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Noise and vibration from the engine! Why does knocking occur?


If there are unusual sounds that have never been heard from the engine or vibrations that are different from usual, it makes us very uneasy, right?
Perhaps the engine is causing "knocking".

What kind of phenomenon is knocking?

Knocking refers to a phenomenon in which abnormal noise and vibration are generated from the engine.
These sounds can be heard as a kind of "rattle rattle", "cancan cancan" or "kinkin kinkin".
It is called knocking because noise sounds like the sound of knocking on a door.

Why knocking happens

There are several types of knocking. For each type, I will introduce the cause and the measures to prevent it.

Car knock (slow knocking)

Car knock is not caused by engine trouble or mechanical malfunction, it is caused by driver-error.

In a manual car, when accidentally starting off in 3rd gear, or turning corners without shifting down from 4th or 5th gear, the engine speed is too low for the speed the car is travelling. In this case, the car body vibrates and rattles when traveling.

Most car knocking is not a malfunction of the engine itself, it is caused by the driver's mistake in operation.
The way to solve this is to ensure proper driving technique.

Spark (ignition) knock

A gasoline engine compresses a mixture of gasoline and air, which ignites and explodes from the spark of the spark plugs to make power.

The combustion of the compressed air-fuel mixture will spread mainly around the spark plug.

It is necessary to burn all the air-fuel mixture cleanly, but if the mixture far from the spark plug remains unburned, it is pressed against the piston and cylinder.
As a result, it heats to a very high temperature and high pressure. The mixture then self-ignites independently of the spark of the spark plug, and at the same time it generates a shock wave.

This shock wave causes knocking and generates a metallic sound.
This can be troublesome as this spark knock may cause engine parts to be damaged.

Since the shock wave destroys the heat insulating layer on the piston and cylinder wall surface, the piston and the cylinder become hot and the engine becomes useless.

In order to prevent spark knock, you need to ensure that unburned air-fuel mixture does not remain. This is to prevent self-ignition of unburned air-fuel mixture.

In particular,

  1. Use fuel that is unlikely to self-ignite
  2. Slow the ignition timing
  3. Reduce the cylinder diameter

It is difficult to change the size of the cylinder, but it is not difficult to use a fuel that is unlikely to self ignite.
Fuel that is difficult to self ignite is fuel with high octane number, or “high-octane” gasoline.

skirt knock

The lower part of the cylinder has a shape like a skirt, and that part is called a piston skirt.
Knocking that is generated by this piston skirt colliding with the cylinder wall is called a skirt knock.

The cause of the skirt knock is that the gap between the cylinder and the piston widens, the piston swings more than necessary, and the piston hits the cylinder wall.

Since the gap becomes wider when the engine is cold, the knocking sounds will remain until the engine warms up and the gap becomes narrow.
The knocking may disappear once the engine is warm, but it could also remain.

To prevent skirt knocking, ensure proper clearance between the piston and the cylinder wall.

If knocking subsides as the engine gets warm it will have little effect on the engine, however if it is at a level that there is always noise, even if the engine is warm, then the piston and cylinder will be scratched because the gap has been considerably widened.

Also, in order to make the clearance proper, it is necessary to overhaul (disassemble and repair) the engine, so it will be quite expensive.

Thus, knocking has various causes.

If you are concerned, you should have your vehicle checked at the maintenance shop.

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