Diesel combustion — ignition source In a compression-ignition (Diesel) engine, the injected fuel is ignited primarily by:

Introduction / Context:
Ignition physics distinguishes Diesel from petrol engines. In exams and troubleshooting, recognizing the correct ignition source explains why Diesel engines need high compression ratios and why glow plugs are used only for cold starts.

Given Data / Assumptions:

• Conventional four-stroke Diesel engine.
• Fuel is injected near the end of the compression stroke.
• No spark plugs are present for normal operation.

Concept / Approach:
Air is compressed to a high pressure and temperature. When finely atomized Diesel fuel is injected, it vaporizes, mixes with hot air, and auto-ignites after a short delay. The primary ignition energy comes from the high temperature created by compression work, not from an external spark.

Step-by-Step Solution:
Air only is drawn in and compressed.Compression raises temperature sufficiently for auto-ignition.Injector sprays fuel; a delay period occurs; then combustion propagates.

Verification / Alternative check:
Starting a cold Diesel often requires glow plugs or intake heaters to raise initial air temperature, confirming dependence on temperature rather than spark.

Why Other Options Are Wrong:
Electric spark is used in spark-ignition engines, not Diesels. Hot exhaust does not flow into the cylinder during compression and cannot be an ignition source.

Common Pitfalls:
Assuming glow plugs ignite fuel directly; they only aid auto-ignition by preheating air and surfaces. Excessive injection advance or poor atomization can lengthen delay and cause harsh combustion.

Final Answer:
the heat of compression