T7 Tuning - BoostCal

how to get the Trionic7 boost controller to work with your new shiny turbo.

 

 

The goal of this class:
To be able to bolt on a turbo with unknown characteristics and properly calibrate Trionic7 for this particular turbo.

Prerequisite hardware:
Anything that let's T7suite make realtime logs (mandatory)
EGT-sensor (optional but useful)

Prerequisite knowledge:
Understanding of how the mechanical boost control works
Understanding of how the boost control valve works, and how it affects the mechanical boost control

Useful optional knowledge:
Control theory, and understanding of PID-controllers


Lets get to it.
What is BoostCal.?
It is a group of symbols in Trionic7 that contains calibration data for the boost controller.
At first, it might look daunting. There are many symbols under BoostCal. Rest assured, we will only need to touch a handful of these.

The most important ones are:
BoostCal.P_LimTab
BoostCal.I_LimTab
BoostCal.PMap
BoostCal.IMap
BoostCal.DMap
BoostCal.RegMap
BoostCal.ST_Enable
BoostCal.EnableBoostAdapt
BoostProt.PWM_Calc


The boost controller is what we need to control the turbo boost so that the engine gets the correct amount of air at all times. We have several inputs and one output. The most important inputs are mreq, mAir and engine RPM. once everything is processed by the boost controller, we get a PWM signal output that is sent to the boost control valve.
High output duty cycle leads to increase in boost pressure
Low output duty cycle leads to decrease in boost pressure

mreq is the equivalent of the setpoint (SP) when speaking in industrial terms. mAir is the process value (PV). When mreq changes because of a sudden change in driver pedal input, we want mAir to follow as quickly as possible, while remaining stable. As always when dealing with control systems, there's a compromise to be made between quickness and stability.

When does the boost controller go active?
The boost controller is not always active. This is because your turbo also has traditional mechanical boost control. The maximum boost of the mechanical boost controller is determined mostly by the spring in the wastegate actuator, and is called the base boost. Up until you reach base boost, there is no electronic boost control active. The electronic boost controller only goes active once it determines that base boost is not enough to achieve the airmass demand.

The base boost is really important, because it will affect everything we do. A significant change in baseboost means that you will have to go back and redo everything regarding boost controller calibration. Therefore, you need to choose your baseboost before starting out. Choosing to low baseboost will lead to problems reaching max boost, and can potentially make for unstable boost control under max engine load. Choosing too high baseboost means you have less of an area where the electronic boost controller is active. This can have negative consequences, especially on a daily driver.

A good baseboost will be something between 1/2 and 1/3 of your estimated maximum boost pressure.

So now we have a new turbo, with much higher baseboost than stock. How do we tell the boost controller to not activate too early? First we need to turn the boost controller off and then drive around on baseboost only, while logging at least mAir and engine RPM.
BoostCal.ST_Enable = 0 (turns off the boost controller)
BoostCal.EnableBoostAdapt = 0 ( turns off boost adaption)

Drive around and make some full throttle accelerations while logging mAir and engine RPM. You need to go through the entire RPM range, from idle to redline so that we get a nice mAir curve in our logs. After getting some nice plots, you should go home and sit down at your computer to analyse the data and update your bin accordingly.

The symbols we need to edit are:
BoostCal.P_LimTab
BoostCal.I_LimTab

The way this works is quite simple. When mreq goes higher than the values in these maps, the boost controller goes active. Therefore we need to see how much airmass your engine managed to get on baseboost. Look in the logs you made, and fill in these tables with the airmass you got at full throttle.



Note the high numbers at the lower part of the I_LimTab. These are to make sure the I-part of the controller is not active at RPM's where the turbo is not fully spooled. This is important because the I-part of the controller would otherwise accumulate an error if you floor the pedal at low RPM's where the turbo can't physically keep up with what the controller is asking of it. If it is allowed to accumulate such an error, then it will take some time before it stabilizes once the turbo is fully spooled. This leads to unstable boost.

When you have filled in everything according to your logs, you might want to decrease the numbers slightly. Subtract 10 or 20 mg/c to create an overlap so that the boost controller kicks in just slightly before maximum base boost is reached. The reason for this is too minimize the transition from mechanical to electronic boost control. If the transition is not smooth, then you might feel a pause during turbo spool-up, which is really annoying.

Now the boost controller knows when it's needed and when it's not. Time to move on to other things!