Water-in-diesel emulsion (WiDE) is an alternative fuel for CI engines that can be employed with the existing engine setup with no additional engine retrofitting. It has benefits of simultaneous reduction of both NOx and particulate matters in addition to its impact in the combustion efficiency improvement, although this needs further investigation. This review paper addresses the type of emulsion, the microexplosion phenomenon, emulsion stability and physiochemical improvement, and effect of water content on the combustion and emissions of WiDE fuel. The review also covers the recent experimental methodologies used in the investigation of WiDE for both transport and stationary engine applications. In this review, the fuel injection pump and spray nozzle arrangement has been found to be the most critical components as far as the secondary atomization is concerned and further investigation of the effect of these components in the microexplosion of the emulsion is suggested to be center of focus.

Microexplosion phenomenon is attributed for achieving simultaneous reduction of particulate matter and nitrogen oxides in diesel engine exhaust when water in diesel emulsion is used as fuel. In this work, an emulsion droplet suspended on a wire-type thermocouple on a hot plate as the heat source was used to study the evolution of microexplosion phenomenon of emulsions prepared by two different methods. Microexplosion behavior of emulsions produced by a homogenizer and mechanical stirrer with 5, 10, and 20% water by volume was visualized. A high-speed camera synchronized with a data-logging system was used to capture the events. The results show that the waiting time, puffing frequency, initial temperature drop, and microexplosion temperature were affected by the size and distribution of the dispersed water droplets. No microexplosion was observed for all of the homogenized emulsions, while all of the mechanically stirred emulsions developed microexplosions.

The microexplosion evolution phenomenon of single droplets of water in pure diesel emulsion under Leidenfrost effect has been studied. The tested emulsions were stabilized with a blend of commercial surfactants with three different water contents of 9%, 12% and 15%. A high speed camera synchronized with backlight technique was used to capture the evolution of microexplosion and puffing. Three different droplet diameters of approximately 2.6mm, 2mm and 0.2mm were analysed. It was found that the tendency of microexplosion and puffing frequency was influenced by the droplet diameter. Coalescence was the dominating factor in inducing microexplosion in bigger droplets. It was observed that the child droplets ejected from the parent droplet undergoes further puffing processes.The size of the secondary droplets after microexplosion were also found to be slightly influenced by the parent droplet size.The waiting time for microexplosion and puffing were compared for different droplets size.

Limited studies had been conducted using water-in-diesel emulsion produced from ultrasonic homogenizing method. In this study, Water-in-Biodiesel Emulsions (WiBE) produced using ultrasonic homogenizer were characterized and studied for their physical and chemical properties through various laboratory investigations. The data were then compared with WiBE produced using mechanical homogeniser by the current researchers. Physical characterization tests were carried out on 24 WiBE emulsions produced using an ultrasonic bath, with water percentage of 9%, 12% and 15%, HLB value of 6, 7, 8 and 9, and surfactant dosage of 5% and 10%. The water droplets produced using ultrasonic homogeniser were found to be evenly distributed and generally smaller in size. The density and viscosity values of these emulsions were found to be uniformly larger than WiBE produced using mechanical homogeniser. Emulsions with 15% water exceeded the density threshold, indicating the limits of the amount of water which can be added to the biodiesel fuel. Also, it was found that for viscosity, the HLB is limited to HLB 9 for higher surfactant dosage.

This experimental study endeavors to investigate the evolution of microexplosion phenomenon of water in biodiesel emulsion droplets with the base fuel (B5) containing 95% diesel and 5% of palm oil methyl ester (POME). Parameters such as water content varied from 9%, 12%, and 15%, surfactant dosages of 5%, 10%, and 15% and the hydrophilic–lipophilic balance (HLB) values of 6, 7, 8, and 9 were varied to study its impact on microexplosion phenomenon. Three different sizes of emulsion droplets of approximately Ø2.8 mm, Ø2.2 mm, and Ø0.3 mm were visualized for the evolution of microexplosion phenomenon under the Leidenfrost effect using hot plate as a heat source. The evolution of microexplosion phenomenon of parent droplets, puffing behavior, and waiting time was visualized with high-resolution images. It was observed that the coalescence process was the dominating factor in inducing the microexplosion, and the coalescence process can either be advanced or be delayed by the surfactant dosage. The waiting time for the microexplosion was found to be influenced by the surfactant dosage and the droplet size. The rate of phase change of emulsions and puffing was found to be influenced by the surfactant dosage. By analyzing the postbehavior of the child droplets formed after the microexplosion of the parent droplet, it was observed that the child droplets undergo a series of puffing process and eventually microexplosion phenomenon also. The size of the parent droplets has a significant influence on the size of the child droplet.