contains a cerous compound for increasing the combustion rate of the fuel in the internal combustion engine in another embodiment of the composition the combustion modifier may include only a single ferric iron containing organometallic soap selected from among those described herein in another embodiment the organometallic soap can be cerium 2 ethylhexanoate only although the combustion modifier preferably also contains a ferric compound for increasing the combustion rate of the fuel in the internal combustion engine by preventing the accumulation of carbon residues on the internal surface of the combustion chamber of the internal combustion engine in another embodiment of the composition the combustion modifier may include only a single cerium containing organometallic soap selected from among those described herein in another embodiment the combustion modifier can include a mixture of one or more ferric compounds selected from among those described herein and one or more cerous compounds selected from among those described herein the combustion modifier may include the ferric compound or mixture of compounds in a range of about 10 to 100 percent by weight or about 60 to 80 percent by weight and the cerous compound or mixture of compounds in a range of about 10 to 100 percent by weight or about 20 to 40 percent by weight the combustion modifier can also include the ferric compound or mixture of compounds in a range of about 15 to 85 about 35 to 75 or about 65 to 75 percent by weight and the cerous compound or mixture of compounds in a range of about 15 to 85 about 25 to 65 or about 25 to 35 percent by weight the combustion rate and combustion efficiency are most improved when the combustion modifier contains about 70.