Distributor-type fuel pump
The distributor-type fuel pump, perhaps more accurately described as an axial piston injection pump, was the technology that enabled Audi to start series production of the TDI engine. It was a compact unit, driven by toothed belt from the crankshaft. The central element was a slotted piston that delivered fuel at high pressure to the pipes that led to the injectors. Two-spring injector holders opened the injector needles in two stages: first with a smaller, then with a larger stroke. The axial piston pump could build up a maximum pressure of 900 bar at the injector nozzles; a further evolutionary stage in 1994 increased this to about 1,000 bar. For the 1997 V6 TDI, Audi switched to a radial piston injection pump. This operated according to a similar but more advanced principle, and was able to generate a pressure of 1,500 bar, later 1,850 bar at the injector nozzles. The amounts of fuel injected were between two cubic millimeters for the pre-injection stage and 50 cubic millimeters, equivalent to one droplet of fuel, at full load.