What is Engine Balancing

Educational

Nov 15th, 2019

What is Engine Balancing

What is Engine Imbalance?

Several components make up the car engine. Of these, some are stationary, while others have a specified motion. The motion of parts such as the pistons and connecting rods create an imbalance that causes vibrations when the engine is running. This automotive condition is what we refer to as engine imbalance.

Keeping the engine balanced is crucial. Balancing an engine means offsetting the weight of the pistons and rods.

Different Motions in an Engine

The moving components of an engine have different motions. To help analyse imbalance, you can categorise these motions broadly - no matter how complex - into either reciprocating or rotational motion.

For the inline engines, which have vertical pistons, the components that move in a reciprocating manner are the pistons and the connecting rods. In this case, the connecting rods have both an up/down and a left/right action. However, the up/down movement causes more significant vibrations compared to the left/right motion.

On the other hand, the rotating components are the crankshaft and camshafts. The connecting rods also display a rotating motion when they move around piston ends.

Causes of Engine Imbalance

The following factors affect engine balance and are the major causes of imbalance.

1. Mass of the Engine Components

Uniform distribution of weight will help balance the forces caused by the different moving components. The movement of parts generates out-of-balance forces that result in vibrations.

Case in point, pistons or connecting rods that have varying weights across the cylinders produce vertical forces as a result of the reciprocating motion. Likewise, an imbalance will result from the rotation of the crankshaft or flywheel if the web weight (of the crankshaft) and weight distribution (flywheel) are uneven.

At low speeds, the vibrations may not be noticeable but will be as you accelerate.

2. Cylinder Layout

In some cases, even with a perfectly-balanced mass of components, imbalance may still occur as a result of cylinder layout. When the forces from respective cylinders fail to cancel each other out, it can cause a resultant force or vibration. 

An inline-four engine, for instance, has intrinsic vertical vibrations which create an imbalance. This imbalance is unavoidable but manageable with the application of balancing shafts.

Types of Engine Imbalance

Engine imbalance can be any of the following:

  1. Reciprocating plane imbalance such as opposite parallel forces (called a couple) caused by the offset distance between the crankpins in a boxer-twin engine
  2. Reciprocating phase imbalance such as unevenly-spaced phases associated with V6 engines that lack an offset crankshaft
  3. Rotating plane imbalance associated with a boxer-twin engine
  4. Rotating phase imbalance which occurs if the flywheel has irregularly-placed mass
  5. Torsional imbalance

These imbalances can lead to reciprocating, rocking, and torsional vibrations.

Engine Balancing

Engine balancing has the following benefits:

  1. Increases the engine efficiency
  2. Provides tolerance to higher engine speeds
  3. Ensures smooth running of the engine
  4. Maximises the engine lifetime

Mechanics and automotive engineers try to achieve engine balancing. The fact, however, is that no engine can be perfectly balanced, due to various complex factors involved in the process.

Factors to Consider in Engine Balancing

Engine balancing involves the design, production, operation, and maintenance factors of the engine, amongst others, each benefiting from being balanced. Some of the factors to consider are:

  1. Balancing of the structural and operational elements of an engine
  2. Efficiency - cost and power
  3. Performance 
  4. Physical aspects - weight and size

Steps to Balancing an Engine 

There are two steps involved in correcting an imbalance.

1. Static Balancing

The first step aims to ensure that the piston and connecting rod weights are even. Even loads result in equal inertia when these parts move.

2. Crankshaft Balancing

This step involves spinning the crankshaft independently to ascertain if there is a rotational imbalance. In case there is a rotational imbalance, weight is added or removed until a smooth rotation is achieved. Smooth rotation eliminates the vibration caused by rotational imbalance.

A balanced engine runs smoothly and can sustain higher engine speeds without getting damaged.

Primary and Secondary Balance

1. Primary Balance 

Also known as the ‘first-order' imbalance, this balance designation refers to the balancing of items with a shaking engine frequency equal to one crankshaft rotation. In other words, this designation refers to balancing of elements that shake the engine once per crankshaft rotation.

2. Secondary Balance

‘Second-order' balance refers to items that shake the engine twice in one crankshaft rotation. The parts have a frequency of twice per single crank rotation. 

Internal and External Engine Balancing

Engine balancing can be internal and external. It involves offsetting the weights of different components of the engine. 

1. Internal Balance - the counterweights are on the crank while the external parts such as flywheel have a neutral balance which means they can’t affect the other moving components.

2. External Balance - when the crank's counterweights are not heavy, the engine has to be balanced by adding weight to the external part, such as the flywheel.

Of the two, internal balance is the preferred option since external counterweights have a flexing effect on the crankshaft at high engine speeds, which can cause engine damage. Most engines, however, can be balanced both internally and externally.

Switching from external to internal balance is, however, costly as it needs you to acquire a new crankshaft, harmonic balancer, or flywheel. It is recommended to balance the engine in the same way as the factory did.

Component Balance

To reduce vibrations in the engine, consider first the components that rotate at high speeds when balancing. It is crucial especially for large, heavy parts such as the flywheel.

When balancing parts individually, it is essential to mate each of the two components with the axis of the crankshaft.

Balance the crankshaft and flywheel assembly as one unit to eliminate tolerance build-up. Besides, adjusting them as one unit is cheaper than balancing them one at a time.

To attain the right primary balance, the parts that have reciprocating motion should have nearly or the same weight. Component balancing should entail both static and dynamic balance.