WO2008037318A1

WO2008037318A1 - Method of operating an exhaust gas recirculation system

Info

Publication number: WO2008037318A1
Application number: PCT/EP2007/006985
Authority: WO
Inventors: Douglas Trombley; Thomas Wagner
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2006-09-28
Filing date: 2007-08-08
Publication date: 2008-04-03
Anticipated expiration: 2009-03-28
Also published as: DE102006046341A1

Abstract

The present invention comprises a method of operating an exhaust gas recirculation (EGR) system of a combustion engine of a vehicle, said EGR system comprising said combustion engine having an intake manifold and an exhaust manifold, an EGR conduit fluidly coupled between said intake and exhaust manifolds, an EGR cooler for cooling of exhaust gas disposed in-line with said EGR conduit such that exhaust gas flowing through said EGR conduit also flows through said EGR cooler, and a flow rate control valve for determining a flow rate of exhaust gas through said EGR conduit, said flow rate control valve being controlled by an electronic control unit, said electronic control unit being provided with a relationship between different EGR cooler ages and exhaust gas flow rates such that a concentration of a selected exhaust gas component is below a selected threshold value, said method comprising the steps of : determining of the age of a same EGR cooler, and modifying an exhaust gas flow rate based on said relationship and in response to the determined age of the EGR cooler.

Description

METHOD OF OPERATING AN EXHAUST GAS RECIRCULATION SYSTEM

FIELD OF THE INVENTION

The present invention generally relates to control strategies for electronically controlled exhaust gas recirculation systems (EGR systems) such as those used within Diesel combustion engines and particularly relates to a method of operating such an EGR system.

BACKGROUND ART

When combustion occurs in an environment with excess oxygen, peak combustion temperatures increase which leads to the formation of unwanted emissions, such as oxides of nitrogen (NO_x) .

In most industrialized countries, NO_x emissions of combustion engine driven vehicles must be below certain threshold values as specified in various government emission standards to reduce harmfull effects of exhaust gas on the environment.

In order to fulfill government regulations, in recent years, more and more cars have been equipped with electronically controlled EGR systems operable to selectively introduce (recirculate) exhaust gas from the exhaust manifold into the fresh air stream flowing to the intake manifold via a controllable EGR valve that regulates the amount of exhaust gas to be mixed with the engine induction air. More specifically, exhaust gas that contains unburned fuel and other impurities is mixed with the fresh engine induction air to reduce the peak combustion temperature to thereby reduce the formation of NO_x. EGR systems optionally also include an EGR cooler for cooling the exhaust gas before mixing it with the engine induction air to thereby achieve further NO_x reduction. EGR coolers typically include a water-to-gas heat exchanger using the coolant water of the combustion engine.

However, in durability vehicles, it has been observed that EGR coolers experience a reduction in efficiency as they age. More particularly, exhaust gas outlet temperature of exhaust gas to be mixed with fresh engine induction air increases due to a reduction of the heat transfer between the exhaust gas and the coolant water. As is known from those of skill in the art, such reduction of heat transfer is caused by a buildup of soot on the internal surfaces of the EGR cooler which reduces the heat transfer coefficient.

Reference is now made to FIG. 1 depicting a diagram which illustrates an exemplary relationship of exhaust gas outlet tempera- tures varying with combustion engine running time as measured by operating hours of the engine. FIG. 1 illustrates three different calibrations of the EGR system having different exhaust gas flows. As can be taken from FIG. 1, for each calibration, as operating time of the engine increases, an increase of the exhaust gas outlet temperature can be observed. Such increased exhaust gas temperature causes an increase in the overall charge temperature of the exhaust gas mixed with the engine induction air thus resulting in an undesired increase in NO_x emissions (see also FIG. 2) .

Reference is now made to FIG. 2 depicting a diagram which illustrates an exemplary relationship between NO_x emissions (arbi- can be taken from FIG. 2, NO_x emissions generally increase with increasing mileage of the vehicles. As given by the arrow, in one of the vehicles, a new EGR cooler has been installed after a mileage of 50 000 kilometers resulting in a reduction of the NO_x emissions to an initial value as observed at a mileage of 0 (zero) kilometer. This clearly shows that the increase of NO_x emissions with mileage of the vehicle is overwhelmingly due to the aging of the EGR cooler.

Conventionally, in order to safely fulfill government regulations at the mileage requirement, in view of the increasing NO_x emissions due to aging of the EGR cooler, calibration of any new EGR system is made using a mileage requirement aged EGR cooler. Using such a fully-aged EGR cooler in EGR system calibration, it can be ensured that NO_x emission is below specific governmental NO_x emission regulations at the vehicle mileage requirement of for example 100000 km (as in the "Euro 4" regulation of the European Community) or 160 000 km (as in the "Euro 5" regulation of the European Community) . In order to compensate for the higher EGR exhaust gas temperatures experienced with the aged EGR cooler, an increased EGR exhaust gas flow compared with a fresh (new) EGR cooler is required to thereby further decrease oxygen content necessary for the combustion of fuel.

