🔍 Key Concepts

• Helix‑type fuel injection pump plunger and how rotation changes fuel quantity • The difference between the plunger’s top land (straight edge) and the helical control edge with the vertical groove • How port closure and port reopening (spill) define the beginning and end of the effective stroke

💭 Think About

• Look at when, during the upward motion of the plunger, the inlet/spill port is first completely covered. Does rotating the plunger change that closure point, or does it always occur at the same plunger position? • Now look at the helical edge and vertical groove: when that helix uncovers the port, pumping stops. Does changing plunger rotation move that helix higher or lower relative to the port, and what does that do to the point where injection ends? • Based on the illustration, decide whether the start of discharge and the end of discharge each stay fixed or move when you rotate the plunger with the control rack.

✅ Before You Answer

• Identify which edge of the plunger causes start of injection (straight upper edge covering the port vs. helix). • Identify which edge of the plunger causes end of injection (helix and vertical groove uncovering the port). • Decide whether port closure height is independent of rack position while port opening height depends on rack‑controlled rotation of the plunger.