Understanding Engine Firing Order

The firing order of an engine is the sequence in which the cylinders fire. In an internal combustion engine, cylinders must fire in a specific order to ensure smooth operation, balance the forces generated by the moving parts, and deliver power effectively.

For multi-cylinder engines like a six-cylinder diesel engine, a carefully chosen firing order is crucial for minimizing vibration and distributing the power strokes evenly throughout the engine's rotation cycle. Without the correct sequence, the engine would experience excessive shaking and wear.

Six Cylinder Diesel Engine Firing Sequence

Six-cylinder engines, especially inline-six configurations common in diesel applications, have several possible firing orders. However, certain sequences are preferred because they provide optimal balance and power delivery.

The question asks for the firing order of a six cylinder diesel engine and provides several options. Let's look at the options:

• 654321: This is simply reverse order and would lead to poor balance.
• 124356: This sequence groups firing events closely at one end (1-2) and then mid-engine (4-3) before the other end (5-6), which would likely result in significant vibration.
• 153624: This sequence tends to distribute the firing pulses more evenly along the crankshaft, helping to balance the engine dynamically. It often alternates between the front/rear and inner/outer pairs of cylinders (when viewed from the front).
• 123456: This is a sequential firing order which would also result in poor balance as adjacent cylinders fire one after another.

Analyzing Common Firing Orders: Why 1-5-3-6-2-4 is Preferred

For many inline six-cylinder engines, including diesel ones, a common and highly effective firing order is 1-5-3-6-2-4. This sequence helps to counteract the natural vibrations produced by the engine's moving parts.

In a typical four-stroke engine cycle, each cylinder goes through Intake, Compression, Power (Firing), and Exhaust strokes. With six cylinders, arranged in a line (inline-six), the crankshaft completes two full rotations (720 degrees) for all cylinders to fire once.

With the 1-5-3-6-2-4 firing order, the power strokes are spaced 120 degrees of crankshaft rotation apart. This even spacing ensures continuous power delivery and contributes significantly to the characteristic smoothness of well-balanced inline-six engines.

Consider the sequence:

This sequence avoids consecutive firing of adjacent cylinders (like 1-2, 2-3, etc.) which would generate imbalanced forces and vibrations. It skips cylinders, balancing the load on the crankshaft and engine mounts.

Based on standard engineering practices for inline-six engines aiming for optimal balance, the firing order 1-5-3-6-2-4 is a widely adopted and effective sequence.

Revision Table: Six Cylinder Engine Firing Orders

Additional Information: Engine Balancing and Firing Orders

Engine balance refers to how well the forces generated by the moving parts (pistons, connecting rods, crankshaft) are cancelled out. These forces can be primary (at crankshaft speed) or secondary (at twice crankshaft speed).

• Inline-six engines are often inherently well-balanced for both primary and secondary forces when using firing orders like 1-5-3-6-2-4 or 1-4-2-5-3-6. This is why they are known for smooth operation.
• V-engines (like V6 or V8) require different firing orders and sometimes balance shafts to achieve smooth operation due to their cylinder arrangement.
• The specific firing order can sometimes be found marked on the engine's intake manifold or cylinder head.
• Understanding the firing order is important for mechanics for tasks like setting ignition timing (in spark-ignition engines) or injector timing (in diesel engines) and for troubleshooting engine misfires.