Doctor Diesel

Balancing Act

IconFrom time to time, the technical reports mention balancer shafts which obviously exist to make the engine run smoother, but it would be interesting to know a bit more about these components, what sort of engines they specifically apply to, and how they actually work. Perhaps the good Doctor could offer us some words of wisdom on the subject?
Rob Norris, Southampton

Thanks for the letter Rob. It probably is time that we opened up on what these items are all about. All conventional internal combustion engines involve the conversion of the energy released by a controlled ignition of fuel and air into rotary motion, using reciprocating pistons and connecting rods to transfer the energy to the crankshaft. Whilst good in terms of primary balance, four-cylinder engines, which are most widely used in cars, have an inherent second order imbalance, creating vibrations that can’t be eliminated by simply balancing rotating engine components. This vibration is generated because the movement of the connecting rods in a four-cylinder in-line engine is not symmetrical throughout crankshaft rotation, as the rising and descending pistons are not always identically opposed in their accelerations. They thus produce unbalanced vertical forces twice in each revolution which increase with the square of the engine speed; so at 4,000rpm the forces are four times those at 2,000rpm, and increasing engine speeds compromise component strength as well as vibration and smoothness. As diesel engines have become faster-revving, these secondary imbalance forces have become more significant, initiating the adoption of the balancer shafts that were invented and used by Englishman Frederick Lanchester way back in 1911. With an in-line four-cylinder engine, two balance shafts, carrying identical weights in opposed positions, are geared to rotate in opposite directions at twice the engine speed; these weights are sized and phased so that their inertial counter-rotation forces cancel out in the horizontal plane but add together in the vertical plane, thereby cancelling out the undesired second-order vibrations of the engine. The positioning of such balancer shafts is optimally close to the centre of gravity of the engine, and recent Volkswagen Group four-cylinder engines have brought their balancer shafts within the cylinder block, also reducing the power losses, which can be significant when they are rotating at 8,000rpm and more.

We’ll now consider the three-cylinder engine, which is becoming increasingly popular for reasons of compactness and cost. Its balance might appear to be inferior to a four cylinder engine, as the pistons and con rods are never moving in exactly the opposite direction and cancelling out some of their imbalance forces, but with the con-rods connected to the crankshaft at 120 degree intervals to produce equal firing intervals, there are, in contrast, never any pairs of heavy pistons moving together in the same direction. So a three-cylinder engine with a fairly heavy flywheel is actually fundamentally as well or better balanced than any four. Any impressions of roughness are generated more by the reduced number of firing impulses for any given engine speed than by any imbalance, just as any four never sounds as sweet as a six at the same engine speed. In three-cylinder (and five-cylinder) engines, a single balancer shaft can be used to cancel out the rocking end-to-end forces from which they suffer, although the chain driven balancer shaft on Volkswagen 1.4 TDI units and the now defunct 1.5 cdi engine used in the smart forfour remain the sole examples to date of balancer shaft use in three-cylinder diesels. Of greater significance are V6 diesel engines, like Volkswagen Group’s 3.0-litre TDI V6, where the layout is essentially two three-cylinder engines in “V” form. In an in-line six-cylinder, the rocking forces of the two sets of three cylinders cancel each other out, which is why straight sixes are particularly smooth. But with a V6 this is not so whatever the angle between the two banks of three cylinders, but the rocking forces can be cancelled out with a single counter-rotating balance shaft suitably installed within the angle of the V.

In conclusion, jungle drums carried rumours back in 2013 of the twin-cylinder 0.8-litre diesel engine from the super-eco Volkswagen XL1 being engineered with balancer shafts for the up! city car, which would in some ways mirror Fiat’s use of the similarly sized TwinAir petrol engine with balancer shaft, but we’ve still no official confirmation of this possibility. You’ll notice that I’ve not mentioned the V4 engine… but I think I’ve said enough!

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