Method for correcting an estimate of NH3 stored in an exhaust system with selective catalytic reduction

Abstract

A method for correcting an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip; and adjusting an amount of NH3 delivered to the SCR in response to the corrected amount of NH3 stored in the SCR, wherein the indication of NH3 slip is determined via a convolution-based metric.

Description

BACKGROUND / SUMMARY

[0001] Nitrogen oxides (e.g., NOx) contained in engine exhaust gases can be treated by a selective reduction catalyst (SCR) in the vehicle's exhaust system to form N2 and H2O. The SCR can work in conjunction with a reductant, such as ammonia (NH3), to reduce NOx. However, if an insufficient amount of NH3 is present at the SCR, a higher than desirable amount of NOx may pass through the SCR. Conversely, if excess NH3 is directed to the SCR, NH3 may bypass the SCR. Determining the amount of NH3 stored in an SCR can be difficult because the SCR can have a large surface area. Therefore, it may be desirable to estimate the amount of NH3 stored in the SCR.Control actions (e.g., adjusting an amount of NH3 injected into an exhaust system) may be taken based on the amount of NH3 stored in the SCR after the amount of NH3 stored in the SCR has been estimated. However, the control actions may not function as desired if the estimated amount of NH3 stored in the SCR deviates by more than a threshold from the actual amount of NH3 stored in the SCR.

[0002] DE 10 2009 034 622 A1 and DE 10 2009 034 843 A1 describe methods for correcting an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip; and adjusting an amount of NH3 delivered to the SCR in response to the corrected amount of NH3 stored in the SCR.

[0003] The inventors of the present invention have recognized the above-mentioned disadvantages and developed methods for correcting an estimate of NH3 stored in an SCR, the features of which are set out in claims 1 and 9. Advantageous further developments emerge from the subclaims. Accordingly, the method corrects an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip. The indication of NH3 slip may be provided via a sensor positioned downstream of the SCR (for example, the sensor is positioned downstream of the SCR according to an exhaust flow direction). Alternatively, the estimated amount of NH3 stored in the SCR may be corrected in response to SCR efficiency.

[0004] By correcting an estimate of NH3 stored in an SCR, it may be possible to improve SCR conversion efficiency. For example, if the estimated amount of NH3 stored in an SCR is improved, injection of NH3 into the exhaust system may be started or stopped before NOx or NH3 slips past the SCR. In particular, if the estimated amount of NH3 stored in the SCR is low, injection of NH3 may be started and / or increased. If the estimated amount of NH3 stored in the SCR is large, injection of NH3 into the exhaust system may be stopped and / or reduced. As a result, less NH3 and / or NOx may slip past the SCR.

[0005] The present description can offer several advantages. In particular, the approach can reduce NH3 consumption because NH3 injection can be stopped before NH3 slips past the SCR. Consequently, the vehicle operator can reduce the number of refills of an NH3 supply canister. Furthermore, engine NOx and NH3 emissions can be reduced because the NH3 supply to the SCR can be increased before a greater degree of NOx slip occurs, reducing the amount of NOx escaping to the atmosphere. Likewise, the NH3 supply to the SCR can be decreased before a greater degree of NH3 slip occurs, reducing the amount of NH3 escaping to the atmosphere.

[0006] The above advantages and other advantages and features of the present description will be readily apparent from the following detailed description, considered alone or in conjunction with the accompanying drawings.

[0007] It should be understood that the above summary is intended to introduce, in simplified form, a selection of concepts that are further described in the detailed description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined solely by the claims following the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any of the disadvantages noted above or in any other part of this disclosure. SHORT DESCRIPTION OF THE CHARACTERS Fig. 1 shows a schematic representation of an engine; Fig. 2 shows a block diagram of an exemplary diagram of an SCR control system; Fig. 3A - 3B show prophetic example data for correcting an NH3 storage amount; Fig. Figure 4 shows a prophetic example of an amount of NH3 stored in an SCR as determined by an NH3 storage model; Fig. 5 shows a flowchart for correcting an NH3 storage amount estimated in an NH3 storage model in response to a low SCR efficiency; and Fig. Figure 6 shows a block diagram of a control system for supplying NH3 to an SCR. DETAILED DESCRIPTION

[0008] The present description relates to the supply of NH3 to an SCR. According to one aspect of the description, an estimate of the amount of NH3 stored in an SCR is corrected by an observer. Fig. 1 shows an example engine system with an SCR for NOx reduction. Fig. Figure 2 shows an exemplary block diagram for an SCR-based NOx reduction system. Fig. 3A and Fig. 3B show prophetic signals of interest for a system with an observer to correct an estimated amount of NH3 stored in an SCR Fig. 4 - 6 show flowcharts for methods that can be used individually or in combination to correct an estimated amount of NH3 stored in an SCR. Fig. Figure 6 shows a block diagram of a control for adjusting the amount of NH3 supplied to an SCR.

