Exhaust system having mid-reducer NOx sensor

Abstract

An exhaust system for a combustion engine is disclosed. The exhaust system may have a housing connected to receive a flow of exhaust. The exhaust system may also have a first catalyst disposed within the housing to reduce the concentration of an exhaust constituent, and a second catalyst disposed within the housing downstream of the first catalyst to further reduce the concentration of the exhaust constituent. The exhaust system may further have a sensor disposed between the first and second catalysts to generate a signal indicative of the concentration of the exhaust constituent at the location between the first and second catalysts. The exhaust system may additionally have a controller in communication with the sensor to receive the signal. The controller may determine, based on the received signal, the concentration of the exhaust constituent at a location downstream of the second catalyst.

Description

TECHNICAL FIELD

The present disclosure is directed to an exhaust system and, more particularly, to an exhaust system having a NOx sensor disposed between two separate NOx reducers.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants may be composed of gaseous compounds such as, for example, the oxides of nitrogen (NOx). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of gaseous compounds emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to demonstrate compliance with the regulation of these compounds, manufacturers have incorporated the use of sensors located at the tail pipe opening of the engine.

Although sensors for measuring the gaseous emissions of engines are currently available in today's market, the sensors may be insufficiently sensitive to measure the low levels of NOx remaining in the engine's exhaust after treatment of the exhaust in compliance with the emission regulations. That is, although current NOx sensors may be capable of detecting low concentrations of NOx, the emission regulation for NOx may be even lower. Therefore, currently available NOx sensors may be of no use in detecting the concentration of NOx emitted to the environment by today's engines.

One method that may be implemented by engine manufacturers to dynamically determine the amount of NOx emitted to the environment includes measuring the concentration of NOx prior to the regulation required reductions. This measured value may then be used to predict the concentration of NOx in the subsequently treated exhaust. One such method is described in U.S. Pat. No. 6,701,707 (the '707 patent) issued to UPADHYAY et al. on Mar. 9, 2004. Specifically, the '707 patent discloses an exhaust emission control system that utilizes an upstream oxidation catalyst and a downstream SCR catalyst to reduce NOx in a lean exhaust gas environment. The exhaust emission control system includes a first NOx sensor located upstream of the oxidation catalyst, and a second NOx sensor located downstream of the SCR catalyst. During operation of an associated engine, NOx at the tailpipe opening may be measured directly by the second sensor. In order to detect degradation of the system, the output and sensitivity of the first NOx sensor is used to estimate the NOx concentration downstream of the SCR catalyst. If the estimated NOx concentration is greater than the measured NOx concentration, a routine indicates degradation of the system.

Although the exhaust emission control system of the '707 patent may suitably estimate a NOx concentration downstream of the SCR catalyst based on an upstream measurement, the accuracy of the system may be limited. Specifically, without a dynamic indication of the SCR catalyst's effectiveness, there may be no way to determine how much of the upstream NOx is being removed. It may be possible for the SCR catalyst to remove less than or more than an assumed removal amount, thereby negatively affecting the accuracy of the routine.

The exhaust system of the present disclosure solves one or more of the problems set forth above.

SUMMARY OF THE INVENTION

One aspect of the present disclosure is directed to an exhaust system. The exhaust system may include a housing connected to receive a flow of exhaust. The exhaust system may also include a first catalyst disposed within the housing to reduce the concentration of an exhaust constituent, and a second catalyst disposed within the housing downstream of the first catalyst to further reduce the concentration of the exhaust constituent. The exhaust system may further include a sensor disposed between the first and second catalysts to generate a signal indicative of the concentration of the exhaust constituent at the location between the first and second catalysts. The exhaust system may additionally include a controller in communication with the sensor to receive the signal. The controller may determine, based on the received signal, the concentration of the exhaust constituent at a location downstream of the second catalyst.

Another aspect of the present disclosure is directed to a method of determining the concentration of an exhaust constituent. The method may include receiving a flow of exhaust, and reducing the concentration of the exhaust constituent within the flow of exhaust. The method may also include measuring the reduced concentration of the exhaust constituent, and further reducing the concentration of the exhaust constituent within the flow of exhaust. The method may further include determining the further reduced concentration of the exhaust constituent based on the measured reduced concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and diagrammatic illustration of an exemplary disclosed power unit and exhaust system.

