Engine balance


Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and balance shafts, to prevent unpleasant and potentially damaging vibration. The strongest inertial forces occur at crankshaft speed and balance is mandatory, while forces at twice crankshaft speed can become significant in some cases.

Causes of imbalance

Although some components within the engine have complex motions, all motions can be separated into reciprocating and rotating components, which assists in the analysis of imbalances.
Using the example of an inline engine, the main reciprocating motions are:
  • Pistons moving upwards/downwards
  • Connecting rods moving upwards/downwards
  • Connecting rods moving left/right as they rotate around the crankshaft, however the lateral vibrations caused by these movements are much smaller than the up–down vibrations caused by the pistons.
While the main rotating motions that may cause imbalance are:
  • Crankshaft
  • Camshafts
  • Connecting rods
The imbalances can be caused by either the static mass of individual components or the cylinder layout of the engine, as detailed in the following sections.

Static mass

If the weight— or the weight distribution— of moving parts is not uniform, their movement can cause out-of-balance forces, leading to vibration. For example, if the weights of pistons or connecting rods are different between cylinders, the reciprocating motion can cause vertical forces. Similarly, the rotation of a crankshaft with uneven web weights or a flywheel with an uneven weight distribution can cause a rotating unbalance.

Cylinder layout

Even with a perfectly balanced weight distribution of the static masses, some cylinder layouts cause imbalance due to the forces from each cylinder not cancelling each other out at all times. For example, an inline-four engine has a vertical vibration. These imbalances are inherent in the design and unable to be avoided, therefore the resulting vibration needs to be managed using balance shafts or other NVH-reduction techniques to minimise the vibration that enters the cabin.

Types of imbalance

Reciprocating imbalance

A reciprocating imbalance is caused when the linear motion of a component is not cancelled out by another component moving with equal momentum, but opposite in direction on the same plane.
Types of reciprocating phase imbalance are:
Types of reciprocating plane imbalance are:
  • The offset distance between crankpins causing a rocking couple on the crankshaft from the equal and opposite combustion forces, such as in a boxer-twin engine, a 120° inline-three engine, 90° V4 engine, an inline-five engine, a 60° V6 engine and a crossplane 90° V8 engine.
In engines without overlapping power strokes, the pulsations in power delivery vibrate the engine rotationally on the X axis, similar to a reciprocating imbalance.

Rotating imbalance

A rotating imbalance is caused by uneven mass distributions on rotating assemblies
Types of rotating phase imbalance are:
  • Unbalanced eccentric masses on a rotating component, such as an unbalanced flywheel
Types of rotating plane imbalance are:
  • Unbalanced masses along the axis of rotation of a rotating assembly causing a rocking couple, such as if the crankshaft of a boxer-twin engine did not include counterweights, the mass of the crank throws located 180° apart would cause a couple along the axis of the crankshaft.
  • Lateral motion in counter-moving pairs of assemblies, such as a centre-of-mass height difference in a pair of piston–connecting-rod assemblies. In this case, a rocking couple is caused by one connecting rod swinging left while the other is swinging right, resulting in a force to the left at the top of the engine and a force to the right at the bottom of the engine.

    Torsional vibration

develops when torque impulses are applied to a shaft at a frequency that matches its resonant frequency and the applied torque and the resistive torque act at different points along the shaft. It cannot be balanced, it has to be damped, and while balancing is equally effective at all speeds and loads, damping has to be tailored to given operating conditions. If the shaft cannot be designed such that its resonant frequency is outside the projected operating range, e.g. for reasons of weight or cost, it must be fitted with a damper.
Vibration occurs around the axis of a crankshaft, since the connecting rods are usually located at different distances from the resistive torque. This vibration is not transferred to outside of the engine, however fatigue from the vibration could cause crankshaft failure.
Radial engines do not experience torsional imbalance.

Primary imbalance

Primary imbalance produces vibration at the frequency of crankshaft rotation, i.e. the fundamental frequency of an engine.

Secondary balance

Secondary balance eliminates vibration at twice the frequency of crankshaft rotation. This can be necessary in larger straight and V-engines with a 180° or single-plane crankshaft in which pistons in neighbouring cylinders simultaneously pass through opposite dead centre positions. While it might be expected that a 4-cylinder inline engine would have perfect balance, a net secondary imbalance remains.
This is because the big end of the connecting rod swings from side to side, so that the motion of the small end deviates from ideal sinusoidal motion between top and bottom dead centre on each swing, i.e. twice per crank revolution, and the distance the small end has to travel in the top 180° of crankshaft rotation is greater than in the bottom 180°. Greater distance in the same time equates to higher velocity and higher acceleration, so that the inertial force through top dead centre can be as much as double that through bottom dead centre. The non-sinusoidal motion of the piston can be described in mathematical equations.
In a car, for example, such an engine with cylinders larger than about 500 cc/30 cuin requires balance shafts to eliminate undesirable vibration. These take the form of a pair of balance shafts that rotate in opposite directions at twice engine speed, known as Lanchester shafts, after the original manufacturer.
In V8 engines, the problem is usually avoided by using a cross-plane crankshaft, and a 180° or single-plane crankshaft is used only in high-performance V8 engines, where it offers specific advantages and the vibration is less of a concern.

