia.org/wiki/Boost_controller

 

The Basics:


So in the simplest case the actuator nipple is connected with a hose that leads directly to a source of boost pressure, this means that the boost pressure is fixed to whatever the "spring pressure" of the actuator is. Actuatuors are avialble in different psi ratings. For example many actuators will open at 7 psi.


If one wants to achieve a higher boost pressure than the spring is set for, then one must reduce the amount of boost pressure that actually reaches the actuator. Imagine it this way if you disconnect the hose to the actuatuor it will never opeen thus the boost level will go very high.
So if we  have an actuator that is set to open  at 7 psi and we want to run 10 psi boost then we need have only 7 psi reach the actuator when the boost is at 10 psi. This 7 psi is called the "Pilot" pressure as it is in charge so to speak. Dumbing it down, the actuator "thinks" there is 7 psi boost when really there is 10 psi....we have "fooled" it.

So how do we actually get the pilot pressure to be different than the boost pressure, we have to "lose" 3 psi somehow.
Here is one way:


What happens in the method above is that say 10 psi boost pressure is on the left side of the restrictor, and due to the restictor there is a pressure drop across the restrictor of 3 psi . This lets the wastegate feel 7 psi when boost is actually 10 psi.  The restrictor must be sized to equate to a 3 psi pressure drop.  The bleed outlet restrictor will always need to  be slightly larger (less restrictive) than the boost restrictor .  Mig welding nozzles come in convenient sizes to use as restrictor "pills".    0.035" orfices are about the sizes commonly used.  If there was no outlet restrictor there would just be zero pilot pressure because everything would be effectively open to the atmosphere.

It is true that it is only the ratio of the resriction between the boost restrictor and the outlet restrictor that gives us the pilot pressure that we want, but we choose to use the smalles orifices that won't clog easily so that we are not creating a large boost leak and robbing the engine of significnant boost.

Either restrictor can be replaced with a needle valve to make the sytem adjustable.

The above system gives one a feel for the fundamentals of boost control, but there are better ways of doing it. For example instead of the boost restictor a pressur relief valve can be fitted in it's place. The relief valve blocks all pressure until the setpoint, this way the wastegate does not start to open at all until very close to the final boost setpoint. This decreases turbo lag. Usually by about 500 engine  rpm.

Electronic "active" boost controllers:

A stand alone boost controller or ECU can  Pulse Width Modulate (PWM) a solenoid valve to open many times a second (typically 10-40 Hz /cycles per second). When a valve is open or shut this fast it acts like and adjustable orifice that can be set fully shut or fully open or anywhere in between. Duty cycle means the percentage of time the valve is held open for one cycle.  For example a valve cyling at 10 Hz takes 1/10 or 0.1 seconds to do one cycle.  In that one cycle if the duty cycle is 75% it woul be open for .075 seconds and closed for 0.025 seconds and then start over again on the next cycle. So whatever is controlling it can change the duty cycle to a different value only when a new cycle starts,in this case every 0.1 seconds.  Therefore the higher the Frequency the finer the control (higher resolution).  Many ecu's will let the user select the frequency. There are mechanical limits to different types of solenoid valves which determine the frequencies they can operate at.

Spool valves, pneumatic logic diagrams :

A solenoid operated 3 port spool valve is commonly used for high performance boost control.
Generally any ecu can drive any normal boost solenoid, but if in doubt one must make sure the coil impedence (ohms/watts) is compatible with the driver in the ecu .
Too high of an impedence may or may not lead to slow valve response, but too low of an impedence may burn out the driver in the ECU.




There are 3 ports , one on the left (port 2),one in the middle(port 3),one on the right(port1).

Somewhere on the valve will be a logic diagram like this:


Since there are two blocks/squares this valve has two possible positions.
The two squares with the arrows in them represent the two flow positions of the valve ,energized and non energized.
The numbers on the right side square represent the ports, the numbers are not repeated on the left side (for simplicity).
The arrow shows which ports are connected at the given state. So in the state on the right side (spring position means unenergized) ports 2 & 3 can flow to each other (in either direction) and port 1 is blocked (dead ended).

In the left case (solenoid slash symbol means energized)  1 and 2 are connected and port 3 is blocked.

The rectangle with the diagonal slash "/" means this valve is solenoid operated.

The spring symbol on the right defines the "at rest" (unenergized) position of the solenoid valve.  So the right side block is what happens when the solenoid is un-energized.


Typical Installations:

You are responsible for determing what is correct for you application , doing it wrong wtithout verifying it is correct could result in expensive and dangerous consequences (blown engines etc).


A typical AEM installation may look like this:    





 

Turbosmarts instructions are similiar to AEM's but with other configurations possible:   http://www.turbosmartonline.com/index.php?s=downloads





Solenoids will "lose control" over the valve movement above and below certain duty cycles (pwm%) too high a percent or too low and the "dead time" of the solenoid will cause the valve not to cycle (stay open or closed), there may also be non-linear characteristics near the limits. The lower the frequency,the wider the usable duty range BUT  there is more time lag to the next possible adjustment cycle. Compromise.
I tested various solenoids at different PWM frequencies.   


valve

freq

approx Duty range %

3port MAC-5.4 watt

10

7-90

 

15

10-89

 

20

15-86

 

25

17-80

 

31

20-80

 

40

26-68

3 port MAC-C7-HI-FREQ

31

15-85

 

40

17-82

4 port 5.4 watt

10

9-87

 

20

16-70

 

30

22-62

 

40

25-40

4 port C7-HI-FREQ

10

6-90

 

20

10-80

 

30

14-70

SAAB  900 classic

10

7 - 84

 

15

10-77

 

20

15-70

 

25

18-60

 

31

20-50

 

 

 

 

 

 

 




































 



















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