Why Do Diesel Engines Have A Depression In The Piston For The Combustion Chamber While Gasoline Engines Use Flat Pistons And Have A Depression In The Head Instead?

Petrol engines mix the fuel and air together and THEN ignite it

Diesels have the fuel burning as it is injected

This means that the fuel and the air have to find each other as it ignites - there is no time for it to mix

That bowl in the piston is designed to make the air swirl - like a smoke ring -

The injector is in the center and it will blast the fuel out of about six tiny nozzles to meet with the air swirling past

That way the air and the fuel find each other in order to burn