What rule is used to determine firing order for an 18-cylinder radial engine?

What rule is used to determine firing order for an 18-cylinder radial engine?

• A. Begin with cylinder 1, and add 11 or subtract 7, whichever keeps the numbers between 1 and 18.
• B. Begin with cylinder 1 and then go in simple sequential order.
• C. Begin with cylinder 18 and work backward.
• D. Use a random order recommended by the manufacturer.

Answer: (A)

Radial engine firing order is set by a fixed skip pattern around the cylinder circle so that each power stroke is spaced evenly and the crank experiences balanced impulses. For an 18-cylinder radial, you start with cylinder 1 and move by a constant offset that keeps you inside 1–18 by adding 11 or, equivalently, subtracting 7 (since 11 ≡ -7 mod 18). This method visits every cylinder exactly once per firing sequence and avoids clustering of firings. Following that rule, the sequence goes: 1, 12, 5, 16, 9, 2, 13, 6, 17, 10, 3, 14, 7, 18, 11, 4, 15, 8. This pattern distributes the firing events evenly around the crank, reducing vibration and preventing excessive loads on any part of the crankshaft. If you tried simple sequential order, firings would be too close together on neighboring cylinders, increasing vibration and stress. A random order won’t provide the balanced distribution needed for smooth operation. Reversing direction from 1 would just produce a different sequence but isn’t the standard firing-order method.

Radial engine firing order is set by a fixed skip pattern around the cylinder circle so that each power stroke is spaced evenly and the crank experiences balanced impulses. For an 18-cylinder radial, you start with cylinder 1 and move by a constant offset that keeps you inside 1–18 by adding 11 or, equivalently, subtracting 7 (since 11 ≡ -7 mod 18). This method visits every cylinder exactly once per firing sequence and avoids clustering of firings.

Following that rule, the sequence goes: 1, 12, 5, 16, 9, 2, 13, 6, 17, 10, 3, 14, 7, 18, 11, 4, 15, 8. This pattern distributes the firing events evenly around the crank, reducing vibration and preventing excessive loads on any part of the crankshaft.

If you tried simple sequential order, firings would be too close together on neighboring cylinders, increasing vibration and stress. A random order won’t provide the balanced distribution needed for smooth operation. Reversing direction from 1 would just produce a different sequence but isn’t the standard firing-order method.