Patent Application
20200157994
This application claims the benefit of the filing date of U. K. Patent Application 1818633.8, “NOX SLIP DETECTION,” filed Nov. 15, 2018; and U.K. Patent Application 1820820.9, “NH3 SLIP DETECTION IN EXHAUST SYSTEMS, filed Dec. 19, 2018; the entire disclosures of each of which are incorporated herein by reference.
The present disclosure relates to a method of determining the ratio of NH3 to NOx in exhaust gases leaving an exhaust system of an internal combustion engine.
There is a rising demand for cleaner and more efficient internal combustion engines, especially diesel engines. Diesel engines emit particulate matter as well as nitrogen oxides (NOx) from their exhaust systems.
The exhaust systems of working vehicles such as tractors often include one or more systems to clean the exhaust gases, such as a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF) for particulate matter treatment, a selective catalytic reduction (SCR), and/or a downstream ammonia oxidation catalyst. A urea solution (CO(NH2)2 and water), also known as diesel exhaust fluid (DEF) is often used to dose (i.e., treat) exhaust gases, and a chemical reaction in the presence of catalysts causes a reaction of NOx and NH3 (ammonia, which is formed by hydrolysis of the urea) to form elemental nitrogen (N2), water, and carbon dioxide (CO2).
In general, there is a correlation between the urea dose and NOx reduction efficiency in that a larger dose of urea delivered to the exhaust gas corresponds to a greater the reduction in NOx in the gases released to the atmosphere.
However, under varying engine conditions, unreacted NH3 can pass or “slip” through the catalytic converter. As such, NH3 concentration in the output gases can vary, and detection of the slip is typically required as part of the system control. Detection of the slip allows the urea to be correctly dosed for the engine operating conditions. Unreacted NH3 exhausted to the atmosphere may be an environmental concern and/or an extra expense that provides no benefit, and therefore, it may be beneficial to limit unreacted NH3.
It is an object of the present disclosure to provide a method for detecting NH3 slip using NOx sensors.
Accordingly, there is provided a method of determining relative ratio of NOx to NH3 in exhaust gases, which includes providing an exhaust system for an agricultural vehicle, the exhaust system comprising: an upstream NOx sensor; a downstream NOx sensor; a catalytic converter; a urea injector; and an exhaust control unit. The method further includes measuring a first NOx level at the upstream NOx sensor, administering a first predetermined dose of NH3 to the exhaust system, measuring a second NOx level at the downstream NOx sensor; administering a second predetermined dose of NH3 to the exhaust system; measuring a third NOx level at the downstream NOx sensor; calculating the difference between the second NOx level and the third NOx level; and comparing the difference to a lookup table to determine the relative ratio of NH3 and NOx.
Advantageously, the second predetermined dose of NH3 may be larger than the first predetermined dose of NH3. Using a larger dose makes the relative differences of the second and third dose levels easier to detect. Because the normal dosage levels are much smaller, this means less accurate sensors may be used, which are generally cheaper than those that are more accurate.
Advantageously, the exhaust system may include more than one catalyst to further reduce NOx levels and therefore improve the chemical efficiency of the exhaust system.
Advantageously, the catalytic converter may include a selective catalytic reduction catalyst. In alternative embodiments, the catalytic converter may include an ammonia oxidation catalyst.
Advantageously, the exhaust system may further include a temperature sensor. If the temperature of the exhaust gases is known, the accuracy of estimations and predicted chemical reactions may be improved because reaction kinetics and equilibrium vary with temperature.
Advantageously, the exhaust system may further comprise a diesel particulate filter. Including a diesel particulate filter helps reduce particulate matter that passes to the atmosphere from the internal combustion engine, reducing pollutants from the exhaust system.
Advantageously, an agricultural vehicle may include an exhaust system configured to carry out the method. An agricultural vehicle including the system would be more efficient and have reduced pollutant output from the exhaust system at a lower cost than traditional known methods, such as using NH3 sensors.
Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:
The drawing is provided by way of reference only, and is not necessarily to scale.
The exhaust system 10 includes an internal combustion engine 12, a catalytic converter 14, a urea tank 16, a urea injector 18, an upstream NOx sensor 20, a downstream NOx sensor 22, a temperature sensor 24, and an engine control unit 26.
