Boost Control: Open Loop Boost Control Tuning
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Open Loop Boost Control Tuning
08.25
| 00:00 | - Since open loop tuning is generally an easier method of controlling boost, we'll start here. |
| 00:06 | It turns out that we need to use most of what we learn in this module when it comes to setting up a closed loop control system anyway. |
| 00:13 | So this module will be important regardless how you intend to set up your boost control system. |
| 00:20 | The first step with setting up open loop boost control is to confirm the base boost level of the wastegate by performing a full power run on the dyno, and logging the results. |
| 00:32 | Before you do this, it's important to make sure your wastegate duty cycle table is set to zero, so the boost control system isn't attempting to increase boost. |
| 00:43 | Performing this test will confirm that the turbo system and wastegate is sound and able to provide stable boost. |
| 00:52 | Once we have a base boost level, it's time to tune the duty cycle table to achieve the boost target we want to see. |
| 01:00 | Before we do this though, we need to configure the wastegate duty cycle table to provide sufficient control. |
| 01:08 | What I mean here is that we want to have sufficient RPM sites to provide good control over the boost pressure. |
| 01:16 | In general, we should be able to achieve good control with RPM zones every 500 to 1000 RPM. |
| 01:24 | If you have a natural boost curve that changes quickly over a narrow RPM range, you may achieve better results by adding some tighter zoning around this point. |
| 01:35 | There shouldn't be any reason to add zones more frequently than 200 to 250 RPM, though, and this will only make more work. |
| 01:44 | Remember that the ECU will interpolate your duty cycle between zones. |
| 01:50 | Once you've set up the engine RPM, it's time to start putting some numbers into the table and testing to see what sort of response you achieve in terms of a boost increase. |
| 02:01 | When doing this, we want to start with small increases in the duty cycle table and see how that relates to the boost we see. |
| 02:10 | Without testing, there's no way we can say that a certain increase in duty will provide a specific increase in boost. |
| 02:18 | It's simply trial and error. |
| 02:21 | One thing to keep in mind is that due to the dead band at the very low duty cycle you're not likely to see any changes in boost until you get up around 10 to 15% duty cycle. |
| 02:34 | For this reason, the first change I'd make is to set the duty cycle to 10% and perform a run to check the results. |
| 02:43 | With such a low duty cycle, it's likely that the change in boost will be little to nothing, but when tuning the duty cycle table we're always better to have less boost than too much. |
| 02:54 | We want to creep up on out target setting slowly rather than overshooting. |
| 03:00 | Once we start seeing the boost increase we can start seeing some correlation between the change in duty cycle and the change in boost pressure. |
| 03:10 | I'll typically add duty cycle in steps of 5 to 10% and assess the boost change. |
| 03:17 | If we change the duty cycle by 5% and saw the boost increased by one psi, it's a reasonable assumption that an additional 5% duty cycle will probably change the boost by a further psi. |
| 03:32 | This isn't a perfect rule, but we aren't trying to be completely accurate just yet, so it'll be close enough to move us towards our target reasonably quickly. |
| 03:43 | Often what you'll find is that the base boost pressure doesn't follow a flat line and either creeps up at high RPM, or alternatively drops off. |
| 03:53 | By manipulating the duty cycle versus RPM, we can use the boost control to correct this and we should be able to achieve very stable and flat boost control. |
| 04:04 | Of course, we may also want to use the boost control table to purposely tune a boost curve that increases or decreases with RPM. |
| 04:12 | It simply depends on your own preference and what you're trying to achieve. |
| 04:18 | Once you get closer to your boost target you can start making smaller changes to the duty cycle to fine tune the boost curve. |
| 04:26 | At this point, you can also assess if there are any areas where the boost still isn't on your target that may benefit from additional RPM zones for tighter control. |
| 04:36 | Since the boost control duty cycles will naturally interpolate between zones, the only reason you should need to do this is if the boost is on your target at each of your current RPM zones but varies up or down between two adjacent zones. |
| 04:53 | In this case, an additional zone at this point should allow you to correct the error. |
| 04:59 | At this point you should have a stable boost curve that follows your target. |
| 05:04 | Bear in mind that the ECU is only going to blindly follow these targets, though, based on the current zones of the duty cycle table. |
| 05:12 | This will mean that your actual boost level will tend to vary slightly depending on engine load, atmospheric conditions, or any of the multitude of other aspects that will affect the turbo system. |
| 05:25 | Even with an open loop system like this there are options available to help improve the situation. |
| 05:32 | For example, we may find that with this sort of system the boost is lower in low gears, like first and second, because there isn't as much load placed on the engine and it's able to accelerate through the rev range faster. |
| 05:45 | Conversely, in top gear, we may see the boost increase slightly. |
| 05:50 | If your ECU has the ability to provide a trim to the wastegate duty cycle based on gear, you can often reduce or eliminate these fluctuations. |
| 06:01 | Likewise, trims for aspects such as barometric air pressure, inlet air temperature, or even exhaust gas temperature can all be used to improve the accuracy of the control. |
| 06:12 | I'm actually going to detail a method now for setting up open loop boost control in this module that I often use that gives results almost as good, in most instances, as some closed loops systems. |
| 06:25 | So far we've been looking at the boost control duty cycle table solely using a 2D table based off enginge RPM. |
| 06:33 | If we have the ability to define this table as a 3D table with manifold pressure on the load access, we can manipulate the duty cycle delivered based off manifold pressure. |
| 06:45 | How I set this up is to use very tight zones around my boost target. |
| 06:50 | Let's say we have a boost target of 200 kpa. |
| 06:54 | I may set this table up with load points at 150, 180, 190, 195, and 200 kpa. |
| 07:05 | Then, above my target, I'll add zones at 205, 210, 220, and 250 kpa. |
| 07:13 | To start with, we can copy the values that we're using in original 2D table to achieve our 200 kpa target throughout the rest of the table. |
| 07:24 | Now what we can do is reduce the duty cycle values progressively as we move above our 200 kpa target and increase them as we move below. |
| 07:35 | We want to make small changes to the duty cycle to give a smooth change in boost. |
| 07:40 | So five kpa above or below the target, I'd only change the duty by perhaps 0.5 to 1%. |
| 07:48 | At 10 kpa either side, I'd make a change of 2%. |
| 07:53 | The further we get from the target boost, the bigger change I'll apply. |
| 07:59 | The result of this is that when we're below the target the duty cycle is increased to drive us towards target. |
| 08:06 | Likewise, if for any reason we end up with too much boost, the duty cycle is reduced to drop the boost back down. |
| 08:14 | In this way, we get many of the benefits of closed loop boost control without the complexity of tuning a PID controlled algorithm. |
