One of the main reasons for the increasing popularity of CRDi is its performance and fuel economy. A CRDi engine is based on direct injection technology and has common rails i.e. tubes which inject pressurised fuel directly into the engine.
The common rail connects all the injectors and contains fuel at a constant high pressure. A small amount of fuel is supplied to the engine before and after the actual supply of charge. The high pressure in the common rail ensures that upon injection, the fuel atomises to a very high degree and mixes consistently with the air, thereby leaving minimal unburnt fuel.
In a CRDi engine, fuel quantity, engine pressure and timing of fuel injection are controlled electronically. The injectors have variable control heads which ensure that a highly precise amount of diesel goes into each cylinder. The onboard computer makes sure that the fuel is injected at the just the precise moment. This significantly improves engine efficiency and reduces noise and vibrations as compared to the conventional diesel engines.
This is for the CRDi. But needless to say that you will understand it better if you know the basic functioning of an internal combustion engine.