NL2021315B1

NL2021315B1 - SCR conversion efficiency testing method

Info

Publication number: NL2021315B1
Application number: NL2021315A
Authority: NL
Inventors: De Graaf John; Godefridus Matheus Vallen Rob
Original assignee: Daf Trucks Nv
Priority date: 2018-07-16
Filing date: 2018-07-16
Publication date: 2020-01-24

Abstract

According to the invention, a method and system is provided for a vehicle comprising: an engine having an engine inlet and exhaust producing an exhaust gas; an exhaust gas recirculation (EGR) system having an EGR valve that determines an amount of exhaust gas to be recirculated into the engine inlet; an aftertreatment system for treating the exhaust gas including a selective catalytic reduction (SCR) catalyst; at least one NOX sensor operatively connected to the aftertreatment system and adapted to determine an amount of NOX in the exhaust gas; an SCR efficiency monitor controller communicatively coupled to the NOX sensor; that is able to measure an SCR efficiency based on the NOX sensing values; the SCR efficiency monitor having a defined efficiency threshold below which an SCR fail criterion is raised. When the NOX sensor detects an efficiency level below a threshold, the EGR valve controller switches to a low EGR mode; and wherein subsequently, in the low EGR mode the NOX sensor measures a sensing value with an enhanced resolution to confirm the SCR fail criterion

Description

BACKGROUND

Turbocharged diesel engines involve combustion processes that generate exhaust gas. Equipped with high pressure exhaust gas recirculation the engines are known to have a trade-off between fuel economy and NOx emissions. The after treatment system strictly limits the engine-out NOx emissions to tailpipe emissions to comply with legislation. During combustion, an air/fuel mixture is delivered through an intake valve to cylinders and is combusted in the cylinders. After combustion, a piston forces the exhaust gas in the cylinders through an exhaust valve and into an exhaust system. The exhaust gas may contain emissions such as oxides of nitrogen (NOx) and carbon dioxide (CO2). Exhaust treatment systems monitor and treat the exhaust gas to meet emissions requirements. Treatment of exhaust gas may include the use of particulate filters, catalysts such as diesel oxidation catalysts (DOC) and/or selective catalytic reduction (SCR) catalysts, hydrocarbon injection and/or other devices. The efficiency of the SCR catalyst is usually monitored to ensure that emission levels remain acceptable during operation.

Due to the kinetics of the combustion process of the diesel engine, a trade off exists between CO2 reduction and NO_X reduction. Reference is made to Figure 2 where exemplary curves are shown illustrating a connection between SCR efficiency (in %) and NOx engine out level (that is NOx concentration before SCR conversion). If a low NOx out level is desired, this goes hand in hand with a higher CO2 level. Generally, reducing engine out NOx increases CO2 and increasing engine out NO_X reduces CO2.

Thus, an improved conversion efficiency of the SCR catalyst is desirable when reducing the CO2 levels, and higher engine out NOx levels are produced. This in turn demands a lower fault threshold for failed part detection, to timely identify fault states.

SUMMARY

According to the invention, a method and system is provided for an engine having an engine air inlet and exhaust producing an exhaust gas. The engine comprises: an exhaust gas recirculation (EGR) system having an EGR valve that determines an amount of exhaust gas to be recirculated into the engine air inlet and an aftertreatment system for treating the exhaust gas including a selective catalytic reduction (SCR) catalyst. At least one NOx sensor is operatively connected to the aftertreatment system and adapted to determine an amount of NOx in the exhaust gas. An SCR efficiency monitor controller is communicatively coupled to the at least one NOx sensor; that is able to measure an SCR efficiency based on a NOx sensing value of the at least one NOx sensor and a reference value, obtained from a second sensor. The SCR efficiency monitor has a defined efficiency threshold below which an SCR fail criterion can be raised. An EGR valve controller controls the EGR valve and is configured to provide high EGR mode wherein a first fraction of exhaust is recirculated into the engine inlet. The valve controller is further configured to provide a low EGR mode wherein a second fraction of exhaust gas is recirculated; the second fraction being smaller than the first fraction. The EGR valve controller switches to a low EGR mode; and subsequently, in the low EGR mode the NOx sensor measures a sensing value with an enhanced resolution.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

In a more general sense it is thus an object of the invention to overcome or reduce at least one of the disadvantages of the prior art. It is also an object of the present invention to provide alternative solutions which are less cumbersome in assembly and operation and which moreover can be made relatively inexpensively. Alternatively it is an object of the invention to at least provide a useful alternative.

