Boost Control: Open Loop vs Closed loop
Open Loop vs Closed loop
- When we're using an ECU to control a specific parameter on the engine, we have the option of using open loop or closed loop control.
I briefly mention the term, closed loop, in the PID module, but now I will discuss what the terms open loop and closed loop mean.
Open loop is the simplest system to configure and tune so we'll deal with this first.
In an open loop control system, the ECU will operate an output, such as the boost control solenoid at a set duty cycle.
In this sort of system, there is no feedback to tell the ECU whether the boost pressure is higher or lower than our target.
An irrespective of the actual boost pressure, the ECU will keep providing the same duty cycle to the solenoid.
The result is that we may see the boost pressure very slightly from our target, based on engine load or atmospheric conditions.
This defeats some of the advantages that electronic boost control can offer, but to see these advantages, we need to use closed loop control.
Closed loop control will look at the current boost pressure as well as our desired boost target.
Any time the measured boost pressure doesn't match the target, there will be an error and the ECU will use a PID controlled algorithm to actively alter the duty cycle being supplied to the waste gate solenoid, in order to reduce the error to zero.
Closed loop control has the ability to provide more stable and consistent boost levels, as atmospheric conditions change or engine load and exhaust temperature change.
This can give superior results but also requires more work from the tuner in order to optimise the PID control settings and achieve stable boost.
This course will show you how to use open loop and closed loop techniques to control boost.