Autoignition (Self-Ignition) Temperature Comparison For typical automotive fuels, how does the self ignition temperature of diesel fuel compare with that of petrol (gasoline)?

Introduction / Context:
Compression-ignition (diesel) engines rely on the fuel autoigniting in hot compressed air, so the self ignition temperature is critical. Spark-ignition (petrol) engines avoid uncontrolled autoignition (knock). Knowing the relative autoignition temperatures explains why diesels use much higher compression ratios than petrol engines.

Given Data / Assumptions:

• Typical commercial diesel and petrol formulations.
• Self ignition temperature refers to the temperature at which fuel vapour ignites without an external spark under specified conditions.
• Pressure and mixture effects exist, but we compare general typical values.

Concept / Approach:
Diesel fuel has a lower autoignition temperature (around the low 200s °C) compared with petrol, which often requires roughly 280–450 °C depending on test method. A lower autoignition temperature facilitates ignition in diesel engines when air is heated by compression. Petrol’s relatively higher self ignition temperature helps prevent spontaneous combustion in SI engines until a spark occurs.

Step-by-Step Solution:
Recall typical ranges: diesel ≈ 210–260 °C; petrol ≈ 280–450 °C.Compare: diesel threshold is lower than petrol’s.Conclude: diesel has lower self ignition temperature than petrol.

Verification / Alternative check:
Engine design corroborates this: diesels depend on compression heating alone; petrol engines need a spark and use lower compression ratios to avoid knock.

Why Other Options Are Wrong:
Higher/same: contradicts well-documented property ranges.Depends on fuel quality: though formulation shifts values slightly, the general ordering (diesel lower) holds.Cannot be compared: they are routinely compared in engineering data.

Common Pitfalls:
Confusing octane (anti-knock) with autoignition temperature; they are related phenomena but different measures.

Final Answer:
lower