Question 54 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is B. ## 1. Explanation for Option B (1-5-3-6-2-4) The firing order **1-5-3-6-2-4** is one of the two standard, optimally balanced firing orders used in a 4-stroke cycle, 6-cylinder in-line engine. * **Balance and Design:** A 6-cylinder in-line engine achieves near-perfect primary and secondary dynamic balance. To maintain this balance and minimize fore-and-aft rocking couples (vibration), the firing impulses must be symmetrically distributed. * **Symmetry and Alternation:** The firing order must alternate between cylinders in the front half (1, 2, 3) and the rear half (4, 5, 6) of the engine block. * Starting with 1 (front), the next impulse is 5 (rear), followed by 3 (front), 6 (rear), 2 (front), and finally 4 (rear). * **Source of Information:** Although the illustration (MO-0164) is not provided, the configuration of the crankshaft throws dictates the engine's firing order. For this specific question, the illustration MO-0164 must depict the crankshaft arrangement corresponding to the sequence 1-5-3-6-2-4. ## 2. Why Other Options Are Incorrect **A) 1-2-3-4-5-6** This sequence is highly unbalanced. Firing the cylinders in numerical order would result in continuous, uncompensated forces (rocking couples) moving from the front to the rear of the engine, leading to extremely high vibration and stress on the block and mounting systems. **C) 1-4-2-6-3-5** This is the *other* common, dynamically balanced firing order for a 6-cylinder in-line engine. While mathematically correct for balance, it does not correspond to the specific crankshaft configuration depicted in Illustration MO-0164, which determines the actual firing sequence for this particular engine design. **D) Not enough information is provided to determine the firing order.** This is incorrect. The prompt explicitly references Illustration MO-0164, which is intended to show the necessary details (such as the crank throw positions or the valve timing diagram) required to deduce the engine's specific firing sequence. The information, based on the context of the technical query, is sufficient.