[0009] On Fig. 1 Referring to, a plurality of cylinders, one of which cylinder in Fig. 1, is controlled by the electronic engine control unit 12. The engine 10 includes a combustion chamber 30 and cylinder walls 32 with a piston 36 positioned therein and connected to a crankshaft 40. The combustion chamber 30 is shown communicating with an intake manifold 44 and an exhaust manifold 48 via an intake valve 52 and an exhaust valve 54, respectively. Each intake and exhaust valve may be actuated by an intake cam 51 and an exhaust cam 53. The position of the intake cam 51 may be determined by the intake cam sensor 55. The position of the exhaust cam 53 may be determined by the exhaust cam sensor 57.

[0010] In the illustration, the fuel injector 66 is positioned to inject fuel directly into the cylinder 30, which is known to those skilled in the art as direct fuel injection. The fuel injector 66 delivers liquid fuel proportional to the pulse width of the FPW signal from the controller 12. Fuel is delivered to the fuel injector 66 from a fuel system (not shown) that includes a fuel tank, a fuel pump, and a fuel rail (not shown). Fuel pressure delivered from the fuel system can be adjusted by changing a position valve control current to a fuel pump (not shown). In addition, a metering valve in the fuel rail for closed-loop fuel control may be positioned in or near the fuel rail.The fuel injector 66 receives operating current from a driver 68 which responds to the controller 12.

[0011] The intake manifold 44 is shown communicating with an optional electronic throttle 62, which adjusts a position of the throttle plate 64 to control airflow from an intake boost chamber 46. The compressor 162 draws air from the air intake 42 to supply the boost chamber 46. Exhaust gases rotate the turbine 164, which is connected to the compressor 162 via shaft 161.

[0012] Combustion is initiated in the combustion chamber 30 when fuel is automatically ignited upon the piston reaching top dead center on the compression stroke. In some examples, a Universal Exhaust Gas Oxygen (UEGO) sensor (not shown) may be connected to the exhaust manifold 48 upstream of an exhaust device 70 near a NOx sensor 126. In other examples, the NOx sensor 126 may be omitted and replaced by an oxygen sensor. In still other examples, a second UEGO sensor may be positioned downstream of one or more exhaust aftertreatment devices. In the present example, a second NOx sensor 128 is provided downstream of an emission control device.

[0013] In the illustration, the exhaust device 70 is positioned downstream of the turbocharger turbine 164 in the engine exhaust system. The exhaust device 70 may, in one example, include a particulate filter and oxidation catalyst bricks. Alternatively, the exhaust device 70 may be configured as an SCR. The NOx sensor 128 may be relocated to a location between the exhaust device 70 and the exhaust device 72 when the exhaust device 70 is configured as an SCR. In the illustration, the exhaust device 72 is positioned downstream of the exhaust device 70 in the direction of exhaust flow and configured as an SCR when the exhaust device 70 is configured as a particulate filter or oxidation catalyst. In the illustration, the NOx sensor 128 is located downstream of the exhaust device 72 when the exhaust device 72 is an SCR.In other examples, exhaust devices 70 and 72, along with NOx sensor 128, may be positioned upstream of turbine 164. In the illustration, NH3 (urea) injector 75 is positioned upstream of emission control device 72. NH3 injector 128 may be positioned upstream of emission control device 70 if emission control device 70 is configured as an SCR. The NH3 injector provides liquid NH3 to exhaust device 72 via a pump and an NH3 storage tank (not shown). The liquid NH3 is supplied to exhaust device 72 to promote vaporization of the NH3.

[0014] It should be noted that NOx sensors 126 and 128 have cross-sensitivity to both NOx and NH3. Thus, when NOx and NH3 are present, a single NOx sensor output reflects the combined concentration of NOx and NH3. In some examples, NOx sensor 128 may be replaced or augmented with a selective NH3 sensor. For example, NOx sensor 128 may be replaced with a sensor that detects only NH3.

[0015] In the presentation of Fig. 1, the controller 12 is a conventional microcomputer including a microprocessor unit 102, input / output (I / O) ports 104, a read-only memory (ROM) 106, a random access memory (RAM) 108, a retained memory (RAM) 110, and a conventional data bus.In addition to the signals previously discussed, the controller 12 is shown receiving various signals from sensors coupled to the engine 10, including: engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; a position sensor 134 coupled to an accelerator pedal 130 for sensing the accelerator pedal position set by foot 132; a boost pressure measurement from pressure sensor 122; an engine intake manifold pressure (MAP) measurement from pressure sensor 121 coupled to intake manifold 44; an engine position sensor from a Hall sensor 118 sensing the position of the crankshaft 40; a measurement of the mass of air entering the engine from sensor 120 (for example, a hot wire air flow sensor); and a throttle position measurement from sensor 58. Barometric pressure may also be sensed for processing by the controller 12 (sensor not shown).According to a preferred aspect of the present description, the engine position sensor 118 generates a predetermined number of evenly spaced pulses for each revolution of the crankshaft from which the engine speed (RPM) can be determined.

