STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not Applicable BACKGROUND OF THE INVENTION Field of the Invention The present invention is in the field of processes for controlling the operation of a diesel engine, specifically in the area of limiting the production of nitrogen oxides.

Background Art During the combustion of fuel in a diesel engine, nitrous oxides are commonly formed as a by-product. Nitrous oxides are detrimental to the environment, so it is desirable to limit their formation. Methods for accomplishing this are known, but each such method typically has drawbacks such as reduced engine power, reduced efficiency, or increased production of other pollutants such as particulate matter.

It is known that the production of NOx in a typical diesel engine combustion cycle begins shortly after the injection of fuel into a cylinder of the engine. Specifically, the production of NOx begins shortly after combustion of the injected fuel actually begins. So, upon injection of fuel into a given cylinder during a given engine rotation, a combustion cycle begins in that cylinder. Shortly after the combustion cycle begins in that cylinder, an associated NOx formation cycle begins in localized areas of the cylinder where the local temperature rises above the temperature at which NOx forms.

The formation of NOx then continues in some regions of the cylinder where the local temperature is above the level at which NOx forms.

As the combustion cycle advances during the power stroke of a given cylinder, the piston in the cylinder is driven downward to produce power. As the piston continues to be driven downward and the combustion chamber expands, the pressure and temperature in the combustion chamber both eventually drop. This drop in the temperature causes localized temperatures to fall below the temperature which will support NOx formation. When there are no localized areas with temperatures high enough to support NOx formation, the NOx formation ceases. As the temperature drops off, the level of NOx concentration in the chamber increases until there are no local areas in which the temperature is high enough to support additional formation of NOx, and the NOx concentration stays at that level until the combustion products are exhausted. This NOx concentration can be thought of as the level at which NOx concentration levels off or naturally"freezes"during a combustion cycle. This freezing or leveling off of the NOx concentration caused by expansion in a given combustion cycle in a given cylinder of an engine can be thought of as the"natural" freeze of NOx concentration during the"natural"combustion cycle in that cylinder.

Each NOx formation cycle in each cylinder of an engine typically stops before the associated fuel combustion cycle in that cylinder is complete. As the crankshaft continues to rotate, the piston rises during the exhaust stroke to expel the combustion products from the cylinder to the atmosphere. Additional fuel combustion cycles and their associated NOx formation cycles follow in each cylinder.

It would be desirable to alter the operation of the engine to further reduce the level at which the NOx concentration freezes, below the"natural"NOx freeze level, to thereby limit the overall rate of production of NOx, without significantly affecting the efficiency of the combustion cycle.

BRIEF SUMMARY OF THE INVENTION The purpose of the present invention is to reduce the rate of formation of NOx in the combustion process of a diesel engine by altering the in-cylinder temperature after the start of combustion in a given combustion cycle and a given cylinder, and before the "natural"freeze, due to expansion, of NOx concentration during the NOx formation cycle associated with that combustion cycle. The manipulation of the NOx mechanism, therefore, comes after the time at which the combustion cycle has begun, and before the time at which the NOx level would have"naturally"frozen during the associated NOx formation cycle.

The NOx formation cycle is manipulated by injecting into the cylinder a heat sink fluid. This fluid may be air, nitrogen, water or any other fluid that can act as a heat sink once introduced into the combustion chamber. An added benefit can be increased soot oxidation within the combustion chamber, if turbulent mixing is promoted. Depending on the amount of NOx reduction required, the heat sink fluid may be introduced into the combustion chamber during every cycle, every other cycle, every third cycle, etc. The heat sink fluid is injected after the beginning of the combustion cycle, with the result that the beginning of the combustion process is left undisturbed. Unlike exhaust gas recirculation or water injection at the same time as the fuel, this process will allow the"natural"combustion to proceed until it is time to shut down or slow down the NOx formation mechanism.

The heat sink fluid is also injected before the"natural"freeze of NOx concentration, in order to slow down or stop the NOx formation process. This alters the NOx formation process prior to its"natural"freeze, unlike injection of fluid after the "natural"freeze, which can not reduce the formation of NOx.

The novel features of this invention, as well as the invention itself, will be best understood from the attached drawing, taken along with the following description, in which: BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Figure 1 is a combination of two graphs illustrating a combustion cycle and the manipulation of its associated NOx formation cycle in a cylinder of an engine, according to the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION As shown in the upper graph of Figure 1, cylinder pressure increases during a compression stroke in a given cylinder of an engine, as the piston moves toward a crankshaft angle at Top Dead Center (TDC). At some point, fuel is injected into the cylinder, with this Start of Injection (SOI) being, in this example, at a crank angle just before TDC. Shortly after SOI, the Start of Combustion (SOC) occurs, and cylinder pressure continues to increase for a short time as the piston passes TDC. Then, as the piston moves downward after TDC and the fuel combustion cycle continues, the cylinder pressure eventually decreases. This is the pressure cycle seen during a single fuel combustion cycle in the"natural"combustion process in a given cylinder of an engine.

The lower graph of Figure 1 shows a plot of the level of concentration of NOx in the cylinder, during the fuel combustion cycle represented in the upper graph. The vertical lines for the crank angles at which SOI, TDC, and SOC occur are at the same locations as in the upper graph, to illustrate the time relationship, relative to crank angle, between the fuel combustion cycle and its associated NOx formation cycle. It can be seen that the solid line representing NOx concentration increases from near zero, at approximately the time of SOC. In the normal combustion process currently known in the art, the NOx concentration increases continuously until it levels out or "freezes", as a result of the expansion of the combustion chamber caused by downward movement of the piston. This"natural freeze"of the NOx level is shown by the upper solid line indicated on the graph.

According to the method of the present invention, a predetermined quantity of heat sink fluid is injected into the cylinder at a time after the point at which SOC occurs, but before the point at which the"natural freeze"of the NOx level would normally occur. This heat sink fluid may be air, nitrogen, water, or any other fluid that can act as a heat sink once introduced into the combustion chamber. The exact timing of the heat sink fluid injection, and the quantity of fluid to be injected will vary from one engine to another. Therefore, these parameters will have to be determined experimentally, and they will be dependent on the individual diesel engine.

Depending upon the amount of NOx reduction required, the heat sink fluid may be introduced into the combustion chamber during every fuel combustion cycle, every other cycle, every third cycle, etc.

Since the heat sink fluid is injected after SOC, the initial portion of the fuel combustion cycle is left undisturbed, until the heat sink fluid is delivered at a controlled time to the combustion chamber. Unlike the EGR process, or water injection at the same time as the fuel, this process will allow the first part of the "natural"combustion process to proceed until it is time to shut down or slow down the associated NOx formation cycle.

As shown in the lower graph in Figure 1, a relatively larger quantity of heat sink fluid can be injected to completely stop the NOx formation cycle, causing an immediate leveling off or"freeze"of the NOx concentration. This higher quantity injection option is represented by the lower, horizontal, solid line in the lower graph.

Alternatively, it may in some cases be sufficient to inject a relatively smaller quantity of the heat sink fluid, resulting in a slowed, rather than stopped, production of NOx.

When this relatively smaller quantity of heat sink fluid is injected, the level of NOx concentration increases at a slower rate than before the injection, then it ultimately freezes at a level between the"natural freeze"level without heat sink fluid injection and the"immediate NOx freeze"level with the larger heat sink fluid injection. This lower quantity injection option is represented by the intermediate dashed line in the graph.

While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.