Sensor cleaning apparatus for use in a motor vehicle

Abstract

An apparatus 100 for use in an internal combustion engine 102 comprising a sensor 116 placed within an exhaust passage 108. The apparatus also has an ultrasonic transducer 124 arranged within the exha

Description

TECHNICAL FIELD This disclosure relates generally to NOx (nitrogen oxide) sensors used in internal combustion engines. More particularly, examples provided in this disclosure address issues related to the performance and accuracy of NOx sensors in engine systems, with a focus on addressing sensor contamination and maintaining optimal sensor function over time. BACKGROUND NOx sensors are critical components in modern internal combustion engines. They are used primarily to monitor and measure the levels of nitrogen oxides (NOx) in exhaust gases. NOx is a pollutant that contributes to smog and acid rain and its regulation is a key factor in meeting emissions standards. The sensors provide feedback to an engine control unit (ECU), which is used to adjust the fuel-air mixture and optimise engine performance, ensuring that NOx emissions are kept within acceptable limits. NOx sensors are typically placed in the exhaust system, before or after the catalytic converter, to measure the concentration of NOx in the exhaust gases. However, a problem with NOx sensors is their susceptibility to contamination during use. Over time, NOx sensors can become covered in soot, carbon and other particulate matter commonly found in exhaust gases, leading to a buildup of contaminants on the sensor surfaces. This contamination can affect the ability of the sensor to accurately detect NOx levels, which can result in inaccurate readings and improper engine adjustments. Consequently, this can lead to increased emissions and decreased engine efficiency and can potentially also cause damage to other components in the exhaust system. Various approaches have been proposed in the prior art to address this issue. One common solution is the use of a heating element integrated into the NOx sensor itself, which is activated periodically to burn off accumulated contaminants. While this method can help maintain sensor accuracy, it often requires additional power consumption and may not be fully effective in all conditions, especially when the accumulation of 1 contaminants is extensive. Another approach involves the use of protective coatings or physical barriers that reduce the accumulation of soot and carbon on the NOx sensor. However, these coatings may degrade over time and / or may not provide a sufficient level of protection, particularly in high-performance or high-temperature environments. Despite the existing solutions, the issue of NOx sensor contamination remains a challenge in maintaining the long-term reliability and accuracy of NOx sensors. Therefore, there is a need for a more effective solution that addresses sensor contamination while minimising the impact on engine performance, system complexity and energy consumption. It is an aim of the invention to address these issues, at least in part. SUMMARY OF THE INVENTION According to an aspect of the invention there is provided an apparatus for use in an internal combustion engine. The apparatus comprises an exhaust passage; a sensor arranged with respect to the exhaust passage; an ultrasonic transducer arranged with respect to the exhaust passage; and a control unit coupled to the ultrasonic transducer and configured to activate the ultrasonic transducer to clean the sensing part of the sensor. The sensor is configured so that a sensing part of the sensor protrudes into the exhaust passage. The ultrasonic transducer is configured such that a probe end of the ultrasonic transducer protrudes into the exhaust passage and is opposed to the sensing part of the sensor. The control unit is configured to be responsive to an engine overrun state to monitor gas flow through the exhaust passage; monitor oxygen concentration in the gas flowthrough the exhaust passage; monitor nitrogen oxide concentration in the gas flow through the exhaust passage; and activate the ultrasonic transducer in dependence on the monitored gas flow, oxygen concentration and nitrogen oxide concentration to direct ultrasonic energy at the sensing part of the sensor. Beneficially, the examples of the invention provide for an advanced activation scheme for an ultrasonic transducer to clean a sensing part of a sensor. In the advanced activation scheme, the ultrasonic transducer is operated in dependence on measured 2 variables indicative of a blocked sensor such that the likelihood of operating the ultrasonic transducer when the sensor does not require cleaning is reduced, thus the sensor cleaning system is more efficient. In some examples, the control unit may be configured to activate the ultrasonic transducer under conditions in which, the monitored gas flow rate is greater than about 150g per second; the