The internal combustion engine 12 in this example is specifically a diesel engine. The internal combustion engine 12 is configured to drive a transmission of the tractor. In alternative embodiments, the internal combustion engine 12 is configured to drive a generator that produces electrical power for charging batteries or driving motors. The internal combustion engine 12 burns diesel fuel with atmospheric air and produces power and waste gases, the waste gases commonly being known as exhaust gases. The exhaust gases are rich in particulates and NOx.
The internal combustion engine 12 is in fluid communication with the catalytic converter 14 via conduit 28 (e.g., an exhaust manifold). The exhaust gases from the internal combustion engine 12 exit the internal combustion engine 12 at A, pass via conduit 28 and enter the catalytic converter 14 at B. The conduit 28 between the internal combustion engine 12 and the catalytic converter 14 may be formed by any suitable means, for example a tube or conventional exhaust pipe constructed from mild steel or stainless steel. In some embodiments, the conduit 28 may be ceramic-coated to provide better thermodynamic (e.g., insulative) properties.
The catalytic converter 14 may include one or more catalysts, such as a selective catalytic reduction catalyst, a diesel oxidation catalyst, an ammonia oxidation catalyst, or a hybrid of different catalyst types.
Exhaust gases exit the catalytic converter 14 at C and enter another conduit 30, which may be referred to in the art as a tail pipe or stack. The exhaust gases exit the conduit 30 at D, which may be open to the atmosphere. The conduit 30 may be of any conventional design.
The urea tank 16 is provided as a reservoir for urea (as a source of NH3 for reduction of NOx). The urea is used as a dosing agent for the exhaust system 10. The urea tank 16 is in fluid communication with the urea injector 18 via line 32.
The urea injector 18 is of suitable type and is provided to dose the exhaust gases in the exhaust system 10 with urea from the urea tank 16. In this example, the urea injector 18 is provided in the conduit 28 between A and B such that urea may be administered to the exhaust gases prior to their entering the catalytic converter 14.
The upstream NOx sensor 20 is provided in the conduit 28 upstream of the urea injector 18 and is configured to provide measurements of the NOx levels in the exhaust gases leaving the internal combustion engine 12.
The downstream NOx sensor 22 is provided in the conduit 30 between C and D. The downstream NOx 22 sensor is thus situated downstream of the catalytic converter 14, the urea injector 18, and the upstream NOx sensor 20. The downstream NOx sensor 22 is configured to measure the levels of NOx in exhaust gases in the conduit 30, i.e. downstream of the catalytic converter 14.
The temperature sensor 24 in this example is situated in the conduit 28 between A and B. The temperature sensor 24 is also situated between the urea injector 18 and the catalytic converter 14. In other examples, the temperature sensor 24 may be omitted. The temperature sensor 24 is of any suitable type, such as a thermocouple. The temperature sensor 24 is configured to measure the temperature of exhaust gases flowing in the conduit 28. That is, the temperature sensor 24 is used to measure the temperature of exhaust gases as they exit the internal combustion engine 12 and before they enter the catalytic converter 14. In alternative embodiments, it will be understood that the temperature sensor 24 may be upstream of the urea injector 18 and/or further temperature sensors may be positioned throughout the exhaust system 10.
The engine control unit (ECU) 26 is provided as part of the exhaust system 10. However, it will be understood that the engine control unit 26 may be a main or auxiliary control unit provided as a standalone control unit on a vehicle or in data communication with another control unit on the vehicle.
In this specific example, the engine control unit 26 is a main engine control unit for the vehicle and is configured to receive measurements from vehicle sensors and send messages to various vehicle components and devices.
The engine control unit 26 is in data communication with each of the internal combustion engine 12, the urea injector 18, the upstream NOx sensor 20, the downstream NOx sensor 22, and the temperature sensor 24.
When the exhaust system 10 is in use, exhaust gases containing NOx are expelled from the internal combustion engine 12 and are vented to the atmosphere at D after passing through the catalytic converter 14.