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figure 1 a schematic overview of the system 100 layout is depicted;

Figure 2 shows exemplary curves illustrating a connection between SCR efficiency (in %) and NOx engine out level;

Figure 3 shows an exemplary trace of NOx concentration measured with upstream NOx;

Figure 4 shows an exemplary trace, where the filtered SCR efficiency (%) does not improve;

Figure 5 shows a schematic diagram of the exemplary method;

Figure 6 shows an exemplary flow chart of the SCR monitor operation.

In Figure 1 a schematic overview of the system 100 layout is depicted. An air path is disclosed of air circulating and flowing in the engine system 100. In the air path an amount of fresh air Wfresh 201 is introduced,

i.e. the mass flow of fresh air into the engine system 100, and mixed with EGR mass flow W_(!gr 208 of an EGR cooler 106.

In the system layout, a compressor 101 is located in an inlet flow path of the engine. For the purpose of the present invention, the compressor is optional. The compressor 101 may be propelled by a turbine 102, that may be mechanically coupled. In another form, multistage turbochargers are envisioned. A compressor rotational speed sensor ni_ur 204 may be provided.

In one form, the engine 105 is a six cylinder four-stroke internal combustion engine. Estimation of the injected fuel mass flow Wfuei 205 may be available. In another form, the engine has a different number of cylinders or a different number of operating cycles. Furthermore, to reduce the engine out NOx mass flow to legal limits, the engine system is equipped with an after-treatment system 108 which includes a catalyst, known as a selective catalytic reduction filter and optionally a particle filter.

Recirculated exhaust gas may be cooled in EGR cooler 106 and an EGR valve 107 might be employed to regulate the recirculated mass flow W_egr 208. In the system 100 a controller 300 is arranged to control the air path of the diesel engine, in particular, by control of the EGR valve 107. The controller may be arranged in hardware, software or combinations and may be a single processor or comprise a distributed computing system. Typically, a controller operates in time units such as (numbers of) clock cycles that define a smallest time frame wherein data can be combined by logical operations. Controller 300 may be coupled or may comprise an SCR efficiency monitor controller 310 part communicatively coupled to the NOx sensor 211. SCR efficiency can be measured based on the NOx sensing values of NOx sensor 211. For example, SCR efficiency monitor controller 310 is able to measure an SCR efficiency based on the NOx sensing values; and the SCR efficiency monitor has a defined efficiency threshold below which an SCR fail criterion is raised. This fail condition may be detected a number of times, e.g. three times, in order to activate the ‘intrusive step’ mode as further described below.

Measurement of the oxygen concentration of the exhaust gas 02% 209 can be performed by various methods. A reference NOx value could be based on offline tuned look-up tables which are parameterized by engine speed 206 and fuel mass flow (205). In the system, NOx sensors 210, 211 may be present that are optionally available both up- and downstream of the after treatment system 108. Accordingly, it is shown that Figure 1 illustrates an engine 105 having an engine inlet 201 and exhaust 202 producing an exhaust gas. Exhaust gas recirculation (EGR) system 106 has an EGR valve 107 that determines an amount of exhaust gas to be recirculated into the engine inlet 201. Specifically EGR valve controller 300 controls the EGR valve 107 and provides a ‘high EGR mode’ wherein an first, higher fraction of exhaust is recirculated into the engine inlet; likewise, the valve controller 300 provides a low EGR mode’ wherein a second, smaller, fraction of exhaust gas is recirculated. In the low EGR mode, the NOx values in the exhaust are raised due to increased oxygen content.

Aftertreatment system 108 is constructed for treating the exhaust gas including a selective catalytic reduction (SCR) catalyst; and at least one NOx sensor 210 is operatively connected to the aftertreatment system 10 and adapted to determine an amount of NOx in the exhaust gas.

