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Principles of Gasoline And Diesel Fuel Systems
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6

e. Other Causes for Knocking.

(1) Low-octane fuel is not the only reason for knocking.

Anything

that adds heat or pressure to the last part of the mixture to burn within a

cylinder will aggravate detonation and also result in knocking. That is why

the compression ratio of a gasoline engine has an upper limit.

When the

ratio is raised too high, the immediate result is detonation caused by

excessive heat from the additional compression. Under certain conditions,

excessive spark advance, lean fuel mixtures, and defective cooling systems

are some of the many causes of detonation.

(2) Preignition is another cause for knocking.

Though its symptoms

are similar, it is not to be confused with detonation. Preignition is an

igniting of the air-fuel mixture during compression before the spark occurs;

it is caused by some form of hot spot in the cylinder, such as an overheated

exhaust valve head or spark plug, or a glowing piece of carbon. Preignition

can lead to detonation, but the two are separate and distinct events.

Conclusion

With a basic understanding of the characteristics of gasoline, it is a

logical progression to learn about the function and construction of gasoline

fuel systems which will be covered in the next task.