Question 178Motor Plants - QMED

The correct answer is D) a bypass valve. In a common rail fuel system, the injection pressure is controlled by a bypass valve. This valve regulates the pressure in the common rail by allowing excess fuel to be recirculated back to the fuel tank. The bypass valve adjusts its opening based on the engine's fuel demand, maintaining the desired injection pressure regardless of engine speed (A is incorrect) or other factors. The other options are incorrect because they do not directly control the injection pressure. Varying the fuel pump piston stroke (B) or the injector needle valve clearance (C) may affect the fuel delivery, but they do not specifically regulate the common rail pressure as the bypass valve does.