Internal balancer

An internal balancer, more formally known as a balance shaft, is a weighted, engine-driven shaft fitted inside an engine specifically to counteract the vibrations that its layout inherently produces. It exists because certain cylinder arrangements, most notably the in-line four, generate forces during operation that the crankshaft and counterweights alone cannot cancel. Left unchecked these forces would reach the cabin as an intrusive buzz, so the balance shaft is added to smooth the engine's running and bring it closer to the refinement of inherently balanced designs.

The vibration it addresses is the secondary imbalance of a four-cylinder engine. As the pistons rise and fall, they do not move in perfect harmonic motion because the connecting rod swings at an angle, causing the pistons to accelerate slightly faster near top dead centre than near the bottom. The net effect is an up-and-down shaking force that occurs at twice engine speed and cannot be eliminated by crankshaft counterweights. The classic solution, refined by Mitsubishi in the 1970s from an earlier Lanchester patent, is a pair of balance shafts spinning at twice crank speed in opposite directions, their eccentric weights timed so their vertical force components add to oppose the imbalance while their horizontal components cancel each other.

Driving and phasing are critical. The shaft, or pair of shafts, is turned by the crankshaft through gears, a chain or a toothed belt, and it must run at a precise multiple of engine speed and at exactly the right angular position relative to the pistons. If the timing is even slightly out, the shaft will add vibration rather than remove it. The weighted lobes are usually carried low in the crankcase, often partly immersed near the oil, and their mass and offset are calculated to match the magnitude of the force they must cancel.

Balance shafts are most commonly found on larger four-cylinder engines, where the secondary forces grow with bore and stroke, and on some V6 engines, where an uneven firing layout creates a rocking couple that a single balance shaft can tame. Smaller four-cylinders sometimes omit them to save cost, weight and friction, accepting a little more vibration in return. By contrast, inherently balanced layouts such as the inline-six, the flat-six and many V12s have no significant primary or secondary imbalance and therefore need no balancer at all.

In practice a balance shaft is a quiet, maintenance-light component, but it is not free. It absorbs a small amount of power through friction and the inertia of being spun at high speed, and on belt- or chain-driven designs its drive must be serviced like any other timing component. Some engines have suffered premature wear in the balance-shaft bearings or chain, which can lead to oil-pressure problems if neglected. The component therefore represents an engineering compromise: a deliberate addition of mass and complexity, accepted because the gain in smoothness from an otherwise economical layout is judged worth the cost.