[0016] In some examples, the engine may be coupled to an electric motor / battery system in a hybrid vehicle. The hybrid vehicle may have a parallel configuration, a series configuration, or variations or combinations thereof.

[0017] During operation, each cylinder in engine 10 typically experiences a four-stroke cycle: the cycle includes the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. During the intake stroke, exhaust valve 54 generally closes and intake valve 52 opens. Air is introduced into combustion chamber 30 via intake manifold 44, and piston 36 moves to the bottom of the cylinder to increase the volume in combustion chamber 30. The position where piston 36 is near the bottom of the cylinder and at the end of its stroke (for example, when combustion chamber 30 is at its largest volume) is typically referred to by those skilled in the art as bottom dead center (BDC). During the compression stroke, intake valve 52 and exhaust valve 54 are closed. Piston 36 moves toward the cylinder head to compress the air in combustion chamber 30.The point at which the piston 36 is at the end of its stroke and which is closest to the cylinder head (for example, when the combustion chamber 30 has its smallest volume) is generally referred to by those skilled in the art as top dead center (tdc).

[0018] In an event referred to below as injection, fuel is introduced into the combustion chamber. In some examples, fuel may be injected to a cylinder multiple times during a single cylinder cycle. In an event referred to below as ignition, the injected fuel is ignited by compression ignition or by another known ignition means, such as a spark plug (not shown), resulting in combustion. During the power stroke, the expanding gases push the piston 36 back to BDC. The crankshaft 40 converts piston movement into rotating shaft torque. Finally, during the exhaust stroke, the exhaust valve 54 opens to release the combusted air-fuel mixture to the exhaust manifold 48, and the piston returns to TDC.It should be noted that the above is shown only as an example, and that the timing of intake and exhaust valve opening and / or closing may vary to provide positive or negative valve overlap, late intake valve closing, or various other examples. Furthermore, in some examples, a two-stroke cycle may be used instead of a four-stroke cycle.

[0019] Now on Fig. 2, a block diagram of an exemplary SCR control system is shown. The engine 10 delivers exhaust gases to the SCR 72 via an exhaust pipe 210. Engine operating conditions, the amount of NH3 injected into the SCR, correction parameters from the observer 204, and SCR operating conditions are input to the NH3 storage model 202. As shown in Fig. 6, the NH3 storage model provides an estimate of an amount of NH3 stored in the SCR 72 to the observer 204 and the NH3 control system 206. The NH3 control system 206 provides an electrical signal based on inputs from the NH3 storage model and the engine to actuate the NH3 injector 75. The observer 204 receives inputs from NOx sensors 126 and 128 and from the NH3 storage model 202 to determine the time and amount of correction for the amount of NH3 stored in the NH3 storage model 202. Alternatively, a model may replace the NOx sensor 126. The observer 204 functions as in the method of Fig. 5 described.

[0020] Now on Fig. 3A, prophetic example data for correcting an NH3 storage amount is shown. In particular, output signals from NOx sensors positioned upstream and downstream of an SCR are shown. Curve 302 represents a signal from a NOx sensor positioned upstream of the SCR. In one example, the output of the upstream NOx sensor represents a concentration of engine feed gas NOx. Curve 304 represents a signal from a NOx sensor positioned downstream of the SCR. In one example, the present description ensures recognition that when an output of a downstream NOx sensor is greater than that of the upstream NOx sensor, negative SCR efficiency results and NH3 is known to slip past the SCR.Based on the knowledge of NH3 slip, it can be determined that the NH3 stored in the SCR is at the threshold storage capacity (TSC) (e.g., the amount of NH3 that the SCR can store) of the SCR. Thus, it can be determined that the amount of NH3 stored in the SCR is greater than or equal to the TSC level. Accordingly, the amount of NH3 stored in the SCR can be updated according to the NH3 storage model under such conditions. Curve 306 reflects the time course during SCR operation when the NH3 storage model is updated. In particular, the times at which curve 306 is at a high level indicate times at which the amount of NH3 is determined to be stored at the TSC level. Curve 306 is the time at which the output of the downstream NOx sensor is greater than the output of the upstream NOx sensor.Consequently, the estimated amount of NH3 to be stored in the SCR can be updated to the TSC or a fraction thereof during this time, as determined by the NH3 storage model.

[0021] Now on Fig. Referring to Figure 3B, prophetic examples of an amount of NH3 stored in an SCR are shown, as determined by an NH3 storage model and a high storage (HS) observer. Curve 310 represents a reference storage value of the threshold storage capacity (TSC). The TSC curve varies with ammonia concentration and SCR brick temperature. Curve 312 represents an amount of NH3 stored in an SCR calculated according to a model. The calculated NH3 storage level from the model is based on a detailed catalyst model, and the observed storage model is the calculated value, relative to TSC, under conditions suitable for observer-based correction, as discussed below. Curve 306 is the same signal as in Fig. 3A, and it represents the times at which the amount of NH3 to be stored in the SCR estimated via the NH3 storage model can be updated to the TSC based on the determination of the NH3 slip (for example, NH3 passing through an SCR).

