How a 4-Stroke Engine Works

Concept — one angle θ runs the whole engine

A four-stroke engine turns fuel into rotation in a fixed four-step sequence. The key idea: a single crankshaft angle θ drives everything at once — the piston position (through the connecting rod), which valve is open, and when the spark fires. Because the cam turns at half crank speed, the sequence repeats every 720° = two crankshaft revolutions, one full cycle.

What the animation shows

The crankshaft rotates; the connecting rod converts that rotation into the piston's up-and-down motion using honest crank-slider geometry (not a fake sine). Each 180° is one stroke: Intake (intake valve open, piston down, charge drawn in) → Compression (both valves shut, piston up, charge squeezed; spark fires at the top) → Power (burnt gas expands and forces the piston down — the only stroke that does work) → Exhaust (exhaust valve open, piston up, spent gas pushed out). Valves lift ONLY on their own stroke; the spark flashes once per cycle at the end of compression.

Quantities

• θ — crank angle, 0–720° (two revolutions = one cycle).
• Piston s — displacement from top dead centre: s = (r+l) − (r·cosθ + √(l²−r²·sin²θ)), with crank throw r = 43 mm, rod l = 143 mm (rod ratio n = 3.33).
• Volume V — cylinder volume V = V_c + A·s; bore 86 mm → area A = 5808 mm² = 5.808 cc/mm; clearance V_c set by the compression-ratio slider.
• Pressure P — a simplified polytropic teaching model (γ = 1.3, P₁ = 100 kPa), with a combustion rise on the power stroke. Not a datasheet trace.
• Compression ratio — CR = (V_c+V_d)/V_c; displacement V_d = 499 cc.

Sources

J.B. Heywood, Internal Combustion Engine Fundamentals (1988); R. Stone, Introduction to Internal Combustion Engines. Square 86×86 mm gasoline geometry; P–V is the Otto air-standard idealisation (explicitly a teaching model).