Identify the component that does NOT belong to a compression-ignition (diesel) engine fuel/air system.

Introduction / Context:
This question checks recognition of key subsystems in compression-ignition (CI) engines versus spark-ignition (SI) engines. CI engines rely on fuel injection into hot compressed air, while SI engines premix fuel with air via a carburettor (traditional) or port/direct injection with throttling.

Given Data / Assumptions:

• CI engines need high-pressure fuel delivery at precise timing.
• Air is drawn in separately; fuel is not premixed upstream in a carburettor.
• Speed/load are regulated by controlling injected fuel quantity.

Concept / Approach:
Components typical for CI engines include the fuel pump (high-pressure), injectors (atomization and spray), and a governor (controls fuel quantity for speed regulation). A carburettor, whose job is to meter petrol into the intake air stream by pressure drop at a venturi, is characteristic of old-style SI engines and is not used on diesels.

Step-by-Step Solution:

Verification / Alternative check:
Modern common-rail diesels still use high-pressure rails and electronic injectors, not carburettors, confirming the functional distinction.

Why Other Options Are Wrong:

• Fuel pump: Essential to pressurize diesel fuel for injection.
• Fuel injector: Creates spray for mixing and ignition.
• Governor: Maintains speed by modulating fuel delivery in many diesel systems.

Common Pitfalls:
Confusing fuel system components across SI and CI engines; note that some small diesel generators still have mechanical governors and pumps but never a carburettor.

Final Answer:
Carburettor