Dew point activation method for exhuast gas sensors

An emission control system, such as an emission control system for a diesel engine, which includes both a NOx sensor and an electrostatic Particulate Matter (ePM) sensor, and uses the signal from the ePM sensor to determine when it is safe to activate and heat up the NOx sensor after engine ignition. This is performed as soon as moisture clears the exhaust, without having to wait any additional time as a safety factor to maximize the reliability of the NOx sensor against damage from water thermal shock. It also allows for a higher degree of application flexibility for a specific engine and aftertreatment combination to be used in a variety of vehicle applications, environmental conditions, and vehicle operating profiles.

FIELD OF THE INVENTION

The invention relates generally to an emission control system, such as an emission control system for a diesel engine, which includes both a NOx sensor and an electrostatic Particulate Matter (ePM) sensor, and uses the signal from the ePM sensor to determine when to activate the NOx sensor after engine ignition to reduce or eliminate the risk of thermal shock.

BACKGROUND OF THE INVENTION

One of the most common field failure modes for nitric oxide (NOx) sensors is cracking of the ceramic sensor element due to thermal shock caused by impingement of water droplets on the element. This failure mode is catastrophic, causes the emission control system to fail, and sets an on-board diagnostic code that requires the vehicle to be serviced. This often occurs early in the vehicle life, and thus incurs significant warranty cost and customer dissatisfaction. It is also a possible failure mode for Oxygen, Hydrocarbon, or Particulate Matter sensors which use a ceramic sensor element, and are used in the exhaust system.

In the case where a NOx sensor is used in an exhaust system, the greatest risk of failure of the sensor typically occurs during cold start when clouds of water vapor in the exhaust gas condense and travel through the exhaust pipe and impact the sensors and other exhaust system components. Because the NOx sensor is heated to a high temperature (approximately 800° C.) to optimize its operation, any water impact on the hot ceramic sensor element may present a risk of thermal shock, which may damage and cause the sensor element to fail. The risk may be amplified when the vehicle is operated in cold ambient temperatures. Because a large fraction of the regulated and harmful exhaust gas emissions occur during cold start, it is considered desirable to activate the NOx sensor as soon as possible to facilitate control of the emissions, but it is also considered desirable to prolong activating the sensor a significant amount of time until water has cleared the exhaust system to reduce risk of failure.

This situation is typically addressed during the development of the vehicle engine control and exhaust aftertreatment control system. Application studies are performed on a vehicle or engine which attempt to replicate the environment and application that the sensor is used in over most operating conditions. Measurements are taken on the engine or vehicle to assess how various sensors on the end-use application vehicle may be used to predict when any water in an exhaust gas system has been eliminated. This may include temperature measurements, along with visual observation via remote camera to determine if water is present in the system. A model is then developed for an engine and aftertreatment control algorithm that uses on-board sensors and other information to predict when water in the exhaust system is most likely to be eliminated, and thus minimize risk to the NOx sensor. However, it is not possible to predict and replicate 100% of all possible conditions in testing that the sensor may be exposed to in various driving conditions; therefore there is always some degree of risk of failure of the sensor. This type of risk may be increased in the situation where the sensor is sold to an engine or exhaust system supplier, who then provides their system to an OEM, or end user, such as a vehicle manufacturer. The supplier does not necessarily always know, or have control over, the applications where their exhaust system, which includes the NOx sensor, may be used.

Thus, there is value in providing a system, method, or component which may directly detect the presence of water in the exhaust system so that the NOx sensor is not exposed to thermal shock after activation, and placed at risk of failure. However, current systems which have these features also must provide some margin of error, and typically may wait longer than necessary before activating the NOx sensor. After the sensor is sold to a vehicle manufacturer, it may not be possible to determine what all of the end-user applications will be, or all of the environments in which the NOx sensor may be used.

Accordingly, there exists a need for a more accurate water detection method, such that the NOx sensor may be used as part of any type of exhaust system, and activated as soon as it is safe to do so, the emissions control system may engage, and cold start exhaust emissions are reduced as much as possible without risking failure of the sensor.

SUMMARY OF THE INVENTION

This proposed method of the present invention optimizes the design trade-off between emissions control performance and component reliability risk.

