TM 10-3930-671-24 pressure   on   the   piston   is   opposed   by   the   regulating spring,   the   delivery   pressure   of   the   transfer   pump   if controlled  by  the  spring  rate  and  size  of  the  regulating slot "A." Therefore, pressure increases with speed. A  high-pressure  relief  slot  ’B"  is  incorporated  in  some regulators  as  part  of  the  pressure  regulating  slot  to  pre vent   excessively   high   transfer   pump   pressure,   if   the engine or pump is accidentally overspeeded. FIGURE 1.6. VISCOSITY COMPENSATION The DB2 transfer pump works equally well with different grades  of  diesel  fuel  and  varying  temperatures,  both  of which     affect     fuel     viscosity.     A     unique     and     simple feature     of     the     regulating     system     offsets     pressure changes  caused  by  viscosity  difference.  Located  in  the spring   adjusting   plug   is   a   thin   plate   incorporating   a sharpedged  orifice.  The  orifice  allows  fuel  leakage  past the  piston  to  return  to  the  inlet  side  of  the  pump.  Flow through a short orifice is virtually unaffected by viscosity changes. The biasing pressure exerted against the back side  of  the  piston  is  determined  by  the  leakage  through the clearance between the piston and the regulator bore and  the  pressure  drop  through  the  sharpedged  orifice. With cold or viscous fuels, very little leakage occurs past the  piston.  The  additional  force  on  the  back  side  of  the piston  from  the  viscous  fuel  pressure  is  slight.  With  hot or  light  fuels,  leakage  past  the  piston  increases.  Fuel pressure  In  the  spring  cavity  increases  also,  since  flow past   the   piston   must   equal   flow   through   the   orifice. Pressure   rises   due   to   increased   piston   leakage   and pressure rises to force more fuel through the orifice. This variation in piston position compensates for the  leakage which  would  occur  with  thin  fuels  and  design  pressures are maintained over a broad range of viscosity changes. E. CHARGING AND DISCHARGING Charging Cycle As the rotor revolves (Figure 1.7), the two inlet passages in  the  rotor  register  with  ports  of  the  circular  charging passage.  Fuel  under  pressure  from  the  transfer  pump, controlled  by  the  opening  of  the  metering  valve,  flows into the pumping chamber forcing the plungers apart. FIGURE 1.7. F-218