Figure 2 as mentioned shows exemplary curves illustrating a connection between SCR efficiency (in %) and NOx engine out level (that is NOx concentration before SCR conversion). For higher engine out NOx levels (reducing CO2 emission) a higher engine out NOX level is produced that demands higher target SCR efficiency to meet the emission targets and a lower fault threshold for fault detection. That is, when a SCR efficiency monitor determines the SCR efficiency is below a certain threshold, the NOx reduction will be produced by increasing the EGR recirculation by switching the EGR valve controller 300 switches to a low EGR mode. Efficiency can be measured as conversion efficiency, e.g. a number between 80 and 100%.

Figure 3 shows an exemplary trace of NOx concentration measured with upstream NOx sensor 210 illustrated in Figure 1. As the NOx concentration in high EGR mode has a certain base level, in the intrusive step’ mode the NOx concentration is increased to about 1.5 times the base level concentration, or even to more than 2 times of the base level, by adjustment of the EGR valve 107 via controller 300 to the low EGR mode. The adjustment is e.g. carried out in such a way that the amount of NOx in the low EGR mode is increased to a level above 1.2 times the high EGR mode or even increased to a level of above 2 e.g. 3 times the high EGR mode. SCR efficiency monitor 310 is programmed to have a defined efficiency threshold below which an SCR fail criterion is raised. However, by comparing the NOx sensor values of upstream (210) and downstream (211) sensors an SCR efficiency is traced from 88 % in standard NOx mode, to about 96 in the ‘high efficiency mode’. In this exemplary trace, the high efficiency mode is sufficient detecting an efficiency level above a threshold; and a pass level is signaled by efficiency monitor 310 to controller 300.

Figure 4 shows an exemplary trace, where the filtered SCR efficiency (%) does not improve. The NOx sensor 211 may in a passive mode have previously identified that the Normalized Catalyst InEfficiency (NCIE) is above a threshold. The NCIE is calculated as:

NCIE =[100%-SC/? efficiency ^[Normalization Factor#] wherein (#) the normalization factor is a function of SCR bed temperature, exhaust gas space velocity and amplitude to noise ratio. It is noted that the NCIE value is one of the possible ways of expressing the efficiency or inefficiency of the SCR filter, and the described method is not limited thereto.

In an exemplary method, after multiple (e.g. 2, 3, 4 or 5) times passively measuring an NCIE value above a Passive Fault Threshold, starting an intrusive step, and measuring the NCIE value to conform or reject a passive efficiency test result.

Similarly, the catalytic filter in aftertreatment system 108 can be monitored by the SCR efficiency monitor 310 by having a number of times, e.g. .3-6 times passed the NCIE value < Passive Fault threshold criterion.

In Figure 4 it is shown to remain at about 84 %, irrespective of increased engine out NOx level. Thus, it is shown that in the intrusive step, wherein the EGR valve controller switches to a low EGR mode, the SCR out NOx level (NOx sensor 211 in Figure 1) detects an increased level which may be a level above the NOx tolerance level. In this intrusive step accordingly, in this example, the NOx sensor measures a sensing value with an enhanced resolution e.g. to confirm a SCR fail criterion. Alternatively, the EGR valve controller can be configured to provide the intrusive, low EGR mode in a time based manner. For example the controller can be configured to trigger the intrusive mode for every new key-cycle or drivecycle, every x hours or another time based program.

Figure 5 shows a schematic diagram of the exemplary method described previously. In the ‘SCR Eff monitor individual update’ phase, three times in a row a fail criterion is measured in the standard NOx mode, with high EGR F(p) recirculation in passive mode. This fail criterion is confirmed (or, as the case may be: rejected) in a subsequent intrusive step F(i), wherein the fail criterion is measured in the SCR high efficiency mode, i.e. with low EGR circulation. When a fail criterion is established, a malfunction signal MIL is activated and the ‘in use performance ratio’ (IUMPR) is adjusted in the on board diagnostic system.

