🔍 Key Concepts

• Start of injection in a jerk-type fuel pump is determined mechanically by when the plunger is lifted by the cam profile • The cam or cam ring position relative to the plunger determines when pressure first rises high enough to open the injector nozzle • Other components (like the helix on the plunger or control rack) mainly change end of injection or fuel quantity, not the initial timing

💭 Think About

• Which labeled part actually decides WHEN the plunger first begins its upward stroke to build injection pressure? • Look at which component’s angular position on the drive shaft would advance or retard the start of plunger motion relative to crankshaft position. • Among C, H, K, and I, which one is clearly associated with the cam/drive that lifts the plunger rather than with metering or nozzle operation?

✅ Before You Answer

• Identify which letter marks the cam or cam ring that rides with the drive shaft at the pump’s lower end. • Confirm that this component’s rotation would advance/retard the start of plunger lift, and therefore the start of injection. • Eliminate any parts that are primarily involved with fuel quantity control or nozzle opening/closing rather than timing.