As acknowledged by Needham, Doyle, and Nicol, in "The Low NO.sub.x Truck Engine", Society of Automotive Engineers (SAE), Paper No.  910731, the reduction of nitrogen oxides emissions from diesel engines can be achieved by retarding the injection timing of the engine.  Although there is little doubt that such timing changes are effective at reducing NO.sub.x emissions, they carry with them the disadvantage that fuel consumption rates and emission of carbon monoxide and unburned hydrocarbons suffer as a trade-off for the NO.sub.x reductions achieved. 
These results are confirmed by the work of Wasser and Perry in "Diesel Engine NO.sub.x Control: Selective Catalytic Reduction and Methanol Emulsion", EPRI/EPA Joint Symposium on Stationary NO.sub.x Control, New Orleans, La., March, 1987.  Engine timing modification, therefore, is not a complete answer to the emissions of pollutants such as nitrogen oxides from diesel engines. 
The desirability of improving the efficiency of combustion in a vehicle's engine to, inter alia, reduce NO.sub.x emissions, has long been recognized.  For instance, Lyons and McKone in U.S. Pat.  No.  2,086,775, and again in U.S. Pat.  No.  2,151,432, disclose a method for improving combustion efficiency in an internal combustion engine by adding to the fuel what is described as "relatively minute quantities" of catalytic organometallic compounds.  The Lyons and McKone patents, though, are directed solely to gasoline-fueled engines and do not address the problem of NO.sub.x emissions from diesel engines. 
In a unique application of catalytic technology described in International Publication No.  WO 86/03492 and U.S. Pat.  No.  4,892,562, Bowers and Sprague teach the preparation of diesel fuel containing fuel soluble platinum group metal compounds at levels of from 0.01 to 1.0 parts per million.  Epperly and Sprague disclose a further advance in the field in U.S. Pat.  No.  5,034,020. 
Moreover, in "Assessment of Diesel Particulate Control--Direct and Catalytic Oxidation", SAE Paper No.  81 0112, 1981, Murphy, Hillenbrand, Trayser, and Wasser have reported that the addition of catalyst metal to diesel fuel can improve the operation of a diesel trap.  Among the catalysts disclosed is a platinum compound, albeit one containing chlorine, which is known to reduce the effectiveness of a platinum group metal catalyst. 
What is desired therefore, is a system which enables the reduction of NO.sub.x from a diesel engine without the trade-off of reduced fuel consumption and increased emissions of carbon monoxide and unburned hydrocarbons by a process which makes use of a fuel-borne catalyst.