DETAILED DESCRIPTION

FIG. 1 illustrates a power system including a power unit 10 and an exhaust system 12. In one embodiment, the power system may be associated with a mobile vehicle such as a passenger vehicle, a vocational vehicle, a farming vehicle or a construction vehicle. Alternatively, the power system may be associated with a stationary machine such as an industrial power generator or a furnace.

For the purposes of this disclosure, power unit 10 is depicted and described as a four-stroke diesel engine. One skilled in the art will recognize, however, that power unit 10 may be any other type of internal combustion engine such as, for example, a gasoline engine, a gaseous fuel-powered engine, a 2-stroke engine or a turbine engine. Power unit 10 may include an engine block 14 that at least partially defines a plurality of combustion chambers (not shown). In the illustrated embodiment, power unit 10 includes four combustion chambers. However, it is contemplated that power unit 10 may include a greater or lesser number of combustion chambers and that the combustion chambers may be disposed in an “in-line” configuration, a “V” configuration, or any other suitable configuration.

As also shown in FIG. 1, power unit 10 may include a crankshaft 16 that is rotatably disposed in engine block 14. A connecting rod (not shown) may connect a plurality of pistons (not shown) to crankshaft 16 so that a sliding motion of each piston within each respective combustion chamber results in a rotation of crankshaft 16. Similarly, a rotation of crankshaft 16 may result in a sliding motion of the pistons. Rotation of crankshaft 16 may function as output from power unit 10 for effecting a desired work such as rotation of a generator or rotation of one or more drive axles of an associated vehicle.

Exhaust system 12 may include an exhaust manifold 18 having exhaust passageways, each passageway being in fluid communication with an associated combustion chamber of power unit 10. Exhaust manifold 18 may expel exhaust flow away from power unit 10 towards a housing 20 located downstream from exhaust manifold 18.

Housing 20 of exhaust system 12 may be a cylindrical or tubular conduit for directing exhaust gasses and particulates away from power unit 10 for processing by various emission controlling devices. In one embodiment, exhaust manifold 18 may be integral with housing 20 of the exhaust system. For example, an exhaust manifold and housing may be integrally stamped or forged from metal. Alternatively, exhaust manifold 18 may be secured to housing 20 by one or more fasteners (e.g. rivets, nuts and bolts, etc.) or by deformation (e.g. hemming). Housing 20 may also constitute structural support for the various emission controlling devices of the system.

The emission controlling devices of exhaust system 12 may include a first catalyst 22 and a second catalyst 24 located downstream from first catalyst 22 for reducing the concentration of a constituent within the exhaust from power unit 10. Second catalyst 24 may be incorporated so as to further reduce the concentration of the exhaust constituent beyond that achieved by first catalyst 22. Both catalysts may be disposed across the cylindrical width (i.e., cross section) of housing 20. Furthermore, catalysts 22 and 24 may be either removably or fixedly secured at their respective perimeters to housing 20. Moreover, it is contemplated that exhaust system 12 may include other components such as, for example, a turbine, an exhaust gas recirculation system, a particulate filter, or any other exhaust system component known in the art.

In one exemplary embodiment of the present disclosure, catalysts 22 and 24 may be SCR catalysts. Alternatively, catalysts 22 and 24 may be NOx reducers. Specifically, catalytic conversion of exhaust across catalysts 22 and 24 may involve the reduction of nitrogen oxides to nitrogen gas and water so as to reduce the concentration of oxides of nitrogen in the exhaust flow. Reductants contemplated for use in this conversion may include urea and ammonia. Accordingly, first catalyst 22 may be a first stage of NOx reduction capable of effecting a first reduction in NOx concentration. Second catalyst 24 may be a second stage of NOx reduction capable of effecting a second reduction in NOx concentration. In one embodiment, the concentration of NOx downstream from second catalyst 24 may be too low for detection by known NOx detection devices and methods. Thus, it may be possible that nitrogen oxides are only sufficiently concentrated for detection in the location downstream from first catalyst 22 and upstream from second catalyst 24 (i.e., between catalysts 22 and 24).

