Compression-ignition (diesel) principle: In diesel engines, ignition occurs due to the high temperature attained at the end of compression without using a spark plug.

Introduction / Context:
Diesel engines are compression-ignition (CI) machines. Unlike spark-ignition (SI) petrol engines, they do not use a spark plug to initiate combustion. Instead, fuel auto-ignites when injected into hot, highly compressed air.

Given Data / Assumptions:

• Air is compressed to a high compression ratio (typically 15–25).
• Fuel is injected near the end of the compression stroke.
• No external spark source is provided.

Concept / Approach:
High compression raises the air temperature substantially (well above the auto-ignition temperature of diesel fuel). When finely atomized diesel is injected, it vaporizes and ignites after a short ignition delay. This fundamental mechanism defines CI operation.

Step-by-Step Solution:

Verification / Alternative check:
Engines equipped with glow plugs still rely on auto-ignition; glow plugs only assist cold starting by preheating air or combustion chamber surfaces.

Why Other Options Are Wrong:
“Incorrect” would imply a need for a spark similar to petrol engines, which contradicts the CI definition and practical engine design.

Common Pitfalls:
Confusing glow plugs with spark plugs; glow plugs are heaters, not ignition triggers in normal operation.

Final Answer:
Correct