monitored oxygen concentration in the gas flow is indicative of an air-fuel ratio of greater than 3; the monitored nitrogen oxide concentration in the gas flow is less than 150 parts per million. Moreover, the control unit may be further configured to activate the ultrasonic transducer in response to monitoring the operation of a diesel particulate filter associated with the apparatus. Monitoring of the diesel particulate filter may include checking for a predetermined number of failed filter regeneration events. In addition to the above, the control unit may be further configured to monitor temperature of the gas flow and vehicle speed and to activate the ultrasonic transducer in dependence also on the temperature of the gas flow and engine speed. More specifically, the control unit may be configured to activate the ultrasonic transducer under conditions in which the gas flow temperature exceeds about 200°C and the vehicle speed exceeds 50km / h. In additional examples of the invention, the ultrasonic transducer and / or the control unit may be configured to emit ultrasonic energy at a frequency range suitable to remove soot particles from the sensing part of the sensor. More specifically, the ultrasonic transducer and / or the control unit may be configured to emit ultrasonic energy at a frequency range between 20kHz and 50kHz. When operating the ultrasonic transducer, the control unit may be configured to activate the ultrasonic transducer for a time period exceeding 20 seconds and preferably for a time period exceeding 30 seconds. In specific examples of the invention, the ultrasonic transducer and / or the control unit may be configured to emit ultrasonic energy on the basis of one or more of: a multifrequency regime, a frequency sweep regime and a frequency pulse regime. It will be appreciated that the various features of each aspect of the invention are equally applicable to, alone or in appropriate combination with, other aspects of the invention. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an engine system, including an exhaust system showing a plurality of common exhaust system components connected via an exhaust passage, to which the examples of the invention may be applied; Figure 2 is a diagram of a cross-section of the exhaust passage of Figure 1 showing an example arrangement of a sensor and an ultrasonic transducer; Figure 3 is a schematic view of an engine control unit, to which examples of the invention may be applied; and Figure 4 is a flow chart showing a method in accordance with an example of the invention. DETAILED DESCRIPTION In general terms, examples of the invention provide methods and activation schemes for maintenance of sensors suitable for use in an exhaust system of an internal combustion powered vehicle. With reference to Figure 1, an internal combustion engine system 100 comprises an engine 102, an air intake 104, a mass air flow (MAF) sensor 106 and an exhaust system. The exhaust system includes an exhaust passage 108 which extends from the engine 102 such that exhaust gases flow therethrough to a plurality of exhaust system components arranged along the exhaust passage 108. The plurality of exhaust system components includes a diesel particulate filter (DPF) 110, diesel exhaust fluid (DEF) supply 112, a selective catalytic reduction (SCR) device 4 114 and a plurality of sensors. In exhaust systems configured for use with different engines, the exhaust system arrangement may differ from that shown in Figure 1. In particular, the DPF and the SCR may be provided as a single unit comprising both components. The DPF and SCR may be combined into one unit to more efficiently utilise the available space. The plurality of sensors comprises at least one lambda sensor 116, at least one nitrogen oxide (NOx) sensor 118 and at least one temperature sensor 120. Each of the exhaust system components 110, 112, 114, the MAF sensor 106, and the plurality of sensors may be electrically connected to an engine control unit (ECU) 122 and may provide the ECU 122 with data. The data provided to the ECU 122 may describe a property of the exhaust gases, an operational condition of a component and / or any suitable data relating to any component, the exhaust gases, or measurements thereof. As would be understood by a skilled person, the exhaust system may comprise additional sensors configured to measure various properties of the exhaust gases. Lambda sensors 116, also known as oxygen sensors, are configured to measure the oxygen concentration of the exhaust gases flowing through the exhaust system. In order to measure the oxygen concentration of the exhaust gases, lambda sensors 116 comprise at least one small opening, described in more detail below with reference to Figure 2, at a sensing part of the lambda sensor 116 such that a small volume of the exhaust gases will enter the lambda sensor 116 and be measured. Typically, lambda sensors 116 comprise a