Abstract:

Premixed combustion and diffusion combustion are the basic combustion modes of diesel engines. In this paper， the two typical combustion chambers of the reentrant combustion chamber and flared combustion chamber of high-pressure common-rail direct injection diesel engine are selected to study the mixture formation mechanism based on the entrainment effect under different background airflow conditions by simulation calculation. The results show that the entrainment effect caused by the small-scale vortex on the surface of the oil spray is the main reason for the formation of the pre-mixture when the oil spray penetrates the background air flow at the initial stage of injection. The entrainment effect is directly affected by different background airflow under certain injection condition. For the reentrant combustion chamber， at the initial stage of injection， there is strong entrainment effect and a fast premixing rate. Most of the fuel is rapidly diffused and combusted under the action of strong tumble inside the combustion chamber. Therefore， it belongs to the premixed diffusion combustion mode （PDC）. Although the proportion of premixed combustion is small， the high-temperature zone is wide. For the flared combustion chamber， although there is relatively weak entrainment effect and a low premixed combustion rate at the initial stage of injection， a secondary premixing process is formed after the oil spray impinges on the wall convex platform， which extends the formation duration of the pre-mixture and increases the proportion of premixed combustion to form a double-premixed diffusion combustion mode （DPDC）. Through the DPDC combustion mode， the high-temperature area is reduced， thus effectively inhibiting the generation of NO _x .

Key words: combustion mode, entrainment effect, vortex intensity, NO _x generation mechanism