In the presented study, the effects of light fuel fumigation on the diesel engine cycle and characteristics have been examined theoretically. For this purpose, a computer program has been developed for prediction of the diesel engine cycle and engine characteristics in the cases of neat diesel fuel (NDF) and light fuel (LF) fumigation. Here, ethanol is used as the fumigating (supplementary) light fuel. For calculation of the diesel engine cycle, the quasi-dimensional phenomenological combustion model developed by Shahed and then Ottikkutti has been used and developed with new assumptions. Firstly, the accuracy of the developed engine cycle model for NDF and LF fumigation has been controlled by using various experimental and theoretical results. Then, the effects of 2.5-20% ethanol fumigation have been investigated at variable equivalence ratios (VER) and constant equivalence ratios (CER). It can be said that for increasing ethanol fumigation at VER, the effective power, effective efficiency and carbon monoxide (CO) ratios increase and the specific fuel consumption (SFC) and nitric oxide (NO) concentration decrease. Effective power, effective efficiency and NO concentration increase, generally, with increasing ethanol fumigation ratios at CER. In this case, SFC and CO ratio exhibits an incremental tendency generally. (c) 2007 Elsevier Ltd. All rights reserved.