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-How the Stanadyne DB2 Fuel Injection Pump Works-

by

Neil Beadle

photos courtesy of Peter Bierman

End view of rollers, roller spring, cam ring, light load advance rocker lever and servo advance piston The Stanadyne DB2 rotary distributor pump has a single pumping element that feeds all injectors. The high pressure pumping action is carried out in a single barrel, which houses two opposing plungers operated by two rollers. These plungers are pushed together by a cam ring with internal lobes acting on the rollers and it is this action that pressurizes the fuel.

The number of lobes on the cam ring corresponds to the number of cylinders in the engine, so each thrust of the plungers by a pair of cam lobes results in a pulse of fuel to one particular cylinder injector.

The injection pump controls fuel delivery by:

  • accurately metering the fuel as demanded by the operator
  • automatically pressurizing the fuel to match the injector pop off pressure to open the nozzles
  • delivering the pulses of fuel in the required firing order
  • controlling injection timing

Injection Pump Operation

Fuel enters the pump inlet under lift pump pressure from the fuel filter, where a vane type transfer pump further pressurizes it. From the transfer pump, the fuel is then routed to three separate places:

  • Some is sent to the automatic advance mechanism.
  • Some fuel goes through a vent wire assembly to the housing reservoir and then out of the HPCA check valve return fitting to the fuel tank.
  • Most of the fuel is sent to the metering valve.

Hydraulic head with throttle linkage attached (at top) and drive shaft engage (right) The metering valve is actuated by the throttle lever at all engine speeds above idle and by the governor linkage, when the engine is at idle or maximum speed. 

The metering valve flows a measured fuel quantity into the center of the hydraulic head where the distributor rotor is turning and from there the fuel goes to the center of the rotor, which has two inlets at one end and an outlet at the other end.

As the rotor spins, the inlets line up with ports in the hydraulic head and fuel is forced out into the pumping chamber.

End view of rollers, roller spring, cam ring, light load advance rocker lever and servo advance pistonThe pumping chamber consists of two sets of rollers, shoes, and plungers, which rotate inside a cam ring. The cam ring has lobes for each injector on the inside.

The force of the fuel entering the pumping chamber, along with the centrifugal force generated by the spinning rotor, moves the plungers apart. The plungers move outward a by an amount directly proportional to the volume of fuel passed by the metering valve.

As the rotor turns, the cam ring lobes, squeeze the rollers together, which in turn push in the rollers. As the plungers are pushed together, they pressurize fuel until the injectors nozzles open at their preset pop pressure.

View of hydraulic head with injector outlets on the bottom and the throttle assembly on the right The fuel at injection pressure is forced back along the center passage of the rotor and into the discharge port and released when the discharge port lines up with a injector line outlet in the hydraulic head.

The discharge ports can be seen inside the hydraulic head's bore at leftAs the single discharge port rotates with the rotor it lines up with each of the hydraulic head injector line outlets in a sequence matching the engine’s firing order.

Each injector outlet is connected to an injector nozzle by a high-pressure line. The injector pump varies the rate and quantity of fuel delivered to the nozzles as demanded by the throttle and/or governor.

The DB2’s Components and Functions

Vent Wire Assembly. The vent wire assembly controls the amount of fuel returned to the fuel tank from the injection pump, and is located in a short passageway behind the metering valve bore.

Excess fuel from the transfer pump flows past the vent wire, carrying any air which might have entered the transfer pump.

After the excess fuel passes through the vent assembly, it goes to the governor compartment and then back the pump through the return line.

The vent wire’s size controls the amount of fuel that enters the return line. So if the amount of return fuel doesn’t meet specification, the vent wire can be swapped for one of a different size.

A delivery valve, in the center of the distributor rotor, assists the injector nozzles to rapidly reseat and prevents unatomized fuel from dribbling into the precombustion chambers. Pressurized fuel from the pumping chamber forces the delivery valve plunger slightly out of its bore so fuel flows past the plunger and then out the discharge port. When fuel pressure drops, the delivery valve plunger immediately reseats, causing a rapid drop in injection line pressure.

There are three timing advance mechanisms on the fuel injection pump:

  1. An automatic advance system
  2. A mechanical light load advance
  3. The housing pressure cold advance (HPCA) solenoid.

The automatic advance mechanism advances and retards the start of fuel delivery. This mechanism starts working as the engine speed increases to ensure that the injector nozzle opens just before the piston reaches top dead center, when compression is at its highest point. Otherwise fuel wouldn't be injected before the piston had started moving downward on it’s power stroke.

The mechanism comprises a power piston, servo valve, servo spring, servo piston and a cam advance pin. The cam advance pin connects the advance mechanism to the cam ring. When the power piston moves, it rotates the cam ring so that fuel is delivered earlier.