plurality of small openings. The oxygen concentration is typically provided to the ECU 122, which may control operation of the internal combustion engine 102 to optimise a fuel-air ratio. Optimisation of the fuel-air ratio may reduce the concentration of pollutants in the exhaust gases, thus reducing the environmental impact of the internal combustion engine 102 and improving engine performance. The optimisation of the air-fuel ratio would be understood by the skilled person to be a standard functionality of an ECU 122 and so will not be discussed in further detail here. As is known, NOx sensors 118 are configured to measure the NOx concentration of the exhaust gases flowing through the exhaust system. In order to measure the NOx concentration of the exhaust gases, NOx sensors 118 comprise at least one small opening at a sensing part of the sensor, described in more detail below with reference 5 to Figure 2, such that a small volume of the exhaust gases will enter the NOx sensor 118 and be measured. Typically, NOx sensors 118 comprise a plurality of small openings. The NOx concentration is typically provided to the ECU 122 as an input signal. In example exhaust systems, the NOx sensor 118 and lambda sensor 116 may be provided as a single exhaust gas sensor. In such an example, the single exhaust gas sensor is configured to measure both the NOx and oxygen concentrations of the exhaust gases. The single exhaust gas sensor may comprise a plurality of small openings in a manner similar to that of a standard lambda or NOx sensor 116, 118. It is noted that example exhaust systems may comprise a plurality of exhaust gas sensors configured to monitor the NOx concentration, oxygen concentration and, optionally, additional exhaust gas variables at a variety of locations throughout the exhaust system. In the illustrated example engine system 100, NOx concentration may be reduced by injecting urea solution such as ‘DEF’, into the exhaust gas as it enters the SCR device 114 to initiate a chemical reaction where the NOx in the exhaust gas is converted into nitrogen, water and amounts of carbon dioxide (CO2). Therefore, NOx sensor readings are used to determine the volume of DEF that is injected into the exhaust gases when entering the SCR device 114. In many exhaust systems, NOx sensors 116 are provided before and after the SCR device 114. A difference between a measurement taken at a first NOx sensor 116 and a measurement taken at a second NOx sensor 116 can then be used to indicate a performance rating of the SCR device 114. By maintaining optimal performance of the SCR system and optimal DEF injection, NOx sensors 116 help reduce the environmental impact of the internal combustion engine 102 and enhance engine efficiency. This functionality would be understood by the skilled person as being known a functionality of a conventional ECU 122. The DPF 110 is configured to reduce the concentration of particulate matter in the exhaust gases. The DPF 110 is disposed in the exhaust system such that the exhaust gases travel through it. When the exhaust gases travel through the DPF 110, particulates such as soot and other fine particles created during combustion, are trapped in the DPF 110 and, therefore, prevented from being released into the atmosphere. Over time, the accumulated particulates must be removed to maintain filter efficiency. This is achieved through a process called DPF regeneration, where high temperatures are used to burn off the particulates, converting it to ash. A challenge associated with such an engine system 100 is to ensure that the NOx sensors 116 and / or lambda sensors 118 are operating properly and providing valid signals to the ECU 122. It is known that such sensors 116, 118 may become contaminated over time which may influence the sensor readings adversely. With additional reference to Figure 2, there is provided an ultrasonic transducer 124 disposed adjacent to one of the plurality of sensors. In specific embodiments of the invention the ultrasonic transducer 124 may be proximal to one or more NOx sensor 116 and / or lambda sensor 118. As such, in Figure 2 the sensor will be referred to more generally as the reference numeral 202, but it will be noted that it may be any or all of the NOx sensors 116 or Lambda sensors 118 of the exhaust system. Note that in Figure 1 ultrasonic transducers 124 are shown positioned next to both NOx sensors 118. However, it is also applicable for an ultrasonic transducer 124 to be positioned adjacent one or more NOx sensors 118 and / or adjacent one or more Lambda sensors 116. As will be appreciated from Figure 2, the sensor 202 is configured such that a nose end of the sensor, or in other words, a sensing part 204 of the sensor 202, protrudes into the exhaust passage 108 for the purposes of sensing the relevant properties of the exhaust gases. As described previously, the sensor 202 comprises at least one small opening 206 at the