The Crank and the Flywheel: Smoothing One Bang in Two Turns

Cut a plywood base. Pivot a plywood crank disc on a bolt, with a crank pin 40 mm off centre. Cut a connecting rod 140 mm between hole centres, and a slider that runs in a slot or between two strips — the 'piston'. Glue a protractor under the crank disc. Turn the crank in 30° steps and measure the piston's distance from the crank centre with the steel rule. Compare with the notebook's formula. Notice that the piston moves further during the half-turn near top dead centre than near bottom — the connecting rod's angle causes it, and it is why the torque trace is lopsided.

A practice rung: the slider-crank goes back to the rotative steam engine in this catalogue, and the flywheel to the potter's wheel and the spindle whorl. The torque trace is generated from an ideal cycle; a real engine's is less peaky because combustion takes time. **Honest limits of the model.** No friction, no inertia of the piston itself (which matters at high speed), heat added instantly at top dead centre, and a single cylinder. Multi-cylinder engines are smoothed by overlapping power strokes; balance shafts deal with the shaking the piston's own motion causes.