Cam ring with servo piston and light load advance mechanism

Housing pressure and transfer pump pressure behind the power piston influence the action of the servo piston. When the engine is cranking, the fuel behind the servo piston is at housing pressure, and the power piston is seated against the housing. As the engine speed increases, transfer pressure rises and the subsequent increase in transfer pump pressure forces fuel into a chamber behind the power piston.

When transfer pressure in chamber behind the power piston exceeds housing pressure, the servo piston acts against the servo spring, and the power piston pushes the cam advance pin which rotates the cam ring in the opposite direction to the distributor rotor’s rotation and so the rollers contact the cam lobes earlier and injection timing is advanced.

When engine speed decreases, transfer pressure drops, the cam ring rotates in the other direction retarding injection timing.

A light load advance mechanism provides advance when the engine is operating at low speed or under light load, when the transfer pressure is too low to move the advance piston.

The light load advance is actuated by an external face cam and rocker lever assembly when the throttle shaft rotates (on the 6.2L and 6.5L engines, this mechanism is on the passenger side of the pump). The lower end of the rocker lever pushes on the end of the servo advance plunger.

As the throttle shaft rotates, the face cam pushes on the rocker lever using a “see-saw” action, which depresses the servo plunger and advances the timing through the power plunger’s linkage to the cam ring. At a predetermined angle, the face cam flattens out, so that additional throttle movement does not affect the servo.

After the light load advance mechanism ceases to act on the servo plunger, advance action is regulated by transfer pump pressure.

Picture of HPCA, comprising: check valve solenoid needle (left), solenoid body and check valve (on right)The housing pressure cold advance (HPCA) solenoid is one of three solenoids that affect the operation of the injection pump. The HPCA solenoid makes it easier to start a cold engine by reducing housing fuel pressure in the advance mechanism.

The HPCA solenoid is located under the fuel return outlet, under the pump housing cover. It is activated by the coolant temperature switch, which is mounted on the rear of the passenger side cylinder head. When coolant temperature is low the temperature switch is closed, energizing the HPCA solenoid (rear pump terminal connected with a green wire), which lifts the check ball off its seat in the return outlet. This reduces housing pressure to near zero, so that the transfer pump pressure behind the power advance piston can easily advance the cam ring.

In addition to the housing pressure cold advance (HPCA) solenoid, there is a fuel shut-off solenoid (front terminal connected with a pink wire) located inside the pump housing cover that stops the engine by cutting off the fuel flow.

The fuel shut-off solenoid moves the governor linkage, which in turn rotates the metering valve.Shutoff solenoid in housing (top) engaging the shutoff rack on top the hydraulic head.  To the left of the hydraulic head is the governor assembly

When the ignition is off, the solenoid is no longer energized and the return spring pulls the shut-off rack to the "OFF" position, which through the governor linkage rotates the metering valve to cut off fuel.

The minimum/maximum engine speed governor, located under the governor cover maintains idle speeds under varying engine loads and limits the maximum speed of the engine.

The governor assembly comprises weights, the governor arm, low idle spring, idle spring guide, main governor spring, main governor spring guide, and the guide stud.

Governor weightsThe governor weights are rotated by the drive shaft. Their centrifugal force controls the metering valve at minimum and maximum engine speeds.

At idle speed, the governor weights don’t exert much force, so the spring on the governor keeps the metering valve nearly closed.

At high engine speeds, the centrifugal force of the governor weights moves a pivot arm, compressing the spring, and rotating the metering valve to an almost closed position.

At engine speeds other than idle or maximum, the driver directly controls the metering valve through the accelerator/throttle linkage. At those engine speeds the force of the governor weights and the governor spring tension are balanced, so that neither can influence the metering valve.

A pressure regulator protects the transfer pump from excessive output pressure caused by high engine speeds or because of a restricted fuel return line.

When the valve is closed during normal operation, the valve spring holds the piston forward, blocking the regulating slot in the valve thus rendering it inactive.

As output pressure increases, the valve opens. High-pressure fuel pushes the valve piston, which compresses the spring. If the pressure is high enough to overcome the spring’s force, the piston will be pushed back, uncovering the regulating slot in the valve. This will allow fuel to flow back to the input side of the pump, thus relieving output pressure.

A viscosity-compensating device maintains the constant fuel pressure, so that fuels with differing viscosity levels due to composition or temperature may be used. The compensator is part of the design of the pressure regulator mechanism.

GLOSSARY

Picture of HPCA comprising: check valve solenoid needle (left), solenoid body and check valve (on right)

HPCA: High Pressure Cold Advance; this mechanism advances the timing by about 6 degrees to compensate for the slower burning injected diesel charge until the engine warms up to around 125F.

 

 

Sources:

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