sensing part 204 of the sensor 202 such that a small volume of the exhaust gases will enter the sensor 202 and be measured. The ultrasonic transducer 124 is configured such that a probe end 124’ protrudes into the exhaust passage 108 and is opposed to the sensing part 204 of the sensor 202. The ultrasonic transducer 124 is configured to produce high frequency ultrasonic energy and direct it at the sensing part 204 of the sensor 202, which may be clogged with particulate matter. The high frequency ultrasonic energy will interact with the sensing part 204 of the sensor 202 and cause it to vibrate. When the sensing part 204 of the sensor 202 is induced to vibrate, the particulate matter may be dislodged as to clean the sensor 202 of particulate matter. In specific examples of the invention, the ultrasonic energy is produced at a frequency range between 20kHz and 50kHz. As is the case for many specific values provided herein, the frequency range recited is intended to serve as an example only and it is envisaged that the optimum frequency for the ultrasonic energy may be implementation-specific and, therefore, may be optimised in each specific use case. In examples of the invention comprising a plurality of sensors, an ultrasonic transducer 124 may be provided adjacent to a number of sensors 202 of the plurality of sensors. In some cases an ultrasonic transducer 124 may be provided adjacent to each sensor 202 of the exhaust system. Having described the general overview of the engine system 100, the discussion will now focus on a methodology that may be embodied as suitable algorithms and software processes implemented on the ECU 122 for controlling the ultrasonic transducer 124 and processing data received from the sensor 202. At this point, it should be appreciated that the ECU 122 may be the main ECU of the vehicle or a dedicated control unit for control of the ultrasonic transducer 124 based on signals received from the various sensors of the engine system 100. The ECU 122 is adapted to receive data input to sense operational parameters of the engine 102 to provide suitable control output signals to the ultrasonic transducer 124, as discussed in more detail below. With reference to Figure 3, the ECU 122 includes a memory component 302, for data processing purposes. The memory component 302 stores data such as self-learnt control parameters and operating history data, as will be described in more detail later, which can be retrieved by the ECU 122 for processing. The ECU 122 may be operable to perform various engine monitoring and control objectives to manage the performance of the vehicle or engine system 100 into which it is installed. It should be appreciated that the ECU 122 may be any suitable control environment provided by the engine system 100. The ECU 122 may be the ECU 122 of the engine system 100 or another control unit that is configured to carry out other performance and monitoring tasks within the engine system 100 of the broader vehicle. In particular, the ECU 122 may define a suitable control environment provided specifically for the purposes of performing the method as discussed in this disclosure. Irrespective of the functionality of the ECU 122, it will be appreciated that the ECU 122 has the necessary memory 302, processing environment 304 and communications interface 306 to be integrated into the engine system 100 and the broader system of an associated vehicle. With reference to Figure 4, a method 400 in accordance with an example of the invention provides an activation scheme for the ultrasonic transducer 124 to clean the sensor 202. At step 402, the method 400 initiates in a suitable manner. For example, the method may be initiated in response to ignition of the internal combustion engine 102, the vehicle travelling a specified number of miles after a previous sensor cleaning event, data received from one or more of the plurality of sensors of the exhaust system, data received from one or more sensors of other vehicle systems (not shown) and / or any other suitable vehicle event. Following step 402, the method 400 monitors for a deceleration phase of the vehicle at step 404 in order to determine whether the main processing routine of the method 400 may be commenced. The monitoring may take place as a one-time check for a suitable deceleration phase, or the monitoring may take place over an extended time period, for example of a few seconds. Step 404 may comprise receiving, at the ECU 122, data indicative of a set of vehicle speeds. Based on the speed data, the current acceleration of the vehicle can be determined. If the current acceleration of the vehicle is greater than or equal to zero, the method 400 comprises returning to step 402. However, if the current acceleration of the vehicle is less than zero, i.e. the vehicle is decelerating, the method 400 proceeds to step 406. Whilst this process has been described as interpretating vehicle speed