Self Ignition Temperature:
The self-ignition temperature, also known as the autoignition temperature, is the minimum temperature at which a substance spontaneously ignites without an external ignition source, such as a spark or flame.

Comparison of Self Ignition Temperatures:
Let's compare the self-ignition temperatures of the given options:

- Gasoline: Gasoline typically has a self-ignition temperature of around 495-536°C (923-997°F).
- CNG (Compressed Natural Gas): CNG has a self-ignition temperature of approximately 540°C (1,004°F).
- Petrol: Petrol, which is another term for gasoline, has a similar self-ignition temperature range of 495-536°C (923-997°F).
- Diesel: Diesel has the lowest self-ignition temperature among the given options, typically ranging from 210-260°C (410-500°F).

Reason for Diesel having the lowest self-ignition temperature:
Several factors contribute to the lower self-ignition temperature of diesel compared to gasoline and CNG:

- Chemical Composition: Diesel fuel is composed of heavier hydrocarbon molecules compared to gasoline and CNG. These heavier molecules have higher boiling points and are more prone to ignition at lower temperatures.

- Cetane Number: Diesel fuel is assigned a cetane number, which represents its ignition quality. A higher cetane number indicates better ignition quality. Diesel fuels with lower cetane numbers tend to have lower self-ignition temperatures.

- Air-Fuel Mixture: Diesel engines operate on the principle of compression ignition, where air is compressed to a high temperature before fuel is injected. This high compression ratio generates the necessary heat to ignite the fuel. The compression process in diesel engines creates higher temperatures compared to the spark ignition process in gasoline engines, making diesel more prone to self-ignition.

Conclusion:
In conclusion, among the given options, diesel has the lowest self-ignition temperature. Its lower self-ignition temperature can be attributed to factors such as the chemical composition, cetane number, and the high compression ratio in diesel engines.