Wow....This could be a long one! In a nutshell:
- Most large truck/machinery diesels are, and some early passenger diesels were, direct injection
- "direct injection" means just that...the diesel is atomised by the injector and sprayed directly into the combustion space offered by the ascending piston
- direct injection diesels are generally noisier (more "diesel knock") than IDI (InDirect Injection) diesels
- indirect injection was adopted by engine developers who saw it as a way of offering more petrol engine-like characteristics to buyers of diesel engined passenger cars and light commercials... eg less noise, smoother running and a more compact engine
- direct injection is generally more fuel efficient than indirect injection as the fuel isn't forced to "swirl" in the pre-combustion chamber, a process that saps some of its potency
- glow plugs to preheat the fuel in the pre combustion chamber for easier starting aren't so important in direct injection diesels (although they are still used to make the car more petrol engine-like)
- direct injection became applicable in passenger car diesel only after computerisation of the fuel system. This combined with modifications to the pump(s) and injectors solved all of the problems of earlier direct injection engines
There's a whole lot more to this than this brief account, however it may help your understanding.
- until VW brought in the PD engines of the later TDIs, all diesels had a separate injection pump to provide pressurised diesel to each injector. These were either multi-element (in-line) pumps or distributor pumps like the Bosch VE used on VWs until PD was introduced
- injection pumps are expensive and complex with very fine tolerances and are only serviceable by people who know what they're doing (having said that they are extremely durable and should outlast the engine if the fuel is kept clean and water-free)
- common rail injection involves a high pressure fuel pump delivering extremely high pressure fuel to a pipe (the "common rail") that delivers fuel to the injectors. Unlike other systems, the fuel is under a constantly high pressure.
- Each injector is computer controlled in order to deliver the exact amount of fuel to the cylinder at the exact time it's needed. This negates all of the "guesswork" that evolves no matter how well the fuel injecting system is designed or maintained in a mechanical system
I stand fully prepared to edit and amend any inaccuracies in the account above
I'll move this thread to the "Diesels" forum