data, it should be noted that a deceleration phase may be identified by other means, for example by monitoring for a drop in torque demand signal, which would indicate that the drive of the vehicle has released the acceleration pedal, thereby causing a deceleration of the vehicle, or that a cruise control function causes the torque demand signal to fall to zero. This may be useful in the event that the vehicle may be travelling downhill in a no-load drive condition which could mean that the acceleration is zero or positive non-zero, but there is no torque demand. At step 406, the main processing steps of the method 400 begins by receiving and analysing, at the ECU 122, a plurality of sensor readings indicative of characteristics of the exhaust gases. In specific embodiments of the invention, the plurality of sensor readings comprises at least a MAF sensor reading, indicative of the mass of air flowing into the internal combustion engine per second; a lambda sensor reading, indicative of an oxygen concentration of the exhaust gases; and a NOx sensor reading, indicative of a NOx concentration of the exhaust gases. Suitable signal filtering can be applied to the input signals, the specifics of which is beyond the scope of this discussion. At step 408, the method 400 compares the MAF sensor reading to a MAF threshold ‘A’. As indicated previously, the MAF sensor 106 monitors a rate of airflow in the engine system 100, accordingly, the MAF threshold ‘A’ is a threshold airflow rate. The comparison is done to ensure that the flow of fresh air through the exhaust system is sufficiently high so that the measurements taken from the lambda sensor / NOx sensor 116, 118 are predictable with a high oxygen concentration and low NOx concentration. The MAF threshold ‘A’ may therefore be set accordingly. For example, the MAF threshold ‘A’ may be in the range 150 g / s (grams per second) to 200 g / s and even between 175 g / s to 185 g / s. It is envisaged that a MAF threshold ‘A’ of about 180 g / s may be appropriate, in one example. However, the specific thresholds provided may be implementation dependent and the optimal values may vary from engine to engine. If the MAF sensor reading is less than the MAF threshold ‘A’, the method 400 returns to step 404, where it continues monitoring for a deceleration phase. However, if the MAF sensor reading is greater than or equal to the MAF threshold ‘A’, the method 400 then flows to step 410. At step 410, the method 400 checks the readings from the NOx sensor 118 and the Lambda sensor 116 to determine whether they are indicative of properly functioning sensors, given the high rate of fresh air flow through the exhaust system, as has been verified at step 408. Accordingly, at step 410, the method 400 compares the lambda sensor reading to a lambda threshold ‘B’ and the NOx sensor reading to a NOx threshold ‘O’. In examples of the invention, the lambda threshold ‘B’ may be in the range 3 ppm to 8 ppm and the NOx threshold ‘C’ may be in the range 80 ppm to 120 ppm. In specific examples of the invention, the lambda threshold ‘B’ may be about 5ppm and the NOx threshold ‘C’ may 10 be about 100ppm. The thresholds are configured to indicate the readings to be expected with a high fresh air flow, during zero engine load (torque demand), resulting in high oxygen concentration and low NOx concentrations. The thresholds may be set during manufacture, as appropriate and may be configurable by way of a suitable service centre computer system, or other appropriate computing device that can interface with the vehicle control systems. Advantageously, by requiring that the MAF sensor reading is above the MAF threshold ‘A’ and the vehicle is decelerating, the method 400 may ensure that the engine 102 is in an overrun state and, therefore, the exhaust system is purged with air, before comparing the lambda sensor reading and the NOx sensor reading to their respective thresholds. If the lambda sensor reading is greater than or equal to the lambda threshold ‘B’ and the NOx sensor reading is less than or equal to the NOx threshold ‘O’, it can be determined at step 412 that the sensor 202 is blocked due to particulate matter buildup in the small openings 206 formed in the sensing part 204 of the sensor 202. At step 412, a suitable internal software flag may be set as being indicative of a faulty sensor. Following the determination that the sensor 202 is blocked, the method then proceeds to start a cleaning cycle for the sensor 202, using the ultrasonic transducer 124. As such, at step 414, the method 400 verifies that the engine 102 is in a suitable operating condition to start a cleaning cycle. In general, a relevant operating condition is one where the engine 102 is running hot enough to ensure that the contaminants on the surface of the sensor 202 are more likely to be cleaned successfully. In principle, this step is optional, but it is useful since it improves the success