In. the present experimental investigation, waste frying oil anon-edible vegetable oil was used as an alternative fuel for diesel engine. The high viscosity of the waste frying oil was reduced by preheating. The properties of waste frying. oil such as viscosity, density, calorific value and flash point were determined. The effect of temperature on the viscosity of waste frying oil was evaluated. It was determined that the waste frying oil requires a heating temperature of 135 degrees C to bring down its viscosity to that of diesel at 30 degrees C. The performance and exhaust emissions of a single cylinder diesel engine was evaluated using diesel, waste frying oil (without preheating) and waste frying oil preheated to two different inlet temperatures (75 and 135 degrees C). The engine performance was improved and the CO acid smoke emissions were reduced using preheated waste frying oil: It was concluded from the results of the experimental investigation that the waste frying oil preheated to 135 degrees C could be used as a diesel fuel substitute for short-teen engine operation . (c) 2005 Elsevier Ltd. All rights reserved.