Different diesel engines use different fuels.
Fuel can be delivered to the chamber in different ways.
Direct injection means fuel is injected directly into the chamber. The cylinder head usually has a flat surface and the combustion chamber is formed in the piston crown. At top dead center, there is very little clearance between the cylinder head and the top of the piston.
Another method of injecting fuel is Indirect injection.
Fuel is sprayed into a smaller, separate chamber in the cylinder head.
This chamber can have various designs.
A glow plug is not a spark plug. It is a small electric heater that pre-heats the separate chamber as an aid to cold starting. It helps the combustion which then spreads to the main chamber.
