- Apr 4, 2018
There is a physical piston moving up and down compressing a hydraulic fluid. This pressure expands the bellows forcing them to expand to displace the fuel. Because it's coupled hydraulically, it needs a spring to assist in returning it back down. It is physically the same as a wobble plate hydraulic pump. The bellow just separates the two fluids because gasoline is not a good enough lubricant to lubricate this style pump. A true bellow pump would have the plunger mechanically coupled to the top plate of the bellow and would not be able to deal with the pressure needed for direct injection. Almost all true bellow pumps are low pressure. The lobe driven pumps you are referring to have very little thrust load on the cylinder wall and do not require that much lubrication. This is why they can get away with directly pumping the fuel.The piston just expands the bellows. Compared to a piston pump like on an F series or the majority of other DI cars where an actual piston compresses fuel directly.