Volkswagen 1.9-Liter TDI Owner's Manual page 16

Fuel Supply
Chamber 4
Chamber 1
Rotor
Chamber 1
Rotor
12
Chamber 3
Chamber 2
SSP209/052
Chamber 4
Chamber 3
Chamber 2
SSP209/051
Function
The fuel pump operates by taking fuel in as
the pump chamber volume increases and
pushing the fuel out under pressure as the
chamber volume is reduced.
The fuel is drawn into two chambers and
pumped out from two chambers. The
intake and delivery chambers are separated
from one another by the spring-loaded
blocking vanes and the pump rotor lobes.
Fuel drawn into chamber 1 is pushed out at
chamber 2. Fuel drawn into chamber 3 is
pushed out at chamber 4.
The rotation of the rotor increases the
volume of chamber 1 while the volume of
chamber 4 is simultaneously reduced.
Fuel is pushed out of chamber 4 to the
fuel supply line in the cylinder head.
The rotation of the rotor increases the
volume in chamber 3 as it reduces the
volume in chamber 2. Fuel drawn in at
chamber 1 is forced out of chamber 2 to
the fuel supply line in the cylinder head.