Scroll Type Fuel Injection Pump:
(a) Varying Delivered Fuel Quantity:
Unlike jerk-type pumps, scroll type pumps achieve fuel metering through a fundamentally different mechanism. Here’s how the delivered fuel quantity is varied:
- Scroll Design: The core of the pump is a spiral-shaped scroll housed within a cylindrical body.
- Inner Rotor: An eccentrically positioned inner rotor with blades rotates within the scroll.
- Fuel Chambers: As the rotor rotates, crescent-shaped cavities are formed between the rotor blades and the scroll wall, acting as fuel chambers.
Fuel Metering Principle:
- Fuel Intake: During rotation, the expanding volume of the fuel chambers on the suction side draws fuel in through an inlet port.
- Fuel Trapping and Compression: As the rotor continues to rotate, the scroll’s shape progressively reduces the volume of the fuel chambers, trapping and compressing the fuel.
- Delivery Valve and Outlet Port: A delivery valve is located at the outlet of the pump, connected to the high-pressure fuel line.
- Fuel Delivery: When the pressure in a chamber exceeds the delivery valve’s opening pressure, the valve opens, allowing the high-pressure fuel to flow through the line to the injector.
- Fuel Delivery Control: The amount of fuel delivered per cycle is primarily determined by the geometry of the scroll and the rotor. However, some designs might incorporate a bypass valve or variable control sleeve to influence the filling and discharge processes, allowing for limited adjustment of the delivered quantity.
(b) Purpose of the Delivery Valve:
Similar to jerk-type pumps, the delivery valve in a scroll type pump serves a crucial purpose:
- Location: The delivery valve is positioned at the outlet of the pump, separating the high-pressure fuel section from the discharge line.
- Function: The valve remains closed by a spring until the pressure within the pump chamber reaches a predetermined level.
- Operation: As the trapped fuel gets compressed during rotor rotation, the pressure within the chamber builds up.
- Opening and Injection: Once the pressure reaches the set limit, the delivery valve opens, allowing the high-pressure fuel to flow through the line towards the injectors for fuel injection.
- Safety Function: The delivery valve acts as a safety mechanism by preventing excessive pressure buildup within the pump and fuel lines that could lead to component damage.
- Closing and Cycle Repeat: After fuel injection occurs and the pressure drops below the set point, the delivery valve closes again, and the cycle repeats for the next injection event.
(c) Preventing Fuel Spray in Case of High-Pressure Pipe Failure:
Scroll type pumps typically incorporate a safety feature to prevent uncontrolled fuel spraying in case of a high-pressure pipe failure:
- Overflow Valve or Bypass Port: Some designs might use an overflow valve or bypass port located within the pump housing.
- Operation: In case of a high-pressure pipe rupture, the sudden pressure drop would cause the overflow valve to open or fuel to bypass an internal passage.
- Fuel Return: This allows the pressurized fuel to return back to the pump’s low-pressure side (suction side) through this overflow or bypass mechanism.
- Preventing Spray: By redirecting the fuel back to the low-pressure side, the system prevents uncontrolled spraying of high-pressure fuel, minimizing the risk of fire or injury.
Additional Notes:
- Scroll type pumps are generally simpler in design compared to jerk-type pumps.
- Their flow rate is typically less sensitive to variations in engine speed compared to jerk pumps.
- However, their fuel metering capabilities might be less precise than those of jerk-type pumps.