However, using a fresh EGR cooler in an EGR system that has been calibrated using a fully-aged EGR cooler, a problem occurs that the increased EGR exhaust gas flow as set by calibration in combination with the low exhaust gas temperatures due to the yet high efficiency of the fresh EGR cooler results in unstable combustion in some engine operating conditions. Due to incomplete combustion, this can result in an undesired high content of hydrocarbons and CO in the exhaust gas. In some cases, the driver or jerking. While this condition will go away with some mileage of the vehicle as the soot buildup in the EGR cooler reduces its efficiency and causes the EGR exhaust gas temperature to rise, the situation until this happens is not acceptable.

In order to overcome such a drawback, according to the present invention, a method of operating an EGR system is disclosed.

SUMMARY OF THE INVENTION

In light of the above, according to the present invention, a method of operating an electronically controlled exhaust gas recirculation system of a combustion engine of a vehicle is disclosed.

The EGR system comprises a combustion engine having an intake manifold and an exhaust manifold and an EGR conduit fluidly coupled between the intake and exhaust manifolds. The EGR system further comprises an EGR cooler for cooling of exhaust gas disposed in-line with the EGR conduit such that exhaust gas flowing through the EGR conduit also flows through the EGR cooler. The EGR system further comprises a flow rate control valve for determining a flow rate of exhaust gas through the EGR conduit with the flow rate control valve being controlled by an electronic control unit. The electronic control unit is provided with (e.g. stores) a relationship between various EGR cooler ages and critical (minimum) exhaust gas flow rates such that a concentration of a selected exhaust gas component, which preferably is chosen to be oxides of nitrogen (NO_x) , is below a selected threshold value.

In the method of the present invention, for a same EGR cooler is determined (or at least estimated) using an aging determination means and a critical (i.e. minimum) exhaust gas flow rate is modified (for instance periodically) based on the established relationship in response to the determined age of the EGR cooler.

According to a preferred embodiment of the invention, for establishing above relationship between various EGR cooler ages and critical exhaust gas flow rates, for each of a plurality of differently aged EGR coolers, a critical (i.e. minimum) exhaust gas flow rate is determined such that a concentration of a selected exhaust gas component is below a selected threshold value. The differently aged EGR coolers preferably include a "fully-aged" EGR cooler, that is defined to be one that has been run on an engine with a representative calibration out to the maximum mileage requirement as specified by government regulations, and less aged EGR coolers down to a fresh EGR cooler.

According to another preferred embodiment of the invention, in response to the determined or estimated age of the EGR cooler, a critical exhaust gas flow rate preferably is modified by multiplying the critical gas flow rate of a selectably maximum aged EGR cooler (fully-aged EGR cooler) with an EGR exhaust gas flow multiplication factor, with the EGR exhaust gas flow multiplication factor being obtained by dividing critical exhaust gas flow rates of EGR coolers being younger than the fully-aged EGR cooler by the critical exhaust gas flow rate of the fully-aged EGR cooler.

According to yet another preferred embodiment, the age of the EGR cooler is determined based on a determination of opera- According to yet another preferred embodiment, the age of the EGR cooler is determined based on a determination of vehicle mileage .

According to yet another preferred embodiment, the age of the EGR cooler is determined based on a determination of the EGR cooler outlet gas temperature since cooler outlet gas temperature typically increases with cooler age. To this aim, the EGR system comprises an exhaust gas temperature sensor for sensing of EGR cooler outlet gas temperatures. In such embodiment, the electronic control unit, apart from being provided with a relationship between various EGR cooler ages and critical (minimum) exhaust gas flow rates such that a con- centration of a selected exhaust gas component is below a selected threshold value, is also provided with a relationship of different EGR cooler ages and corresponding cooler outlet gas temperatures. Alternatively, the electronic control unit may be provided with a relationship of different EGR cooler outlet gas temperatures and critical (minimum) exhaust gas flow rates such that a concentration of a selected exhaust gas component is below a selected threshold value. In such embodiment, for a same EGR cooler that is used in the EGR system, the cooler outlet gas temperature is determined and a critical (minimum) exhaust gas flow rate is modified based on the established relationship in response to the determined EGR cooler outlet gas temperature. In order to establish above relationship between various EGR cooler outlet gas temperatures and critical exhaust gas flow rates, it may be preferable to determine cooler outlet gas temperatures and critical (minimum) exhaust gas flow rates such that a concentration of a selected exhaust gas component is below ently aged EGR coolers. Upon doing so, the differently aged EGR coolers preferably include a fully-aged EGR cooler and less aged EGR coolers down to a fresh EGR cooler. In such embodiment, it may also be preferred, that, in response to the determined cooler outlet gas temperature, a critical exhaust gas flow rate is modified by multiplying the critical gas flow rate of a selectably maximum aged EGR cooler (fully-aged cooler) with an EGR exhaust gas flow multiplication factor, with the EGR exhaust gas flow multiplication factor being obtained by dividing critical exhaust gas flow rates of EGR coolers being younger than the fully-aged EGR cooler by the critical exhaust gas flow rate of the fully-aged EGR cooler.

