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Abstract. Water-in-diesel emulsion fuel (W/D) is considered to be a potential alternative fuel that can reduces nitrogen oxides (NO x ) and particulate matter (PM). W/D is normally produced with addition of surfactant to prolong its stability. However, the dependency on surfactant leads to higher production cost. A concept that can eliminate dependency on the surfactant was introduced by strong mixing concept and direct supply the non-surfactant W/D to a diesel engine. Therefore, the objective of this study is to investigate the stability behavior of non-surfactant W/D using microscopic observation. Water and diesel fuel were mixed using a combination of high-shear mixer and an ultrasonic transducer. The amount of water injected into the system are 5% and 10% by volume, which were labelled as E5 and E10 respectively. The produced non-surfactant W/D was then placed into a petri dish for microscopic observation. The results showed that the average diameter of water droplets observed in E5 and E10 at sedimentation stage were 15.38 and 22.41 μm respectively. The stability period of E5 and E10 before it sediment were 25 and 67 seconds correspondingly. Overall, it is observed that microscopic observation is a reliable approach to determine the stability behavior of non-surfactant W/D.The interest in water-in-diesel emulsion fuel (W/D) for diesel engine application has been growing worldwide as it is regarded as an effective way to reduce the harmful pollutants such as nitrogen oxide (NO x ) and particulate matter (PM) simultaneously. Moreover, it also improves the combustion efficiency [1][2][3][4]. An emulsion is a mixture of two or more liquids that are immiscible in nature where one present as droplet, or the dispersed phase throughout another liquid known which acts as the continuous phase [3]. Emulsion is produced with the assistance of some external means, either by mechanical or ultrasonic agitation; and eventually, with an addition of surfactant to help bind both diesel and water together as one solution [5][6][7][8]. The mechanical homogenizing machine is the most used emulsifying device in emulsion preparation; it stirs up the mixture of continuous and

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