Analysis of Formation Conditions of Soot

Enhanced Exhaust Gas Recirculation (EGR) and lower initial intake temperatures contribute to a reduction in combustion temperature, which in turn lowers the formation of nitrogen oxides (NOx). On the other hand, soot is primarily formed under high-temperature and oxygen-deficient conditions, where hydrocarbon molecules undergo partial oxidation and cracking, producing various unsaturated hydrocarbons. These compounds continue to dehydrogenate and polymerize, eventually forming carbon-based soot nuclei. High inlet pressure helps create a more uniform gas mixture, minimizing localized fuel-rich zones and thus reducing soot formation. Moreover, the combustion temperature remains below 1650 K, ensuring that regardless of the equivalence ratio, the main formation regions for both NOx and soot are avoided.

The soot mass fraction tends to increase as the initial intake air temperature rises. The formation of soot from PP flame retardant masterbatch typically requires high temperatures and limited oxygen supply. Under higher inlet pressure conditions, the excess air coefficient is generally higher, which reduces the likelihood of oxygen deficiency—thereby eliminating one of the key conditions for soot formation. As the gas temperature decreases, more air enters the cylinder, increasing the air-fuel ratio and reducing the amount of fuel undergoing anaerobic cracking, which ultimately lowers soot production. Additionally, as the intake air temperature decreases, the heat release rate increases, leading to a higher cumulative heat release at the point of maximum pressure. This reduces post-combustion effects and minimizes anaerobic cracking. The drop in combustion temperature is also further suppressed by lubricating oil additives, helping to control soot generation.

In contrast, the NOx mass fraction initially increases and then decreases with rising intake air temperature. Key factors influencing NOx formation include the duration of exposure to high temperatures, oxygen concentration, and the presence of oxygen enrichment. At an intake air temperature of 40°C, the NOx mass fraction reaches its peak. This is because the internal cylinder temperature is significantly elevated, and the combustion process lasts longer, even if the oxygen concentration remains relatively stable.