ICE-02 — The Four-Stroke Engine

ASSUMPTION — This lesson explains the physical flow at a basic level. Crank-angle values are illustrative; real valve timing depends on the cam profile.

In this lesson you will learn:

• What happens in each of the four strokes
• Piston and valve positions
• Crank angle and dead-center concepts
• The basic difference from a two-stroke engine

1. Why Four Strokes?

Combustion in an ICE is a repeating cycle. For efficient, controlled combustion, gas must be drawn in, compressed, burned, and expelled. The four-stroke cycle spreads these four tasks over two crankshaft revolutions (720°).

FACT — Each stroke corresponds to 180° of crank angle. Intake and exhaust are gas-exchange strokes; compression and power take place with the valves closed.

2. The Four Strokes

Intake — the piston descends from TDC with the intake valve open; the cylinder fills with fresh air (or air-fuel).

Compression — both valves closed; the piston rises from BDC to TDC, compressing the gas and raising its temperature and pressure.

Power — combustion begins near TDC; the expanding gas pushes the piston down and produces work. This is the only stroke that produces net work.

Exhaust — the piston rises from BDC to TDC with the exhaust valve open, expelling the burned gas.

EXAMPLE — In a four-cylinder engine the power strokes are phased 180° apart (720° / 4), giving a much smoother torque flow than a single cylinder.

3. Valve Timing and Dead Centers

• TDC (Top Dead Center) — highest piston position.
• BDC (Bottom Dead Center) — lowest piston position.
• Valve overlap — the brief period when exhaust closing and intake opening overlap to improve gas exchange.

FACT — Real valves do not open/close exactly at the dead centers; gas inertia means intake opens before exhaust closes, etc. Timing is set by the cam profile (details: ICE-17).

4. Four-Stroke vs Two-Stroke

FACT — A two-stroke completes the cycle in one revolution (360°), overlapping the strokes. It is simpler and power-dense but harder to control for emissions and lubrication, so four-stroke dominates in automobiles.

5. Summary

• Cycle: intake → compression → power → exhaust (720°).
• Intake/exhaust exchange gas; compression/power have valves closed.
• TDC/BDC define piston extremes.
• Valve timing is optimized by gas dynamics.

6. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch Automotive Handbook

Next Lesson

• ICE-03 — The Otto and Diesel Cycles: thermodynamic comparison of the two ideal cycles.