Why Does Engine Knocking Occur?



Why Does Engine Knocking Occur

During normal combustion, the air-fuel mixture in the combustion chamber starts burning from the spark plug as soon as the sparking takes place. The mixture burns smoothly from beginning to end, providing an even and powerful thrust to the piston. The pressure inside the cylinder increases evenly. As soon as the sparking takes place, a wall of flame spreads out in all directions from the spark. It travels rapidly outward through the mixture until all the charge is burned. 


Under certain conditions, part of the air-fuel mixture explodes before the flame wall reaches it. The temperature of the last part of the mixture increases as the flame progresses. If the temperature reaches the critical point, the last part of the mixture will explode before the flame reaches it. This causes a sudden increase in pressure which imposes a sudden heavy load on the piston that is almost like a hammer blow. 

Knocking and Detonation- 

It should be noted that knocking and detonation are synonymous terms with the same meaning. Detonation is the name given to the sudden and violent knocking experienced inside the engine cylinder. detonation is not pre-ignition. If the detonation is allowed to continue for a long time it may overheat the cylinder and spark plug so as to ignite the charge even before the sparking, this will cause pre-ignition. Thus, detonation follows the spark while pre-ignition precedes it. 

Characteristics of Gasoline.

Characteristics of gasoline are discussed in this article.

Characteristics of gasoline-

Gasoline is a mixture of hundreds of different hydrocarbons. Gasoline is also called Gas or Petrol. The major characteristics of gasoline are the following –

  1. Volatility.
  2. Antiknock quality.
  3. Sulfur content.
  4. Gum content.
  5. Purity.
  6. Calorific value.
  7. Operating Economy.

1. Volatility-

Volatility is one of the most important characteristics of gasoline. It has been defined as the tendency of gasoline to pass from the liquid into the vapor state at any given temperature. It refers to the base with which the gasoline vaporizes. Gasoline is highly volatile because it vaporizes at a relatively low temperature. At 40 degrees Celcius it vaporizes with a vapor pressure of 5 N/cm to 10 N/cm.
Because gasoline is a mixture of different hydrocarbons, each having a different volatility of boiling point, the proportions of low velocity and high volatility hydrocarbons must be correct for the different operating conditions as follows :

(i) Easy Starting-

For easy starting with a cold engine, the gasoline must be highly volatile, so that it will vaporize readily at a low temperature. Thus, percentage of the gasoline must be highly volatile. This percentage must be higher for colder Northern states than that for the South.

(ii) Quick Waring-

For quick warming of the engine, after it has been started from cold, the gasoline must be fairly volatile. The speed with which the engine warms up depends upon the volatility of the gasoline. The volatility for this purpose does not be as easy starting.

(iii)Good economy-

For a good economy or maximum kilometers per liter, gasoline must have low volatility and high heat content. High volatility reduces the economy and science it may produce an over-rich mixture under many operating conditions. The gasoline’s low volatility tends to burn more effectively.

(iv) Smooth acceleration-

For smooth acceleration, a sufficient proportion of the gasoline must be sufficiently volatile. During the acceleration, an extra amount of gasoline and also the air is delivered to the engine. If this gasoline does not vaporize quickly, the air-fuel mixture will be too lean momentarily which will cause the engine to hesitate. But immediately after as the gasoline begins to evaporate, the mixture will become temporarily too rich. Thus, for smooth acceleration, a sufficient proportion of the gasoline must be sufficiently volatile.

(v) Freedom from vapor lock-

To prevent vapor lock the gasoline must have low volatile. If it is too volatile, the engine heat will cause it to vaporize in the fuel pump. This can cause a vapor lock which prevents normal fuel delivery to the carburetor. Thus, the percentage of highly volatile gasoline must be kept low to prevent vapor lock.

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