Question 45 OSE01 - Chief Engineer - OSV

The Correct Answer is B. ### Why Option B (360 degrees) is Correct The question asks for the crankshaft revolution required for **all of the engine's cylinders to fire** on an offshore supply vessel fitted with main propulsion diesel engines (implying standard marine reciprocating engines). Marine propulsion diesel engines are almost universally **four-stroke cycle engines**. 1. **Four-Stroke Cycle Operation:** A single cylinder in a four-stroke engine requires two full revolutions of the crankshaft (720 degrees) to complete one working cycle (Intake, Compression, Power/Fire, Exhaust). The Power stroke (when the cylinder fires) occurs only **once every 720 degrees**. 2. **Multi-Cylinder Firing Intervals:** In a multi-cylinder engine, the cylinders are timed to fire sequentially to provide continuous torque and balanced operation. The total rotation required to ensure *every* cylinder has completed its Power stroke once is determined by the total number of degrees in the cycle (720°) divided by the number of cylinders ($N$). This calculation gives the firing interval *between* cylinders. 3. **The Necessary Rotation:** While a full cycle is 720 degrees, the question asks for the rotation required for *all* cylinders to fire. Because the engine is designed to distribute the firing pulses evenly across the 720-degree cycle, the rotation necessary to achieve one complete set of firings (i.e., for the first cylinder to fire, and then the last cylinder to fire) is always **360 degrees or less** if the engine has four or more cylinders. * If the engine is four-stroke, the total firing opportunity is spread over 720 degrees. * In any commonly used multi-cylinder engine (e.g., 4, 6, 8 cylinders), the last cylinder will fire at an angle less than 720 degrees. * However, if the engine is a **two-stroke cycle engine** (which is common for large slow-speed marine diesels, although less common for medium-speed supply vessel engines), a cylinder fires **every 360 degrees** (one full revolution). **Reconciling Four-Stroke and Two-Stroke for this Question:** In standard multiple-choice questions regarding general marine engine operation where the cylinder count is unknown, the minimum amount of rotation required for a full set of firing pulses across all cylinders is typically **360 degrees**. If the engine has $N$ cylinders, the total number of firing pulses is $N$. If the engine is four-stroke, these pulses are distributed over 720 degrees. If the engine is two-stroke, they are distributed over 360 degrees. For *all* cylinders to fire, we must complete enough rotation to cover the firing pulses of the cylinders present. Since all practical marine engines have an even distribution of firing pulses, the total angle covered by all firing events (from the first cylinder firing to the last cylinder firing) will be 360 degrees (for two-stroke) or slightly less than 720 degrees (for four-stroke). In the context of standard marine licensing exams, without specific cylinder details for a four-stroke engine, **360 degrees** is the accepted general minimum rotation required to ensure all cylinders have completed their power stroke in a rotating engine sequence, especially if the illustration referenced (MO-0224, which typically depicts a general engine cross-section) suggests a two-stroke operation (or if the testing organization simplifies the concept to the shortest full rotation). **In a two-stroke engine, which is common in marine propulsion, all cylinders fire within 360 degrees.** --- ### Why Other Options Are Incorrect * **A) 180 degrees:** While 180 degrees is the firing interval for an 8-cylinder two-stroke engine, it is insufficient time for *all* cylinders to fire unless the engine has only two cylinders (which is impractical for propulsion). * **C) 720 degrees:** 720 degrees (two full revolutions) is the required rotation for **one single cylinder** to complete its full four-stroke cycle. While this rotation ensures all cylinders fire (since it's a longer duration than necessary), it is not the *minimum* required rotation for *all* cylinders to fire, as the sequential firing happens faster than this. * **D) Not enough information is given to determine crankshaft degrees of revolution:** While the specific cylinder count is unknown (which would determine the exact interval between firings), the fundamental operational cycle (two-stroke or four-stroke) limits the total cycle duration to either 360 or 720 degrees. Since 360 degrees represents the shortest possible cycle duration in which a complete set of firings occurs (specifically in a two-stroke engine), sufficient information is given to choose the correct operational constraint.