Boost Management

Boost management

The replacement for displacement is boost. We can achieve impressive power levels in small displacement motors by putting turbos on them. We'll assume a working knowledge of forced induction... If you don't already have that, there's plenty of places to look.

Once the ecu has calculated the amount of torque the driver wants, it'll use that to dictate how much boost it should try and create. The engine will convert the driver request torque value into an airflow volume. I.e. to make T torque at R rpm we need V volume of air.

Modeled tables: The ecu has no way to calcute actual airflow. There's no MAF. what it does have is PUT and MAP sensors ("PUT" stands for pressure upstream throttle, vs "MAP" which you should already be familiar with.), Baro sensor, temperature sensors, etc. If you know the airflow characteristics of the turbo, you can calcute the volume of air that it flows given its output pressure. So to target V volume of air, the ecu assumes that it should target B boost pressure (PUT set point or PUT_SP)

But there's more involved in choosing a PUT_SP than just modeled airflow tables. We also have tables that define the maximum turbo pressure ratio. At certain target airflow volumes and RPMs, we can tell the ecu to limit the PUT.

And once we've got our PUT_SP, how do we limit the boost? Wastegate duty cycle (or wastegate_sp).

The Simos family of ECUs use electronic wastegate control rather than a traditional boost actuated wastegate. Typically there's a spring that will operate at a fixed boost pressure, and a boost controller will bleed off a certain amount of pressure so the spring operates at a higher boost. An electronic wastegate works by having a target position at a certain boost level.