Originally posted by varg
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The controller is electronic, with a built-in closed loop control - but the closed loop does the job of matching commanded vane position with actual vane position. It doesn't determine what the appropriate commanded position should be, so I had to design the system to figure it out. The vane position gets sent to the controller over CAN by sending a 10-bit value (in practice, the fully open position is a command value of 50, and fully closed is a command of 960).
I start with the Megasquirt boost target map to determine a "demand" for more boost based on boost error, or target boost minus MAP - in general, the bigger the error, the more I should close the vanes. As boost builds and error decreases, the vanes open, until they're fully open by the time I hit the boost target from the Megasquirt map. So it's a mostly closed loop, but ultimately boost is controlled by the wastegate unless I'm well below the cam's operating range (below 3000rpm). Even with vanes fully open, at higher engine speeds it'll build at least 15psi with a closed wastegate with my engine (haven't tried to go higher, and even that "test" was an accident! I forgot to reconnect my wastegate boost signal at one point).
My goal with the VGT is twofold:
- Top priority, take advantage of my 280/280/11mm turbo cam to make as much top-end power as I can at a given boost level.
- Having the VGT fully open once turbo is spooled to wastegate pressure will minimize exhaust manifold pressure and increase flow. No point in having the vanes closed any amount if the wastegate is controlling boost.
- Tubine housing is something like 26cm^2 when vanes are fully open; for a given boost level, a bigger turbine housing (A/R, nozzle, etc) should make more power, eventually plateauing due to other factors.
- Second, offset the relatively large wheels and housings by using the VGT function to get the turbo to spool more rapidly.
- There's definitely a balance in the tuning here: get too aggressive with closing the vanes, and the nozzle gets small enough to choke the flow through the head - leading to less torque for sure, but because you have less flow, slower spool too, because less power into the turbine! Minimum nozzle size is something like 2.5cm^2, so this is a definite possibility!
- So the VGT, if it's working right, just gets the turbo up to speed sooner, it doesn't affect ultimate boost level unless it's way too timid at low RPM (turbo won't spool at all).
- The VGT can choke the engine enough to significantly decrease torque, so to this effect, I've built-in a map on my microcontroller that limits the minimum turbine size based on RPM. This should also help avoid WOT surge at low RPM due to big turbo (stock on 6.7L Cummins 24-valve) on small engine:
So an example of this can be seen especially in the second log: I went from low throttle to WOT at 4000rpm, and the target boost (165kPa) jumped significantly higher than my MAP value, so I closed the nozzle "fully", or to about 7.6cm^2 for this RPM.
My current control algorithm is purely proportional to boost error (target minus MAP), but the scaling is set so that I get the minimum allowable nozzle size until I'm some percentage of the way between baro (80kPa where I live) and my boost target; this threshold is currently set to 65%. So the nozzle stays at 7.6cm^2 until I reach 65% of gap between baro and boost target, in this case, until MAP gets to 135kPa. Then between 135kPa to 165kPa, I linearly open the nozzle so it's fully open when I reach boost target. Right now my wastegate is giving me between 167-170kPa pretty consistently. Tuning the minimum allowable nozzle size, the fully-closed transition to proportional control (i.e. the current 65% value), and the boost target map *should* provide enough tuneability to hit that balance between boost pressure and mass flow.
Originally posted by varg
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Substituting for those sensors, I can see an immediate flow impact by watching AFR. I'm running open-loop fueling, with a pretty good fuel map for wide-open vanes at the moment: so if I close the nozzle enough to choke the flow significantly, I see richer running. Also, the butt dyno doesn't lie if I'm way too aggressive - a slight throttle lift will drop the boost target, causing the vanes to open, but MAP hardly changes and I can instantly feel more torque! Currently this is the case around 2000rpm.
Here are a couple datalogs to compare. I think my minimum nozzle size is a little too aggressive when I go WOT at 3000rpm, but I also start opening the vanes too soon as boost builds (MAP is below the target for far too long). I should probably increase the minimum nozzle size there, but let it stay at that minimum size for longer. You can see I got quite a bit richer than target with the vanes closed, but once the vanes started opening, I leaned out immediately even though VE didn't change much.
Second datalog I went WOT at 4000rpm, less difference in AFR based on vane position (although I need better AE tuning there, too). I think it's pretty close to ideal at 4000rpm (but my wastegate is getting blown open from the exhaust manifold pressure, slowing the spool significantly).
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