Final answer:
In a CR fuel system with a stuck nozzle valve, the rail pressure regulator valve is responsible for limiting fuel to the affected cylinder. It controls the fuel pressure within the rail, adjusting in response to the fault to reduce fuel flow.
Step-by-step explanation:
In the scenario where a nozzle valve is stuck open in an Electro-Hydraulic Injector (EHI) that's used in a Common Rail (CR) fuel system, the component that ensures fuelling to the affected cylinder is limited is the rail pressure regulator valve. Unlike a rail pressure sensor, which measures the pressure of the fuel within the rail, or an unintended fuel sensor which is not standard in diesel fuel systems, the rail pressure regulator valve plays an active role in controlling the pressure and the flow of the fuel within the rail. When a nozzle valve is stuck open, the regulator valve can adjust to reduce the pressure and attempt to limit the excess fuel entering the cylinder.