Fundamentals Multec 3.5 Fuel Injector Application Manual
2-22 Delphi Energy and Chassis Systems
Revision: 11/05-1
consult the Delphi Fuel Pump Application manual.
2.3.3 Fuel Pump Check Valve Requirements
To protect the rail assembly and fuel supply subsystem from exposure to
extreme pressure during pump operation, a pressure relief valve (check
valve) is incorporated in the fuel pump. For failure modes where the fuel
return line is blocked causing a dead headed pump condition, the check
valve will open to minimize the maximum pump generated pressure. This
check valve relief pressure must be selected to meet the following
conditions:
• Check valve relief pressure is always less than the maximum
sealability and burst pressure for the fuel rail assembly and fuel supply
subsystem.
• Check valve relief pressure is greater than the maximum required fuel
pump pressure to achieve regulated system pressure in worst case
conditions.
Note: The maximum relief pressure recommended for the Multec 3.5
injector is 1000 kPa.
2.3.4 Pressure Regulator Gain/Considerations (Vacuum Biased)
A typical vacuum biased fuel pressure regulator contains a vacuum
chamber that is connected to manifold vacuum that is separated from the
fuel by a diaphragm and valve assembly. The diaphragm has fuel on one
side and engine manifold pressure (vacuum) on the other (vacuum biased
recirculating fuel systems). A calibrated spring is located in the vacuum
chamber side. Fuel pressure is regulated as pressurized fuel, acting on the
bottom side of the diaphragm, works against the spring action and
manifold absolute pressure (MAP) on the top side. When this happens,
the diaphragm relief valve moves, changing the size of the flow orifice.
This controls the amount of fuel returning to the fuel tank. Fuel rail
pressure is controlled by the return spring calibration, as well as by engine
manifold pressure acting on the top side of the diaphragm.
Fuel pressure varies as a function of fuel pump recirculation flow due to
the gain of the regulator. Regulator gain is the slope of the “pressure
versus recirculation” curve. It occurs because of flow passage pressure
drops and the increase in valve lift required for increases in recirculation
rates. Refer to Figure 2-5 for help in understanding and calculating
pressure regulator gain. Other considerations include:
• Fuel pump check valve setting hysteresis failure modes
• Fuel pump impact on time-to-regulated pressure
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