Abstract: A process for reducing carbonaceous deposits caused by the combustion of heavy fuel oil in high-compression, spontaneous-ignition internal combustion engines involves adding an additive such as ferrocene or ethyl ferrocene to a heavy fuel oil having a density of 0.9 to 1.01 kg/dm.sup.3 in an amount of 1 to 100 ppm prior to combustion of the fuel oil and combusting the fuel oil in the high-compression, spontaneous-ignition internal combustion engine. The additive is dissolved in the heavy fuel oil.