Effective transport rate: A novel metric for quantitative evaluation of air-fuel mixing in diesel engines

Air-fuel mixing exerts a decisive influence on combustion and emission characteristics. It is essential to understand the underlying mixing mechanisms and quantitatively analyze the mixing process. However, most existing studies quantify mixing using global statistical indicators, which are insufficient to achieve these objectives. Therefore, a new metric, termed the effective transport rate, is proposed, which is derived from mass transfer principles and based on the concept of effective transport (defined as the portion of fuel mass transport that alters the local equivalence ratio). The metric identifies the key physical fields governing the mixing process and clarifies the mechanisms by which they exert influence. An evaluation framework based on the effective transport rate is established and embedded into a CFD post-processing program, enabling spatially resolved analysis of the mixing rate and quantification of the respective contributions from diffusion and convection. Moreover, CFD simulations of a double-swirl combustion chamber are performed, in which the proposed evaluation framework is applied. It is found that the cumulative effective transport exhibits a strong correlation with the equivalence ratio standard deviation and combustion duration, validating the model. In the double-swirl configuration, high-speed mixing zones are guided by the chamber walls toward the center and liner, and subsequently toward the piston. Convective transport plays a dominant role in the overall mixing process. From the start of injection to near the end of combustion, the cumulative effective convective transport is 4.9 times that of the cumulative effective diffusion transport.