To reduce the NOx content in the exhaust gases, the exhaust gases are dosed with urea (which is hydrolyzed to form NH3) at the urea injector 18 before entering the catalytic converter 14. This process is referred to in the art as Selective Catalytic Reduction (SCR), is well known in the art, and is not described in detail herein.
During the SCR process, the NH3 formed from the dosed urea is not necessarily entirely consumed by reacting with the NOx.
It is useful to know the ratio of NOx to NH3 in the exhaust gases at D so that the urea dosing, operating temperature, or other parameters may be adjusted to improve the removal of NOx and/or particulate matter from the exhaust gases.
It is possible to include a further sensor configured to measure NH3 concentration into the system in the conduit 30 between C and D. Combined with the data measured from the downstream NOx sensor 22, it would then be possible to calculate the ratio of NOx and NH3 in the exhaust gases leaving the exhaust system 10.
Disadvantageously, the addition of a sensor to directly measure the levels of NH3 in the exhaust gases increases the system cost and complexity. Instead, the NOx sensors 20, 22 provided in the exhaust system 10 shown may be used to calculate the relative ratios of NOx and NH3 in the exhaust gases without measuring the concentration of NH3, as described below.
To calculate the relative ratios of NOx and NH3 in the exhaust gases when the exhaust system 10 is in use, a baseline concentration of NOx (i.e., a first NOx level) is measured by the upstream NOx sensor 20 and communicated to the ECU 26. The exhaust gases are dosed with a known quantity (i.e., a first predetermined dose) of urea (NH3 source) at the urea injector 18, which is downstream of the upstream NOx sensor 20 (and therefore, the injection of urea does not perturb the first NOx level at the upstream NOx sensor 20). The exhaust gases, now enriched with NH3, pass into the catalytic converter 14. In the catalytic converter 14, a chemical reduction reaction takes place, whereby the NH3 reacts with the NOx to convert some of the NOx to H2O (water), N2 (nitrogen gas), and optionally CO2 (carbon dioxide). However, the NOx and NH3 do not necessarily entirely react, and as such the exhaust gases exiting the catalytic converter 14 at C may contain H2O, O2, N2, NH3, and NOx, among other gases and particles. The concentration of NOx (i.e., a second NOx level) is measured by the downstream NOx sensor 22.
The preceding process occurs continually when the exhaust system is in use and when NOx reduction is required. The reaction in the catalytic converter 14 generally produces stable results and conversion ratios, and thus, the second NOx level measured by the downstream NOx sensor 22 is generally stable.
To calculate the relative ratios of NH3 the NOx and thereby check that the process is running correctly, the dose of urea applied to the exhaust gases at the urea injector 18 is spiked to a second predetermined dose for a period of time. That is, the dosing of urea is normally stable and relative to the measured NOx levels and is applied at a first predetermined level, which is appropriate to the measured NOx levels at the upstream NOx sensor 20, and is set according to a predetermined dosing map. The second predetermined dose or spiked level is significantly higher than what would be required according to the aforementioned dosing map. The NOx concentration following the spike is measured in the exhaust gases in the conduit 30 by the downstream NOx sensor 22 (i.e., a third NOx level).
The difference between the second and third NOx levels can be computed by the engine control unit 26 and compared to a stored map or table to determine the relative ratio of NH3 and NOx. The stored map is typically constructed from results acquired from testing (i.e., calibration).
The temperature sensor 24 can be used to provide further data and more precisely estimate the amount of urea to apply because reaction kinetics and equilibrium vary with temperature.
In alternative embodiments, a further catalyst may be provided in the exhaust system 10. In a specific embodiment, this further catalyst is a DOC catalyst and is positioned upstream of the upstream NOx sensor 20.
In further alternative embodiments, the upstream NOx sensor 20 is positioned anywhere in the exhaust system 10 upstream of the urea injector 18.
Furthermore the exhaust system 10 may include a diesel particulate filter. This diesel particulate filter may for instance be situated downstream of the catalytic converter 14.
The disclosure is not limited to the embodiments or examples described herein, and may be modified or adapted without departing from the scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1818633.8 | Nov 2018 | GB | national |
| 1820720.9 | Dec 2018 | GB | national |
