Why does a car engine vibrate when starting?

Car engines vibrate due to unbalanced forces arising from the translation and rotation of engine parts .

Why does a car engine vibrate when starting?

Although we may seem oblivious to the vibrations in the car, they are a subtle reminder that the car is running. When something goes wrong, one of the most common signs is a deviation from this 'unnoticed' running vibration.

Vibration is often attributed to the movement of components within the powertrain. However, this is only partially true because electric vehicles also have moving parts inside (motors) but it does not cause such vibrations. The answer to this conundrum lies in the design of the internal combustion engine.

Vibration in a car is caused by moving parts in the transmission system.

Internal combustion engines are piston engines. They consist of a reciprocating part (piston and connecting rod) to produce rotational motion (crankshaft). The combination of translational and rotational motion creates cyclic and unbalanced forces. These forces are considered vibrations that we feel when starting/running the car.

What are unbalanced forces? Unbalanced forces are created when all forces in a moving system are not canceled by opposing forces. Residual forces cause various disturbances, in this case vibrations. Vibration in the engine mainly arises from two sources: static imbalance and dynamic imbalance.

Static imbalance refers to the inequality in weight and center of gravity of different reciprocating components with respect to each other. Whereas dynamic unbalance refers to the presence of an eccentric rotating mass that results in the generation of unbalanced centrifugal force when the engine is in motion.

The translational and rotational movements of engine parts generate unbalanced forces that cause vibration.

What have manufacturers done to reduce engine vibration?

The first step to engine balancing is to resolve any static imbalance of the machined parts to extremely close and identical tolerances. Engineers seek to solve most unbalance problems by static balancing.

What cannot be solved by static balance will be solved by dynamic balance. Here, unbalanced forces are resisted by balance axes placed opposite them.

However, it is important to note that it is not possible to completely eliminate vibration-causing imbalances in piston engines. They can only be reduced to an acceptable level in terms of mechanical limits and passenger comfort.

The use of counterweight shafts is important to reduce the effects of unbalanced forces.

Dynamic balance is associated with engine running time and includes primary balance and secondary balance. Primary balancing is performed for unbalanced forces acting according to the rotation speed of the crankshaft. Secondary balancing is performed for unbalanced forces acting at twice the rotational speed. A connecting rod longer than the crankshaft radius will produce these unbalanced forces.

As a general rule, a larger number of cylinders is beneficial to engine balance. Additional pistons improve the ability to generate opposing forces that can eliminate unbalanced forces. In addition to the number of cylinders, their arrangement and firing order also contribute to reducing engine vibration. Below are some multiple cylinder arrangements and their equilibrium states.

1. Inline engine 4

The inline 4-cylinder configuration is the most common type in modern cars. Thanks to the layout and firing order, its generated main forces are completely balanced. Conversely, the secondary forces must be balanced by the balance shaft. To prevent the secondary force from exceeding the allowable limit, 4-cylinder engines are often smaller in size and capacity than other types of engines.

2. Inline 6 engine

This engine is one of the most balanced configurations in piston engines. Since each piston has an opposing "partner," it represents a well-balanced setup without the need for a balance shaft or large counterweight.

3. V6 engine

The V6 engine consists of two banks of 3 cylinders connected to a common crankshaft. Because the number of cylinders in each bank is an odd number, these engines are fundamentally unbalanced. To address this imbalance, the angle between the cylinder banks can be varied to create the optimal setup. Common angles include 60 degrees, 90 degrees. and 120 degrees, 60 degrees is the most balanced layout.

4. V8 engine

The V8 engine is a combination of 2 rows of 4 cylinders placed at 90 degree angles. The crankshaft is designed so that the two pistons can share one bearing. It also incorporates heavy counterweights to balance secondary forces. 6 and 8 cylinder engines are commonly found in sports cars.

5. V12 engine

The v12 engine can be considered a combination of two inline 6-cylinder engines and exhibits a remarkable level of refinement. V12 engines have a very large capacity and are used in powerful sports cars and luxury cars.

Inline 6 and V12 engine configurations are the best balanced piston engines.

An unwanted change in normal vibrations can signal some type of mechanical failure or impending failure. Engine vibrations and road noise transmitted into the cabin also help the driver establish a connection with the road surface. This is essential for safety on the road.