Spark-Ignition (Petrol) Engines — When does the spark occur? In an SI engine, ignition is initiated relative to the piston’s movement near top dead centre (T.D.C.) on the compression stroke. Identify the most accurate description.

Introduction / Context:
This question checks understanding of ignition timing in spark-ignition (petrol) engines. Because combustion takes a finite time, the spark cannot be triggered exactly at top dead centre (T.D.C.); it is advanced so that peak pressure occurs slightly after T.D.C. for best torque and efficiency.

Given Data / Assumptions:

• Conventional four-stroke spark-ignition engine.
• Normal gasoline fuel and typical flame speeds.
• No abnormal conditions such as knock control retarding timing.

Concept / Approach:
Flame propagation across the combustion chamber requires milliseconds, which correspond to several crank degrees at typical engine speeds. To align maximum cylinder pressure just after T.D.C. (often 10–20 crank degrees ATDC depending on design), the spark must be fired before T.D.C. while the piston is approaching T.D.C. on the compression stroke. This is called spark advance and is adjusted with speed and load.

Step-by-Step Solution:
Goal: achieve peak pressure slightly after T.D.C. for best leverage on the crank.Constraint: combustion duration is not instantaneous.Action: trigger spark BTDC (before T.D.C.) as piston approaches T.D.C. on compression.Therefore, the correct description is “approaching T.D.C. on the compression stroke.”

Verification / Alternative check:
Service data and timing marks show positive BTDC values at idle and greater advance at higher speeds, confirming that ignition occurs before T.D.C., not at or after it.

Why Other Options Are Wrong:
Exactly at T.D.C.: too late for optimal pressure phasing.Leaving T.D.C.: much too late; power and efficiency fall.Exhaust stroke: wrong stroke altogether.Mid-stroke: far too early; would cause severe knock.

Common Pitfalls:
Confusing spark timing (BTDC) with the crank angle of maximum pressure (typically ATDC).

Final Answer:
approaching the T.D.C. position on its compression stroke