Mechanical Engineering - Automobile Engineering - Discussion

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.

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.

Agreed @Dixit.

Given Answer is right. But the confusion is created due to the fact that we have seen petrol catching fire easily whereas diesel doesn't.

This is because of their chemical properties. Petrol having simpler formula and highly flammable /vapourable, gets atomised and sustains ignition (combustion) upon firing.

But diesel on the other hand, is heavier/viscous than petrol. This is therefore, in spite of having a lower self-ignition temperature it does catches fire earlier but can't sustain combustion.

Guys, understand that self-ignition temperature measured at a Normal atmospheric pressure, in that case, diesel burn at 210°c whereas petrol at 280°c.

Due to low self-ignition temp, we can burn diesel without sparkplug at high compression ratio.

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.

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.

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

Self-ignition temperature of petrol is 247-280°C. Self-ignition temperature of diesel is around 210°C.

B is correct, I agree with @Retheesh.