Combustion

Combustion turns fuel's chemical energy into pressure. Learn air–fuel ratio and lambda, spark-ignition flame propagation and knock, compression-ignition diesel combustion, heat-release and mass-fraction-burned (Wiebe), octane and cetane, and airflow-to-power calculations.

Inside a cylinder, fuel burns in a few milliseconds. How quickly and cleanly it burns — and whether it burns smoothly or explodes (knocks) — decides power, efficiency and emissions.

Petrol and diesel engines burn fuel in fundamentally different ways: a spreading flame versus a spray of self-igniting droplets.

An analogy

SI combustion is like a grass fire spreading from one match; CI combustion is like throwing fuel onto hot coals, where it ignites wherever it lands.

Air–fuel ratio

Complete burning of petrol needs ~14.7 kg of air per kg of fuel (stoichiometric, λ = 1). Rich mixtures (λ < 1) give maximum power but waste fuel and raise CO/HC; lean mixtures (λ > 1) improve efficiency but can misfire and raise NOx. Three-way catalysts need λ ≈ 1.

Spark-ignition combustion and knock

A spark starts a turbulent flame that sweeps the chamber in ~40–60° of crank angle. If the unburned end gas auto-ignites before the flame arrives, pressure waves 'knock' — damaging pistons. Higher octane fuel, cooler charge, lower compression or retarded spark prevent it.

Diesel combustion

Air is compressed to ~15–20:1 (≈ 700–900 K); injected fuel evaporates, ignites after an ignition delay (premixed burn), then burns as a diffusion flame controlled by injection. Cetane number measures ignitability. Always lean overall, so efficient but prone to NOx and soot.

Heat release and advanced modes

Cylinder-pressure analysis gives heat-release rate and CA50 (crank angle of 50 % burn, ideally ≈ 8–10° ATDC). Advanced modes — HCCI, GCI, lean-burn with pre-chamber ignition, EGR dilution, Miller cycle — push efficiency above 45 % while cutting emissions.

Formulas

• Air mass flow (4-stroke): ṁ_air = VE · ρ_air · V_d · N / 120
• Fuel flow: ṁ_f = ṁ_air / (λ · AFR_st), AFR_st ≈ 14.7 petrol, 14.5 diesel
• Brake power: P = η_b · ṁ_f · LHV
• Wiebe function: x_b = 1 − exp[−a((θ − θ₀)/Δθ)^(m+1)] (a ≈ 5, m ≈ 2)

Worked example: Airflow to power

Given: 2.0 L, 3000 rpm, VE = 0.9, ρ = 1.18 kg/m³; λ = 1 (AFR 14.7), η_b = 35 %

Find: Air and fuel flow, fuel power, brake power.

• ṁ_air = 0.9 × 1.18 × 0.002 × 3000/120 = 0.0531 kg/s
• ṁ_f = 0.0531/14.7 = 3.61 g/s (13 kg/h)
• Fuel power = 0.00361 × 43 × 10⁶ = 155 kW
• Brake power = 0.35 × 155 = 54 kW

Answer: ≈ 54 kW

Worked example: Lambda check

Given: Measured AFR = 16.2 on petrol

Find: λ and mixture type.

• λ = 16.2/14.7 = 1.10 → lean

Answer: λ = 1.10 (lean) — TWC NOx conversion will drop

Common mistakes

• Wrong: Thinking richer always means more power. — Right: Best power is slightly rich (λ ≈ 0.85–0.9); richer wastes fuel and fouls.
• Wrong: Advancing spark to fix low power. — Right: Beyond MBT or into knock, power falls and damage risk rises.

Applications

• Engine calibration: Spark timing, AFR and EGR maps set with combustion analysis.
• Turbocharged downsizing: Boost raises air density (VE > 1 effective) for more power per litre.
• Alternative fuels: Ethanol, CNG, hydrogen and e-fuels change AFR, octane and flame speed.

Key takeaways

• AFR_st ≈ 14.7; λ = AFR/AFR_st.
• SI: flame propagation, knock limits compression.
• CI: auto-ignition, lean, cetane matters.
• Power = η × ṁ_f × LHV; air flow sets fuel flow.

Going further

• Pre-chamber ignition: Turbulent jets from a small pre-chamber ignite very lean mixtures rapidly.
• Knock control: Accelerometer knock sensors retard timing cylinder-by-cylinder in milliseconds.
• CFD combustion: 3D simulation of sprays, turbulence and chemistry guides chamber design.