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Why do diesel engines produce particulate matter called soot?

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diesel engine and precombustion chamber — diesel engine Diesel engine equipped with a precombustion chamber.

Diesel engines produce soot—a form of particulate matter—primarily because their combustion process is heterogeneous, meaning the fuel and air are not premixed before ignition.1

In a diesel engine, fuel is sprayed directly into compressed, hot air within the cylinder. Because the fuel and air are not premixed, the fuel must rapidly vaporize and find oxygen atoms to complete the combustion process. Since oxygen makes up only about 20 percent of the air, each fuel atom has a limited chance of encountering an oxygen atom. If the fuel does not mix thoroughly with the available oxygen, it results in improper burning, which generates soot.12

To mitigate this, engineers focus on fuel-injection nozzle design and techniques to improve air-fuel mixing, such as creating air swirls or radial movements (squish) within the cylinder to help the fuel better locate oxygen. Despite these efforts, the emission of soot remains a principal drawback of diesel technology compared to spark-ignition engines.12