ICE-04 — Engine Geometry

ASSUMPTION — This lesson models the crank mechanism as a simple slider-crank. Because the connecting rod is finite, piston motion is not purely sinusoidal; this is described qualitatively.

In this lesson you will learn:

• Bore, stroke, TDC and BDC
• Piston motion vs crank angle
• The bore/stroke ratio effect

1. Basic Dimensions

• Bore (D) — the cylinder’s internal diameter.
• Stroke (S) — the piston’s travel between TDC and BDC.
• Crank radius (r) — half the stroke: S = 2r.
• Connecting rod length (l) — distance between crank pin and piston pin.

FACT — Stroke equals twice the crank radius (S = 2r). One full piston cycle (TDC→BDC→TDC) corresponds to one full crank revolution (360°).

2. Piston Motion

FACT — With a finite connecting rod, the piston is not exactly mid-way at 90° crank angle; it moves faster near TDC. This asymmetry grows with the r/l ratio and affects side loads and vibration.

3. Bore/Stroke Ratio

• Oversquare (bore > stroke) — short stroke, wide bore; tends toward high RPM and power.
• Undersquare (bore < stroke) — long stroke, narrow bore; tends toward better low-end torque.
• Square (bore = stroke) — a balanced middle ground.

EXAMPLE — A long-stroke engine has a longer crank radius, giving gas force a larger moment arm on the crank — usually improving low-speed torque.

4. Why It Matters

Geometry underpins displacement (ICE-05), compression ratio (ICE-06), torque/power character (ICE-07) and NVH (ICE-43).

FACT — Bore/stroke choice affects packaging, friction, mean piston speed and thermal efficiency together; there is no single “best” ratio — it is a trade-off.

5. Summary

• Bore = cylinder diameter; stroke = TDC–BDC distance.
• Stroke = 2 × crank radius.
• Finite rod length skews piston motion.
• Bore/stroke ratio shapes power/torque character.

6. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch Automotive Handbook

Next Lesson

• ICE-05 — Displacement: calculating engine swept volume.