Engine NVH control — primary purpose of a balancer shaft in automotive engines In modern engines, what is the main function of a balancer shaft with counterweights?

Introduction / Context:
Engines generate unbalanced forces and moments that lead to vibration and noise. Balancer shafts are used in many inline-3 and inline-4 engines (and others) to improve refinement (NVH: noise, vibration, harshness).

Given Data / Assumptions:

• Shaft has counterweights rotating at specific speeds and phases.
• Purpose is related to vibration mitigation, not valve timing.
• Applies to production automotive engines.

Concept / Approach:
Balancer shafts create controlled counter-inertial forces that oppose primary or secondary shaking forces produced by reciprocating masses and crankshaft rotation. Proper phasing reduces the amplitude of felt vibrations transmitted to the vehicle body.

Step-by-Step Solution:
Identify the problem: crankshaft and reciprocating mass imbalance.Balancer shaft(s) rotate(s) to generate opposing forces at the same frequency.Result: reduced vibrations perceived by occupants and reduced stress on mounts.

Verification / Alternative check:
Engine design texts specify single or twin balance shafts (e.g., Lanchester shafts) turning at crankshaft speed or twice speed to cancel primary/secondary forces.

Why Other Options Are Wrong:

• Timing of piston groups or valve events is handled by crank/valvetrain, not balancer shafts.
• Increasing inertia is the opposite of the goal; added mass is used only to counter forces.

Common Pitfalls:
Confusing cam phasing (VVT) with balance shafts; they serve entirely different purposes.

Final Answer:
reduce vibrations caused by the crankshaft rotation