Effect of cylinder layout

For engines with more than one cylinder, factors such as the number of pistons in each bank, the V angle and the firing interval usually determine whether reciprocating phase imbalances or torsional imbalances are present.

Straight engines

s most commonly use the following configurations:
  • 360° crankshaft: This configuration creates the highest levels of primary and secondary imbalance, equivalent to that of a single cylinder engine.; but the even firing order provides smoother power delivery.
  • 180° crankshaft: This configuration has primary balance but an uneven firing order and a rocking couple; also, the secondary imbalances are half as strong compared with a 360° straight-twin engine.
  • 270° crankshaft: This configuration minimises secondary imbalances; however, a primary-rotating-plane imbalance is present and the firing order is uneven. The exhaust note and power delivery resemble those of a 90° V-twin engine.
Straight-three engines most commonly use a 120° crankshaft design and have the following characteristics:
  • Firing interval is perfectly regular.
  • Primary and secondary reciprocating-plane balance is perfect.
  • Primary and secondary rotating-plane imbalances are present.
Straight-four engines typically use an up–down–down–up 180° crankshaft design and have the following characteristics:
  • Firing interval is perfectly regular.
  • Primary and secondary reciprocating-plane imbalances are present.
  • Secondary reciprocating forces are high, due to all four pistons being in phase at twice the rotating frequency.
  • Counterweights have been used on passenger car engines since the mid-1930s, either as full counterweight or semi-counterweight designs.
Straight-five engines typically use a 72° crankshaft design and have the following characteristics:
  • A perfectly regular firing interval with overlapping power strokes, resulting in a smoother idle than engines with fewer cylinders.
  • Primary and secondary reciprocating-plane balance is perfect.
  • Primary and secondary rotating-plane imbalances are present.
Straight-six engines typically use a 120° crankshaft design, a firing order of 1–5–3–6–2–4 cylinders and have the following characteristics:
  • A perfectly regular firing interval with overlapping power strokes. The use of two simple three-into-one exhaust manifolds can provide uniform scavenging, since the engine is effectively behaving like two separate straight-three engines in this regard.
  • Primary and secondary reciprocating-plane balance is perfect.
  • Primary and secondary rotating-plane balance is perfect.

    V engines

s have the following characteristics:
  • With a V angle of 90 degrees and offset crank pins, a V-twin engine can have perfect primary balance.
  • If a shared crank pin is used, the 360° crankshaft results in an uneven firing interval. These engines also have primary reciprocating-plane and rotating-plane imbalances. Where the connecting rods are at different locations along the crankshaft, this offset creates a rocking couple within the engine.
V4 engines come in many different configurations in terms of the 'V' angle and crankshaft configurations. Some examples are:
  • The Lancia Fulvia V4 engines with narrow V angle have crank pin offsets corresponding to the V angles, so the firing interval matches that of a straight-four engine.
  • Some V4 engines have irregular firing spacing, and each design needs to be considered separately in terms of all the balancing items. The Honda RC36 engine has a 90° V angle and a 180° crankshaft with firing intervals of 180°–270°–180°–90°, which results in uneven firing intervals within 360 degrees and within 720 degrees of crankshaft rotation. On the other hand, the Honda VFR1200F engine has a 76° V angle and a 360° crankshaft with shared crank pins that have a 28° offset, resulting in 256°–104°–256°–104° firing interval. This engine also has an unusual connecting rod orientation of front–rear–rear–front, with a much wider distance between cylinders on the front cylinder bank than on the rear, resulting in reduced rocking couples.
V6 engines are commonly produced in the following configurations:
  • 60° V angle: This design results in a compact engine size, and the short crankshaft length reduces the torsional vibrations. Rotating plane imbalances. The staggering of the left and right cylinder banks makes the reciprocating plane imbalance more difficult to be reduced using crankshaft counterweights.
  • 90° V angle: This design historically derives from chopping two cylinders off a 90° V8 engine, in order to reduce design and construction costs. An early example is the Chevrolet 90° V6 engine, which has an 18° offset crankshaft resulting in an uneven firing interval. Later, the Honda C engine used 30° offset crank pins, resulting in an even firing interval. A newer example, the Alfa Romeo 690T engine, uses three crankpins 120 degrees apart, and has an uneven firing interval with a harmonic order of 1.5. As per V6 engines with 60° V angles, these engines have primary reciprocating plane and rotating plane imbalances, staggered cylinder banks and smaller secondary imbalances.