More specifically, the present invention is directed to a secondary injection of HC into the exhaust stream using a fuel injector of a lean burn engine.

Background of the Invention The need for reducing NOX emissions from internal combustion engines is highly desirable because of ever more stringent environmental laws.

Currently, many manufacturers are working on selective catalyst reduction (SCR) to reduce NOX emissions from engines, especially diesel and other lean burn engines.

An SCR catalyst is designed to reduce NOX emissions in an oxygen-rich or fuel-lean environment. In order to provide good conversion efficiency, the catalyst prefers an HC-to-NOx feed gas ratio of between 4: 1 and 6: 1 at an optimal temperature of between 200-300°C.

The main problem in diesel engines is that the typical HC/NOX ratio of the engine exhaust is less than one (1); thus, making the SCR catalyst very inefficient.

There have been prior art disclosures teaching the addition of hydrocarbons into an exhaust stream to increase the efficiency of a reducing catalyst to remove oxides of nitrogen from the exhaust gas stream. For example, US patent 5,524,432 to Hansell for Catalytic Reduction of Nitrogen Oxides in Methane-Fueled Engine Exhaust by Controlled Methane Injections-

teaches injecting control amounts of methane into an exhaust gas stream in order to reduce nitrogen oxides through use of an additional valve.

Similarly, US patent 5,364,606 to Hung for NOX Reduction Through the Use of Methane discloses introducing small quantities of methane to provide a selective catalyst reduction of the NOX.

Additionally, US patent 5,266,083 to Peter-Hoblyn et al. for Method for Reducing Pollution Emissions From a Diesel Engine teaches retarding the timing of a diesel engine in order to reduce the NOX emissions.

However, the prior art does not disclose an efficient structure to introduce the additional hydrocarbon into the exhaust stream in a vehicle application except through the use of an additional valve or injector. The need for an additional valve or injector or additional mechanical equipment impacts the cost, reliability, and desirability of such a system. Therefore, a system that does not require any additional mechanical elements beyond those already existing in the engine would be highly desirable.

Brief Description of the Drawings FIG. 1 is a block diagram showing a system in accordance with the present invention; FIG. 2 is a block diagram disclosing another embodiment of the present invention; FIG. 3 is a typical diesel engine combustion timing diagram illustrating a preferred secondary fuel injection phase; FIG. 4 is a typical diesel engine NOX map as a function of load and injection timing; and FIG. 5 is a typical diesel HC map as a function of load and injection timing.

Description of a Preferred Embodiment A selective catalyst reduction system 10 in accordance with the present invention is shown in FIG. 1. An internal combustion engine 12 is preferably a diesel engine or other lean-burn type of engine and includes a plurality of fuel injectors 14. Fuel injectors 14 in the engine 12 inject a primary amount of hydrocarbon into the engine 12 at primary intervals for combusting the hydrocarbon, which combusts and creates nitrogen oxide (NOx) in an exhaust stream 16 within exhaust pipe 18. The hydrocarbon is typically diesel fuel or any other fuel being used to power the engine 12. The system 10 further includes a fuel injector controller 20 coupled to the fuel injectors 14 through interface 22. The controller 20, controls an introduction of the hydrocarbon into the engine 12 at a time other than the primary intervals thereby injecting a secondary amount of hydrocarbon into the exhaust stream 16. The secondary amount of hydrocarbon combines with the NO x in the exhaust stream 16 to increase a HC-to-NOx ratio thereby allowing a catalyst 24 to efficiently reduce an emission of NOX from exhaust pipe 18.

The catalyst 24 is preferably of the platinum type. Reduction of NO, under lean conditions occurs most favorably when the catalyst temperature is between 200-300°C and the HC/NO ratio is 4-6 (Cl basis).

Controller 20 preferably includes a map memory 26 for storing an emissions engine map of HC-to-NOx emissions as a function of the engine 12's operating conditions. The operating conditions of engine 12 preferably include a load, a main injection timing, an engine speed, a water temperature, and intake air temperature and pressure. The map memory 26 also preferably includes a temperature engine map to control when the secondary amount of HC is injected into the engine. The temperature engine map is preferably of the burned gas temperature as a function of engine 12's operating conditions and engine 12's crank angle such that the injection of the secondary amount of HC occurs at a crank angle where the secondary amount of HC will not combust

but before an exhaust valve (not shown) of the fuel injectors 14 close. It is noted that as little as one fuel injector 14 needs to be used to inject the secondary amount of HC into the exhaust stream 16.