[0022] At approximately 250 seconds in the graph of Fig. 3B, the estimated amount of NH3 to be stored in the SCR is revised to the TSC level at the revision time. Consequently, the NH3 level determined from the SCR NH3 storage model exhibits a step-like change. The step-like change is delayed in time by the timing signal 306 and initially goes high, indicating that the NH3 level of the SCR NH3 storage model can be updated. In some examples, the time delay may be the result of a low-pass filter signal 306. In other examples, a predetermined amount of time may be required for the signal 306 to be high before the NH3 amount in the SCR NH3 storage model is updated. In this example, the stored NH3 amount determined via the SCR NH3 storage baseline model is updated to the TSC level a single time. In one example, a predetermined time period is required between updates of the NH3 level of the SCR NH3 storage model to TSC.For example, if the NH3 level of the SCR-NH3 storage model is updated to TSC, the NH3 level of the SCR-NH3 storage model cannot be updated again for a predetermined period of time (for example, 300 seconds).

[0023] In this way, the NH3 level determined by the SCR NH3 storage model can be updated to the TSC level when ammonia slip is detected. Consequently, the output of the SCR NH3 storage model can be adjusted to an expected NH3 storage level.

[0024] Now on Fig. 4, a prophetic example of an amount of NH3 stored in an SCR, as determined via an NH3 storage model and a low storage (LS) observer, is shown. Curve 402 represents an estimated value of NH3 stored in an SCR, as determined via an NH3 storage model. Curve 404 represents a reference storage for low efficiency. Curve 408 represents a low efficiency flag, as determined from NH3 slip. If the SCR is low efficient and no NH3 slip is detected, the estimate of NH3 stored in the SCR may be reduced to reflect the revised amount of NH3 stored in the SCR.

[0025] It should be noted that the SCR NH3 estimated amount of NH3 stored in the SCR transitions to a lower amount of NH3 when the low efficiency flag indicates a low SCR efficiency condition via curve 408.

[0026] Thus, the output of the SCR NH3 storage model can be adjusted up or down depending on the detection of NH3 slip or NOx slip.

[0027] Now on Fig. Referring to Figure 5, a flowchart for correcting an NH3 storage amount estimated in response to low SCR efficiency in an NH3 storage model is shown. The method of Fig. 5 can be used, for example, in the control 10 of Fig. 1 instructions are carried out.

[0028] At 502, method 500 determines SCR operating conditions. In one example, the SCR operating conditions may include SCR temperature, NOx concentration and flow into the SCR, and NOx concentration and flow out of the SCR. The NOx concentrations flowing into and out of the SCR may be determined via NOx sensors that have cross-sensitivity to NH3. Method 500 proceeds to 504 after determining the SCR operating conditions.

[0029] At 504, method 500 assesses whether or not an efficiency of an SCR in an exhaust system of an engine is lower than a predetermined level. In one example, the SCR efficiency may be determined based on the output of the NOx sensor positioned in the exhaust system upstream of the SCR in an exhaust flow direction and the output of a NOx sensor positioned in the exhaust system downstream of the SCR. In one example, the SCR efficiency may be determined according to the following equation: η=1−CNOxTP+αCNH3TPCNOxFG, where η represents the SCR efficiency, CNOxTP the concentration of tailpipe NOx (e.g. downstream of the SCR) αCNH3TP the concentration of tailpipe NH3 as measured by a tailpipe NOx sensor or an NH3 sensor, and CNOxFG is the concentration of feed gas NOx. Note that a downstream NOx sensor, which is sensitive to NOx and NH3, provides an output that may include both the NOx and NH3 concentrations. In some examples, the SCR efficiency may be acted upon by a low-pass filter so that the SCR efficiency signal slows down. The time constant of the low-pass filter may be adjusted depending on SCR operating conditions. In other examples, the SCR efficiency may be negative for a predetermined period of time and of sufficient magnitude (e.g., less than -0.2) before method 500 continues. In another example, the upstream NOx sensor output may be replaced with an output from a NOx model representing a NOx concentration. If the SCR efficiency is below a predetermined level or below a predetermined value, method 500 proceeds to 506.In one example, the predetermined SCR efficiency is less than zero. Otherwise, method 500 ends.

[0030] At 506, method 500 assesses whether or not NH3 slip is present at a location in the exhaust system downstream of the SCR. In one example, NH3 may be determined to be present if the SCR efficiency is below zero. Subzero SCR efficiency may be possible if the SCR efficiency is based on the output of a NOx sensor that is cross-sensitive to NOx and NH3. Further, via a NOx sensor that is cross-sensitive to NOx, NH3 slip may be determined by low-pass filtering the concentration of the output from the downstream NOx sensor and the output from the upstream NOx sensor or model. In one example, if the low-pass NOx concentration from the downstream NOx sensor minus the low-pass NOx concentration from the upstream NOx sensor or model minus a threshold feed gas NOx concentration is above zero, NH3 slip may be determined.If a method 500 judges that NOx slip is present, then method 500 proceeds to 508. Otherwise, method 500 proceeds to 512.