Figure 6 shows an exemplary flow chart of the SCR monitor operation. After engine start 610 a counter is reset of previous test events in 611. Subsequently in 612 it is determined in a passive mode, with normal (high) EGR conditions; whether enable conditions are met to carry out the subsequent efficiency measurements. This is for example determined by measuring a catalytic bed temperature or measuring normal operating conditions. Then, in 613 NOx samples are collected until a certain number is achieved; e.g. 400 samples. If the number of samples is enough, in 613 a passive NCIE value is determined 614 and compared in 615 with a threshold criterion. The counter is updated 615. If the count reaches a decision threshold, the threshold is compared with a pass-threshold 620 or with a fail threshold. If the no fault was detected in the previous cycle 622 a pass decision is declared in 623. Accordingly, it is determined in 624 whether a new drive cycle is started - if yes, than a new cycle starts in 612.

Conversely, if the decision equals the fail-threshold in 621, the intrusive mode is triggered in 630. In the intrusive mode a similar cycle is started as in the passive mode 612 but now with the EGR mode in low condition, that is, with increased NOx production. Step 632 determines whether testing conditions are met; and if so, NOx samples are collected in 633 and a NCIE value is calculated with an enhanced signal to noise ration in 634. This NCIE value is compared with the fail threshold in 635; if a fail condition is declared 636 than the cycle ends in 637. When declaring the fail condition, this may affect updating the IUMPR and MIL status. If no fail condition is declared, the process directs to 623 where a pass decision is declared.

It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which may be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and can be within the scope of the invention. In the claims, any reference signs shall not be construed as limiting the claim. The terms 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus expression as 'including' or ‘comprising’ as used herein does not exclude the presence of other elements, additional structure or additional acts or steps in addition to those listed. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may additionally be included in the structure of the invention without departing from its scope. Expressions such as: means for ...” should be read as: component configured for ... or member constructed to ... and should be construed to include equivalents for the structures disclosed. The use of expressions like: critical, preferred, especially preferred etc. is not intended to limit the invention. To the extend that structure, material, or acts are considered to be essential they are inexpressively indicated as such. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the scope of the invention, as determined by the claims.

Claims

Conclusions

An engine with an engine air inlet and an outlet for producing an exhaust gas comprising:

- an exhaust gas recirculation system (EGR) with an EGR valve that determines an amount of exhaust gas to be recirculated in the engine inlet;

- a post-treatment system for treating the exhaust gas comprising a selective catalytic reduction (SCR) catalyst;

- at least one NOx sensor operatively connected to the after-treatment system and adapted to determine an amount of NOx in the exhaust gas;

- an SCR efficiency monitor controller that is operatively coupled to the at least one NOx sensor; capable of measuring an SCR efficiency based on an NOx measurement value of the at least one NOx sensor and a reference value obtained from a second sensor wherein the SCR efficiency monitor has a defined efficiency threshold value below which an SCR failure criterion is flagged up;

- an EGR valve controller that controls the EGR valve; wherein the controller is adapted to provide a high EGR mode in which a first fraction of exhaust gas is recycled into the engine inlet and configured to provide a low EGR mode in which a second fraction of exhaust gas is recycled; wherein the second fraction is smaller than the first fraction;

- wherein the EGR valve controller switches after a low EGR mode and where in the low EGR mode a NOx sensor measures a measured value with an improved resolution.

An engine according to claim 1, wherein the EGR valve controller

5 is configured to provide a low EGR mode for confirming the SCR failure criterion when the SCR efficiency monitor controller detects an efficiency value below a threshold.

A motor according to claim 1, wherein the EGR valve controller

10 is configured to provide the low EGR mode in a time based manner.

A motor according to any one of the preceding claims, wherein the second fraction is smaller than the first fraction in such a way that the amount of NOx in the low EGR mode is increased to a

15 level above 1.2 times the high EGR mode.

A motor according to any one of the preceding claims, wherein the second fraction is smaller than the first fraction in such a way that the amount of NOx in the low EGR mode is increased to a level above 2 times the high EGR mode.

1/6

202

NOx

Compressor (101)

π (206)

System (wg)

Aftertreatment (108)

EGR cooler

EGR valve (107)

11 / / SCR efficiency monitor 310 NCIE pass / fail