Mechanical Engineering - IC Engines and Nuclear Power Plants - Discussion

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.
So as seen, diesel has low self-ignition temperature, and that's why diesel engine doesn't have a spark plug

For a comparison, the substance like white phosphorus will catch fire as low of 34°C.

In a petrol engine, it is imperative that the autoignition temperature (the temperature at which the fuel will ignite itself) of the fuel stays high enough.

Because, if it is very low, then as the mixture is being compressed, it will ignite itself as soon as its auto-ignition temperature is reached. This ignition may happen even before the spark plug shoots a spark.

This phenomenon is called auto-ignition and results in engine knocking (a very distinct metallic knocking sound).

The Branching habit and tendency to form intermolecular bonds reduce with the growth of chain length in alkanes as per the fact that the Gibbs Free Energy is lowered with increasing molecular weight. So, diesel having higher chain length will have a lower tendency for branching and hence break easily due to the application of heat as compared to petrol. Hence we can correctly assume the correct answer is C. And petrol will have higher autoignition temperature than that of diesel.

Substance
Autoignition point (°C)

Butane
405 °C

Carbon disulfide
90 °C

Diesel or Jet A-1
210 °C

Diethyl ether
160 °C

Ethanol
363 °C

Gasoline (Petrol)
247-280 °C

Hydrogen
536 °C

Magnesium
473 °C

Paper
218-246 °C

Silane
21 °C

Triethylborane
-20 °C

White phosphorus
34 °C

Self ignition temperature is that temperature at which fuel starts burning without any assistance as flame or burner. And we also know that the calorific value of petrol is more than the diesel, and will easily burn compared to diesel hence we can say that the self ignition temperature of petrol is less than the diesel.

Option C is correct.

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

So as seen, diesel has low self ignition temperature, and that's why diesel engine doesn't have a spark plug.

Petrol engine as compared to a diesel engine has a lower compression ratio. If the autoignition temp of petrol is higher than diesel then why is the compression ratio of petrol engines lower than diesel engines?

Auto-ignition temperature of Gasoline (petrol) is high as compared to Diesel that's the reason of using spark plug in the Gasoline Engine. If it is not then we don't use to spark plug in Gasoline Engines.

Auto-ignition temperature of Gasoline (petrol) is high as compared to Diesel. And spark plug is used because we can't compress air within the cylinder about 250° Celsius.

In the gasoline, petrol engine self-ignition temp is low when compared with diesel engines because if petrol has high ignition temp then no need for spark plugs right.