rate of a cleaning cycle. In verifying the suitability of the engine operating condition, at step 415, the method 400 checks data indicative of a current temperature of the exhaust gases and / or the current vehicle speed. The current exhaust gas temperature data and the current vehicle speed data received in step 414 are compared to respective thresholds (see temperature threshold ‘D’ and speed threshold ‘E’), at the ECU 122. The respective thresholds may be stored in the memory 302 of the ECU 122 and may be configurable. In examples of the invention, the temperature threshold ‘D’ may be in the range 200°C to 300°C and the speed threshold ‘E’ may be in the range 30km / h to 70km / h. In specific examples of the invention, the temperature threshold ‘D’ may be in the range 240°C to 260°C and the 11 speed threshold ‘E’ may be in the range 45km / h to 55km / h. In further specific examples of the invention, the temperature threshold may be about 250°C and the speed threshold may be about 50km / h. If the current temperature is less than the temperature threshold ‘D’ and / or the current speed is less than the speed threshold ‘E’, indicating that the vehicle is not in a suitable operating condition for a sensor cleaning cycle, the decision step moves to step 416 at which the method 400 terminates. However, if the current exhaust gas temperature is greater than or equal to the temperature threshold and the current vehicle speed is greater than or equal to the speed threshold, the ECU 122 is operable to perform step 418. In specific activation schemes, the engine parameters considered at step 414 may vary. In some cases, alternative engine parameters such as the revolutions per minute (rpm) of the engine (taking into account a current gear position) or the engine torque may be used to monitor the gas flow through the exhaust system and the operational state of the engine 102. It is noted that the skilled person would be aware of numerous additional alternative parameters which may be used in place of the exhaust gas temperature and vehicle speed to verify that the operating condition of the engine is suitable for initiation of a cleaning cycle. If step 414 determines that the vehicle is in a suitable operating condition for a cleaning cycle to begin, at step 418, the ECU 122 is operable to activate the ultrasonic transducer 124 such that high frequency ultrasonic energy is incident on the sensing part 204 of the sensor 202 to dislodge the accumulated particulate matter on the sensor 202. It is currently envisaged that the ultrasonic transducer 124 may be activated for a time period in the range 20 seconds to 50 seconds. In particular, it has been found that an activation time exceeding 30 seconds (though not exclusively) may achieve acceptable cleaning results. Unblocking the small openings 206 improves the accuracy of the subsequent sensor data provided to the ECU 122, thus reducing the environmental impact and improving the efficiency of the internal combustion engine 102. High exhaust gas temperatures and high exhaust gas flow rate, associated with an increased vehicle speed in a deceleration phase, are highly effective for detaching soot particles from the sensor 202.Therefore, providing exhaust gases exceeding approximately 250°C and ensuring that the vehicle speed exceeds approximately 12 50km / h improves the efficiency of the cleaning process and avoids the activation of a cleaning cycle in suboptimal conditions. After activating the ultrasonic transducer 124, the method 400 terminates at step 416. Optionally, at this point further steps may be introduced to verify that the sensor 202 is operating with acceptable accuracy. For example, steps 408, 410 and 412 may be repeated. Further, a following on cleaning cycle may also be executed by repeating steps 414 and 418. If the sensor 202 is determined to be operating with acceptable accuracy, a suitable internal software flag indicative of a faulty sensor may be deactivated. Returning now to step 410 of the method 400, it will be noted that the previous discussion was based on the result of the decision step 410 indicating that the sensor 202 was not operating correctly. Now, in the case that the decision step 410 indicates that the sensor 202 was operating correctly, in that either the lambda sensor reading is less than the lambda threshold ‘B’ or the NOx sensor reading is greater than the NOx threshold ‘C’, the method proceeds to steps 422 to 426, as an optional leg of the process, to base the need for a sensor cleaning cycle on monitoring of a regeneration process for the DPF 110. It is also envisaged that the method may terminate at this point. As such, if decision step 410 determines that the sensor 202 is functioning within nominal limits, the method 400 proceeds to step 422 where it begins to monitor a DPF regeneration cycle. At step 424, the method 400 checks for a predetermined number of failed DPF regeneration