Four strokes. Two turns.: sources & model

Open a detailed engine, turn its crank, and take the assembly apart. Follow four different jobs hidden inside two very similar-looking turns.

What supports the explanation?

What this model assumes

1. Original generic air-cooled OHV teaching assembly. Dimensions, finishes, cam profile, fasteners and inspection cutaways are not a manufacturer CAD model or repair instructions.
2. The crank and cam gears show the 20:40 tooth-count relationship. Their simplified teeth do not establish manufacturing contact, backlash, lubrication or durability.
3. Ideal 180° stroke boundaries and a spark at the start of expansion. Real engines can use valve overlap and ignition advance; specific service values do not transfer between engine variants.
4. Crank motion is prescribed. No cylinder pressure, airflow, torque, fuel consumption, temperature, stalling or flywheel acceleration is calculated. The gas color and spark flash are qualitative cues.
5. The model includes the slider-crank and rigid pushrod/rocker constraints. Rocker contact pads, bearings and seals remain simplified; independent engineering and visual review is pending.
6. Slider-crank geometry: The crank pin follows a circle, but the piston must remain on a straight guide. With crank radius r and rod length l, downward travel from the top is r+l−r cos θ−√(l²−r² sin² θ). Here r=30 mm and l=120 mm. At 90°, travel is about 33.81 mm, slightly more than halfway down.
7. Same piston position, different state: At 0° and 360° the piston is at the top. At 0° intake is about to begin; at 360° expansion is about to begin. The complete ideal sequence repeats at 720°. The study diagram separates crank angle from stroke identity.
8. Motion is not an engine test: You supply the crank motion in this model. Removing the spark removes the combustion cue while the externally turned mechanism still moves. Actual speed, torque, pressure, fuel consumption and stalling require combustion and rotational-dynamics models.
9. The cam, follower and rocker agree: The visible cam surface is derived from a radial roller-follower pitch curve. Its prescribed lift rises and returns smoothly during one ideal stroke. A fixed-length pushrod and rocker determine the valve lift. Roller size, cam lift and link dimensions are teaching design choices, not measurements from a commercial engine.

What has been checked

Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.

Each source supports the associated claim. Sources do not certify this implementation or its visuals.

About the cover illustration

The topic card uses an AI-generated editorial illustration. It introduces the subject; it is not a validated anatomical reference or a measured landscape. The experiment’s diagrams, readouts and assumptions explain the model separately.