The transesterification reaction of two different types of raw materials, a refined cooking oil and a used cooking sample, was performed at small scale (10 L) at the constant temperature of 65 degrees C. The effects of several reaction parameters, such as KOH wt% with respect to the oil weight, methanol/oil molar ratio and reaction time, were investigated. Biodiesel yields as good as 97.5 and 93.2% were achieved for the refined and the cooking oils, respectively, in the following conditions: 1.2 wt% of KOH as catalyst, a methanol/oil molar ratio of 6:1 and reaction time of 60 min. The properties of the biodiesel obtained starting from the used cooking oil are as good as those of the biodiesels obeying the European standards. The resulting product was used in a diesel electricity generator engine, which operated in real conditions. The results showed that biodiesel combustion leads to higher concentration of CO and to a lower emission of NOx as compared to a petrodiesel-fueled engine. An optimization of the operating parameters of the engine would guarantee lower CO emissions in conformity with the regulation. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.