Under the Engine: how a motorcycle actually works

An interactive, open explorer for the parts of a petrol motorcycle and the cycle that drives it. Toggle between four-stroke and two-stroke, change the bore, stroke, compression and RPM, and watch the piston, valves, ports and spark do their work. Electric is next.

1 November 2026 · 3 min read

Launch Under the Engine → The car tool that inspired this

Most people who ride a motorcycle, including most people in the Lab's e-mobility fieldwork, have never seen the four-stroke cycle actually walked through. Petrol engines are described as magic that happens somewhere under the fuel tank; electric motors are described as magic of a different kind. Both are mechanisms you can draw, label and animate — and once you can, the differences between them become much easier to talk about.

Under the Engine is the first piece of that: an interactive, light-theme explorer of a petrol motorcycle. There is no sign-in, no account; everything runs in the browser.

What is in version 0.1

  • Toggle four-stroke / two-stroke. The same stage shows a different mechanism: a four-stroke has an intake and an exhaust valve in the head and takes two crank revolutions per cycle; a two-stroke uses ports in the cylinder wall and fires every revolution.
  • Animated cross-section. Piston, connecting rod, crankshaft, cylinder, head, spark plug, valves (4T) or exhaust and transfer ports (2T). Gas colour in the cylinder follows the stroke: cool blue for the fresh charge, warm orange for combustion, grey for exhaust.
  • Timing strip. A 720° (4T) or 360° (2T) strip at the bottom of the stage shows which stroke you are in and marks the current crank angle.
  • Parameters you can change. RPM, bore, stroke, compression ratio. Displacement is recomputed live, with educational estimates of peak torque and power at the current RPM.
  • Exploded motorcycle. A full side-view diagram with every major part labelled — frame, engine, carburetor or injector, air filter, fuel tank, exhaust, battery, forks, rear shock, swingarm, brakes, chain, sprockets, wheels, lights, instruments. Click a label to read what it does.

What is coming

  • Electric mode. The same stage, swapped: motor, controller, battery pack, throttle, BMS. A side-by-side comparison with the ICE drivetrain, so you can see which parts fall away and which new ones appear.
  • Save your own configurations. Dial a 650 cc single or a 125 cc two-stroke and share the URL.
  • Richer sections. Separate stages for the ignition system, the fuel system, and the transmission.

Credits

The idea is adapted from the excellent Under the Hood by @gristavo, which does this for a car. This version is a from-scratch build by the Lab, aimed at motorcycles and (soon) at electric drivetrains, as part of the Lab's wider work on E-Mobility & Transport.

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