Advance arrangement

An advance arrangement for use in controlling timing of fuel delivery, the advance arrangement comprising;

• an advance piston which is moveable within a first bore to adjust the timing of fuel delivery, a surface associated with the advance piston being exposed to fuel pressure within an advance piston control chamber,
• a servo-piston which is slidable within a further bore provided in the advance piston to control the pressure of fuel in the advance piston control chamber, a surface associated with the servo-piston being exposed to fuel pressure within a servo control chamber,
• a light load piston movable relative to the advance piston against the action of a light load control spring by means a force due to fuel pressure within a light load control chamber, and
• a damping arrangement for damping movement of the servo-piston in a direction to retard timing which occurs as consequence of movement of the advance piston in the advance timing direction at the end of a pumping event;
• wherein the advance piston is provided with a fill passage which is brought into communication with the servo control chamber upon movement of the servo-piston in a direction to advance timing, thereby to permit fuel within the servo control chamber to flow into the advance piston control chamber;
• and
• wherein the non-drift arrangement is arranged to maintain a negative net leakage flow into the advance piston control chamber for any position of the servo-piston in which the servo control chamber communicates with the advance piston control chamber through the fill passage.

An advance arrangement for use in controlling timing of fuel delivery by a fuel pump, the advance arrangement comprising;

• an advance piston which is moveable within a first bore to adjust the timing of fuel delivery, a surface associated with the advance piston being exposed to fuel pressure within an advance piston control chamber,
• a servo-piston which is slidable within a further bore provided in the advance piston to control the pressure of fuel within the advance piston control chamber, a surface associated with the servo-piston being exposed to fuel pressure within a servo control chamber,
• a non-drift arrangement for preventing fuel pressure variations in the advance piston control chamber due to fuel leakage causing the advance piston to drift whilst the servo control piston remains in a substantially fixed position relative to the advance piston or the cam box housing;
• and
• a dashpot arrangement provided at an end region of the servo-piston, the dashpot arrangement including a restricted flow path for fuel flow which serves to limit the rate of flow of fuel out of the servo control chamber in the event that the servo control piston is urged in a direction to retard timing;
• wherein the servo-piston is provided with a damping arrangement for damping movement of the servo-piston in a direction to retard timing which occurs as a result of movement of the advance piston in a direction to advance timing at the end of a pumping event.