As illustrated in FIG. 1, a sensor 26 may be disposed between first catalyst 22 and second catalyst 24. In particular, sensor 26 may detect a concentration of an exhaust constituent which, in one example, is an oxide of nitrogen. A controller 28 may be disposed in communication with sensor 26. Various circuits may be associated with controller 28 such as, for example, power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other appropriate circuitry. Moreover, because sensor 26 may be in communication with controller 28 by either wired or wireless transmission, controller 28 may be disposed in a location remote from housing 20, if desired.

In the event that it may be desirable to accurately predict the concentration of an exhaust constituent downstream from catalyst 24 based on the conversion efficiency of catalyst 22, several embodiments of the present disclosure may provide a reliable model. For example, catalysts 22 and 24 may be selected so as to have substantially similar conversion efficiencies. In a still further embodiment, catalysts 22 and 24 may be identical components. The accuracy and precision involved in modeling second catalyst behavior based on known first catalyst behavior may be improved because the second catalyst can be expected to have substantially similar specifications and characteristics as the first catalyst. That is, they may be expected to remove equivalent proportions of an exhaust constituent from the gaseous flow. Identical catalytic components may provide further economic advantages in that similar parts may result in higher part volume, lower component cost and lower stocking expenses.

In a simplified example, if first catalyst 22 is known to remove 50% of NOx from the exhaust directly expelled from power unit 10, second catalyst 24, having similar properties as the first, may be expected to remove 50% of the NOx from the exhaust downstream from first catalyst 22. In this implementation, a net conversion of 75% would result (50% of the original exhaust constituents from the first stage; 25% of the original exhaust constituents from the second stage).

Assuming that catalyst 24 can be expected to remove the same proportion of exhaust constituent from the flow as can catalyst 22 (e.g., when catalysts 22 and 24 are identical components), it may be possible to predict the concentration of an exhaust constituent downstream from catalyst 24 based on information from sensor 26 combined with an estimated or calculated conversion efficiency of catalyst 22. Specifically, the concentration downstream from catalyst 24 may be approximated as the concentration resulting from the effect of catalyst 22 acting on the exhaust flow as measured at sensor 26. The concentration downstream from catalyst 24 may be determined according to Eq. 1 below.

$\mathrm{Eq}.1\text{:}$ $\mathrm{DownstreamConcentration}=\mathrm{SensorConcentration}-\Delta \mathrm{Concentration}$ $\mathrm{wherein}\text{:}$ $\mathrm{DownstreamConcentration}\mathrm{is}\mathrm{the}\mathrm{constituent}\mathrm{concentration}\mathrm{predicted}$ $\mathrm{to}\mathrm{be}\mathrm{downstream}\mathrm{from}\mathrm{catalyst}24;$ $\mathrm{SensorConcentration}\mathrm{is}\mathrm{the}\mathrm{constituent}\mathrm{concentration}\mathrm{measured}\mathrm{at}$ $\mathrm{sensor}26;\mathrm{and},\Delta \mathrm{Concentration}\mathrm{is}\mathrm{the}\mathrm{estimated}\mathrm{change}\mathrm{in}\mathrm{concentration}\mathrm{across}$ $\mathrm{catalyst}22\mathrm{and}\mathrm{therefore}\mathrm{the}\mathrm{expected}\mathrm{change}\mathrm{across}\mathrm{catalyst}24.$

Alternatively, even in situations where the conversion efficiency of the second catalyst 24 is different than the conversion efficiency of the first catalyst 22, the total system conversion may be predicted based on complex catalyst models incorporating parameters, such as, known temperature gradients, reaction properties and/or intermediate sensor measurements. Various embodiments for estimating or measuring the conversion efficiency of catalyst 22, and therefore of catalyst 24, are contemplated by the present disclosure; however, several exemplary embodiments are recited below for the purpose of aiding the reader in understanding the concepts herein.

According to one embodiment of the present disclosure, the conversion efficiency of catalyst 22 may be calculated by comparing the expected production of exhaust constituent from power unit 10 to the concentration measured by sensor 26. Estimated production of the exhaust constituent from power unit 10 may be accomplished by one or more controllers, including controller 28, as a function of variables relating to power unit 10 and/or a vehicle associated therewith.

