Quote:
Originally Posted by swiss
What would you call what a Diesel does?  |
Diesels are much less sensitive to knocking, although it is also an undesired effect.
Knocking in Diesel engines occurs when there are either low temperatures inside a cylinder, there is a low load on the engine (I think this is a proper translation) or with fuel that has very low cetane number. Cetane number is a measure of a fuel's ignition delay - the time period between the start of injection and start of combustion (ignition) of the fuel. If the delay is too long, the fuel will ignite when there is already too much fuel inside a cylinder - resulting in a sudden reaction accompanied with strong pressure waves and a very loud noise.
The air in Diesel engines has to be compressed below the minimum level (when compressing air its temperature rises) in order for the fuel to ignite when entering a cylinder. Before modern fuel injection, early Diesels had a combustion chamber inside a cylinder where Diesel fuel would first ignite and spread across the rest of the cylinder - this is the reason why they were so loud (very high pressures inside the cylinder and overall unbalanced/unequal burning process). In early '90-s fuel injector pumps capable of rapid dual injection in one process were introduced. This lowered the noise, and improved the burning process significantly.
Nowadays with Common rail systems (which compress the fuel up to 2000 bar) the fuel gets injected up to 5 times when the piston is at/or close to the TDC, further improving the burning process inside the cylinder. This is one of the reasons why they are so quiet and even more efficient today.
12-18-2010, 01:06 PM
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