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What is Engine Knock and What Causes Engine Knocking Noise?

car engine

Engine knock, also known as Engine detonation, refers to an abnormal combustion phenomenon in which the fuel-air mixture in the engine's combustion chamber ignites prematurely. This phenomenon typically occurs in gasoline engines, especially under high pressure and high temperature conditions. Knock can lead to a decrease in engine performance and may also cause damage to the engine's mechanical components. By understanding and addressing the issue of engine knock, we can further improve vehicle performance, extend engine life, reduce fuel consumption, and lower emissions, thereby contributing to the creation of a greener transportation environment.

1. What is engine detonation?

Before we delve into the concept of engine knock, it's important to understand three things:

(1) In a gasoline engine, the air-fuel mixture (a thorough blend of air and fuel) enters the combustion chamber during the intake stroke. The piston then compresses this mixture during the compression stroke. Once the spark plug ignites the high-pressure mixture, the pressure generated by the combustion is converted into the power that drives the engine.

(2) When the air-fuel mixture burns in the combustion chamber, the flame spreads from the ignition point in a wave-like pattern towards the periphery. Therefore, it takes a brief moment from ignition to complete combustion of the fuel-air mixture.

(3) Although the air-fuel mixture requires a spark plug for ignition, an excessively high-temperature, high-pressure environment can also cause the mixture to ignite spontaneously.

Engine knock

Typically, engine knock occurs when, after the air-fuel mixture in the combustion chamber is ignited, the flame has not yet fully spread, and the unburned mixture at a distance ignites due to high temperature or pressure. The flame from this spontaneous combustion collides with the flame from the regular combustion, generating a significant amount of pressure and causing an abnormal knocking in the engine.

2. what does engine knock sound like?

During engine knock, the engine produces a knocking sound. A slight, intermittent knock sounds quite crisp, somewhat similar to the sound of lightly tapping a triangle. In cases of severe and continuous knock, the engine will produce a "rattling" sound, and the engine's performance will noticeably decrease, appearing to lack power.

3. What are the Dangers of Engine Knock?

(1) It accelerates the damage of engine components.

(2) It causes the engine to overheat.

(3) It leads to excessive carbon buildup.

(4) It results in a decrease in engine power and an increase in fuel consumption.

(5) If continuous and severe knock occurs at high engine speeds and loads, within a minute, the spark plugs and pistons could melt at the least. In severe cases, even the cylinder and the engine body could be punctured.

4. Causes of Engine Knock

4.1 Overly advanced ignition timing

To ensure that the piston can immediately gain power once it enters the power stroke after the compression top dead center, ignition usually occurs before the piston reaches the top dead center (because it takes some time from ignition to complete combustion). However, overly advanced ignition can cause most of the fuel-air mixture to burn while the piston is still in the compression stroke. The unburned fuel-air mixture will then be subjected to extreme pressure and self-ignite, causing knock.

4.2 Excessive carbon buildup in the engine

Engine Carbon Buildup

Excessive carbon buildup in the engine combustion chamber not only increases the compression ratio (creating high pressure) but also generates high-temperature hot spots on the carbon surface, causing engine knock.

4.3 Overheating of the engine

If the engine operates in an excessively hot environment, causing the intake air temperature to be too high, or if the engine cooling water circulation is poor, this can lead to engine overheating and knock.

4.4 Incorrect air-fuel ratio

An overly lean fuel-air mixture can raise the combustion temperature, and an increase in combustion temperature can raise the engine temperature, making knock more likely.

4.5 Low octane rating of gasoline (gasoline grade), knock resistance

The octane rating is an indicator of fuel's resistance to knock. The higher the octane rating, the stronger the resistance to knock. Engines with a high compression ratio have higher combustion chamber pressure. If fuel with low knock resistance is used, knock is more likely to occur.

Different grades of gasoline

Under normal gasoline octane ratings, engine knock is often caused by an overly large (or too early) ignition advance angle. Therefore, controlling the ignition advance angle is one of the measures to prevent engine knock.

5. What should I do if the engine knocks?

5.1 Choose gasoline suitable for the engine compression ratio

Generally, different gasoline grades correspond to different engine compression ratios (for engines with a compression ratio of 8.6:110.0:1, use 92 octane gasoline; for engines with a compression ratio of 10.1:111.5:1, use 95 octane gasoline; for engines with a compression ratio higher than 11.5:1, use 98 octane gasoline).

5.2 Regularly clean the oil circuit and fuel injectors

Cleaning the oil circuit and fuel injectors, removing engine carbon deposits, and changing the engine lubricating oil can keep the engine in good condition. It is recommended to maintain the engine every 5000km to avoid problems such as engine knock and high fuel consumption.

5.3 Properly choose high-quality fuel additives

High-quality fuel additives can solve the problem of engine knock to a certain extent, but how to choose a reliable additive is a question. It is recommended to add it at a 4S shop or gas station.

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