Water-in-Diesel emulsion fuel (W/D) is an alternative fuel which progressively intrigues the world attention due to its great impact to the environment as well as energy consumption. However, the high dependency on surfactant for the production of W/D makes it non-effective in terms of cost, and this restricts the commercialization of said fuel. This paper presents the first time ever that W/D without surfactant is being used in a conventional compression ignition engine. Diesel fuel and water, being stored in different units, are quantitatively transferred and instantaneously emulsified by a mixing system combination of high shear mixer and ultrasonic mixer before the produced emulsion fuel is transferred into the engine. The non-surfactant emulsion fuel so called unstable emulsion fuel, labelled as UW/D, is tested in a single cylinder, direct injection diesel engine. The engine is tested under four different load conditions (1 kW (25%), 2 kW (50%), 3 kW (74%), 4 kW (100%)) and with a constant speed of 3000 rpm. As for comparison, a surfactant added emulsion called as the stable emulsion (SW/D), and neat diesel fuel (D2) are also tested. 5% of water is used for both unstable emulsion and stable emulsion. The surfactant used for making the SW/D is SPAN 80 with 0.2% of volume fraction. The engine testing result showed that emulsion fuel without surfactant UW/D does give significant improvement to the engine with a 3.59% increase in brake thermal efficiency (BTE) and 3.89% reduction in brake specific fuel consumption (BSFC) as compared to diesel fuel. In addition, Nitrogen Oxides (NOx) and Particulate Matter (PM) contents in the exhaust emission reduced significantly compared to neat diesel fuel with the average reduction of 31.66% and 16.33% respectively. Overall, the concept of producing and supplying the emulsion fuel in real-time into the engine without having surfactant was proved to be working and functioning through this research while maintaining its benefits; greener exhaust emission and fuel saving.