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Closed loop VS Open Loop

Boost Control

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Discussion and questions related to the course Boost Control

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After finishing these webinars about boost control, I have come to an understanding that the preferred way to tune boost control is the Open Loop type control. Being able to control boost in so many different ways manually on the dyno by part throttling, full throttle to no throttle to full throttle scenarios. or potentially making the turbo boost sooner? Even being able to give the ecu a zone of decreasing boost slowly or rapidly as a engine safety feature. ( As well with the PID controls in closed loop, but seems much more simple to just tune it for open loop controls and boost fuel cut feature in both boost control methods.)

Just curious about when and why would someone prefer to tune boost control in the closed loop function? I cant think of a time closed loop boost control would benefit a driver or an engine in that case.

The amount of time you spend on a dyno by the hour, it does not seem like you would be able to setup the closed loop control feature as quickly or even as best as you like too.

Andre, Great webinar on this topic really enjoyed it, I did like how you showed the full throttle to off throttle to full throttle on the dyno, to show how the turbo will spike above the desired boost level quickly. Thats something that really will play a big role in drifting vehicles since throttle movement in this motorsport is all over the graph. Being able to properly control the boost levels like that on the dyno to simulate real world driving could potentially save many engines.


I wouldn't say that open loop is the preferred way of controlling boost. A properly setup closed loop boost control system will offer more consistent boost levels as operating conditions change so this can be an advantage. The trouble is that setting the closed loop control can be tricky for those who don't understand what they're doing and the interaction of the P, I, and D gains. By completing this course though you should be in a position to achieve good closed loop control.

The key point is that good closed loop control relies on the open loop control being tuned well in the first place. The less work the ECU needs to do in closed loop, the better the results.

If you understand PID controls, and the way that your ECU of choice uses them, you should be able to setup a closed loop boost control feed forward table in less time than a open loop table, and then have a more stable boost control system through the whole of the engines operating range without having to map the whole of the engines boost areas. Having the closed loop setup correctly also means that if you change the Boost Aim (by 20% or so, not doubling), then the ECU will retain control of the boost with only minor tuning needed. This is also where logging of the boost control system assists greatly in setting up the system, as you can look at the boost control duty cycle and PID values, and then adjust the feed forward table to suit.

Where a lot of people fall over (tuners with experience as well) is not understanding the operation of the PID systems (not just for boost, but Cam control and other systems in the ECU) and either going too small or too large in the numbers, making the PID system not do anything, or have it fighting itself as each of the different functions are trying to assert control over the system, thus making the situation worse, and then the person tuning it turns it off and reverts to an open loop system that is only really tuned for the environment that it was in at the time. If the open loop system is then taken to an environment that is different to that that it was tuned in, such as being driven to the top of a mountain, you can then have the system over boosting as it doesn't have the ability to tune the feed forward table to suit the changes.

One of the reasons that I have found for the use of a stand alone boost controller in vehicles that have an aftermarket ECU with boost control available, is that the standalone systems quite often have a PID system enabled in the background of the controller, that has been tuned to give OK results for a large variety of systems, so that that tuner only has to set their boost aim table, and let the controller do the rest.

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