For example, one or more engine performance maps relating a fueling amount, ignition timing, power output, engine speed, boost pressure, engine temperature, an air/fuel ratio, and/or other known parameters may be stored within the memory of controller 28. Each of these maps may be in the form of tables, graphs, and/or equations and include a compilation of data collected from lab and/or field operation of power unit 10. Controller 28 may reference one or more of these maps in order to estimate production of an exhaust constituent of power unit 10 for a given operating condition of power unit 10.

Therefore, the estimated production of exhaust constituent from power unit 10 in combination with the exhaust constituent concentration measured by sensor 26 may be used to indicate the conversion efficiency across first catalyst 22. To the extent that the conversion efficiency of catalyst 24 may be assumed to be substantially equivalent to the above calculated conversion efficiency of catalyst 22, the concentration of the exhaust constituent downstream from second catalyst 24 may be predicted as a function of the estimated constituent production of power unit 10.

In another exemplary embodiment of the present disclosure, the conversion efficiency of catalyst 22 may be calculated by comparing the exhaust constituent concentration measured upstream from catalyst 22 to the concentration measured downstream from catalyst 22. In this implementation, a second sensor 30 may be disposed at a location upstream from catalyst 22 for providing a dynamic exhaust constituent concentration measurement at the location upstream from catalyst 22.

According to this embodiment, second sensor 30 may generate a second signal indicative of the concentration of an exhaust constituent as expelled directly from power unit 10. Second sensor 30 may be in communication with controller 28. Controller 28 may determine the concentration of the exhaust constituent downstream of second catalyst 24 based on signals from both sensor 26 and sensor 30. For example, the concentrations of NOx both downstream and upstream from catalyst 22 may be determined as a function of both signals from sensors 26 and 30, respectively.

Therefore, the relative change in concentration across first catalyst 22, as measured by sensors 26 and 30, may be used to approximate the conversion efficiency of catalyst 22. To the extent that the conversion efficiency of catalyst 24 may be assumed to be substantially equivalent to the above calculated conversion efficiency of catalyst 22, the concentration of the exhaust constituent downstream from second catalyst 24 may be accurately predicted as a function of the concentration measured by sensor 30.

Accordingly, the estimated change in exhaust constituent concentration across second catalyst 24, as applied to the concentration measurement generated by sensor 26, may be used to reliably predict the exhaust constituent concentration downstream from second catalyst 24.

INDUSTRIAL APPLICABILITY

The disclosed exhaust system may be applicable to any combustion-type device such as, for example, an engine, a furnace, or any other device known in the art wherein it is desirable to measure the concentration of pollutant exhaust constituents. In fact, the disclosed exhaust system may be a simple, inexpensive and compact solution for accurately estimating the concentration of NOx within an engine's exhaust flow, even when the concentration of NOx is lower than that detectable by known NOx sensors. The operation of power unit 10 and exhaust system 12 will now be explained.

Referring to FIG. 1, fuel may be injected into the combustion chambers of power unit 10, mixed with the air therein, and combusted by power unit 10 to produce a mechanical work output and an exhaust flow of hot gases. The exhaust flow may contain a complex mixture of air pollutants composed of gaseous material such as oxides of nitrogen (NOx), as well as solid particulates. This NOx laden exhaust flow may be directed from the combustion chambers of power unit 10 to housing 20 by exhaust manifold 18. NOx may be removed from the exhaust flow by first catalyst 22 and second catalyst 24. The reduction in exhaust constituent concentration downstream from catalyst 22 may be measured by sensor 26 and communicated to controller 28. Further reduction in the concentration of the exhaust constituent may occur at second catalyst 24, downstream from first catalyst 22. Controller 28 may determine the further reduction in exhaust constituent concentration downstream from sensor 26 based on the previously measured reduction in concentration.

