Torsional vibration is the twisting oscillation of the crankshaft about its longitudinal axis. It arises because the crankshaft is not infinitely rigid - it has elasticity (torsional stiffness) and the rotating masses (flywheel, propeller, crank throws) have inertia. The periodic torque from the cylinders (each cylinder produces a torque pulse) excites the shaft, which twists and untwists at its natural (torsional) frequency. If the frequency of the exciting torque coincides with the natural frequency of the shaft (resonance), the amplitude of the torsional oscillation becomes very large. The effect on the crankshaft: the large alternating torsional stress can cause fatigue cracking of the crankshaft (usually at the fillets/webs), failure of the coupling, and damage to the driven machinery (e.g. the propeller shaft, gearbox). It also causes vibration and noise. The crankshaft is therefore designed to avoid resonance in the operating speed range, and a torsional vibration damper/detuner is fitted to control it.

A detuner (torsional vibration damper) is fitted to control torsional vibration. It is fitted when:

1. The engine's natural torsional frequency falls within the operating speed range, so resonance would occur at a normal running speed, causing high stresses and possible crankshaft failure.
2. The engine is coupled to a heavy load (e.g. a propeller or generator) that changes the torsional characteristics and brings a critical speed into the operating range.
3. The engine is required to run over a wide speed range (e.g. a variable-speed propulsion engine) where a critical speed cannot be avoided.

The damper (a rubber or viscous damper with an inertia ring) dissipates the vibrational energy, reducing the amplitude of the torsional oscillation and the stress on the crankshaft, so the engine can run safely through the critical speed.

A barred speed range is a range of engine speeds (e.g. 40-60 rpm) in which the engine must not be operated continuously because a critical torsional vibration (resonance) occurs in that range, producing high torsional stresses that could damage the crankshaft. The engine is "barred" from continuous operation in that range. It should not be operated continuously in that range because:

1. The resonance produces large alternating torsional stresses that can cause fatigue cracking of the crankshaft and failure.
2. It causes excessive vibration, noise, and wear of the bearings and driven machinery.
3. It can damage the coupling, the propeller shaft, and the gearbox.

The engine is therefore accelerated through the barred range quickly (so it does not dwell at the critical speed) and is not run continuously within it. The barred range is marked on the tachometer and in the engine instructions.