Combustion and heat transfer characteristics obtained based co-firing LPG with used engine oils (UEO) in a furnace, are investigated experimentally. In an attempt to assess UEO as a fuel, the UEO-based results are compared with results obtained using two other fuels, namely diesel, and a used cooking oil (UCkO). To case its admission to the furnace and its subsequent vaporization and combustion, UEO is preheated by allowing it to flow upwardly in a vertical pipe surrounded by hot gases generated from LPG combustion. UEO that reaches the tip of the pipe un-vaporized, spills and hence has the chance to further heatup and vaporize as it exchanges heat with the upwardly flowing LPG combustion gases, in a counter flow process. Runs are divided into three groups based on the mass ratio of the liquid-fuel/LPG and the mass flow rate of the LPG supplied to the furnace. Ranges of these quantities over which UEO qualify as a good fuel and/or good promoter to radiation have been identified. (C) 2009 Elsevier Ltd. All rights reserved.