The desirability of improving the efficiency of combustion in a vehicle's engine has long been recognized. For instance, Lyons and McKone is 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 internal combustion 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 86/03482 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 (ppm). The Bowers and Sprague results were corroborated and refined by work of Kelso, Epperly, and Hart, described in "Effects of Platinum Fuel Additive on the Emissions and Efficiency of Diesel Engines," Society of Automotive Engineers (SAE) Paper No. 901,492, August 1990. Although the use of platinum group metal additives is effective, further nitrogen oxides reductions are still believed possible.
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, known to reduce catalyst effectiveness. In addition, the regeneration of a diesel trap by the use of a metallic additive which can include copper, nickel, cobalt, and, especially, iron, is discussed by Mueller, Wiedemann, Preuss and Schadlich in "Diesel Particulate Filter System With Additive Supported Regeneration," ATZ Automobiltechnische Zeitschift 91 (1989).
Other researchers have considered the use of water in oil emulsions for improving combustion efficiency in diesel engines. For instance, DenHerder, in U.S. Pat. No. 4,696,638, discusses such emulsions and indicates that the positive effects therefrom include "cleaner exhaust." Although the disclosure of Denherder refers to emulsions containing up to about 40% water, Denherder is primarily directed to emulsions having only up to about 10% water in the form of droplets having a diameter of about 1 to about 10 microns.
Furthermore, Wasser and Parry have reported in "Diesel Engine NO.sub.x Control: Selective Catalytic Reduction and Methanol Emission," EPRI/EPA Joint Symposium on Stationary NO.sub.x Control, New Orleans, La., March, 1987 that NO.sub.x reductions of up to 80%, which are the levels desired for effective emission control, can be achieved in diesel engines using water and oil emulsions. They found, though, that emulsions of at least 60% water in oil are necessary to achieve such reductions. Unfortunately, such high water ratios can lead to increased emissions of carbon monoxide (CO) and unburned hydrocarbons. In addition, such high water levels can also cause problems in emulsion stability and create corrosion and storage volume concerns.
Recently, Dainoff and Sprague, in U.S. patent application entitled "Process for Reducing Nitrogen Oxides Emissions and Improving the Combustion Efficiency of a Turbine:, Ser. No. 07/691,556, filed Apr. 25, 1991, have discovered that water-in-fuel oil emulsions up to about 50% water by weight are useful for reducing nitrogen oxides and particulate emissions from combustion turbines, but they do not address the usefulness of such emulsions in vehicle diesel engines.
There has been a great deal of study of processes and agents for reducing nitrogen oxides emissions from stationery sources, such as boilers, etc. Preferred among such processes are those referred to as selective, non-catalytic reduction (SNCR) NO.sub.x reducing processes. Such processes involve the introduction of a NO.sub.x reducing treatment agent into a combustion effluent to achieve reductions of nitrogen oxides of up to about 50% or greater by a gas phase free radical mediated reaction process. SNCR processes generally utilize a nitrogen containing treatment agent such as ammonia, urea, or other chemicals and compositions which can break down in the effluent to the amidozine radical, which is believed to be the moiety which reacts with and reduces nitrogen oxides to molecular nitrogen (N.sub.2).
Such processes are disclosed by, for instance, Lyon, in U.S. Pat. No. 3,900,554, Arand, Muzio, and Sotter, in U.S. Pat. No. 4,208,386, and Arand, Muzio, and Teixeira, in U.S. Pat. No. 4,325,924. Although effective at reducing nitrogen oxides in effluents from open flame combustion, it has been believed that at the temperatures and pressures which exist in diesel engines, the use of nitrogenous nitrogen oxides reducing agents in such processes may actually create NO.sub.x, thus exacerbating the problem.
Accordingly, a process and composition which is effective at substantially reducing the nitrogen oxides emissions from a diesel engine without the drawbacks of the prior art is extremely desirable.