In this work the effect of the structure of water-in-diesel fuel emulsion (WFE) on a three cylinder diesel engine performance has been investigated. Based on membrane emulsification, two different membranes of pore sizes of 0.2 mu m and 0.45 mu m has been individually used to change the emulsion structure while keeping the same WFE volumetric content (at 17% water volumetric content and 0.5% mixing emulsifier content). The Results showed that emulsions with large size of water droplets resulted in greater reduction in NOx emissions up to 25%. While, emulsions with finer droplets not only gave reductions in engine smoke and unburned hydrocarbons of values greater than 80% and 35% respectively, but also resulted in an increase of the engine effective efficiency up to 20%. (C) 2013 Elsevier Ltd. All rights reserved.