Engine fundamentals — identify the standard firing order commonly used in an in-line four-cylinder internal combustion engine for smoothness and balance.

Introduction / Context:
The firing order of a multi-cylinder engine determines the sequence in which cylinders ignite. Proper selection minimizes vibration, balances primary and secondary forces and couples, and smooths torque delivery to the crankshaft. The in-line four-cylinder configuration is ubiquitous, and its conventional firing order is widely taught in automotive engineering courses and service manuals.

Given Data / Assumptions:

• Four cylinders in a single in-line bank using a four-stroke cycle.
• Uniform 180-degree crank throws between adjacent cylinders.
• Objective of even power strokes and acceptable balance without complex shafts.

Concept / Approach:

For a four-stroke, four-cylinder engine, one cylinder fires every 180 degrees of crank rotation. A firing order of 1-3-4-2 spaces power impulses evenly and yields satisfactory balance with typical crankshaft layouts. Alternatives can introduce undesirable torsional oscillations or uneven exhaust pulsing. The 1-3-4-2 sequence also simplifies exhaust manifold design by pairing cylinders that are out of phase to reduce interference.

Step-by-Step Solution:

Verification / Alternative check:

Many OEM specifications and textbooks list 1-3-4-2 as the canonical firing order for in-line fours, validating the selection.

Why Other Options Are Wrong:

1-2-3-4 and 1-2-4-3 produce poor balance and exhaust interference. 1-3-2-4 disrupts smoothness and can raise torsional vibration.

Common Pitfalls:

Assuming firing order is arbitrary; overlooking the effect on crankshaft torsion, NVH, and manifold tuning.

Final Answer:

1-3-4-2