An environmentally clean dual fuel for an internal combustion engine, comprising acetylene as a primary fuel and a combustible fuel, such as one or more fluids selected from an alcohol such as ethanol, methanol or any other alcohol or alcohols from the group comprising C.sub.1 -C.sub.20 carbon chains, ethers such as from the group comprising dimethyl ether, diethyl ether, methyl t-butyl ether, ethyl t-butyl ether, t-amyl methyl ether, di-isopropyl ether and the like, low-molecular-weight esters such as from the group comprising methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate and the like, or other suitable combustible fluid such as mineral spirits and the like, as a secondary fuel for operatively preventing early ignition and knock arising from the primary fuel. The dual fuel, internal combustion system, which generally utilizes a two-stage process for start-up and operation and can be operated with air- or liquid-cooling, is environmentally clean with hydrocarbon, CO, NO.sub.x, and SO.sub.x emissions substantially eliminated.