Bosch (Jerk Type) Fuel Injection Pump: Fuel Delivery Control and Delivery Valve
(a) Controlling Fuel Delivery Quantity:
In Bosch or jerk-type fuel injection pumps, the delivered quantity of fuel oil is controlled by the interplay of three main components:
- Plunger and Helical Groove:
- The pump has a plunger that moves reciprocally within a barrel.
- The key element for fuel metering is a helical groove machined on the plunger’s surface.
- Spill Port and Control Rack:
- The barrel has a spill port that allows pressurized fuel to return to the suction side of the pump when uncovered.
- A control rack, connected to the engine governor or electronic control unit, positions a sleeve around the plunger.
- Operating Principle:
- During the upward stroke of the plunger, fuel gets drawn into the pump chamber through an inlet port.
- As the plunger continues to move up, the helical groove on its surface progressively covers the spill port.
- The duration for which the spill port remains covered determines the amount of fuel trapped in the pump chamber for injection.
- The control rack position influences the orientation of the plunger and the helical groove relative to the spill port.
- By moving the rack, the governor or ECU can adjust the position of the helical groove, thereby controlling how long the spill port remains covered during the plunger’s stroke.
- A shorter coverage time (less fuel trapped) corresponds to a lower fuel delivery. Conversely, a longer coverage time results in a higher fuel delivery.
(b) Purpose of the Delivery Valve:
The delivery valve, also known as the discharge valve or pressure relief valve, plays a critical role in ensuring proper fuel injection pressure and preventing internal damage:
- Location: The delivery valve is typically located at the outlet of the pump chamber, directly connected to the high-pressure fuel line leading to the injectors.
- Function: The valve remains closed by a spring until the pressure within the pump chamber exceeds a predetermined level.
- Operation: As the plunger moves upwards and the spill port closes, the fuel 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 and reach 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, the delivery valve closes again, and the cycle repeats for the next injection event.
The delivery valve ensures that the fuel injection pump operates within the designated pressure range, delivering pressurized fuel to the injectors for efficient engine operation while safeguarding the system from excessive pressure.