events, i.e. to determine if DPF regeneration is incomplete after a plurality of cycles. As the skilled person will know, a DPF regeneration process comprises steps which increase the exhaust gas temperature to a very high level to burn off the impurities captured by the DPF 110. Typically, this may be achieved by running the engine 102 in an operating regime that increases exhaust gas temperature to above typically 600°C, for example during high speed / high load for an extended period of time. Alternatively, fuel can be introduced into the exhaust system to increase the exhaust gas temperature. Typically, introduction of fuel into the exhaust system is achieved by post-combustion injection of fuel into at least one engine cylinder such that the subsequent airflow through the cylinder carries the unignited fuel into the exhaust system. Still alternatively, a DPF 13 heater may be activated to achieve the high temperatures. The method 400 may obtain the number of failed / incomplete regeneration cycles from the memory of the ECU 122 or a comparable system of the vehicle that is responsible for controlling the DPF regeneration process. The number of cycles that will trigger step 424 may be in the range 3 to 7 cycles and it is envisaged currently that about 5 cycles should provide the right balance between providing sufficient cleaning but also not activating too frequently. If at step 424 it is determined that DPF regeneration completes successfully before reaching the predetermined number of regeneration cycles, then the process is terminated at step 416. However, if it is determined that the DPF regeneration is incomplete after the predetermined number of cycles, then the method 400 moves on to step 426 to determine whether the engine system 100 is in an appropriate condition to perform a cleaning cycle using the ultrasonic transducer 124. Failed DPF regeneration cycles indicate that the temperature of exhaust gases may be too low to detach soot build up on the DPF. Therefore, numerous consecutive failed DPF regeneration cycles indicate that the temperature of exhaust gases may have been too low to detach soot build up for an extended period of time. If the exhaust gases are too cool to effectively detach soot which has built up on the DPF, then it is increasingly likely that the exhaust gases are also too cool to have adequately detached soot from the sensor 202. Therefore, it is advantageous to monitor failed DPF regeneration cycles as they are likely to indicate inadequate sensor cleaning. More specifically, at step 426, the current exhaust gas temperature is compared to a temperature threshold ‘F’. It is envisaged that a suitable temperature range for the temperature threshold ‘F’ may be in the range 250°C to 350°C, for example about 300°C. If the current exhaust gas temperature is less than the temperature threshold ‘F’, step 426 exits and the method 400 terminates at step 416. However, if the current exhaust gas temperature is greater than or equal to the temperature threshold ‘F’, the method 400 moves to step 418 thereby activating the ultrasonic transducer 124 for a cleaning cycle as has been discussed above. It should be noted that the left hand leg of the method 400 covered by steps 422 to 426 is optional, as mentioned above. However, advantageously, this part of the method 400 provides a further measure to ensure the cleanliness of the sensor 202 in the case where the check on the oxygen concentration and NOx concentration of the exhaust gases indicates that the sensor 202 is operating within nominal limits. By providing a further option for running a cleaning cycle with the ultrasonic transducer 124 in circumstances where the DPF regeneration process has failed for a predetermined number of times, the method 400 provides a further opportunity for maintaining the cleanliness and proper operation of the sensor 202. This may be useful because action to perform a cleaning cycle may be taken earlier to clean the sensor 202 before it is contaminated to such a degree that its sensing capabilities have degraded to such an extent that the NOx concentration readings are affected. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. For example, the ultrasonic transducer 124 and / or the ECU 122 may be configured to emit ultrasonic energy on the basis of one or more of: a multifrequency regime, a frequency sweep regime and / or a frequency pulse regime. A multifrequency regime constitutes operating the ultrasonic transducer 124 to excite a plurality of vibration modes simultaneously. A frequency sweep regime constitutes operating the ultrasonic transducer 124 to increase, or decrease, the frequency of the ultrasonic energy from a first to a second value over a set period of time, the first and second value defining a suitable range therebetween. A frequency pulse regime constitutes operating the ultrasonic transducer 124 to emit short bursts of ultrasonic energy.

