Question 30 OSE02 - Assistant Engineer - OSV

The Correct Answer is A ### Explanation of Correct Option (A) Option A states: "The injector is of the closed type and features port and helix metering." This is the correct statement regarding the type of injector commonly used in diesel generators on offshore supply vessels, specifically when referring to the mechanical unit injector (EUI) design associated with port and helix metering (often used in earlier Detroit Diesel, EMD, or similar two-stroke engines, or certain mechanical four-stroke systems). 1. **Closed Type:** A closed-type injector utilizes a spring-loaded needle valve (or differential valve) that opens only when the fuel pressure generated by the pumping element inside the injector reaches a predetermined high pressure. This valve immediately closes once the pressure drops. This design ensures sharp cutoff and precise atomization, preventing fuel drip into the cylinder. 2. **Port and Helix Metering:** This describes the mechanism used to control the volume (metering) and timing of the injected fuel. In a unit injector featuring this design (like the Detroit Diesel Unit Injector or certain mechanical EUI systems), the pumping plunger has a machined helix (or scroll) cut into its surface. As the plunger reciprocates, the rotation of the plunger relative to an intake/spill port determines when the port is covered and uncovered by the helix edge, thereby regulating the effective stroke of the plunger and thus the quantity of fuel delivered. ### Explanation of Incorrect Options **B) The injector is of the open type and features port and helix metering.** This is incorrect because diesel injectors used for high-pressure injection must be the **closed type**. An open injector lacks a needle valve and would allow fuel to constantly drip into the combustion chamber as soon as the pressure builds, leading to poor combustion, excessive smoke, and carbon buildup. **C) The injector is of the open type and features pressure-time metering.** This is incorrect for two reasons: 1. It incorrectly labels the injector as the **open type** (see B). 2. **Pressure-time (PT) metering** is specific to Cummins fuel systems. PT systems regulate fuel quantity primarily based on the duration of time that the metering orifice is exposed to pressurized fuel (T), combined with the system pressure (P), and does not utilize the mechanical port and helix mechanism inside the injector body for metering. **D) The injector is of the closed type and features pressure-time metering.** While the injector is correctly identified as the **closed type**, it incorrectly assigns the metering method. Pressure-time (PT) metering is a separate system design (Cummins) and is distinct from the port and helix metering mechanism found in the type of mechanical unit injector referenced by the illustration and context (e.g., Detroit Diesel or similar mechanical EUI). Since the specific figure 2 (MO-0150) typically depicts a mechanical unit injector utilizing the helix for metering, "port and helix metering" is the characteristic feature of that specific injector type.