Scroll type fuel pumps are positive displacement pumps commonly used in large diesel engines, particularly marine applications. They offer a simple yet efficient method for metering fuel based on engine load.
Components:
- Spiral Groove (Scroll): This is a helical groove machined into the inner surface of the pump housing.
- Rotor: A spool-shaped rotor with a tight fit inside the scroll has an eccentrically positioned cam follower pin.
- Cam Follower: This pin rides inside a groove on a drive sleeve connected to the engine.
- Inlet Port: Located at the base of the scroll, where fuel enters the pump.
- Outlet Port: Located at the top of the scroll, where fuel exits the pump towards the engine injectors.
Metering Mechanism:
- Rotation: The drive sleeve rotates the rotor within the scroll housing.
- Eccentricity Effect: Due to the eccentric cam follower pin, the rotor doesn’t rotate perfectly centered within the scroll. This creates a varying volume between the rotor and the scroll during each revolution.
(a) High Load:
- Large Pockets: As the rotor rotates at high engine load, the eccentric position allows for the formation of larger pockets between the rotor and the scroll.
- More Fuel Delivery: During each rotation, these larger pockets fill completely with fuel on the inlet side and are carried towards the outlet on the discharge side. This results in a higher volume of fuel delivered per revolution, satisfying the increased fuel demand at high engine loads.
(b) Low Load:
- Smaller Pockets: At low engine load, the cam follower pin position creates smaller pockets between the rotor and the scroll.
- Less Fuel Delivery: These smaller pockets fill with less fuel during each rotation, resulting in a lower overall volume of fuel delivered per revolution, matching the reduced fuel requirement at low engine loads.
Key Points:
- The pump doesn’t have an external mechanism for actively controlling fuel flow. The varying volume pockets created by the rotor’s eccentric movement achieve the metering.
- Engine speed remains relatively constant, and the varying fuel delivery based on load is achieved by the scroll pump’s internal design.
Additional Notes:
- The specific shape and dimensions of the scroll and rotor profile determine the pump’s performance characteristics and fuel delivery curve.
- Modern scroll type pumps might incorporate control mechanisms for additional adjustments or integration with electronic engine management systems.