Mechanical Engineering - Automobile Engineering - Discussion

How it is possible that self ignition temp of diesel is lower than petrol as it requires high compression ratio?

The Self Ignition auto-ignition temperature is the lowest temperature at which a fuel such as petrol or diesel spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. Self-ignition temperature of petrol is 247-280°C. Self-ignition temperature of diesel is around 210°C.

The self-ignition Temperature of petrol is higher than that of Diesel because it ignites at a higher temperature than Diesel when subjected to compression by a piston in an engine. A spark plug (external spark) is used to provide faster and better ignition in Petrol engines due to this reason.

Agreed @Dixit.

Diesel - The temperature of fuel-air mixture in diesel engine running at 1:20 compression ratio can go as high as 700°C. Since this temperature is much higher than the STI, diesel automatically catches fire at 210°C.

Petrol-Petrol engines have lesser compression ratios compared to diesel engines(1:8"10) and they relay on spark plugs for the source of ignition.

The self-ignition temperature of diesel is more because its compression ratio is 15 to 20 for diesel & petrol 8 to 10. So more pressure is required to self ignite.