Question 2 MODE02 - Assistant MODU Engineer

The Correct Answer is D ### Explanation for Option D (360 degrees) This question requires calculating the angular separation between two specific firing events within the $720^\circ$ operating cycle of a four-stroke engine. 1. **Determine the Firing Interval (FI):** A four-stroke engine completes its full cycle in $720^\circ$ of crankshaft rotation. Since this is an eight-cylinder (8C) engine, the angle between successive firing events (the firing interval, FI) is: $$\text{FI} = \frac{720^\circ}{\text{Number of Cylinders}} = \frac{720^\circ}{8} = 90^\circ$$ 2. **Trace the Sequence:** The firing order is 1-5-2-6-8-4-7-3. We start counting immediately after No. 4 fires and trace the sequence until No. 5 fires. | Firing Cylinder | Degrees from Previous Firing | Cumulative Crankshaft Rotation | | :------------- | :-------------------------- | :----------------------------- | | 4 (Start) | - | $0^\circ$ | | 7 | $90^\circ$ | $90^\circ$ | | 3 | $90^\circ$ | $180^\circ$ | | 1 | $90^\circ$ | $270^\circ$ | | 5 (Target) | $90^\circ$ | $360^\circ$ | The crankshaft must rotate $360^\circ$ (four firing intervals of $90^\circ$ each) from the moment Cylinder 4 fires until Cylinder 5 reaches TDC and fires. --- ### Explanation of Incorrect Options **A) 120 degrees:** This is incorrect. $120^\circ$ would be the firing interval for a six-cylinder four-stroke engine ($720/6$). In this eight-cylinder engine, $120^\circ$ represents $1.33$ firing intervals, which does not correspond to the separation between 4 and 5. **B) 180 degrees:** This is incorrect. $180^\circ$ represents two full firing intervals ($2 \times 90^\circ$). This would be the rotation required between Cylinder 4 and Cylinder 3 (4 $\rightarrow$ 7 $\rightarrow$ 3). **C) 240 degrees:** This is incorrect. $240^\circ$ would be the firing interval for a three-cylinder four-stroke engine ($720/3$). In this eight-cylinder engine, $240^\circ$ does not align with the required separation of $360^\circ$.