82 lines
4.3 KiB
Markdown
82 lines
4.3 KiB
Markdown
# Wall Wetting (X-tau) Acceleration Compensation
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Wall Wetting (or X-tau as it is sometimes called) is one of the acceleration fuel compensation methods available in rusEFI.
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This method uses a simple model to estimate the amount of fuel that drops out of the intake charge when the manifold pressure changes.
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## The Detail
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There are 3 key factors that go into the X-tau method:
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1. X = the fuel deposited on the wall of the inlet port
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2. Tau = the build up of fuel on the wall and how persistent it is
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3. T = The temperature of the air and surfaces
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The math used is based on: SAE 810494 by C. F. Aquino and SAE 1999-01-0553 by Peter J Maloney
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M_cmd = commanded fuel mass (output of this function)
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desiredFuel = desired fuel mass (input to this function)
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fuelFilmMass = fuel film mass (how much is currently on the wall)
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First we compute how much fuel to command, by accounting for
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a) how much fuel will evaporate from the walls and enter the air charge
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b) how much fuel from the injector will hit the walls amd stay there, thus being deposited
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Next, we compute how much fuel will be deposited on the walls.
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The net effect of these two steps is computed (some leaves the walls, some is deposited)
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and stored back in fuelFilmMass.
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alpha describes the amount of fuel that REMAINS on the wall per cycle.
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It is computed as a function of the evaporation time constant (tau) and
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the time the fuel spent on the wall this cycle, (reciprocal RPM).
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beta describes the amount of fuel that hits the wall.
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TODO: these parameters, tau and beta vary with various engine parameters,
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most notably manifold pressure (as a proxy for air speed), and coolant
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temperature (as a proxy for the intake valve and runner temperature).
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[//]: # "Note - Are we sure these all increase?"
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[//]: # "TAU: decreases with increasing temperature."
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[//]: # "decreases with decreasing manifold pressure."
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[//]: # "BETA: decreases with increasing temperature."
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[//]: # "decreases with decreasing manifold pressure."
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The current implementation of X-tau ignores coefficients (X) below 0.01 and RPM below 100.
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## How to tune it
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[//]: # "need a screen shot of the x-tau TS section"
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### Evaporation time constant /tau (seconds)
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This value is the one that sets how long the fuel takes to re-enter the air charge after being deposited on the manifold walls.
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Increasing this will have the effect of making the enrichment last for a longer time. When tuning increase this value if your logs show that the AFR goes lean towards the end of the acceleration.
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Similarly reduce this value if the AFR becomes increasingly rich during the acceleration.
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### Added to wall coef / Beta (Fraction)
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This value sets how much of the fuel lands on the walls during an acceleration event. Increasing this value will cause a larger quantity of fuel to be added to the inlet charge.
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A large value here implies that a lot of the fuel is landing on the walls of the inlet manifold, requiring a large value may indicate that the spray pattern of the fuel injectors is incorrect for their positioning or the manifold type.
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While tuning if you are failing to prevent the AFR from becoming leaner then increase this value.
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Good tuning of the X-tau system relies on taking logs and some trial and error to tune the coefficients correctly.
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## Old info
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This is about 1 second worth of log, during a 2-3-4 shift at low-ish RPM, but near wide open throttle. Worst case scenario for AE!
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The yellow trace on the top graph, AfrRatio, is a ratio of target vs. actual. If the engine is lean, it goes below 1, and rich above 1. Graph is scaled to +-20%. I'm using this because my actual AFR target varies from 14.7 all the way down to around 11.5. This swing causes the AFR signal to look like the ECU is doing a poor job, when in fact the setpoint is moving by nearly 30%.
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There's a bit of a spike rich-then-lean during the shifts, but it's not bad (results in a nice burble out of the exhaust :lol:), and resolves once back on the throttle. Some of this is caused by the phase shift between the AFR target and measured AFR, but some of it is real.
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[Sample log](Overview/wall_wetting/wall_wetting_2019-01-01_19_modified.msl)
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Based on [https://rusefi.com/forum/viewtopic.php?f=5&t=1481](https://rusefi.com/forum/viewtopic.php?f=5&t=1481)
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[//]: # "OrchardPerformance"
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