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Causes of Engine Knocking on a Car

Cause of engine knocking

Causes of engine knocking is an incorrect air-fuel mixture this will result in the burning of fuel or pre-ignition before the combustion cycle.

An engine knocking occurs if the air-fuel mixture is not correct, this will result in the burning of fuel at a different portion in the combustion chamber if this problem always occurs for a longer period this will damage the combustion chamber specifically the piston and the cylinder wall. Also knocking will deprived the upper cylinder head area of the needed lubrication.

Before the compressed air-fuel mixture gas is ignited by a spark, the gas is partially exploded for certain reasons.

This phenomenon is called knocking, and the explosion speed of this abnormal combustion is very fast, and it may cause serious damage to the piston and cylinder head.

The causes of engine knocking occurrence

(1) The normal combustion speed of the air-fuel mixture in the combustion chamber is 20 to 30 m/s.

(2) The combustion speed during the occurrence of knocking, or during abnormal combustion, will be extremely high, which is approximately 1,500 m/s.

(3) During this abnormal combustion, the impulse wave that is generated from it makes a knocking sound.

(4) For an automobile engine, a bore diameter of approximately 110 mm is the threshold limit for preventing the occurrence of knocking.

Preventive measures for engine knocking

(1) Delay the ignition timing.

(2) Remove the carbon deposit in the combustion chamber to decrease the compression ratio and to remove hot spots.

(3) Use premium high octane gasoline to raise the ignition point of the fuel.
High octane gasoline is generally called premium gasoline with an octane rating of 91 to 94 octane, low octane gasoline is generally called regular gasoline with an octane rating of 87 t0 88 octanes, gasoline with 89 to 90 octane is generally called midgrade gasoline.

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The main factor concerning engine performance is the octane number. The octane number is the antiknock property of gasoline expressed by a number. The larger the figure for this property, the less the gasoline will generate self-ignition and knocking.

The higher the octane rating of the gasoline, the slower it will burn, because high octane gasoline offers greater resistance against early ignition or what is called “pre-ignition” which causes engine knocking.

However, using high octane gasoline only prevents engine knocking not correcting the problem of engine knocking, if your engine already develops engine knocking the correct way of fixing it is to remove carbon deposits accumulated from the combustion chamber.

(4) Change the spark plug heat range (to cold type).
A spark plug has its own heat range at which it works properly. The lower limit is referred to as the self-cleaning temperature and the upper limit is the pre-ignition* temperature.

Causes of engine knocking

Spark plugs can be divided into the hot type and cold type. A cold type has the characteristic of high heat radiation efficiency with the short pole while the hot type has a long distance for the heat to be released to the cylinder head. The type of spark plug to be used is described in your car Service Manual.

Therefore, a spark plug must be selected so that the temperature near the center electrode will be approximately 500 to 950°C. Because engine usage characteristics differ, such as for high-speed high power or mid-speed high torque, the spark plug heat range should be carefully chosen to correspond to the temperature of the combustion chamber.

*Pre-ignition: A symptom that generates the combustion before spark ignition. The temperature that generates the pre-ignition symptom is approximately 950°C.

Using the above measures, knocking can be prevented. In most cases, high-performance engines are designed to use premium gasoline to obtain maximum performance as well as to prevent knocking.

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