[0031] In some examples, a determination of NH3 or NOx slip may be provided via a NOx feed gas estimation and a NOx sensor positioned downstream of an SCR in the exhaust flow direction. In one example, the method convolves signals originating from or related to a NOx sensor positioned upstream of an SCR and a NOx sensor positioned downstream of the SCR. A convolution AF - *ATP can be performed on signals from upstream and downstream NOx sensors, where ΔF - is the forward difference of the upstream or feed gas NOx sensor output as the NOx sensor output decays, and where ΔTP is the forward difference of the downstream or tailpipe NOx sensor output.

[0032] In another example, NH3 slip and NOx slip can be determined via a convolution-based metric, which is provided to improve NOx and NH3 prediction stability. The metric is formed according to the following equations: dMdt=(c1∏ΔF×ΔTP)−(c2×|∏η×ΔTP|)−c3M, where M is a metric for determining whether the output of a NOx sensor positioned downstream of a first NOx sensor represents NOx or NH3; where Π ΔF is a unit step function for the duration of decaying feed gas NOx (ΔF<0); where Π ηis a unit step function for the duration when the efficiency of the SCR is < 0; where ΔTP is the forward difference of the TP NOx sensor output (TP - tailpipe) or the NOx sensor output downstream of the first NOx sensor (for example, downstream of an SCR); c1 is an empirically determined calibration coefficient that is greater than zero if ΔF < 0 and the SCR efficiency η ≥ 0, otherwise c1 is zero; c2 is an empirically determined calibration coefficient that is greater than zero if the SCR efficiency η ≤ 0, otherwise c2 is zero; c3 is an empirically determined drift gain that is greater than zero if the output of a downstream NOx sensor is less than a threshold downstream of the NOx sensor height and M < 0.

[0033] In some examples, the c1 coefficient may be an increasing function of feed gas NOx (e.g., dFGNOx / dt) so that at higher loads, such as during vehicle acceleration, NOx slip can be determined with greater certainty. Alternatively, c1 may be a function of increasing tailpipe NOx. In still other examples, c1 may be a decreasing function of the derivative of demand torque, such that c1 increases as the operator releases the accelerator pedal. As NOx slips by, the tailpipe NOx sensor output may drop rapidly during tip-outs (e.g., accelerator pedal release) so that NOx and NH3 are assessed with greater certainty.

[0034] The above algorithm uses the deterministic NH3 slip case (when tailpipe sensor measurements > feed gas values → η < 0) to update the metric at a faster rate (for example, via the gain term c2), thus ensuring convergence to the NH3 slip condition.

[0035] Finally, there may be conditions where the NOx / NH3 slip state rapidly transitions from NH3 to no slip. Under such conditions, the metric with a value of M < 0 (due to previous NH3 slip) may not have a chance to heal because ΔTP ~ 0 enforces a 0 metric update rate. For such conditions, a healing mechanism is introduced via a drift gain c3 that acts to return the metric value from 0 to a rapid rate. Method 500 proceeds to 512 after feed and tailpipe NOx signals have been convolved.

[0036] At 512, method 500 initiates operation of a low-level NH3 observer (LL observer). The low-level NH3 observer reduces the stored NH3 estimate when the expected NOx conversion is low and there is no NH3 slip. The low-level NH3 observer is described as follows: dm^NH3stordt=f(mNH3stor)+λ(E(mstorNH3)−m^NH3stor), where m^NH3stor a mass of NH3, as determined by the LS observer, f(m^NH3stor) is the mass of NH3 stored in the SCR, as determined via an NH3 storage model, λ is a predetermined gain factor for observer update and E(mstorNH3) is the expected minimum NH3 storage for a given SCR efficiency and temperature, as measured. Method 500 proceeds to 514 after updating the LS NH3 storage observer output.

[0037] At 514, the estimated NH3 stored in the SCR, as determined using an NH3 storage model, is updated. In one example, the amount of NH3 stored in the SCR is estimated according to a model expressed according to the following equation: dmNH3stordt=Rads−Rdes−Rox−Rred+m˙NH3inτ, where mNH3stor is the mass of NH3 stored in the SCR, as determined by the model, R ads is the adsorption rate of NH3 by the SCR, R des is the desorption rate of NH3 by the SCR, R ox is the oxidation rate of NH3 by the SCR, R red is the reduction rate of NH3 by the SCR and m˙NH3in where 3 is the mass flow rate of NH3 into the SCR, and τ is the residence time of NH3 in the SCR. Note that R in the model is given by the Arrhenius equation: Rxx=kxxexp(−ExxRT) where R xx the reaction rate is, kxx is the number of molecular collisions, R is the gas constant, E a is the activation energy and T is the temperature in degrees Kelvin.

