What closes differential type needle valves in diesel engine fuel oil nozzles?

In differential type needle valves used in diesel engine fuel oil nozzles, the closure mechanism is influenced primarily by spring pressure. These valves are designed to regulate the flow of fuel to the nozzle. When the engine operates, the spring applies a force that keeps the valve closed unless the force from the fuel oil pressure exceeds the force exerted by the spring. This design ensures that the valve can open under specific conditions related to fuel flow and engine requirements.

While fuel oil pressure does play a role in opening the valve and allowing fuel to flow, the primary mechanism for closing the valve is the spring pressure. This is critical for maintaining proper operation within the fuel injection system, ensuring that the fuel is delivered at the correct timing and pressure during the combustion cycle. Thus, choice A accurately reflects the mechanism behind the closure of differential type needle valves in these applications.