The article presents the comparative bench testing results of a naturally aspirated four stroke, four cylinder, water cooled, direct injection Diesel engine when running on shale oil that has been treated with multi-functional fuel additives. The purpose of the research is to evaluate the effectiveness of the fuel additives Marisol FT (Sweden) and SO-2E (Estonia) as well as to verify their ability to increase energy conversion and reduce brake specific fuel consumption, contamination and smoke opacity of the exhausts when fuelling the Diesel engine with shale oil. Test results show that application of these additives could be a very efficient means to improve Diesel engine performance on shale oil, especially when operating at the light load range. The brake specific fuel consumption at light loads and speeds of 1400-2000min(-1) reduces by 18.3-11.0% due to the application of the Marisol FT. The additive SO-2E proves to produce nearly the same effect. The total NOx emission from the fully loaded Diesel engine fuelled with the treated shale oil reduces by 29.1% (SO-2E) and 23.0% (Marisol FT). It is important that the lower NO, is obtained due to reducing both harmful pollutants, NO and NO2. The CO emission at rated power increases by 16.3% (SO-2E) and 48.0% (Marisol FT), whereas the smoke opacity of the exhausts increases by 35% and over 2 times, respectively. The effect of the fuel additives on the HC emission seems to be complicated and ambiguous. (c) 2004 Elsevier Ltd. All rights reserved.