FIG. 2 discloses a typical diesel combustion timing diagram, along with a preferred timing of injecting the secondary amount of HC. The start of ~ combustion begins at approximately 30. The primary amount of HC injection typically begins at 32 which is approximately 18 degrees before top dead center of a crank angle of the engine 12. The primary injection ends at 34, which is typically approximately 8 degrees after top dead center (TDC), with the premixed combustion phase being shown generally at 36. The diffusion combustion phase is shown generally at 38, and the end of combustion occurs around 40. The beginning of the injection of the secondary amount of HC preferably begins at 42 and ends at 44. As those skilled in the art will appreciate, the injection of the secondary amount of HC needs to occur at a time sufficiently after the primary injection and combustion such that the cylinder (not shown) associated with the injector 14 is at a temperature sufficiently low so that the secondary amount of HC will not combust but before the exhaust valve of injector 14 opens at 46. Thus, the secondary amount of HC is preferably injected at a crank angle after 80° past TDC and before 120° past TDC. Also, as mentioned above, the secondary amount of HC injected should be sufficient such that the HC-to-NOx ratio is between approximately 4: 1 and 6: 1 to allow the catalyst to efficiently reduce the emission of NOx.

A set of preferred steps for determining the amount of HC to be added to the exhaust stream is as follows: 1) Determine NOX in exhaust from a) an estimate based on a map contained in memory 26; or b) an NOX sensor 2) Calculate amount of HC required in the exhaust stream 16 = (4 to 6) *Step 1)

3) Determine amount of HC in exhaust stream 16 from a) an estimate based on a map contained in memory 26; or b) an HC sensor 4) Determine amount of HC to be added to exhaust stream 16 = Step 2)- Step 3) 5) Determine amount of fuel to be added during the secondary injection as function of an inlet air pressure and temperature (preferably measured); an engine displacement (preferably calculated); an engine speed (preferably measured); the amount of HC in Step 4); and an engine volumetric efficiency (preferably mapped) 6) Determine the desired secondary injection pulse width of injector 14 based on relationship between a fuel pulse width and an injector flow rate.

An alternative embodiment of a system in accordance with the present invention is shown in FIG. 3 at 50. The system 50 is similar to the system 10 of FIG. 1 in that it includes an engine 52, injectors 54, an exhaust stream 56 in exhaust pipe 58, a controller 60 and interface 62, a catalyst 64, and a memory 66. However, system 50 additionally includes an HC sensor 68, an NOX sensor 70, a catalyst temperature sensor 72, and an air injector 74. System 50 preferably directly measures the HC and NOX levels of exhaust stream 56 with sensors 68 and 70 that are coupled to controller 60 via lines 76 and 78. By using sensor 68 and 70, the secondary amount of HC can be calculated from a direct measurement ofthe HC-to-NOx ratio in exhaust stream 56, thereby eliminating the need for the engine maps disclosed above. Preferably sensors 68 and 70 are of the calorimetric or electrochemical type.

In addition, system 50 preferably includes an air injector 74 for injecting a quantity of air into exhaust stream 56 to maintain an exhaust stream temperature allowing for an optimal conversion efficiency of the catalyst 64 wherein the quantity of air injected is based on a catalyst temperature engine

map as a function of engine 52's operating conditions. Engine 52 is preferably a diesel engine or other lean-burn engine. Preferably, the injection of the air is conditioned on the temperature of the catalyst as determined by temperature sensor 72 which is electrically connected to controller 60 via line 8. As those skilled in the art will appreciate the air injection technique would be similar to the secondary air injection method commonly used on contemporary spark- ignition engines.

FIG. 4 discloses a typical diesel engine emission of NOX in parts per million as a function of a primary injection timing and engine load. The diagram of FIG. 4 shows two examples of engine timing at 15° before TDB (BTDC) and 25° BTDC. The engine load is expressed in terms of break mean effective pressure (BMEP) in units of kilo-Pascals.

FIG. 5 is an HC map expressed in parts per million as a function of engine timing and BMEP load.

By comparing FIGS. 4 and 5, it can be seen that a typical engine exhaust stream will include an HC-to-NOx ratio of less than 1.0. As explained above, the SCR catalyst 24 most efficiently reduces NOX emissions when the HC-to- NOX ratio is between approximately 4: 1 and 6: 1. Therefore, by using the present invention disclosed above to inject secondary amounts of HC into the exhaust stream, the NOX emissions can be reduced by 80-90%.

A specific embodiment of the present invention has been shown and described above, however, further modifications and improvements will occur to those skilled in the art. Such modifications may include the use of HC and NOX sensors without the use of air injector or the use of an air injector in conjunction with the engine maps. All such modifications retaining the basic underlying principles disclosed in claims herein are within the scope of the present invention.