First, the basics, starting with terms. "Wastegate" is a device that controls boost. There are internal (actuator or "flap" type) or external ("poppet" type) wastegates. An open wastegate reduces boost. A closed wastegate increases boost. Remember this.
"Duty Cycle" is the duration of valve opening time vs the available opening time, expressed as a percentage. i.e., if the valve is always open, then duty cycle is 100%. Conversely, if the valve is always closed, then duty cycle is 0%. Here's where it can get confusing. Depending on how the boost control is set up, an "open" valve may open or close the wastegate. You must know which way the valve is set up to avoid over-boosting.
On a TYPICAL setup:
INTERNAL Wastegate:
Valve open (100% duty cycle) = More boost
Valve closed (0% duty cycle) = Less boost
EXTERNAL Wastegate:
Valve open (100% duty cycle) = Less boost
Valve closed (0% duty cycle) = More boost
The simplest form of boost control is using open loop wastegate duty. Specify a wastegate duty cycle, and you're done! In fact this is how a manual boost controller works. There's a valve that is controlled by spring pressure, once manifold pressure exceeds a certain level,it diverts the manifold pressure to open the wastegate, reducing boost.
Since we're dealing with electronic boost control, we can add another dimension such as gear, vehicle speed, RPM or other form of load signal or input voltage, to vary the wastegate duty. Obviously this feature is specific to each boost controller, some of the more basic ones do not have this function.
Next up is closed loop control. Instead of blindly following a fixed wastegate duty cycle, or varying it by an arbitrary load value, the boost controller can control boost by looking at the target boost, comparing it with actual boost, and change the boost as appropriate. In essence a PID or PD controller.
The "P" stands for "Proportional" gain. "I", "Integral" gain. "D" is "Derivative" gain.
The proportional gain is used to determine the rate of response to a change in boost pressure. If gain is too low, the targeted boost level will never be reached. If gain is too high, there will be overshoot. If gain is just right, it will reach the boost target but there may be slight fluctuations, depending on how quickly the boost changes. This is where the integral gain comes in. It will smooth out the fluctuations if it is set correctly. If integral gain is too high the boost will oscillate and go out of control. The derivative gain is used to further remove any errors from the "I" term. Depending on the complexity of your boost controller, you may not have to set the "I" and "D" terms.
Here's a practical example: Blitz SBC i-Color. For manual mode, all you need to do is program the "SET" and the "GAIN". "SET" refers to valve opening duty cycle. Again, it is critical that you know what a valve opening of "100" means for your own set-up. If your boost is fluctuating around the target or overshoots, then "GAIN" is probably too high. If you never meet the target boost, then "GAIN" is probably too low. Auto mode is the same except that you don't even need to program "SET", just your desired boost.
In this way, practically all electronic boost controllers are the same. The differences lie in the implementation details e.g. using solenoid valve(s), stepper motor, sampling rate, additional sensors etc.
Legal disclaimer: the information above is for educational purposes only. The author shall bear no liability for the quality of the information presented. Follow the instructions at your own risk.
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