Level 1 · Fundamentals Gasoline Engine (Petrol Engine) Lesson 3/3 beginner 12 min

Fuel Supply and Air-Fuel Mixture

Learn how fuel is delivered and mixed with air in carbureted and fuel-injected engines, and why the air-fuel ratio is critical for performance and emissions.

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Safety notice

Gasoline is highly flammable. Work in a well-ventilated area, keep ignition sources away, and have a fire extinguisher rated for flammable liquids nearby.
Fuel systems may remain pressurized after the engine is shut off. Relieve pressure according to the manufacturer's procedure before servicing any fuel line or component.
Do not attempt to adjust or modify mixture settings on a vehicle equipped with an oxygen sensor and catalytic converter unless you have the proper training and equipment; incorrect mixture can damage the catalytic converter and increase emissions.

What you will learn

Compare carburetor and fuel injection systems.
Explain the concepts of vaporization, atomization, and homogeneous mixture.
Define stoichiometric air-fuel ratio and lambda, and describe rich and lean mixtures.
Understand the role of the ECU and oxygen sensors in mixture control.

Key points

The air-fuel ratio (AFR) is the mass of air divided by the mass of fuel.
Stoichiometric AFR for gasoline is about 14.7:1, but this is a typical value and varies slightly with fuel composition.
Lambda is the actual AFR divided by the stoichiometric AFR; lambda = 1 means stoichiometric.
Carburetors rely on pressure differences and mechanical metering; fuel injection uses pressurized fuel and electronic control.
The ECU uses sensors, especially oxygen sensors, to fine-tune mixture for emissions and driveability.
Proper atomization and vaporization are essential for complete combustion.

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