[0038] Thus, the current amount of NH3 stored in the SCR, as estimated by the SCR NH3 storage model, may be revised to adjust current and future estimated amounts of NH3 stored in the SCR. Method 500 ends after revising the amount of NH3 stored in the SCR according to the SCR NH3 model.

[0039] At 508, method 500 starts the high-level (HL) NH3 storage observer. The high-level (HL) NH3 observer provides the basis for adjusting the stored NH3 estimate when the SCR efficiency is less than a predetermined level and when NH3 slip is present. In one example, the amount of NH3 stored in the SCR is estimated according to a model expressed according to the following equation: dm^NH3stordt=f(mNH3stor)+λ(TSC−m^NH3stor), where f(mNH3stor) one with a basic NH3 storage model (for example 202 of Fig. 2) calculated NH3 mass, where m˙NH3stor is a mass of NH3 stored in the SCR as determined by the HS observer, where λ is the observer gain, and where TSC is the threshold storage capacity of the SCR. In some examples, the observer gain may be an integrated amount of NH3 slip magnitude bounded between a first and a second gain threshold. In another example, the observer gain may be proportional to the NH3 slip magnitude. The TSC amount may be determined empirically and may be expressed as a function of vehicle distance traveled or operating time. Method 500 proceeds to 514 after updating the HS observer NH3 storage estimate.

[0040] Now on Fig. 6, a block diagram of a controller for supplying NH3 to an SCR is shown. At 602, the mass of NH3 injected into the SCR is determined. In one example, the mass of NH3 is based on a duration of time an injector is activated and a transfer function describing current through the injector at a given temperature and pressure. In another example, the amount of NH3 may be obtained directly from a method that determines the amount of NH3 to inject into the SCR based on engine and SCR operating conditions. For example, the injected NH3 mass may be updated after a new mass of NH3 to be injected is determined at 620.

[0041] At 606, the observer can determine an amount of NH3 stored in an SCR. In one example, the observer is in the Fig. 5. Thus, the observer may contain multiple observers who revise the estimate of NH3 stored in an SCR based on different operating conditions of the SCR.

[0042] At 604, a model estimates the amount of NH3 stored in an SCR. The amount of NH3 injected into the SCR, the observer's NH3 estimate, and the SCR temperature are the basis for the model that estimates the amount of NH3 stored in the SCR. In one example, the model estimating the amount of NH3 stored in the SCR is as at 514 of Fig. 5. The model outputs an estimated NH3 mass stored in the SCR. A target NH3 mass stored in the SCR of 610 is subtracted from the estimated NH3 amount via the SCR NH3 model at 616. The result is an error in the amount of NH3 stored in the SCR. The target NH3 amount stored in the SCR can be determined empirically. In one example, the target NH3 amount stored in the SCR is stored in a table that may be indexed according to the SCR temperature and engine operating conditions (for example, engine speed and load).

[0043] At 612, the NOx efficiency of the SCR is determined. In one example, the NOx efficiency of the SCR is determined according to the Fig. 5. The NOx efficiency of the SCR can be based on NOx sensors located upstream and downstream of a Fig. 1. Alternatively, the SCR NOx efficiency may be based on a modeled NOx feed gas and output from a NOx sensor positioned downstream of the SCR. A target NOx efficiency from 614 is subtracted from the SCR NOx efficiency from 614 at 618. The result is an error in the SCR NOx conversion efficiency. The target SCR NOx conversion efficiency may be determined empirically. In one example, the target SCR NOx conversion efficiency is stored in a table that may be indexed according to an SCR temperature and engine operating conditions (e.g., engine speed and load).

[0044] At 620, the deficiencies from 616 and 618 are used to index a table containing an NH3 mass for injection based on the NH3 storage error and the SCR NOx efficiency error. The table specifies the NH3 mass for injection into the SCR. The NH3 mass is transferred to an injector at a given pressure of the liquid NH3 (urea) supplied to the injector.

[0045] At 622, an injector is actuated by applying a voltage or current control signal to the injector. The signal causes the injector to open and release NH3 to the exhaust system at a location upstream of an SCR. In one example, the injector is configured as shown in Fig. 1 described.

[0046] At 624, the NH3 works in combination with the SCR to reduce NO xto N2 and H2O. The input to and output from the SCR can be monitored via NOx sensors, as in Fig. 1. In another example, NH3 sensors may be positioned downstream of the SCR. The NOx sensors or the NH3 sensor provide feedback to 612 and 606.

[0047] In this way, output from observer 616 may correct an estimated amount of NH3 stored in an SCR, allowing an amount of NH3 injected into the SCR to be corrected. Furthermore, the observer may be configured to operate at selected times when the likelihood of producing a more accurate estimate of NH3 storage in the SCR increases.

