Petrol (SI) engines – which description is correct? Choose the statement that correctly describes how a petrol (spark-ignition) engine prepares and ignites the charge.

Introduction / Context:
Petrol engines are spark-ignition (SI) machines. Unlike diesel engines, which rely on compression ignition of a fuel spray, SI engines prepare a fuel–air mixture and then initiate combustion with a timed electric spark.

Given Data / Assumptions:

• Conventional four-stroke petrol engine.
• Mixture preparation via carburettor or fuel injection (port or direct) depending on design.
• Ignition provided by an ignition system and spark plug.

Concept / Approach:
In SI engines, fuel is metered to achieve a target air–fuel ratio (often near stoichiometric at many conditions). The resulting homogeneous or stratified mixture is then ignited by an electric spark near the end of the compression stroke. This differs fundamentally from diesel operation, where auto-ignition occurs due to high temperature from compression.

Step-by-Step Solution:
Identify correct principle: mixture + spark → SI operation.Contrast with CI: diesel uses heat of compression to ignite injected fuel (not correct for petrol).Recognize variations in mixture preparation: carburetion is not the only method; injection systems are common.Therefore, the accurate single statement is that fuel and air are proportioned and ignited by a spark plug.

Verification / Alternative check:
Modern engines employ electronic fuel injection (EFI) and ignition control; the core SI principle remains mixture formation followed by spark ignition.

Why Other Options Are Wrong:

• (a) Describes diesel CI behavior.
• (c) Overgeneralizes carburettors; port and direct injection are widely used.
• (d) Cannot be true because (a) and (c) are not universally correct.
• (e) Self-ignition would be knock or pre-ignition at normal petrol compression ratios, not normal operation.

Common Pitfalls:
Assuming carburettors are mandatory for SI engines; many use EFI. The essential feature is the spark-initiated flame.

Final Answer:
Fuel is proportioned with air to form a combustible mixture and ignited by a spark plug (spark ignition).