Going lean under boost and high load

[jlturpin]

I've got my 944 turbo running great, with one exception right now.

This is sort of a cross post. http://www.microsquirt.com/viewtopic.php?f=87&t=23045

Car goes lean, no matter the VE table setting under high load and boost.

Here is the datalog, http://www.autocrossphoto.com/microsquirt/951072908.xls

See log around 468-470 seconds in log for good example. My target AFR is around 11.5 under higher boost (Higher Compression engine, 9.7:1). It goes to 11.5 or below as it should according to my MAP/VE table, and holds for a few seconds and shoots up. I have checked VE tables, and changed them to with no luck.
Could the autotune function be causing this? EDIT

Attached is my MSQ file.

[Matt Cramer]

I'm wondering if it is a fuel delivery problem with the fuel pump or fuel pressure not being up to sustained loads.

[jlturpin]

If I put the factory ECU back inplace, my afr's are perfect. It is a new High volume fuel pump, new filter and static pressure is set on my adjustable FPR. I'll see if I can find some specs on the siemens injectors today that may help with my injector config.

I did disable the autotune function (I think) on the uS. This made no difference.

[SQLGUY]

then I would guess there's an accel enrichment problem. Sounds like your accelleration enrichment (MAPDOT, TPSDOT, or XTAU) is providing enough fuel to mask a problem in the AFR target table or VE table. You might want to try disabling AE and then tune your targets for correct AFR after the initial lean spike, then reenable AE and see how much of the spikes you can pull back out.

[jlturpin]

I'll give the AE a try, but.... If I go lean, and immidately let off and go back to 50+% Throttle, it is still lean. Should'nt the AE take over and add more fuel for the designated number of cycles?

[grippo]

You are using x-tau with the warmup + accel option. Use the x-tau accel only option and then work on reducing the x-tau decel enleanment. This can often be too much when you pop your foot off the throttle - there are places where this is totally killing fuel (xtcor = 0).

[jlturpin]

It actually goes lean, 100%TPS, and I lift when I see the rise in AFR. I will try that, but even this afternoon all enrichments turned off and tried to dial in by VE, It would hold the correct AFR for a few seconds under 100% TPS and start to drop. The length of time that the AFR's stayed on track, seemed to have something to do with the amount of load, lower gears seemed to hold a little longer, while in higher gear on an incline caused it to lean out much quicker.

I am using Siemens 3102 Low Imp. injectors. I have changed my PWT% to 2, and my open time to 2ms, and this seems to work best.

Thanks for the help

[jlturpin]

I found this post, and feel like it is more than likely my problem. I am running a total of 4, 2.5 ohm injectors. 2 per driver. I'll do some more research and post up any findings.

Yep, this one was tricky! But having the scope on the output available was the help needed....

The driver stage on the MicroSquirt is the VND5N07 from ST. A nice part in that it will current limit at 5 amps and will turn itself off during overtemp. Datasheet indicates that the temp shutdown is for a die temp at 150 DegC, restoring at 135 DegC.

When this part reaches the 5 amp point it goes into a linear mode to hold the current at 5 amps. The best operation is if the load is such that the driver almost reaches the 5 amp point or a bit over, because when the part is in linear mode it is generating more heat.

Here is an example - for a 2.4 ohm load low-Z injector and a 13 volt vehicle battery the ultimate current is 13/2.4 = 5.41. The part will hold the current at 5 amps, so the 0.41 amp remainder is basically dropped by the driver chip. To do this the driver increases its internal resistance such that the equivalent sum of resistance around the loop yields 5 amps at 13 volts. From ohms law it takes 2.6 ohms to give 5 amps at 13 volts (2.6 = 13 / 5). The injector has a 2.4 ohm resistance, so the driver chip introduces another 0.2 ohms to maintain the 5 amp limit. Now, since there is a 0.2 volt drop across the driver and 5 amps is flowing, the power dissipation is P = I * I * R = 5 watts (5Watts = 5 amps * 5 amps * 0.2 ohms). This power dissipation can be handled with uS due to the low duty cycle of injection, and the fact that there is inductance which slows the time it takes to reach current limit.

Now, take the 1 ohm injector case and 13 volts. The ultimate current thru this injector (unlimited) is 13 amps (13 volts/1 ohm). The driver will limit at 5 amps, and at 13 volts again this is a 2.6 equivalent series resistance to maintain this. The injector is 1 ohm, so the driver would have to introduce another 1.6 ohms of equivalent resistance to maintain the 5 amps holding current. The wattage is 5 amps * 5 amps * 1.6 ohms = 40 watts!

It is interesting to compare this to high impedance injectors. In my SBC, I have 4 high-Z injectors per bank in parallel. Each injector is 12 ohms, so four in parallel is 3 ohms load to the driver. At 13 volts the current is 4.3 amps at maximum - this is below the 5 amps current limit point so the driver will never go into current limit mode.

- Bruce

[jlturpin]

OK, hopefully there is someone who can help.

I bought a couple of Ohmite resistors and tested over the weekend. I tried a 10 ohm 25 watt resistor and i could not get the car started. I'll try them some more later this week.

I installed the 3 Ohm, 50 watt, resistors and the car started, but idled around 3k rpm, and it was very rich. I tried changing required fuel, Injector opening time, ve tables, timing and several other settings, no real luck. Anyone have any ideas?

Thanks,

Jonathan

[newtyres1]

You need one resistor per injector, and need to change the PWM inj parameters to suit a hi-Z inj after fitting the resistors. Hope this helps, maybe you already did that.

Ian.