I have searched, mostly unsuccessfully, to try and figure out some of the details of actually getting it working on the vehicle, and working effectively.
First, I'd like to clarify the setup in TunerStudio. Obviously I need to connect the HE351VE signal input to the CAN broadcasting output pin on my Megasquirt, and set the output pin correctly (or do I only have one choice of MS pin?) and I need to enable the HE351VE broadcasting. Once I enable boost control, is that all that's involved in setting up the boost duty -> CAN output? Or am I missing any steps?
Secondly, I'd like to chat a little about actual control method/algorithm. It seems there are many ways to set this up, so I'd like to share my ideas and find out what you think. It can be a complicated topic, so this post will be pretty long, sorry!
Option A: open-loop boost control with a boost duty map.
Set up the duty table to get the desired characteristics, first by finding the max and min duty to give max and min stroke of the vanes (on the bench).
The way I would approach setting up the table is to first consider the extremes, like engine braking or low-load cruising (whichever is preferred), and WOT operation.
Engine braking (i.e. min throttle) could be set by experimentation -- setting the duty cycle vs. engine speed to get the desired subjective deceleration feeling.
If we prefer not to have the turbo making any impact to engine braking, then we can use the low-load cruising condition as our "bottom" extreme.
For best fuel efficiency, it's pretty simple -- open the vanes fully to reduce back pressure and pumping loss. From cruising throttle position and lower, set the map all the way open.
On the other extreme, we have WOT operation. My thinking here is that to get the best power (i.e. best torque, i.e. best V.E.), open the vanes as far as possible while still ensuring you hit your desired boost. This would be fairly easy to do experimentally: set the WOT values to full open, and do a WOT pull. Decrease the values until you hit your desired boost at each engine speed.
So far, things are pretty straightforward. The tricky part is what should be done in between cruising throttle and WOT, and this is the most important part, I think.
The advantage of a VGT turbo is in the transients, as well as improving WOT VE as described above -- but mostly in the transients. Closing the vanes increases boost response. So naturally, we'd like to close the vanes when we stomp the throttle, and have them open as we achieve our target boost. If we just make the partial-throttle column values the same as their corresponding WOT cells, then we will achieve a nice predictable response, but it won't give the fastest possible response. Maybe it would be adequate? But for best response, we would want the vanes to close as we transition out of our cruising throttle, but open again at WOT. If you slowly roll on the throttle, matching your throttle position to the spool, then this would work pretty well -- but that's a pretty inconsistent control method! And if you're going uphill, and your cruising throttle increases with the increased road load, and the vanes are closed, then you'd lose efficiency and build up quite a bit of exhaust temp with the closed vanes.
Another way to possibly set up the open-loop table in between cruise throttle and WOT would be to do a similar process to building the WOT curve -- accelerate at constant partial throttle positions above your cruise area, and close the vanes just enough to hit a target boost (perhaps the same, or less than, WOT boost target). I still think this method has a lot of compromises, but it's the best I can think of for open-loop control.
Option 2: closed-loop boost control with boost target map, and external wastegate with desired maximum boost
This might be much more straightforward to run, but I still have one main concern.
For cruise, just set a boost target lower than manifold pressure at that throttle/engine speed point -- the control loop would open the vanes to bring the pressure down. Perfect!
For WOT, set the boost target just below your wastegate boost. When you stomp the throttle, your boost error increases and the vanes snap open to spool the turbo. Once the boost reaches the target, the vanes open again, and even a big A/R (once spooled) should overboost above any unreasonably-low engine speed, so you will likely have your wastegate open and your vanes open fully, giving you the best VE and a great spool.
You can then set your partial throttle boost targets to whatever characteristic you want, and it should work well.
The main issue I see here compared to open-loop control is that the VE will be much less stable/predictable during transients. I'm not sure how much of an impact this would make in practice, with a well-tuned VGT control, but I think it's something to keep in mind.
I should be swapping the turbo into the car some time in the next few weeks, so I look forward to any discussion in this thread. Even if I posted a big wall of text that no one reads, at least I got my ideas down in one place
