PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 1
Modern automotive engines made for use with leadfree gasoline must,
e. Other Causes for Knocking.
(1) Lowoctane 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 airfuel 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.
With a basic understanding of the characteristics of gasoline, it is a
logical progression to learn about the function and construction of gasoline