I'll try, but don't grade me on accuracy or anything.

The principle of the common-rail is to more accurately measure equal fuel distribution through each injector solenoid connected to the system.
A computer monitors injection pressure at all times ensuring optimal combustion rates and producing less noise than previous non common-rail diesel systems.
The CDI's are electronically fuel injected. Because of electronic fuel injection, and adaption of the common rail, more fuel is able to be used efficiently. Better power, and better MPG are the results.

Hey, I don't know much about the system, so please somebody else please pickup where I left off. Or feel free to correct me. I am trying to understand the concept myself.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)