109 lines
5.6 KiB
Markdown
109 lines
5.6 KiB
Markdown
# YZ125 baseline verification
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Linux Release checks, 2026-10-23. All six CTest suites passed. This is a
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YZ125-sized simulation baseline, not a measured replica of a particular year.
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## Changes and physical basis
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Pipes already supported per-cell areas/lengths and the geometric momentum
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source for varying area. The earlier exhaust was a small expansion chamber,
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but the UI drew all pipes with pressure-dependent thickness. The display now
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uses physical diameter and axial cell length. A shared profile builder preserves
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exact conical-frustum volume across mesh refinements.
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The new estimated racing pipe has these dimensions (metres):
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| Section | Length | Inlet diameter | Outlet diameter |
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| --- | ---: | ---: | ---: |
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| Header | .160 | .036 | .036 |
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| First diffuser | .180 | .036 | .060 |
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| Second diffuser | .190 | .060 | .105 |
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| Belly | .130 | .105 | .105 |
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| Baffle cone | .210 | .105 | .022 |
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| Stinger | .240 | .022 | .022 |
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Default mesh maximum length is 10 mm. The straight comparison has the same
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total length and a uniform 36 mm bore. These are design estimates, not Yamaha
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factory pipe dimensions. The test pulse returned -34.33 Pa from a diffuser and
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+27.52 Pa from a convergent, with no positivity repairs. The intended negative
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scavenging pulse and later positive plugging pulse follow the mechanism in
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[SAE 680469](https://saemobilus.sae.org/papers/investigation-tuning-exhaust-system-small-two-stroke-cycle-engines-680469).
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Two-stroke port areas now come from piston travel past rectangular port roofs,
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replacing the poppet-style curtain-area curve. Assumed transfer/exhaust durations
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are 130/190 degrees and aggregate widths 50/36 mm. Trapped compression is 8.2:1;
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clearance volume is derived at exhaust closure, rather than confusing this with
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full-stroke geometric compression. The exhaust power valve remains fixed open.
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These port dimensions and compression interpretation require measured validation.
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Combustion has an estimated .8 ms minimum burn duration; timing and duration are
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frozen for each active charge. The estimated ignition curve retards above 6000
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RPM rather than continuing to advance toward 40 degrees. Neither is a measured
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factory CDI/flame model. Fuel remains ideally calculated at port closure.
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Mechanical friction now includes a quadratic windage term. The two-stroke uses
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FMEP [bar] = .4 + .09*mean_piston_speed + .0009*mean_piston_speed^2, converted to
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crank torque with one revolution per cycle. Coefficients are generic estimates
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within the ranges discussed in this
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[Chalmers engine-model study](https://publications.lib.chalmers.se/records/fulltext/147782.pdf),
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not a YZ125 coast-down fit. Crankcase pumping work is already modeled separately.
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No speed clamp or rev limiter was added.
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Drivetrain ratios use Yamaha's
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[2022 YZ125 specification](https://global.yamaha-motor.com/jp/news/2021/0730/yz125.html):
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primary 64/19, final 49/13, and six ratios approximately 2.384, 1.933, 1.588,
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1.352, 1.176, 1.055. Nominal rear radius is .3313 m from the 100/90-19 tire;
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loaded rolling radius is unmeasured. Test mass is 95 kg plus an assumed 75 kg
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rider. First/sixth overall reductions are 30.276/13.402, replacing the twin's
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much taller drivetrain. Other road-load/clutch parameters remain estimates.
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The dyno now records brake shaft torque/power before external load reaction;
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previously it displayed acceleration torque/power, which becomes zero at loaded
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steady speed. An analytic load-balance test checks 10 Nm and 1 kW at 100 rad/s.
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## Measurements and limits
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The fixed-speed benchmark imposes speed as an ideal motoring/absorbing dyno and
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records shaft output. It runs one simulated second per point and averages the
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last half. All points fired approximately once per revolution, had zero pipe
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positivity repairs, and cost about .53-.58 wall seconds per simulated second.
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| RPM | Straight kW | Racing kW |
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| ---: | ---: | ---: |
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| 6000 | 4.02 | 6.27 |
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| 8000 | .63 | 5.53 |
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| 10000 | 2.43 | 10.53 |
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| 12000 | 3.96 | 14.32 |
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| 14000 | 3.60 | 12.38 |
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| 17000 | -.80 | -3.94 |
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This coarse sweep peaks near 12000 RPM; it is not a validated Yamaha dyno curve.
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Negative power at 17000 means the virtual dyno must motor the engine. Free-rev
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speed differs from peak-power speed: full pedal with the straight pipe still
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approaches 16950 RPM after eight seconds. This remains an unresolved calibration
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limit. The standard transient reached 16128 RPM, recovered idle at 3159 RPM,
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and launched the motorcycle to 61.3 km/h without repairs or nonfinite output.
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Six-second .6-pedal field captures had no clipping: racing rendered peak .201,
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straight .554. Firing cadence, port timing and geometry are improved, but exact
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YZ125 sound still requires listening and further combustion/scavenging work.
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The first-order pipe flux damps high-frequency content; the compact source,
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constant-gamma gas, ideal fueling, fixed power valve, single-zone mixing and
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empirical monitoring/environment filters limit fidelity. No engine sound sample
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or synthetic two-stroke firing track has been added.
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## Run and compare
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```sh
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cmake --build build-desktop --target fluidsim_desktop -j 4
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./build-desktop/fluidsim_desktop --scenario two-stroke --exhaust racing --scene field
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./build-desktop/fluidsim_desktop --scenario two-stroke --exhaust straight --scene field
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cmake --build build-audit --target fluidsim_two_stroke_benchmark -j 4
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./build-audit/fluidsim_two_stroke_benchmark
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```
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Hold C, shift to first with D, apply W, then release C. A/D shift, Z selects
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neutral. Up/down adjusts persistent pedal request; R changes environment.
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The capture tool accepts the same exhaust option. Current captures are under
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`build/audio/yz125-{racing,straight}-{rendered,dry,unfiltered}.wav`.
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