Transfer fuel pressure during...
Transfer fuel pressure during the charging cycle.
How It WorksThe DB2 is an opposed-plunger, inlet-metering, distributor-type diesel injection pump. It was designed for low-cost production and simplicity. A typical DB2 (there were slight variations on the design for different engines) has approximately 100 components and only four main rotating members. There are no spring-loaded components, none are lap-fitted in manufacture, and there are no ball bearings or gears. The pump has a single pumping chamber in which two opposed plungers are actuated by an internal cam ring.
The shape of the hydraulic head determines the distribution of fuel between cylinders, and because fuel flow can be preset, lengthy periods on the fuel pump test bench are eliminated.
Operation of the pump during...
Operation of the pump during the discharge cycle.
Fuel is drawn from the fuel tank by a mechanical lift pump that is completely independent of the DB2 injection pump, and passes through filters and into the injection pump inlet. From there, fuel flows past the inlet filter screen to the vane-operated transfer pump located in the DB2 pump end cap. The vane-type transfer pump consists of a stationary liner and spring-loaded blades carried in slots at the rear of the transfer pump rotor. As the blades rotate in the liner, they move outward and the volume increases until the leading blade passes out of registry through the inlet slot. The fuel between the blade is carried to the bottom of the transfer pump liner and enters the outlet groove. As a result, pressurized fuel is delivered through the pump into a channel to the hydraulic head passage.
Fuel delivered to the head passage by transfer pump pressure splits in several directions. A portion of the fuel goes to the pressure side of the pressure regulator, while the remainder enters a circular passage in the head from which radial passages lead to the vent wire assembly and then to the governor housing and housing pressure regulator.
The pressure regulator assembly in the transfer pump regulates fuel volume based on changes in pump speed-pressure increases with pump speed.
The vent wire assembly is located behind the metering valve bore, and is made of a J-shaped wire retained in a hollow screw. The wire is free to vibrate and restrict excessive return oil and undue pressure loss.
Fuel entering from the vent fills the housing, lubricates the internal components, and cools and carries away any small air bubbles through the return oil line.
The advance circuit allows...
The advance circuit allows the DB2 to be an excellent performer.
The housing pressure regulator uses a ball-check fitting in the governor cover that maintains even housing pressure, except when the housing pressure cold advance solenoid is operating and allows fuel to return to the fuel tank.
The advance system is a simple, direct-acting hydraulic mechanism. Powered by fuel pressure from the transfer pump, the advance mechanism moves the power piston to rotate the cam to vary the delivery timing. It advances or retards the start of fuel delivery in response to engine speed changes. The advance piston, located in a bore in the housing, engages the cam advance screw and moves the cam in the direction opposite of rotor rotation. When engine speed decreases, hydraulic pressure is reduced and the cam becomes retarded because of low transfer pressure. When engine speed increases, transfer pump pressure increases and moves the advance piston and cam.
In addition to the normal speed advance, a mechanical light-load advance is furnished as a function of throttle angle. It consists of a face cam attached to the throttle lever and an external pivot lever. The mechanical function provides proper advance for light loads when transfer pressure is low and the cam ring is retarded by changing the reference point of the servo valve.