Design Considerations in Diesel Engines

Diesel engines had to have several design considerations taken into account because of the chemical properties of the diesel fuel.  The engine had to be designed around the fuel and because of these chemical properties some considerations were made for the diesel engines.

In order to ignite the diesel engine the cylinder has to be very pressurized because there are no spark plugs.  This means that the engines have to be made stronger in order to not break under such high pressures.  This results in diesel engines being extremely heavy than gasoline engines and they are also much stronger as a result.

Another consideration with diesel engines is the byproducts they create.  There are two main byproducts made from the combustion of diesel fuel, carbon based soot and NOX, which is a nitrogen oxide that can have different amounts of oxygen depending on the conditions.  This is more of a problem with diesel engines than gasoline engines because of the way fuel is injected into the diesel engine.    While a car is accelerating there is a shortage of air in diesel engines, which yields unburned soot.

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Generally when one is decreased, it results in an increase in the other.  As combustion temperatures go down, the amount of NOX created is much less, but that results in an increase in the production of soot.  A reduction in temperature can be accomplished this by injecting the fuel later during the combustion cycle.  However this is bad because if too much soot gets trapped in the oil, around a concentration 3-5%, that will results in increased wear on the engine.  There are also very strict regulations on the amount of soot that diesel engines can make which are enforced by the government.  To reduce the soot and NOX made  there are several filters in an engine that help reduce levels of these harmful compounds.  There is a diesel particulate filters which treats the exhaust of the engine after the combustion is complete.  This is much easier and cheaper than reducing NOX by selective catalyst reduction, however there are such strict regulations on the amount of soot that can be created; both are usually present in modern diesel engines.

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Because of the different properties chemical properties of diesel fuel, different considerations were taken into account in the design of the diesel engine.  These are a few of the design considerations that were made.

Chris Pagliaro

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