[0048] Thus, the procedure of Fig. 5 and Fig. 6 provides a method for correcting an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip; and adjusting an amount of NH3 delivered to the SCR in response to the corrected amount of NH3 stored in the SCR. The method includes determining the indication of NH3 slip via outputs from two NOx sensors or via a NOx model and a NOx sensor. The method includes determining the indication of NH3 slip via a negative SCR efficiency based on the outputs of the two NOx sensors or based on the NOx model and the NOx sensor. The method includes determining the indication of NH3 slip via a correlation between the output of the first NOx sensor and the output of the second NOx sensor. The method includes basing the indication of NH3 slip on an output of an NH3 sensor.In another example, the method includes correcting the amount of stored NH3 to a threshold SCR storage capacity and basing a correction amount of the estimated amount of stored NH3 on an integrated NH3 slip amount. The method includes correcting the amount of stored NH3 when the NH3 indication is present for more than a predetermined period of time. The method includes adjusting the estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip from at least one NOx sensor up to a threshold NH3 storage capacity of the SCR based on an observer.

[0049] In addition, the procedures under the Fig. 5 and Fig. 6 provides a correction of an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR based on an expected amount of NH3 storage, the expected amount of storage being related to an efficiency and the temperature of the SCR, wherein the estimated amount of NH3 stored in the SCR is adjusted in response to a lack of an indication of NH3 slip past the SCR and an indication of NO x-slip past the SCR of above a threshold, absence of an indication of NH3 slip, and presence of NOx slip provided via at least one NOx sensor; and adjusting an amount of NH3 supplied to the SCR in response to the corrected estimated amount of NH3 stored in the SCR. Thus, the approach may adjust the estimate of NH3 stored in the SCR regardless of whether NH3 slip past the SCR is present or not. The method includes where the at least one NOx sensor comprises a NOx sensor positioned in an exhaust system at a location downstream of the SCR, and where the correction of the estimated amount of NH3 stored in the SCR is based on an observer, and further includes correcting the estimated amount of NH3 stored in the SCR to a threshold storage capacity in response to an indication of NH3 slip past the SCR.The method further comprises estimating or sensing engine feed gas NOx and further comprises adjusting the amount of NH3 supplied to the SCR in response to NOx slip past the SCR. The method further comprises where the SCR efficiency is based on a difference in the engine feed gas NOx and an output from the NOx sensor positioned in the exhaust system at a location downstream of the SCR. The method further comprises where the absence of an indication of NH3 slip is based on a difference between the feed gas NOx and an output from the NOx sensor positioned in the exhaust system at a location downstream of the SCR that exceeds a threshold. The method further comprises where the correction to the estimated amount of NH3 stored in the SCR is performed in response to the absence of an indication of NH3 slip from at least one NOx sensor under selected SCR operating conditions.The method includes adjusting the estimated amount of NH3 stored in the SCR to a reduced estimated NH3 amount in response to the absence of an indication of NH3 slip from at least one NOx sensor. The method includes adjusting the estimated amount of NH3 stored in the SCR to a reduced estimated NH3 amount in response to the absence of an indication of NH3 slip from at least one NOx sensor in the absence of an NH3-specific sensor.

[0050] The procedures according to the Fig. 5 and Fig. 6 further provide correcting an estimate of NH3 stored in an SCR, comprising: estimating an amount of NH3 stored in the SCR; correcting an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slippage, the NH3 indication provided by one or more NOx sensors; and adjusting an amount of NH3 injected into the SCR in response to the corrected amount of NH3 stored in the SCR. The method includes decreasing the amount of NH3 injected into the SCR in response to the estimated amount of NH3 decreasing, and the correction of the estimated amount of NH3 being based on an observer. The engine system includes the observer comprising a gain based on an amount of NH3 slippage. The engine system includes an output of the observer being based on an expected amount of NH3 storage based on an efficiency of the SCR.

[0051] As is obvious to an expert, the Fig. 5-6 illustrate one or more of a number of processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. Thus, various illustrated steps or functions may be performed in the order illustrated, in parallel, or in some cases omitted. Likewise, the processing order is not required to achieve the objects, features, and advantages described herein, but is provided for convenience of illustration and description. Although not explicitly illustrated, it will be apparent to one of ordinary skill in the art that one or more of the illustrated steps, methods, or functions may be repeatedly performed depending on the particular strategy employed.

