Who controls air–fuel ratio in a petrol engine? In a conventional spark-ignition petrol engine (non-GDI), which component regulates the air–fuel ratio supplied to the cylinders under steady running?

Introduction / Context:
The air–fuel ratio (AFR) directly affects power, efficiency, emissions, and drivability. In traditional petrol engines without electronic fuel injection, the carburettor meters fuel using the pressure drop created by intake air and provides mixture control across operating regimes.

Given Data / Assumptions:

• Carbureted SI engine (not GDI/port-EFI).
• Throttle controls airflow; carburettor circuits meter fuel accordingly.
• Engine speed and load vary during normal operation.

Concept / Approach:
A carburettor uses a venturi to create a pressure differential that draws fuel from jets into the airstream, with dedicated circuits (idle, main, power, accelerator pump, choke) to maintain suitable AFR under different conditions. While a governor may control speed on stationary engines by adjusting throttle, the metering of fuel relative to air in a carbureted SI engine is the carburettor’s function.

Step-by-Step Solution:

Verification / Alternative check:
Service manuals describe adjustments (jets, mixture screws, choke) on carburettors for AFR tuning; EFI replaces carburettors with injectors controlled by ECUs, not relevant here.

Why Other Options Are Wrong:

Diesel-type injectors belong to CI engines; governors manage speed, not direct AFR metering; turbochargers alter air density, not the metering of fuel per se.

Common Pitfalls:
Equating speed control with mixture control; they are related but not identical.

Final Answer:

carburettor