In Mr Diesel’s original engine the fuel was mixed with the air before it entered the cylinder.  These are found amongst the Compression Ignition types used in model aeroplanes.

The engines you are looking at are the ‘solid injection’ type where fuel is injected into the cylinder at or about full compression.  The real problems are the injector, and the injector pump which contains fuel dosing control to get the fuel correct for the load.

Unless you are using a ‘common rail’ engine with electronic control, which is very different and uses a single pump.  The injector has to stay shut until the fuel reaches a certain pressure when it opens squirts in its load and snaps shut.  It has a ‘dribble pipe’ which then opens to overcome the ‘water hammer effect’ and return the surplus oil to the feed side of the pump.

The injector pump which is controlled by the governor has a rotating cylinder liner. This has a triangular hole in one side.  As the governor decides that more oil is needed it rotates the cylinder exposing a larger opening to the output slot and allowing more oil to be injected.  You needed one pump per cylinder.  These could be either on the cylinder head driven by individual cams or as a block of pumps driven by the camshaft and supplying the injectors by pipes.

By the way do not get confused by the direct/indirect injection systems devised by Ricardo and give better performance by better mixing.