Electronic Boost Control

[avos at cochlear.com.au]

     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.
     
     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.
     
     Unfortunately I am not an expert on these things, and most of my 
     information is based on reading books/mags, but I hope that most of 
     this information is correct.
     
     Adrian


______________________________ Reply Separator _________________________________
Subject: Re: Electronic Boost Control
Author:  diy_efi at coulomb.eng.ohio-state.edu at INTERNET
Date:    04/4/97 13:31


avos at cochlear.com.au wrote:
     
>      Hi Everyone,
>      
>      I am a new comer to this mailing list, and I'm not 100% sure that the 
>      information which I am after is applicable, but I'll tell you anyway. 
>      
>      I own a 1992 Subaru Liberty (or Legacy) RS Turbo, for which I wish to 
>      add an electronic boost controller. The advantage of such a system is 
>      improved boost curve through the use of a PID control system instead 
>      of the current proportinal pnuematic bleed control system so better 
>      bottom end performance, and more accurate control of boost than the 
>      current system which due to its dependance on the flow of gases, can 
>      vary alot with temperature.
     
<good stuff removed>
     
>      Adrian
     
Maybe I'm confused,  but do you plan to improve the bottom-end 
performance?  If the waste gate is closed, how can an electronic 
control improve things?
     
je
jengel at fastlane.net
     
     "I can resist anything but temptation"
                                  Mark Twain