Why do diesel internal combustion engines require no spark plug to ignite the fuel unlike petrol engines?
To know the reason, we should first understand the processes in both engines.
In petrol engine, air-fuel mixture is administrated into the cylinder. The compression ratio of petrol engines is lower than diesel engines, i.e, 6–10. So when the charge is compressed inside the cylinder , the temperature of the charge will be less than the auto ignition point of the petrol and hence will not ignite. So a spark is produced by the spark plug to ignite the charge.
In diesel engines, the compression ratio is much higher,i.e 16–20.
In this engine, only air is administrated into the cylinder. When the air is compressed in the compression stroke, the temperature of the air will be very high. Now when the fuel is injected by the fuel injector under high pressure, the fuel mixes with air in the cylinder and since the temperature of air is sufficiently high, the fuel will ignite without any external aid.