Differential needle valves in fuel injectors are primarily closed by ______.

Differential needle valves in fuel injectors utilize spring force to achieve their primary closing mechanism. In the context of fuel injectors, when the engine is not operating or when the injector is in a resting position, the spring applies a force that keeps the valve closed, preventing fuel from entering the combustion chamber. The spring is designed to hold the needle in a position where it seals the injector nozzle and ensures no fuel leaks out when it is not required for combustion.

When the injector is activated, fuel pressure overcomes the spring force, allowing the needle to lift and fuel to flow into the combustion chamber. The design is critical for controlling fuel injection timing and quantity, as precise closure of the needle valve is essential for effective fuel atomization and combustion efficiency. This mechanism distinguishes differential needle valves from other types that may rely on different forces for closure.