The speed advance device (Figure 4-58) is used to achieve best engine performance through the operating speed range of the engine. Timing advance is needed to compensate for the two delay periods: 1) the injection pressure wave traveling the length of the injection line, 2) the ignition delay period.
The injection timing advance system controls the start of injection proportional to pump speed. This is done by moving the cam ring opposite the direction of rotor rotation. The plunger rollers will then come in contact with the lobes on the cam ring earlier in the cycle.
The power advance piston engages the cam ring through the cam advance pin and moves the cam. Fuel under transfer pump pressure is fed through a passage in the hydraulic head, through the cam advance pin to the servo advance piston valve chamber. As transfer pump pressure is increased the servo advance piston valve uncovers a port which is connected to the advance piston, which advance the cam ring.
Secondary control of the cam ring advance is by a mechanical connection from the throttle shaft thru a face cam and rocker lever to the servo advance piston valve spring seat. This secondary timing control results in injection timing better suited to engine demand.