In the event that a statutorily regulated NOx exhaust concentration is lower than that which is detectable by existing NOx sensors, use of substantially identical NOx reducers along with predictions of estimated NOx production and NOx reduction may be leveraged to obviate the need for sufficiently sensitive NOx sensors. Alternatively, it may be desirable to incorporate this implementation of NOx measurement, even if the regulated level of NOx concentration is high enough for detection by either presently known or future NOx sensors. For example, this implementation may be advantageous in reducing the cost of monitoring NOx concentration, for increasing measurement accuracy, and for incorporation as an after-market NOx measuring system. Moreover, because the NOx sensor is shielded from heat, particulates and other foreign materials by the upstream catalyst, the reliability and/or lifetime of the NOx sensor is increased.

In one exemplary embodiment, the first stage of concentration reduction at first catalyst 22 may be measured to involve removal of about 50% of the constituent from the exhaust flow. If second catalyst 24 is identical to first catalyst 22, the second stage of concentration reduction may be approximated to involve removal of about 50% of the remaining constituent from the already reduced exhaust flow. According to such an exemplary embodiment, a concentration of the exhaust constituent may be reduced from, for example, 120 parts per million (ppm) to 60 ppm by first catalyst 22 (assuming 50% conversion efficiency). If second catalyst 24 is substantially identical to first catalyst 22, an additional 50% reduction in exhaust constituent concentration may be used to estimate an exhaust constituent concentration of 30 ppm downstream from second catalyst 24. Thus, the concentration of NOx downstream from second catalyst 24 may be estimated as a function of the measured NOx concentration between first catalyst 22 and second catalyst 24, as well as the assumed conversion efficiency of second catalyst 24 based on its similarity to first catalyst 22. The conversion efficiency of catalyst 22 may be calculated or predicted by one of several methods.

According to one embodiment of the present disclosure, the conversion efficiency across catalyst 22 may be calculated by comparing the estimated production of the constituent by power unit 10 to the constituent concentration measured between catalysts 22 and 24. In this implementation, the constituent production of power unit 10 may be estimated as a function of variables relating to power unit 10 and/or a vehicle associated therewith.

Alternatively, the conversion efficiency of catalyst 22 may be calculated by comparing the exhaust constituent concentration measured upstream from catalyst 22 to the concentration measured downstream from catalyst 22. In this implementation, a second sensor 30 may be disposed at a location upstream from catalyst 22 for providing a dynamic exhaust constituent concentration measurement at the location upstream from catalyst 22.

By equating the proportion of the constituent reduced by catalyst 22 to the proportion of the constituent reduced by catalyst 24, a reliable estimate of the effect of catalyst 24 may be provided. Accordingly, the net reduction in constituent concentration may be calculated as a function of both the constituent concentration measured between catalysts 22 and 24, as well as the amount of further constituent concentration reduction as approximated by dynamic calculation of catalyst 22 conversion efficiency.

Because the conversion efficiency and therefore the amount of concentration reduction across second catalyst 24 may be modeled based on that of first catalyst 22, the constituent concentration downstream from second catalyst 24 may be estimated as a function of a conversion efficiency prediction and a measurement of the constituent concentration before it reaches undetectable levels, that is, before the second stage reduction. Accordingly, the disclosed power system and exhaust system may obviate the need for developing highly sophisticated and expensive NOx or other exhaust constituent detectors. Moreover, because each stage of exhaust constituent reduction is performed by an identical catalyst component, the characteristics of the second catalyst and its associated effects may be accurately and reliably modeled from the characteristics and associated effects of the first catalyst. Finally, because the two catalysts are identical, there are lower associated component costs, storage costs and specification uncertainties.

Alternatively, even in situations where the conversion efficiency of the second catalyst 24 is different than the conversion efficiency of the first catalyst 22, the total system conversion may be predicted based on complex catalyst models incorporating parameters, such as, known temperature gradients, reaction properties and/or intermediate sensor measurements.

It will be apparent to those skilled in the art that various modifications and variations can be made to the exhaust system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the exhaust system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.

Claims

1. An exhaust system, comprising:

a housing connected to receive a flow of exhaust;

a first catalyst disposed within the housing to reduce the concentration of an exhaust constituent;

a second catalyst disposed within the housing downstream of the first catalyst to further reduce the concentration of the exhaust constituent;

a sensor disposed between the first and second catalysts to generate a signal indicative of the concentration of the exhaust constituent at the location between the first and second catalysts; and

a controller in communication with the sensor to receive the signal and determine, based on the received signal, the concentration of the exhaust constituent at a location downstream of the second catalyst.

