In a modern EFI system, what parameter is kept at a set value by regulation?

• A. Exhaust backpressure
• B. Air-fuel ratio
• C. Engine temperature
• D. Rail pressure

Answer: (D)

In a modern EFI system, the fuel rail pressure is kept at a set value through regulation. The ECU monitors rail pressure with a sensor and adjusts the high‑pressure pump (or a regulator) to maintain that target pressure. Keeping rail pressure constant makes the injector pressure drop predictable for a given injector opening amount, so the engine can meter the exact amount of fuel needed to achieve the commanded air‑fuel ratio across all speeds and loads. Exhaust backpressure, engine temperature, and the air-fuel ratio itself aren’t regulated to a fixed value in the same direct way—the first two depend on hardware and cooling, while the ratio is continually adjusted to meet a target but isn’t a fixed regulated parameter like rail pressure.