Combustion and Emissions Basics

Air-fuel ratio, lambda and the three main pollutants.

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What you will learn

Explain stoichiometric air-fuel ratio
Define lambda and its rich/lean meaning
Link combustion quality to emissions and symptoms

Combustion needs the right balance of air and fuel. The stoichiometric ratio for petrol is about 14.7 kg of air to 1 kg of fuel; lambda expresses the actual ratio divided by the ideal one. Lambda 1 means perfect balance, below 1 is rich, above 1 is lean.

Rich mixtures waste fuel and create CO and HC; lean mixtures can create NOx and misfires. Modern engines use oxygen (lambda) sensors and feedback control to stay close to lambda 1, and DTCs such as P0171 (lean) or P0172 (rich) tell you the control loop is out of range.

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Key points

Fuel trims reveal whether the ECU is adding or removing fuel.
A lean code is a symptom, not a part.
Check for unmetered air before replacing sensors.

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Key points

Lambda 1 means chemically ideal combustion
Rich = too much fuel, lean = too much air
CO, HC, NOx and particulates are the targets

All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.