Self-ignition temperature, also known as auto-ignition temperature or spontaneous ignition temperature, is the minimum temperature at which a substance ignites without an external ignition source, such as a spark or flame. In the context of this question, we are comparing the self-ignition temperature of diesel fuel and petrol (gasoline).

Diesel fuel and petrol have different chemical compositions, which affect their self-ignition temperatures. Below are the key points explaining why the self-ignition temperature of diesel is lower than that of petrol:

1. Chemical Composition:
- Diesel fuel is a heavier and more complex hydrocarbon mixture compared to petrol, which consists of lighter hydrocarbons.
- The complex molecular structure of diesel fuel makes it less volatile and more resistant to ignition.
- Petrol, on the other hand, contains lighter hydrocarbons that are more prone to vaporization and ignition.

2. Flash Point:
- The flash point is the minimum temperature at which a substance gives off sufficient vapor to form an ignitable mixture in the air.
- Diesel fuel has a higher flash point (around 52-96°C) compared to petrol (around -45 to -40°C).
- The higher flash point of diesel fuel indicates that it requires a higher temperature to start vaporizing and form an ignitable mixture.

3. Cetane Number:
- The cetane number is a measure of the ignition quality of diesel fuel, similar to the octane number for petrol.
- A higher cetane number indicates better ignition quality, meaning the fuel ignites more easily under compression.
- Diesel fuel typically has a higher cetane number (40-55) compared to petrol's octane number (around 87-93).
- The higher cetane number of diesel fuel suggests that it ignites more readily under compression, leading to a lower self-ignition temperature.

In conclusion, the self-ignition temperature of diesel fuel is lower than that of petrol due to its heavier and more complex composition, higher flash point, and higher cetane number. These factors make diesel fuel less volatile and more resistant to ignition, requiring a lower temperature to initiate spontaneous combustion.