Cross-section
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The colour in each pipe is the pressure of the gas at that point, right now. On the intake stroke a blue suction wave starts at the valve and runs down the tract; at the open end it reflects and comes back as a red compression wave. Slow the playback down and follow one bounce. The dots are the air itself — they surge, stall and reverse. Each tract has its own colour scale (legends at the sides) because the exhaust swings about three times harder than the intake.
Wave action diagram — an x–t chart of the same waves, for digging deeper
Wave action diagram
Solid across (with a °) — the playhead: where the animation is right now.
Dashed across, IVO/IVC/EVO/EVC — valve opens and shuts. Between them the valve is open, and only then can a wave do anything.
Short slope, top right — the speed of sound here. Any coloured stripe leaning the same way is travelling at that speed; the label gives the crank degrees one trip down the pipe costs.
Dashed V from the playhead — the round trip: a pulse leaving the valve now hits the open end, flips, and lands back at the valve where the V closes. Tuning is landing that point just before IVC.
Bar on the far-left edge — valve lift; brighter is more open.
Intake tract — valve at the left edge
Exhaust tract — valve at the left edge
Pressure through the cycle
Absolute pressure against crank angle, one full four-stroke cycle. The strip below is valve lift. Cylinder pressure runs off the top during combustion — that part of the trace is clipped on purpose so the sub-atmospheric detail stays readable. Hover for values.