ICE-09 — Combustion

ASSUMPTION — Combustion is highly complex; this lesson provides a conceptual framework. Advanced details (heat release, cylinder pressure) are covered elsewhere.

In this lesson you will learn:

• The gasoline vs diesel difference
• Combustion phases
• Stoichiometric combustion

1. What Is Combustion?

Combustion is the exothermic reaction that rapidly converts fuel’s chemical energy into heat, raising gas temperature and pressure to push the piston.

FACT — In ideal complete combustion, hydrocarbon fuel reacts with oxygen to form CO2 and H2O. Real combustion also produces CO, HC and, at high temperature, NOx (ICE-25).

2. Gasoline (Spark) Combustion

The mixture is pre-mixed; the spark ignites it and a flame front propagates across the chamber.

FACT — Spark-ignition combustion is premixed turbulent combustion. If the end-gas self-ignites before the flame arrives, knock occurs (ICE-27).

3. Diesel (Compression) Combustion

Only air is compressed; fuel is injected at high pressure and self-ignites on contact with hot air.

FACT — Diesel combustion has two components: a short ignition delay followed by fast premixed combustion, then diffusion-controlled combustion as injection continues. This phase structure governs NOx and soot formation.

4. Stoichiometry

• Stoichiometric — exactly enough air for complete combustion (λ = 1).
• Rich — excess fuel (λ < 1).
• Lean — excess air (λ > 1).

FACT — Gasoline engines run near stoichiometric (for the three-way catalyst); diesel always runs lean, which requires a different NOx strategy (ICE-24, ICE-26).

5. Summary

• Gasoline: spark-initiated flame propagation.
• Diesel: compression ignition, premixed + diffusion phases.
• Stoichiometry (λ) governs combustion and emissions.

6. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch Automotive Handbook

Next Lesson

• ICE-10 — The Gasoline (Petrol) Engine.