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Link G4 Plus Software Tutorial: Closed Loop Boost Control

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Closed Loop Boost Control

09.16

00:00 - While open loop boost control is quick and easy to set up, if the boost isn't exactly where you want it to be, the ECU won't do anything to try and reach your target.
00:09 The result may be more or less boost than you really want.
00:13 Closed loop uses a target boost table and a PID control algorithm to adjust the boost solenoid output to achieve your desired boost level.
00:22 It's more time consuming to configure but can give more stable results.
00:27 To start, we need to go back to the boost setup parameter in the boost control menu and change to closed loop.
00:34 There is now one small change in that the deactivation time out value is how long the boost control system will remain in stage two after the MAP or RPM values fall below their activation values.
00:47 We will talk about the different stages of control shortly.
00:52 If we now go back into the ECU settings menu, we can see that we now have two more menu options, target and turbo dynamics.
01:01 We will look at these in a little more detail.
01:04 First the target menu allows us to set up our desired boost target.
01:08 But it also provides trim tables to let us adjust the target boost pressure depending on engine temperature, air temperature and gear.
01:17 Before progressing, zero these tables so they won't have any influence over your boost tuning.
01:22 You can come back and configure these after you've set up your boost levels.
01:28 The boost target one table is where we set our boost target level.
01:32 This is the actual boost level we want the engine to run.
01:36 It's a 3D table with RPM on the X axis and throttle position on the Y axis.
01:42 As usual we can change these axes to suit.
01:45 For simplicity, I'm going to remove the load axis and just use RPM as a 2D table.
01:52 When setting this table it's important to be realistic about the target boost levels.
01:57 If you're targeting a boost level at low RPM which the engine can't produce, this can result in overboost when the RPM increases.
02:05 I'm going to enter some values here for our example, targeting 200 kPa from 3000 RPM and above and tapering this down at lower RPM.
02:15 The closed loop system uses the wastegate duty cycle map we already looked at in the open loop boost control module as a starting point for the boost solenoid before the ECU applies feedback.
02:27 For the best results, you need to spend time on this table and make sure that the resulting boost level is as close as possible to your target.
02:35 The less work the PID control needs to do, the better your boost control will be.
02:41 An important point here is that you need to be able to achieve stable boost control in open loop mode using just the wastegate duty cycle table.
02:50 If you can't get stable control here, the closed loop system is unlikely to help you.
02:56 Before you tune the wastegate table though, we need to change the Y axis to boost target and make sure that the zones match whatever boost levels we're requesting in the boost target table.
03:08 Once the wastegate table is tuned, we need to set the PID algorithm to tune the ECU's control if there's an error between the boost level and our target.
03:18 This is done through the turbo dynamics option in the boost control menu.
03:22 When you first set up closed loop boost control, all the PID parameters will be zero, meaning the ECU will only work off the wastegate duty cycle table.
03:32 The closed loop boost control function is split into three separate stages and we're now going to discuss them.
03:39 For a visual representation of how these three stages interact, there is a handy diagram in the Link help file which explains it.
03:48 Stage one is used as the engine is coming on boost and during this stage, a fixed duty cycle is applied to prevent the wastegate creeping open.
03:57 This can improve boost response and a typical value would be 80-90%.
04:03 Stage two becomes active once the boost level comes close to the target.
04:08 This is the first stage of close loop control.
04:11 Stage two uses the numbers from your base duty cycle table as well as applying proportional and derivative gains to any error between the target boost and actual boost.
04:21 Stage two control us used to control the boost pressure as it approaches the target boost to help achieve target boost quickly without overshooting.
04:32 Stage three becomes active once the boost pressure comes within a certain range of the target boost.
04:38 This is normally set quite close to the target and in this case the ECU applies proportional gain and integral gain to any error that remains between the target and actual boost.
04:49 Stage three is the final stage of boost control and is responsible for making small adjustments if there is any error left.
04:57 There is also a hysteresis which means that if the boost pressure sits on the changeover point between stage two and three, the ECU won't constantly shuttle between the two stages.
05:07 Now let's get back to the actual adjustments we need to make.
05:11 The first option we need to configure is the base duty cycle mode.
05:16 There are two options here, stage two or stage two and three.
05:22 In stage two, the ECU will reference the wastegate duty cycle map for a base wastegate duty cycle until it moves to stage three.
05:30 At this point, the ECU will ignore the wastegate duty cycle map and make adjustments based on the final duty cycle.
05:38 If you select stage two and three, the ECU will always base this duty cycle output on the wastegate duty cycle table and then apply corrections if there's an error.
05:49 A helpful tip here is that if your wastegate duty cycle table is relatively flat, using stage two will generally work well.
05:57 If on the other hand you're making large changes to the duty cycle through the RPM range to cope with boost creep or taper, using stage two and three will give better results.
06:09 Now we'll look at the rest of the settings.
06:12 Stage one duty is the duty cycle used during stage one.
06:16 As I mentioned, normally this will be 80-90% to really keep the wastegate closed.
06:22 Stage two on is the point where the ECU will transition from stage one to stage two.
06:27 This is measured in kPa and the value is removed from the target boost.
06:32 A typical value would be 50 kPa.
06:36 Stage three on is the point where the ECU will transition from stage two to stage three.
06:42 Again, this is measured in kPa and is removed from the target boost.
06:47 A typical value here would be 15 kPa.
06:51 Stage three hysteresis is how much the boost must drop after stage three is active for the ECU to drop back to stage two.
07:00 This value is subtracted from stage three on value and a typical value would be 15 kPa.
07:07 Stage three on delay lets you set a small time delay before stage three control will begin.
07:14 This allows time for the boost pressure to stabilise before stage three control begins.
07:19 A typical value would be 0.5 seconds however if you set it to zero, it's effectively disabled.
07:27 Next we have the proportional, derivative and integral gain values.
07:31 These affect how aggressively the closed loop control operates.
07:35 Increasing the proportional gain will improve boost response but if you go too far, the boost will overshoot the target and oscillate.
07:44 The derivative gain has a dampening effect on the boost control, reducing overshoot.
07:49 Too much derivative gain will slow the boost response.
07:53 Integral gain helps the boost pressure achieve the target value.
07:56 If you increase the integral value too far, you will end up with boost pressure oscillating.
08:02 The help file has some good starting points for some common engines however the numbers do need to be optimised for each engine and turbo combination.
08:11 Next we have max duty cycle clamp which puts an upper limit on how far the wastegate duty cycle can be increased.
08:18 This can be used to prevent unstable operation and should be set a little higher than what we need to reach maximum boost pressure.
08:27 The last parameter is integral clamp which provides a limit to the amount of influence the integral gain has.
08:33 This helps increase stability of the boost control system and a typical value would be 15%.