In one embodiment of the invention, an emission control system, such as an emission control system for a diesel engine, which includes both a nitric oxide (NOx) sensor and an electrostatic Particulate Matter (ePM) sensor, and uses the signal from the ePM sensor to determine when it is safe to activate and heat up the NOx sensor after engine ignition. In one embodiment, this is performed as soon as moisture clears the exhaust, without having to wait any additional time as a safety factor to maximize the reliability of the NOx sensor against damage as a result of water thermal shock. It also allows for a higher degree of application flexibility for a specific engine and aftertreatment combination to be used in a variety of vehicle applications, environmental conditions, and vehicle operating profiles.

In one embodiment, the present invention is a combined system with both ePM and NOx sensors, linking the signal from the ePM to a control module which signals the activation of the NOx sensor after moisture has cleared the exhaust.

In an embodiment, the present invention is a method for determining when to activate a sensor. More specifically, an exhaust system for a vehicle includes a first sensor which is part of the exhaust system, and a second sensor, which is also part of the exhaust system. A measured current generated by the first sensor is used to detect the presence of a substance in the exhaust system. The presence of the substance is detected in the exhaust system if the current generated by the first sensor is above a predetermined value, which is typically when the current generated by first sensor is zero, or within some measurable threshold, typically approaching a zero threshold. To prevent the second sensor from being exposed to thermal shock, the second sensor is activated once the current measured by the first sensor is at or below the predetermined value.

In one embodiment, the first sensor is an ePM sensor, which generates current above the predetermined value when the substance is in the exhaust system. The second sensor is a NOx sensor, which is not activated until the current measured by the first sensor is at or below the predetermined value, so as to prevent the NOx sensor from undergoing thermal shock.

The substance is moisture, typically in the form of water, and the NOx sensor is activated once the current measured by the first sensor is at or below the predetermined value, indicating that the water has evaporated from the exhaust system.

In one embodiment, an exhaust pipe is part of the exhaust system, such that at least a portion of the first sensor extends into the exhaust pipe, and at least a portion of the second sensor extends into the exhaust pipe. The first sensor and the second sensor may be mounted to a tail pipe, but it is within the scope of the invention that the first sensor and the second sensor may be mounted to other portions of the exhaust pipe as well.

In another embodiment, an exhaust gas catalyst is part of the exhaust system, and the first sensor and the second sensor are mounted in proximity to the exhaust gas catalyst.

In yet another embodiment, an exhaust gas filter is part of the exhaust system, and the first sensor and the second sensor are mounted in proximity to the exhaust gas filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of an exhaust system incorporating NOx sensor activation according to the present invention is shown inFIG. 1, generally at10. The system10includes an engine, shown generally at10A, which generates exhaust gas, and connected to the engine10A is a turbocharger unit, shown generally at10B. A front exhaust pipe12A is connected to, and in fluid communication with, the turbocharger unit10B, and a diesel oxidation catalyst (DOC)14is connected to, and in fluid communication with, the front exhaust pipe12A. Connected to and in fluid communication with the DOC14is a mid-pipe12B, and also connected to and in fluid communication with the mid-pipe12B is a selective catalytic reduction (SCR) catalyst16. Connected to the SCR catalyst16is a diesel particle filter (DPF)18. Connected to and in fluid communication with the DPF18is a tail pipe12C, from which exhaust gas exits the exhaust system10. The SCR catalyst16and the (DPF)18are part of an SCR system, used for reducing exhaust gas emissions.

Referring toFIGS. 4A-4B, the system10also includes a nitrogen oxide sensor, or NOx sensor, shown generally at20, having a sensor component22connected to a control unit, shown generally at24, where the control unit24is in electrical communication with another controller, such as an engine control unit (ECU), not shown. The sensor component22includes a first housing portion26having several apertures28, and the first housing portion26is connected to a base portion30. Also connected to the base portion30is an inner shield32, where the inner shield32protects an end of a sensing element34. The sensing element34is partially surrounded by an insulator36, which is press-fit into the base portion30. Also partially surrounding the insulator36and the sensing element34is a second inner shield38. The sensing element34is also partially surrounded by a second insulator40, and the second insulator40is held in place by a support mount42. Connected to the sensing element34are several wires44, which are part of a wiring harness, shown generally at46. The sensor20also includes a second housing portion48which is connected to the base portion30and the wiring harness46, such that the second housing portion48surrounds various components of the sensor20, as shown inFIG. 4B. Also connected to the base portion30is a mounting portion50having a threaded section, shown generally at52.

