Simplified method for measuring concentrations of exhaust gas components utilizing differential measurement across an absorber

Abstract

A simplified method for measuring a first property and a second property of an exhaust gas mixture utilizing two sensors manufactured for the purpose of measuring a first property, being cross-sensitive to the second property with an absorber of the second property being placed between two of the sensors in the exhausting circuit of the exhaust gas mixture. Direct differential measurement between the two sensors quantify the concentrations of the first and second property.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

(Not Applicable)

BACKROUND OF THE INVENTION

Previous ceramic NO_x sensors exhibit cross-sensitivities to NH₃. This cross-sensitivity reduces the accuracy of the reported NO_x concentration from a sensor if NH₃ is also present in the exhaust gas mixture. The disclosed invention covers a simplified method for measuring concentrations NO_x and NH₃ in an exhaust gas mixture. Previous inventions have required the use of more than one type of sensor (i.e. NO_x and NH₃ sensors), or other catalytic components. One example of recent prior art (U.S. Pat. No. 7,810,313) uses at least two sensors in a system, but still requires complex algorithms and a decoupling observer module in order to quantify the relative concentrations of NO_x and NH₃ in an exhaust gas mixture. The complexity of the above methods is unnecessary and can be reduced significantly in the non-obvious method of the disclosed invention.

BRIEF SUMMARY OF THE INVENTION

The disclosed invention covers a simplified method for measuring concentrations NO_x and NH₃ in an exhaust gas mixture using NO_x sensors placed before and after an NH₃ absorber.

BRIEF DESCRIPTION OF THE DRAWING

The enclosed drawing is a system level diagram of the preferred embodiment of the disclosed invention. Flow of exhaust gas (indicated with bold arrows) in the system as well as the points used for direct differential measurements in an electrical schematic are shown.

DETAILED DESCRIPTION OF THE INVENTION

Two NO_x sensors having cross-sensitivities to NH₃ are used to determine both NO_x and NH₃ concentrations simultaneously using the disclosed method: NO_x sensors having cross-sensitivities are placed before and after an NH₃ absorber in an exhaust gas system. A difference in readings from a first NO_x sensor (NO_x1) with cross-sensitivity to NH₃ and a second NO_x sensor (NO_x2) with a cross-sensitivity to NH₃, is determined (NO_x1-NO_x2). The resulting value is used to determine the NO_x and NH₃ concentrations in the exhaust gas mixture. For example: Sensor NO_x1 has a known, non-zero, cross-sensitivity to NH₃ of c₁ and sensor NO_x2 has a known non-zero cross-sensitivity to NH₃ of c₂. In this case the possible NH3 cross-sensitivity values range from greater than zero to 1 (100%). A value of 1 would mean that “n” ppm of NH₃ would be reported as “n” ppm of NO_x. A value of 0.5 would mean “n” ppm of NH₃ would be reported as “0.5×n” ppm of NO_x. Possible NH₃ absorber effectiveness values (a_e) are between 0 (100% of NH₃ goes though) and 1 (100% of NH₃ is absorbed). For the case of a_e=0, c₁ cannot be equal to c₂.

Turning now to the enclosed drawing, a system with the following properties is used as an example:

• Exhaust Gas: (50 ppm NO_x & 20 ppm NH₃) with values c₁=0.32, c₂=0.71, a_e=0.87
• Direct differential measurement between Pt. 1 and Pt. 2 reads NO_x=50 ppm
• Direct differential measurement between Pt. 3 and Pt. 4 reads NH₃=20 ppm
• Two NO_x sensors (NO_x1, NO_x2) output current (I_a, I_b) that is translated to voltages (V_a, V_b) that are used as inputs into the system above.
• R₁, R₂, R₃, R₄ chosen so that: R₁=R₂=R₃=R₄
• R_f & R_i chosen so that:

$\left[\frac{1}{\left(c_1-c_2\left(1-a_e\right)\right)}=1+\frac{R_f}{R_i}\right]$

• R_a & R_b chosen so that:

$\frac{R_b}{R_a+R_b}=c_1$

Where sensor NO_x1 having a c₁ value of 0.32 and sensor NO_x2 having a c₂ value of 0.71 and an NH₃ absorber having a_e value of 0.87, then NH₃ is found:

NH₃=(NO_x1−NO_x2)/(c₁−c₂ (1−a_e))

NH₃=(V_A−V_B)/(c1−c2 (1−a_e))

NH₃=(56.4−51.846)/(0.32−0.71 (1−0.87))

NH₃=4.554/0.2277 or 20 ppm

To get NO_x:

NO_x=NO_x1−c₁(NH₃)

NO_x=56.4 ppm−0.32(20)

NO_x=50 ppm

Claims

1. A method for simultaneously measuring two properties associated with an exhaust gas mixture, said method comprising: combining at least two sensors wherein each of said sensors exhibits cross-sensitivities to a first property and a second property in said exhaust gas mixture; an absorption device selective of said second property; placement of a first of said sensors upstream from said absorption device; placement of a second of said sensors downstream of said absorption device; directly reading, between said first sensor and said second sensor, a differential value indicative of the concentration of said first property and said second property.

2. The method of claim 1, wherein said plurality of sensors comprises at least two NOx sensors cross-sensitive to NH3, for detecting said first property, wherein said first property comprises a concentration of NOx, and said second property comprises a concentration of NH3 in said exhaust gas mixture.

3. The method of claim 1 wherein said first property comprises NOx and said second property comprises NH3.

4. A method for simultaneously measuring NOx and NH3 in an exhaust gas mixture, said method comprising: the placement of at least a first NOx sensor upstream of a NH3 absorber; the placement of at least a second NOx sensor downstream of said NH3 absorber, wherein each of said sensors exhibits a cross-sensitivity to said NH3; directly reading, between two of said sensors, a differential value proportional to the amount of said NOx and said NH3 in said exhaust gas mixture.

5. A method for simultaneously measuring NOx and NH3 in an exhaust gas mixture, said method comprising: the placement of at least one sensor upstream of an NH3 absorber; the placement of at least another NOx sensor downstream of said NH3 absorber, wherein each of said sensors exhibits cross-sensitivities to said NOx and said NH3, directly reading, between two of said sensors, a differential value proportional to the quantities of said NOx and said NH3 in said exhaust gas mixture.

6. A method for simultaneously measuring NOx and NH3 in an exhaust gas mixture, said method comprising: the placement of at least one sensor upstream of an NH3 absorber; the placement of at least another NOx sensor downstream of said NH3 absorber, directly reading, between two of said sensors, a differential value proportional to the quantities of said NOx and said NH3 in said exhaust gas mixture.

7. A method for simultaneously measuring NOx and NH3 in an exhaust gas mixture in an exhaust gas system, said method comprising: a first exhausting channel and a second exhausting channel separate from said first exhausting channel in said exhaust gas system; placement of at least one sensor in said first exhausting channel, the placement of a NH3 absorber in said second exhausting channel; the placement of at least a second sensor downstream of said NH3 absorber in said second exhausting channel, wherein each of said sensors exhibits a known cross-sensitivity to said NOx and said NH3; directly reading, between said first sensor and said second sensor, a differential value proportional to the quantities of said NOx and said NH3 in said exhaust gas mixture.

8. The method of claim 2 further comprising: configuring at least one of said NOx sensors to comprise a zirconia-based multilayer sensing element.

9. The method of claim 1 wherein at least one of said sensors among said plurality of sensors comprises an electrically-based sensor.