Engine Balance

When talking in terms of engine balance, there are two main states, primary and secondary balance. In it's simplest form, primary balance refers to counter-acting the inertia forces created by the sudden change of direction when a piston passes both TDC and BDC as the piston momentarily attempts to carry on in the previous direction. The amount of force exerted on the piston increases with engine speed.

On an in-line four-cylinder engine, pistons work in sets and the downward (negative) inertia of one set, counter balances and therefore cancels out the upward (positive) inertia of the other. Counterweights are added to reduce twisting action imposed onto the crankshaft by opposing forces and to reduce stress on it's centre main bearing although most engines nowadays use five instead of three crankshaft main bearings in order to provide a stiffer construction.

Single-cylinder engines obviously are unable to use this method as there is only one piston moving up and down and therefore the force, for this reason a counterweight is used on the crankshaft that rotates in the opposite direction of the piston however, the crank and piston assembly will not be balanced horizontally as both the crankpin and connecting rod are travelling in the same plane.

In-line three cylinder engines have the large force of the piston at TDC or BDC cancelled out by the smaller forces of the other two pistons. This also applies to both in-line and vee six cylinder engines.

Secondary balance refers to forces that occur twice during the same revolution as the primary force, which only occurs once per revolution. The distance travelled by the crank and piston assembly during the same amount of time is greater from the top of the rotation (TDC to 90° and 270° to TDC) than it is between  the bottom of the rotation (90° to 180° and 180° to 270°) meaning that the piston is moving faster during the top parts of the rotation. This is what creates secondary imbalance as there is a stronger upward force at TDC and a weaker downward force at BDC. These forces can, depending on engine layout, be cancelled out by adding extra piston or counterweights. Some engines employ a harmonic balancer for this purpose although soft rubber engine mountings are usually used due to cost.

Diagram displaying engine balance

First used in 1911 by Frederick Lanchester to balance out secondary imbalance on in-line four cylinder engines, Mitsubishi also produced a secondary harmonic balancer in 1975 not dissimilar to that of Lanchester, which is now being made and sold under licence to a variety of vehicle manufacturers and engine builders including Saab and Porsche. V4 engines generally tend to incorporate a harmonic balancer in order to keep vibration at an acceptable level. 

The harmonic balancer itself consists of two counter-weighted shafts both taking drive from and timed to the crankshaft. These shafts turn clockwise and anti-clockwise respectively to one another and exert a downward force at TDC. The opposing force of the balancer must be exerted only when needed i.e, when at TDC the shafts must be pointing downwards and at times of low force they must provide a neutral effect as the engine is already in a satisfactory state of balance. 

Operating principles of a secondary harmonic balancer

Aside from primary and secondary balance, all rotating components/masses must also be balanced in order to minimise any untoward vibration such as the complete crankshaft,flywheel and clutch assembly which although ideally dynamically balanced as one assembly, are usually balanced individually due to reasons of cost and locating devices used to ensure the flywheel and clutch run in-line with the crankshaft axis. Removal of metal from the opposing side of a component or/and drilling holes to reduce weight at the heavy point are common methods of correcting imbalance. Pistons and connecting rods can be balanced by ensuring all of their individual weights are equal.

Power strokes of multi-cylinder engines must be regular in order to reduce vibration and the more power strokes in any given 720°  four-stroke cycle, the smoother the engine, torque output and delivery of power from the engine to the road wheels.