🔍 Key Concepts

• Operation of differential type needle valves in diesel fuel injectors • The roles of spring pressure vs. fuel oil pressure during injection and during closing • Sequence of events: start of injection, full lift, end of injection

💭 Think About

• When the injector stops receiving high-pressure fuel, what force is still acting on the needle that can move it back to the closed position? • During injection, which force is responsible for lifting/opening the needle, and which force must be overcome? • Think about which of these forces (spring or fuel pressure) is always present, even when the pump is not delivering fuel.

✅ Before You Answer

• Be clear about which phase of operation the question asks about: opening, full lift, or closing. • Verify how a differential needle uses two different seat areas to control movement—what closes it when fuel pressure drops? • Confirm which force is independent of fuel flow/pressure and therefore can reliably close the valve each cycle.