🔍 Key Concepts

• Jerk-type fuel injection pump operation and purpose of the plunger helix and spill (release) port • How effective stroke (pumping stroke) is controlled by when the spill port is covered vs uncovered • What happens to pressure buildup and fuel delivery if fuel can escape back to the suction side during the entire plunger stroke

💭 Think About

• Visualize the plunger moving up: if the release (spill) port is uncovered the whole time, where can the fuel go as pressure starts to build? • Ask yourself: does injection into the cylinder happen only when fuel pressure reaches nozzle opening pressure, or does it happen any time the plunger moves? • How does keeping the spill port open affect the pump’s ability to build pressure high enough to open the injection nozzle?

✅ Before You Answer

• Be clear on the difference between plunger motion and effective delivery stroke (only the part of the stroke when the spill port is closed counts for delivery). • Confirm what condition is required at the nozzle for fuel to actually spray into the cylinder (minimum opening pressure must be reached). • Check each option against the idea: if pressure never rises above spill port pressure because the port is always open, what physical result follows inside the injector and nozzle?