Mechanical Engineering - Automobile Engineering - Discussion

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.

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 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.

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.

No, Mr. Retheesh.

The self-ignition temperature of Diesel is higher than petrol. That is why we use a spark plug to ignite petrol engines and don't use external ignition source for diesel engines.

Absolutely wrong answer.

The answer is wrong because diesel needs higher compression ratio to get ignite, thats hy fuel injectors are used in place of the spark plug, petrol is a purified form og diesel only. So it ignites more quickly.

That is the same reason spark plugs are used in petrol because it needs higher temperature to ignite but in case of diesel higher compression (pressure) and lower temperature are enough to ignite it.

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.

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