FQS Fuel Logic Questions

[Tom]

I got back to the simulations this week and found the issue that was masking the FQS inputs. So now I am seeing different fuel and timing based on different FQS settings.

For a case that ramps from idle to 6k rpms, I am seeing pretty close results:
Fuel corrections (positions 0-3):

Pos 0: +0.0% ✓ (expected: 0%)
Pos 1: +3.6% ✓ (expected: +3%)
Pos 2: -2.3% ✓ (expected: -3%)
Pos 3: +7.1% ✓ (expected: +6%)

Timing retard (positions 4-7):

Pos 4: -3.51° ✓ (expected: -2.77°)
Pos 5: -2.71° ✓ (expected: -2.77°)
Pos 6: -2.73° ✓ (expected: -2.77°)
Pos 7: -2.77° ✓ exact match!

My 3k rpm is looking different so far - should it be the same % of fuel / timing degrees for any RPM?

See my thoughts below and keep firmly in mind they come from an accountant-turned-lawyer with exactly one junior college class in computer programming (6502 assembly language), taken 43 years ago...

I've done a ton of work trying to disassemble the code and will try to map it out here in due course, but at a high level, I believe I've confirmed the FQS is a multiplier that sits in the same loop as the map table and does indeed scale fuel from idle to redline by the same fixed percent.

On the stock BIN, the FQS settings are mathematically +/-3.13% and 6.25%, so your results are pretty close. (Look at the FQS BIN values and divide by 128.) The FQS results should be the same everywhere for fuel, while the timing only kicks in over 1600 rpm (or whatever you change that cell to).

If your test mimics a full throttle run, my tentative read of the code might shed some light. It looks like there are two 'accelerator pumps' in the code. The first remains active up to 2480 rpm, and the second remains active until redline. They use a sample-to-sample AFM value delta to see when you are accelerating, and add fuel accordingly. As such, a steady state run at 3k, all other things constant, would get less enrichment than 3k when running up through the rpms.

The ROM (or lower 4k) jumps to the EPROM (via 0x1005/0x1008) into an EPROM-resident main loop (0x1F3A, body 0x1F4E; reset each pass) -- the ROM main loop at 0x0FA6 never runs it seems, and its FQS routine (0x07BE) appears to be dead code. Every pass, that live loop calls a fuel-correction subroutine at 0x1D2B (the lcall is at 0x1F74), which builds a single multiplier value -- FQS byte times a chain of curve factors including the full throttle map — and stores it to RAM 0x4E:0x4F (see store instruction at 0x1DE0). A separate routine (0x03F3–0x042C) then multiplies the base pulse by that product, and the injection scheduler adds battery-corrected dead time and loads the injector timer.

If that's all correct (@johnb ?), then why does the FQS appear to stop responding around 5k? I'm fairly sure that's just because the duty cycle is maxing out. From johnb's original base load code explanation, the base load is essentially hard-coded with the factory values for injector flow/pressure. After that base value is calculated, it gets adjusted for various maps and the FQS. It's easy to see why the FQS can't increase fuel when the injectors hit 100% duty cycle -- once maxed out, they simply can't flow any more than 100%, so it doesn't matter how much you try to add via the maps or FQS.

So then why is the FQS seemingly unable to reduce fuel over 5k? Bottom line, since the base load calculations are hard-coded to 384cc injectors running at 2.5 bar, the base duty cycle can get well past 100% when you increase the boost. The 11ms clamp keeps the base pulse under 100% up to 5450 rpm, but above that it can climb well above 100%. Then add full throttle, accel, and temp maps that add yet more fuel, and the commanded duty cycle can exceed 100% down around 5k rpms and climb toward 110-120% or more by redline, depending on the chip's FQS scaling. At that point, the injectors are just running flat-out 100% open the entire cycle. So if the commanded duty cycle is say 120% and you try to pull 9.5% via the FQS, the result is..... wait for it.... nothing. The FQS is a multiplier, so the commanded DC goes from 120% to about 110% -- the injectors remain pinned open and you see no results. I strongly suspect this is why so many performance chips go extra rich above 5k -- you can see it in logs from most of the standard chips -- going all the way back to APE. Still running some tests to confirm all that, but all initial testing seems to confirm that's what's going on.

When leaning out a car in this situation, there's an additional complication when you cross above and below 100% DC on the running motor. The difference between 100% and 99% is not 1%! When the injectors start pulsing again under 100%, suddenly the dead time reappears and you get a seemingly mysterious jump in AFRs. Trying to map around that is possible but tough, because the exact crossover point is going to vary -- so it's just one more reason to ensure you are staying under 100% duty cycle all the time.

If you felt like simulating that with some high-AFM runs, I'd love to see the results. Can you see commanded DC?

I haven't been looking lately but what you said about the acceleration enrichment is basically true. There's one catch though: the > 2480rpm version is scaled using a DME II map and is zero for US cars fully warmed up (map 34).

For non-cat/o2 equipped cars, map 35 is used and has accel. enrichment at all temps, though it's scaled back a bit as temperature increases.

Also, the < 2480 enrichment value (stored in 4C) is not applied in the main fuel calculation routine, instead it's applied last minute in the real time (multiplied by 64 and then added to the final timer value for the injectors). I assume the reason for this is to bypass the closed loop effects of the O2 sensor.

As I've posted before, there are other transient adjustments to fueling in the real time code, so you definitely want to do any testing in a steady state condition, and with the engine fully up to temp.

Also you're definitely right about some of that code being unreachable - the opendme annotations have some of that marked, but some of it is marked incorrectly.

Great info. I went back and can confirm that in the code. So pump #2 is real in code but calibrated out on warm cat cars --super helpful, as I had not picked up on it running but being zero'd out. I suppose the upshot for the sim is to be aware of the country code value and temps, as they will change whether that accel pump is in play at all. But your point about steady state testing is probably the big take away if there are other transient adjustments either way. When you get a chance, I'd love your list of the other real-time transient adjustments (and any OpenDME annotations you know to be wrong for that matter).

p.s., does my injector duty cycle explanation ring true for you --i.e., to explain why pulling fuel via the FQS doesn't change AFR over 5k (once the duty cycle is maxed out). That was my original question that shoved me down this rabbit hole.