USCG Exam PracticeDiesel engine principles (motor propulsion)
In a common-rail fuel injection system, injection pressure and timing are:
- A. Set mechanically by a jerk-type pump plunger for each individual cylinder
- B. Controlled by the camshaft acting directly on a combined pump-nozzle unit at each cylinder
- ✓ Decoupled from engine speed; a single high-pressure accumulator supplies electronically controlled injectorsCorrect
- D. Determined by the spring tension of the needle valve in each injector, which varies with engine load
Why C is correct
In a common-rail system a single accumulator holds fuel at high pressure and electronically controlled injectors fire on command, decoupling injection pressure and timing from engine speed. This distinguishes it from pump-line-nozzle and unit-injector arrangements.
Cited:DOE-HDBK-1018 Vol.1 §1-3