What is a 4-stroke diesel engine?
As name suggests, these engines work on reciprocating actions of pistons. One stroke means one linear motion of piston in one direction. When piston moves in opposite direction, that is counted as another stroke.
For a 4-stroke engine, piston has to move up, then down, then up again, and then down again to complete one cycle of engine. In that 4 stroke of piston, crankshaft will have turned 2 revolutions.
The 4 strokes of piston is to complete 4 stages of combustion cycle. As you might have known, in order for a fire (or explosion) to occur to power engine, we need a fuel, a heat source, and oxygen coming together at same time.
At start of cycle, air must be introduced into combustion space inside cylinder. In 4-stroke (also called 4-cycle) engine, this is air intake stroke. The piston moving down will suck fresh air into cylinder through air inlet valve(s) that is opened.
The next stroke is compression stroke. With all valves at cylinder head closed, piston moves up again. The air becomes compressed to a very high degree. So high that temperature of air becomes sufficiently hot to ignite a finely sprayed fuel mist on contact.
That is precisely what happens. Before piston reaches top-dead center (the maximum height piston can reach before coming down again), fuel is injected as a very fine mist into combustion space at piston top. In order for this fuel to be injected into highly compressed air inside cylinder fuel itself must be at a higher pressure. This is achieved by a fuel pump.
There is a reason for injecting fuel slightly before piston reaches top dead center. The fuel takes some time to reach temperature that it can burn. So timing of injection of fuel is adjusted so that by time piston reaches slightly beyond top dead center, fuel has acquired temperature high enough for ignition.