The threaded section52is used for mounting the sensor component22into an aperture68formed as part of the tail pipe12C, shown inFIG. 2, such that the first housing portion26, inner shield32, a portion of the sensing element34, and a portion of the base portion30are positioned inside the tail pipe12C.

Referring toFIGS. 5A-5C, the exhaust system10also includes an electrostatic particulate matter (ePM) sensor, shown generally at56. The ePM sensor56also includes a sensor component, shown generally at58, connected to a control unit60, where the control unit60is also in electrical communication with the ECU. The sensor component58also includes a mounting portion62which has a threaded portion64, which is used for mounting the sensor component58to the tail pipe12C, such that a portion, shown generally at66, of the sensor component58is disposed in the tail pipe12C. The sensor component58is mounted to the tail pipe12C in an aperture54formed as part of the tail pipe12C. The portion66of the sensor component58disposed in the tail pie12C includes a housing70, and disposed in the housing70is a baffle tube72. There is a connection74between the housing70and the baffle tube72at an end of the housing70, and there is also a cavity, shown generally at76, between the housing70and the baffle tube72. Disposed inside the baffle tube72is an electrode tube78, and the electrode tube78is positioned such that there is a cavity, shown generally at80, between the baffle tube72and electrode tube78.

There are several outer flow apertures82formed as part of the housing70, and the apertures82surround a venturi portion84, where the venturi portion84includes a flow aperture86, as shown inFIG. 5B. The flow of particulate matter into the sensor component58is represented by arrows88, and the flow of exhaust gas across the tip of the sensor component58is represented by arrows90. The exhaust gas flow90passes by the venturi portion84, creating a vacuum inside the sensor component58. More specifically, the vacuum generated causes particles to flow along the flow path, as shown by arrows88, where the particulate matter particles (which are electrically charged) flow through the outer flow apertures82and into the cavity76, through a plurality of side apertures92formed as part of the baffle tube72, through the cavity80, into the venturi portion84, where the particles then flow through the flow aperture86and exit the venturi portion84.

The cavity80acts as a measuring path along the outer wall of the electrode tube78(which is positively charged) and the inner wall of the baffle tube72(which is grounded). The particles in the exhaust gas are drawn to either the outer wall of the electrode tube78or the inner wall of the baffle tube72, as shown inFIG. 5C.

Referring now toFIG. 3A, an example of the output from the ePM sensor56is shown, which occurs during a cold start of a diesel vehicle equipped with the exhaust system10previously described, which uses a dew point activation method to activate the NOx sensor20according to embodiments of the present invention.FIG. 3Aincludes at least one measured parameter, including measured current94. Within the exhaust system, the NOx sensor20is mounted near the ePM sensor56to assess and compare conditions of the exhaust flow. The measured current94in the ePM sensor56is shown inFIG. 3A, where the measured current94also depicts the shorting that occurs across the components of the ePM sensor56due to the presence of a substance, such as water.

When the vehicle is activated during a cold start condition, the current94that is measured increases, as shown inFIG. 3A, indicating the presence of water in the tail pipe12C in proximity to the ePM sensor56and the NOx sensor20, as shown inFIG. 3B, which depicts a water cloud in the exhaust gas flow. In this example, the shorting of the current94has occurred between five minutes and just after fourteen minutes, as shown inFIG. 3A. The presence of water along the measuring path in the cavity80generates the current94, which is measurable, and is used to determine when the water evaporated, which is shown when the measurement of the current94is reduced. When the current94has decreased such that the current94is at or below a predetermined value, the shorting is no longer occurring in the ePM sensor56. The predetermined value is typically when the current94generated by ePM sensor56is zero, or within some measurable threshold, typically approaching a zero threshold. Once the current94is at or below the predetermined value, water is no longer in the tail pipe12C, as shown inFIG. 3C, and there is minimal risk of the NOx sensor20being exposed to thermal shock from water impingement. Therefore, the ePM sensor56may be used to determine when to activate the NOx sensor20, without the risk of thermal shock from water impingement.

While it has been described above that that NOx sensor20and the ePM sensor56are mounted to the tail pipe12C, it is within the scope of the invention that the sensors20,56may be mounted to other areas along the flow of exhaust gas. In alternate embodiments, the sensors may be mounted in proximity to other exhaust system components, such as an exhaust gas catalyst (for example to the DOC14, or the SCR16) or exhaust gas filter (for example the DPF18).