Why is rapid mixing of fuel and air important in diesel combustion?

In a diesel engine, the combustion process is heterogeneous, meaning that fuel and air are not premixed before ignition begins. Because the engine relies on fuel being sprayed directly into hot, compressed air, rapid vaporization and mixing are essential to ensure the fuel is burned thoroughly.1

Without efficient mixing, the fuel must actively "search out" 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, which can lead to improper burning and the production of smoke and particulate matter. To achieve the necessary mixing, engineers often use fuel-injection nozzle designs that create a cone spray, supplemented by air motion techniques such as induction-produced air swirls or "squish"—a radial movement of air from the piston's edge toward the center.21