2. The exhaust system of claim 1, wherein the constituent is an oxide of nitrogen.

3. The exhaust system of claim 2, wherein the first and second catalysts are SCR catalysts.

4. The exhaust system of claim 3, wherein the first and second catalysts have substantially the same reducing capacity.

5. The exhaust system of claim 1, wherein the concentration of the exhaust constituent at the location downstream of the second catalyst is determined based further on an estimated production of the constituent.

6. The exhaust system of claim 5, wherein the controller includes a map stored in a memory thereof, the map relating a power source parameter to the estimated production of the constituent.

7. The exhaust system of claim 5, wherein the estimated production of the constituent and the signal provide an indication of the effectiveness of the first catalyst.

8. The exhaust system of claim 7, wherein:

the effectiveness of the second catalyst is assumed to be about the same as the effectiveness of the first catalyst; and

the concentration of the exhaust constituent at the location downstream of the second catalyst is determined based further on the assumed effectiveness of the second catalyst.

9. The exhaust system of claim 1, further including a second sensor located upstream of the first catalyst to generate a second signal indicative of a constituent production, wherein the concentration of the exhaust constituent at the location downstream of the second catalyst is determined based further on the second signal.

10. The exhaust system of claim 1, wherein the concentration of the constituent downstream of the second catalyst is less than a sensitivity of the sensor.

11. A method of determining the concentration of an exhaust constituent, comprising:

receiving a flow of exhaust;

reducing the concentration of the exhaust constituent within the flow of exhaust;

measuring the reduced concentration of the exhaust constituent;

further reducing the concentration of the exhaust constituent within the flow of exhaust; and

determining the further reduced concentration of the exhaust constituent based on the measured reduced concentration.

12. The method of claim 11, wherein the constituent is an oxide of nitrogen.

13. The method of claim 11, wherein:

reducing the concentration includes removing about 50% of the constituent from the exhaust flow; and

further reducing the concentration includes removing about 50% of the remaining constituent from the already reduced exhaust flow.

14. The method of claim 11, further including estimating a production of the constituent, wherein determining the further reduced concentration is based further on the estimated production.

15. The method of claim 14, wherein the estimated production and the measured reduction provide an indication of the amount of constituent reduced in the reducing step.

16. The method of claim 15, further including equating the amount of constituent reduced in the further reducing step to the indicated amount of constituent reduced in the reducing step, wherein determining the further reduced concentration is based further on the amount of constituent reduced in the further reducing step.

17. The method of claim 11, further including measuring a production of the constituent, wherein determining the further reduced concentration is based further on the measured production.

18. A power system, comprising:

a combustion engine configured to combust a fuel/air mixture and produce a power output and a flow of exhaust;

a conduit connected to receive the flow of exhaust;

a first NOx catalyst disposed within the conduit to reduce the concentration of NOx in the flow of exhaust;

a second NOx catalyst disposed within the conduit downstream of the first NOx catalyst to further reduce the concentration of NOx in the exhaust flow;

a sensor disposed between the first and second NOx catalysts to generate a signal indicative of the concentration of NOx at a location between the first and second NOx catalysts; and

a controller in communication with the sensor to receive the signal and determine, based on the received signal and a production of NOx, the concentration of NOx at a location downstream of the second NOx catalyst, wherein the concentration of NOx at the location downstream of the second NOx catalyst is less than a sensitivity of the sensor.

19. The power system of claim 18, wherein:

the control is configured to determine an effectiveness of the first NOx reducing catalyst based on the NOx production and the signal;

the effectiveness of the second NOx reducing catalyst is assumed to be about the same as the effectiveness of the first NOx reducing catalyst; and

the concentration of the exhaust constituent at the location downstream of the second NOx reducing catalyst is determined based further on the assumed effectiveness of the second NOx reducing catalyst.

20. The power system of claim 18, further including a second sensor located between the combustion engine and the first NOx reducing catalyst to generate a second signal indicative of the NOx production amount, wherein the concentration of the exhaust constituent at the location downstream of the second NOx reducing catalyst is determined based further on the second signal.