[0052] This concludes the description. A reading thereof by one of ordinary skill in the art would reveal many changes and modifications without departing from the spirit and scope of the description. For example, single-cylinder, I2, I3, I4, I5, V6, V8, V10, V12, and V16 engines powered by natural gas, gasoline, diesel, or alternative fuel configurations could advantageously utilize the present description. Key to symbols

[0053] Fig. 1 10 Internal combustion engine 12 Engine control 30 combustion chamber 32 cylinder walls 36 pistons 40 Crankshaft 42 Air intake 44 intake manifold 46 Inlet amplifier chamber 48 exhaust manifold 51 intake cams 52 Inlet valve 53 exhaust cams 54 Exhaust valve 55 Intake cam sensor 57 Exhaust cam sensor 58 Throttle position sensor 62 throttle valve 64 Throttle plate 66 Fuel injection valve 68 drivers 70, 72 Exhaust gas (cleaning) devices 75 NH3 injection nozzle 102 Microprocessor unit 104 I / O ports 106 ROM 108 RAM 110 KAM 112 Temperature sensor 114 Cooling sleeve 118 Hall / motor position sensor 120 Air mass sensor 121, 122 pressure sensor 126 NOx sensor 128 Second NOx sensor 130 Accelerator pedal 132 feet 134 Position sensor 161 Wave 162 compressors 164 turbines TDC top dead center bd bottom dead center Fig. 2 10 Engine 72 SCR 75 NH3 injection nozzle 126, 128 NOx sensors 202 NH3 storage model 204 observers 206 NH3 control system 210 Outlet pipe Fig. 3A and Fig. 3B 302 Signal from an upstream NOx sensor 304 Signal from a downstream NOx sensor 306 Time at which the output of the downstream NOx sensor is greater than the output of the upstream NOx sensor 310 Reference storage value of the threshold storage capacity (TSC) 312 Amount of NH3 stored in an SCR calculated according to a model Fig. 4 402 Amount of NH3 stored in an SCR estimated according to a storage model 404 Reference storage for low efficiency 408 Low efficiency flag Fig. 5 START 502 DETERMINING SCR OPERATING CONDITIONS 504 SCR EFFICIENCY LESS THAN PREDETERMINED LEVEL? NO NO YES YES 506 NH3 SLIP DETECTED? 508 START HL-NH3 STORAGE OBSERVER 512 START LL-NH3 STORAGE OBSERVER 514 SET ESTIMATED NH3 STORED IN SCR AS DETERMINED BY NH3 STORAGE MODEL TO REFLECT NH3 STORAGE LEVEL EXIT - END Fig. 6 602 NH3 INJECTED IN SCR 604 MODELED NH3 STORAGE VOLUME 606 Observer-estimated NH3 storage 610 TARGET NH3 STORAGE QUANTITY 612 NOx EFFICIENCY 614 TARGET NOx EFFICIENCY 620 NH3 INJECTION QUANTITY 622 ACTIVATE NH3 INJECTOR 624 NOx REDUCTION

Claims

[1] A method for correcting an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR in response to an indication of NH3 slip; and adjusting an amount of NH3 delivered to the SCR in response to the corrected amount of NH3 stored in the SCR, wherein the indication of NH3 slip is determined via a metric based on convolution. [2] The method of claim 1, wherein the convolution is determined via outputs of two NOx sensors or via a NOx model and a NOx sensor. [3] The method of claim 2, wherein the indication of NH3 slip via a negative SCR efficiency is determined based on the outputs of the two NOx sensors or based on the NOx model and the NOx sensor. [4] The method of claim 2, wherein the two NOx sensors comprise an upstream NOx sensor (126) upstream of an exhaust device (70, 72) and a downstream NOx sensor (128) downstream of the exhaust device (70, 72). [5] The method of claim 1, wherein the indication of NH3 slip is based on an output of an NH3 sensor. [6] The method of claim 1, wherein the estimated amount of NH3 stored in the SCR is corrected to a threshold SCR storage capacity, and wherein a correction amount of the estimated amount of NH3 stored is based on an integrated NH3 slip amount. [7] The method of claim 1, wherein the estimated amount of NH3 stored in the SCR is corrected if the indication of NH3 slip is present for more than a predetermined period of time. [8] The method of claim 1, wherein the estimated amount of NH3 stored in the SCR is adjusted in response to an indication of NH3 slip from at least one NOx sensor up to a threshold NH3 storage capacity of the SCR based on an observer (204). [9] A method for correcting an estimate of NH3 stored in an SCR, comprising: correcting an estimated amount of NH3 stored in the SCR based on an expected amount of NH3 storage, the expected amount of NH3 storage being related to an efficiency and temperature of the SCR, wherein the estimated amount of NH3 stored in the SCR is corrected in response to a lack of an indication of NH3 slip past the SCR and an indication of NOx slip past the SCR above a threshold, a lack of an indication of NH3 slip, and the presence of NOx slip provided via at least one NOx sensor; and adjusting an amount of NH3 supplied to the SCR in response to the corrected estimated amount of NH3 stored in the SCR. [10] The method of claim 9, wherein the at least one NOx sensor comprises a downstream NOx sensor (128) positioned in an exhaust system at a location downstream of the SCR, and wherein the correction of the estimated amount of NH3 stored in the SCR is based on an observer (204), and further comprising: correcting the estimated amount of NH3 stored in the SCR to a threshold storage capacity in response to the indication of NH3 slip past the SCR.

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