Question 27

The Correct Answer is C **Explanation of Correct Option (C):** The primary function of an over speed trip (or governor over speed device) on a main engine is safety, specifically to prevent catastrophic engine failure due to excessive RPM. Historically and commonly, mechanical over speed trips utilize a weighted mechanism (often a flyball or plunger) that senses **centrifugal force**. As the engine speed increases, the centrifugal force acting on the weights increases proportionally. When the engine RPM reaches a pre-determined, dangerous maximum speed setting (typically 15% above rated speed), the centrifugal force overcomes the resistance (usually a spring), causing the weights to move outward. This movement mechanically or hydraulically triggers a shut-down mechanism (like closing the fuel racks or cutting ignition), thereby **shutting the engine down** completely and immediately to prevent damage. **Why Other Options Are Incorrect:** * **A) Incorrect:** This option correctly identifies that the trip senses centrifugal force proportional to speed, but incorrectly describes its function. An over speed trip is a safety shutdown device, not a speed limiting or regulating device that allows the engine to continue running at the rated speed. Speed limiting and regulation is the job of the normal operating governor. * **B) Incorrect:** This option incorrectly states the sensing mechanism is oil pressure proportional to speed. While some engine speed governors or monitoring systems utilize oil pressure for certain functions, the traditional, dedicated over speed trip mechanism primarily relies on direct mechanical (centrifugal) sensing for reliability. Furthermore, like option A, it incorrectly states the trip limits speed while allowing the engine to continue running. * **D) Incorrect:** While this option correctly identifies the outcome (shuts the engine down at a pre-determined maximum speed), it incorrectly states that the primary sensing mechanism for a dedicated mechanical over speed trip is oil pressure proportional to engine speed. The standard mechanism is centrifugal force.