Claims

1. An apparatus for use in an internal combustion engine (102), comprising:an exhaust passage (108);a sensor (116, 118, 202) arranged with respect to the exhaust passage (108) and configured so that a sensing part (204) of the sensor (116, 118, 202) protrudes into the exhaust passage (108);an ultrasonic transducer (124) arranged with respect to the exhaust passage (108) and configured such that a probe end (124’) of the ultrasonic transducer (124) protrudes into the exhaust passage (108) and is opposed to the sensing part (204) of the sensor (202);a control unit (122) coupled to the ultrasonic transducer (124) and configured to activate the ultrasonic transducer (124) to clean the sensing part (204) of the sensor (116, 118, 202);wherein the control unit (122) is configured to be responsive to an engine overrun state to:monitor gas flow through the exhaust passage (108);monitor oxygen concentration in the gas flow through the exhaust passage (108);monitor nitrogen oxide concentration in the gas flow through the exhaust passage (108);activate the ultrasonic transducer (124) in dependence on the monitored gas flow, oxygen concentration and nitrogen oxide concentration to direct ultrasonic energy at the sensing part (204) of the sensor (202).

2. The apparatus of Claim 1, wherein the control unit (122) is configured to activate the ultrasonic transducer (124) under conditions in which:the monitored gas flow is greater than about 150g per second;the monitored oxygen concentration in the gas flow is indicative of an air-fuel ratio of greater than 3;the monitored nitrogen oxide concentration in the gas flow is less than 150 parts per million.

3. The apparatus of Claims 1 or 2, wherein the control unit (122) is further configured to activate the ultrasonic transducer (124) in response to monitoring the operation of a diesel particulate filter (110) associated with the apparatus.

4. The apparatus of Claim 3, wherein monitoring of the diesel particulate filter (110) includes checking for a predetermined number of failed filter regeneration events.

5. The apparatus of any one of the preceding claims, wherein the control unit (122) is further configured to monitor temperature of the gas flow and vehicle speed and to activate the ultrasonic transducer (124) in dependence also on the temperature of the gas flow and vehicle speed.

6. The apparatus of Claim 5, wherein the control unit (122) is configured to activate the ultrasonic transducer (124) under conditions in which:the gas flow temperature exceeds about 200 °C;the vehicle speed exceeds 50km / h.

7. The apparatus of any one of the preceding claims, wherein the ultrasonic transducer (124) and / or the control unit (122) are configured to emit ultrasonic energy at a frequency range suitable to remove soot particles from the sensing part (204) of the sensor (202).

8. The apparatus of Claim 7, wherein the ultrasonic transducer (124) and / or the control unit (122) are configured to emit ultrasonic energy at a frequency range between 20kHz and 50kHz.

9. The apparatus of any one of the preceding claims, wherein the control unit (122) is configured to activate the ultrasonic transducer (124) for a time period exceeding 20 seconds and preferably for a time period exceeding 30 seconds.

10. The apparatus of any one of the previous claims, wherein the ultrasonic transducer (124) and / or the control unit (122) are configured to emit ultrasonic energy on the basis of one or more of: a multifrequency regime, a frequency sweep regime and a frequency pulse regime.

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