Internal Combustion Engines – Suction Stroke in a Spark-Ignition (Petrol) Engine During the suction (intake) stroke of a conventional carburetted petrol engine, what working charge is drawn into the cylinder through the intake valve?

Introduction / Context:
This question targets the basics of the spark-ignition (SI) petrol engine induction process. Understanding what enters the cylinder during the suction stroke is essential for concepts like air–fuel ratio, throttling, volumetric efficiency, and combustion quality in automotive and small power-plant applications.

Given Data / Assumptions:

• Conventional carburetted petrol (gasoline) engine or port fuel–injected SI engine.
• Throttle controls the intake manifold pressure and the air–fuel mixture quantity.
• Standard four-stroke cycle: suction (intake), compression, power (expansion), exhaust.

Concept / Approach:

In an SI engine, fuel is premixed with air before ignition by the spark plug. Traditionally, a carburettor atomizes fuel into the incoming airstream, producing a combustible air–fuel mixture. In modern systems, port fuel injection meters fuel into the intake port where it mixes with air before entering the cylinder. Thus, during the suction stroke with the intake valve open, the piston’s downward motion draws an air–fuel mixture into the cylinder.

Step-by-Step Solution:

Verification / Alternative check:

Contrast with compression-ignition (diesel) engines, which draw air only on the intake and inject fuel directly near the end of compression. Also contrast with direct-injection SI engines (GDI), where some fuel may be introduced directly into the cylinder; yet even then, the inducted working fluid is still primarily air, and for most conventional petrol engines the inducted charge is air premixed with fuel.

Why Other Options Are Wrong:

'Air only' describes diesel operation. 'Petrol only' and 'pure fuel vapour' are impossible because combustion requires oxygen. 'EGR only' is not a working charge; EGR, if used, is mixed with fresh charge and never alone.

Common Pitfalls:

Confusing SI and CI engine cycles; overlooking that carburettors/prefueling systems create a combustible mixture before intake valve closure.

Final Answer:

a mixture of petrol and air