New engines are expensive to develop, so successful designs tend to live a long time, especially diesels. The new Bosch electronic injection system helps this engine meet some very strict emissions and fuel efficiency goals, and there will probably be several variations of this system in the future. But its first mission is on the Duramax, and it looks like GM is going to sell a lot of them. Rail pressure is controlled with an electric throttling valve at the inlet of the high-pressure pump. With the injector at rest, the ball is held against the vent orifice by the return spring.
Pressure is equal above the servo piston and in the accumulator volume chamber.
Since the servo piston has a larger bore than the nozzle needle, the pressure holds the needle closed. When the electromagnet lifts the ball off its seat, pressure above the servo piston bleeds through the drain orifice. Pressure in the accumulator volume chamber is greater, pushing the nozzle needle off its seat to open the nozzle. The drain orifice is larger, and the size can be selected to control nozzle opening speed, which contributes to rate control. The quick pressure drop and partial recovery is reflected in the rate of injection curve, which shows a gradual rise and constant rate over the main injection event.