Wednesday, Jun 08, 2005 at 13:41
In a diesel engine, the fuel ignites spontaneously shortly after injection begins.
During this delay, the fuel is vaporizing and mixing with the air in the combustion
chamber. Combustion causes a rapid heat release and a rapid rise of
combustion chamber pressure. The rapid pressure rise is responsible for the
diesel knock that is very audible for some diesel engines.
Increasing the cetane number of the fuel can decrease the amount of knock
by shortening the ignition delay. Less fuel has been injected by the time combustion
begins and it has had less time to mix with air. As a result, the rapid
pressure rise, along with the resulting sound wave, is smaller.
One design approach to reducing combustion noise is to shape the injection –
setting the rate slow at first and then faster – to reduce the amount of fuel
entering the cylinder during the ignition delay period. Another is to use indirect-
injection
(from the diesel bible)
FollowupID:
370684