Question 17 OSE01 - Chief Engineer - OSV
You suspect that a diesel generator set on your general-purpose offshore supply vessel has a defective unit injector because the engine, although warm, is running roughly. The two-stroke engine is fitted with mechanically operated unit injectors. When you push and hold down the #3 cylinder injector follower, there is no real change, as the engine continues to run roughly as before. What does this indicate?
The Correct Answer is B **Explanation of why option B is correct:** In a running multi-cylinder diesel engine, if a cylinder is contributing power (i.e., its fuel injector is working properly), momentarily interrupting or disabling that cylinder's fuel supply will cause a noticeable drop in engine speed and/or a change in the roughness of the engine. This is a common method for cylinder power balance testing or fault isolation. In this scenario, the engine is already running roughly, indicating a misfire or power imbalance. The unit injector is mechanically operated, meaning the follower is pushed by the camshaft to inject fuel. By pushing and holding down the #3 cylinder injector follower, the technician is preventing the unit injector from completing its stroke and injecting fuel into the cylinder, effectively cutting fuel supply to cylinder #3. Since there was "no real change" in the engine's rough running when cylinder #3's fuel was cut, it means that cylinder #3 was *not* contributing power (or very little power) to begin with. Therefore, the **#3 cylinder unit injector is faulty**, and the technician has successfully isolated the non-contributing cylinder. **Explanation of why the other options are incorrect:** * **A) Either the #4, #5, or #6 cylinder unit injector is faulty. The #1 and #2 cylinder unit injectors are functioning properly.** This is incorrect. If the fault were in cylinders #4, #5, or #6, cutting the fuel to a properly working cylinder (#3) would have made the engine run even worse. The fact that cutting #3 had no effect points directly to #3 being the fault. * **C) The #3 cylinder unit injector is functioning properly. One of the other cylinder unit injectors must be faulty.** This is incorrect. If the #3 cylinder injector were functioning properly and contributing power, disabling it would immediately make the engine run rougher or drop speed significantly. Since there was no change, #3 was not functioning properly. * **D) Either the #1 or #2 cylinder unit injector is faulty. The #4, #5 and #6 cylinder unit injectors are functioning properly.** This is incorrect for the same reasons as A. While other cylinders may also have issues, the immediate result of the test on cylinder #3 (no change in engine behavior) confirms that #3 is the cylinder responsible for the observed lack of power/roughness.
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