Electronic Boost Control

[Orin Eman]

>      A standard wastegate has a spring which will set it's response to a 
>      given signal pressure. If the wastegate input pressure line is 
>      connected to manifold pressure, the wastegate will open an amount 
>      directly proportional to the manifold pressure. The amount that it 
>      opens depends on the tension of the spring (which holds the wastegate 
>      flap). I guess that it would be possible for the actual wastegate 
>      opening to be not directly proportional to the pressure if the spring 
>      is non-linear, and/or if limits are applied to the movement of the 
>      flap.

You an put some preload on the spring to raise the pressure at which
the wastegate starts to open... shims, or adjustable spring perch are
often used.

>      The problem with this system is that the wastegate starts to open well 
>      before the set boost level is reached (as it always opens an amount 
>      proportional to the boost level), so at low boost levels when you are 
>      hoping to get every ounce of boost into the manifold, it can be 
>      inefficient. Naturally at high boost levels where you have boost to 
>      burn, it makes no difference. 

>      In a electronic boost control, PID control can be used so that the 
>      Integral term delays the opening of the wastegate, so that it is kept 
>      closed until later on the boost curve, so you get more boost when you 
>      need it most.

Couple of ways this can be done... use a solenoid valve to interrupt boost
to the wastegate and only allow boost to the wastegate when it reaches
the level at which you want the wastegate to open.  You can use a
weak spring on the wastegate this way.

Another way is to add boost pressure on the other side of the wastegate
diaphragm.  This is the way my 86 Audi 5000CS quattro works.
You can find lots of information on this in the audi quattro list archives
at coimbra.ans.net/quattro.html.

Orin.