Conventional diesel combustion is a mixing-limited process that passes through high temperature and fuel-rich zones, leading to oxides of nitrogen (NO_x) and particulate matter (PM) formation. Simultaneous reduction of NO_x and PM is difficult due to NO_x-PM trade-off. As alternative fuels, emulsions of water-in-diesel offer several advantages, including a simultaneous reduction in NO_x and PM formation. There are, however, disparities in the reported engine performance and emission characteristics, as they appear to depend on the constituents and microstructure of the emulsion fuel used and engine conditions. Studies on engine performance and exhaust emissions were often carried out without adequate characterization of the emulsions. Therefore, the paucity of cohesive data can be circumvented by standardizing the protocols for emulsion fuels, tailoring their morphology, structure, and characterization, and optimizing engine conditions. This review article recapitulates the salient features of emulsion fuels, from their synthesis, microstructure, characterization, and macroscopic spray characteristics to performance and emissions in diesel engines. A critical analysis of the current state of knowledge is also presented, emphasising the tunability of droplet size and characterization of emulsion stability. The review concludes by suggesting the path forward to utilizing emulsion fuels.

传统的柴油燃烧是一个混合受限的过程，穿过高温和燃料丰富的区域，导致氮氧化物 (NO _x ) 和颗粒物 (PM) 的形成。由于NO _x -PM 的权衡，同时减少NO _{x和PM 是很困难的。}作为替代燃料，柴油中的水乳液具有多种优点，包括同时减少氮_氧化物和颗粒物的形成。然而，报告的发动机性能和排放特性存在差异，因为它们似乎取决于所用乳化燃料的成分和微观结构以及发动机条件。对发动机性能和废气排放的研究通常是在没有对乳液进行充分表征的情况下进行的。因此，可以通过标准化乳化燃料协议、定制其形态、结构和表征以及优化发动机条件来避免缺乏内聚数据的问题。这篇综述文章概括了乳化燃料的显着特征，从其合成、微观结构、表征和宏观喷雾特性到柴油发动机的性能和排放。还对当前知识状况进行了批判性分析，强调了液滴尺寸的可调性和乳液稳定性的表征。审查最后提出了利用乳化燃料的前进道路。