A diesel engine does not contain a spark plug. Then how does ignition take place in it? Explain.

Hint: This given problem can be solved by seeing the molecular nature of matter and how diesel engines are working and what is used in diesel engines to ignite the diesel fuel.

Complete step by step answer:
Step 1: As we know that a spark plug is used in any engine just to ignite the fuel in that engine by providing ambient temperature up to its flash point.
Flash point: Flash point denotes the lowest temperature at which vapour of the material used for fuel in the engine will ignite, when any ignition source is provided to the engine.
Spark plugs are used in the petrol engines to ignite the air fuel mixture whereas in diesel engines the presence of spark plugs is not necessarily required. Because of this, petrol engines are called spark ignition engines (SI) and diesel engines are called compression ignition engines (CI).
 In SI engines air and fuel (petrol) get mixed in the carburettor and then it is supplied to the engine through an inlet manifold, then the air fuel mixture is compressed inside the cylinder.
But in the case of diesel engines spark plugs are not required. The air from the atmosphere is sucked into the cylinder of the engine and then the air is compressed to high pressure which eventually leads to increase in the temperature, so when the diesel is supplied at the end of compression stroke, the temperature developed is more enough to ignite the diesel. And this makes the fuel (diesel) to burn and then expansion of gases takes place from where the power stroke is obtained.

Note: These points also can be remembered –
1)Diesel has low volatility so it cannot form the desired mixture of air-fuel that is to be ignited by spark plugs.
2)Diesel has a lower self-ignition temperature than petrol/gasoline.