In the current time, sustainability and development of internal combustion engines is subjected to two kinds of threats which are fossil fuel depletion and environmental pollution. So, considerable efforts in the world have been exerted to develop and introduce alternative gaseous fuels to replace conventional fuel by partial or total replacement. In this study, the effect of adding acetylene to LPG-fuelled SI engine has been examined. This addition was with flow rates of 500, 750 and 1000 g/h for 25 % load and 1000 g/h for 50 % load, respectively. An experimental investigation has been carried out on 4 stroke, 4- cylinder, water-cooled spark ignition (SI) engine at constant speed (1500 rpm). Also, performance and emissions parameters have been tested at different excess air coefficients (lambda= 0.8, 1, 1.2 and 1.7). As a result, it has been found optimum conditions which are the addition of 750 g/h acetylene for 25 % load (almost 41 % of the LPG-acetylene mixture at lambda= 1.2) and 1000 g/h for 50 % load (almost 36 % of the LPG-acetylene mixture at lambda= 1.5). In addition, hydrocarbon (HC) and carbon monoxide (CO) emissions decreased, while NO emissions increased when compared to that of baseline LPG operation.