Another unique and very simple feature of the DB2 automotive pump is a viscosity compensating device. This component ensures proper transfer pump pressure regardless of the ambient temperature or grade of fuel used. (Figure 4-51).
When fuel โ thinsโ out due to heat, pressure loss will occur. The thin fuel, however, permits increased leakage past a loose fit at the regulating piston causing an increase of pressure in the spring cavity. This aids the spring and moves the piston to restrict spill, thus correcting the pressure. (Figure 4-52).
When fuel is cold it โ thickens,โ and due to the better sealing this affords: fuel pressure increases. Also due to better sealing, leaking past the regulating piston diminishes. This causes a reduction of spring cavity pressure allowing transfer pump pressure to move the regulating piston outward. This increases spill and corrects the pressure. (Figure 4-53).
A short sharp edged orifice in the adjusting plug controls the leakage from the spring cavity. This orifice is not sensitive to viscosity variation. Consequently, as input to the spring cavity, past the piston, varies with viscosity, pressure in this cavity will also change. This biases the position of the regulating piston over the spill slot and maintains the correct transfer pump pressure. (Figure 4-54).
