In the presented study, the effects of light-fuel fumigation on diesel engine cycle and characteristics have been examined theoretically. For this purpose, a computer program has been developed for prediction diesel engine cycles and engine characteristics in the cases of neat diesel fuel (NDF) and light-fuel (LF) fumigation. Here, gasoline is used as fumigated (supplementary) light fuel. For calculation of diesel engine cycle, quasi-dimensional phenomenological combustion model developed by Shahed and then Ottikkutti has been used and developed with new assumptions. Firstly, it has been proved that the developed engine cycle model for NDF and LF fumigation gives correct results by comparing with relevant experimental and theoretical studies. Then, for selected 2-10% gasoline ratios LF fumigation is investigated at varied equivalence ratio (VER) and constant equivalence ratios (CER). As a result, it has been determined that increasing gasoline fumigation at VER effective power and carbon monoxide (CO) ratio increase however, nitric oxide (NO) concentration decreases. Also, effective efficiency and specific fuel consumption (SFC) remain nearly constant in this case. Effective power, effective efficiency and CO ratio increase generally by increasing the gasoline fumigation ratio but SFC and NO concentration decrease generally at CER. Furthermore, this case is economic at high engine speeds. (c) 2006 Elsevier Ltd. All rights reserved.