The present invention further relates to an electronic control unit for controlling the flow rate control valve of an EGR system installed to run a computer program that includes logic instructions that cause the control unit to perform the above-described method. It, yet further, relates to an exhaust gas recirculation system comprising such an electronic control unit.

The present invention further relates to a computer program for an electronic control unit controlling the flow rate control valve of an EGR system that includes logic instruc- tions that cause the control unit to perform the above-described method. It, yet further, relates to a computer program product storing a computer program as described above.

Other and further objects, features and advantages of the invention will appear more fully from the following description . BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the principles of the invention.

FIG. 1 is a diagram which shows a relationship of exhaust gas outlet temperature varying with combustion engine running time for three different calibrations of the EGR system;

FIG. 2 is a diagram which shows a relationship between NO_x emissions (arbitrary units) and vehicle mileage for two durability vehicles;

FIG. 3 depicts a schematic diagram of a combustion engine exhaust system including an EGR system;

FIG. 4 is a diagram which shows a relationship between EGR exhaust gas flow modifier and combustion engine running time.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIGS. 1 and 2 have already been explained in the introductory portion for which reason, in order to avoid unnecessary repetitions, further explanation thereof is omitted. First, reference is made to FIG. 3 which depicts a schematic dia^¬ gram of an internal combustion engine exhaust system including an EGR system according to the present invention.

Accordingly, the system includes an internal combustion engine 1, for instance Diesel engine, having an intake manifold 2 fluidly coupled to an outlet of a compressor 3 of a turbocharger 4 via an intake conduit 5. The compressor 3 includes a compressor inlet coupled to an ambient air intake conduit 6 for receiving fresh ambient air to be introduced into the intake manifold 2. The system also includes an intake air intercooler 7 disposed in-line with intake conduit 5 between the turbocharger compressor 3 and the intake manifold 2.

The turbocharger compressor 3 is mechanically coupled to a turbocharger turbine 8 via a drive shaft 9, wherein the turbocharger turbine 8 includes a turbine inlet fluidly coupled to an exhaust manifold 10 of combustion engine 1 via an exhaust conduit 11. The turbocharger turbine 8 further includes a turbine outlet fluidly coupled to ambient via an exhaust conduit 12 with a pre-catalyzing unit 13 and a main catalyzing unit 14 disposed in-line with exhaust conduit 12 between the turbocharger turbine 8 and ambient.

An EGR valve 15 includes an EGR inlet fluidly coupled to one end of an EGR conduit 16, wherein EGR conduit 16 has an opposite end fluidly coupled to an exhaust gas outlet orifice of an EGR cooler 17. The EGR valve 15 further includes an EGR outlet fluidly coupled to the intake conduit 5 via another EGR conduit 19. The EGR cooler 17 also includes an exhaust gas inlet orifice fluidly coupled to the exhaust manifold 10 via yet another EGR conduit 18. The EGR cooler 17 is fluidly coupled to the combustion engine cooling system via fluid carrying conduits (not shown in FIG. 3) . Thus, coolant fluid circulating through the combustion engine coolant system circulates through the EGR cooler 17 to cool exhaust gas flowing there through.

The system further includes a microprocessor based engine control unit (ECU) 20 generally operable to control and manage the overall operation of the combustion engine 1. ECU 20, for instance, includes a memory unit (not shown in FIG. 3) that includes various control algorithms for operating the combustion engine 1 as well as a number of inputs and outputs for interfacing with various sensors and system components coupled to the combustion engine 1.

The engine control unit 20 includes a number of inputs for receiving signals from various sensors associated with the system.

For instance, the system includes an air flow sensor 21 disposed in fluid communication with ambient air intake conduit 6 and electrically connected to an air flow input of ECU 20 via signal path 22. Air flow sensor 21 is operable to produce an air flow signal on signal path 22 indicative of the air flow of air flowing through the ambient air intake conduit 6.

The system further includes a temperature sensor 23 disposed in fluid communication with exhaust gas flowing through pre-catalyzing unit 13 and electrically connected to a tempera- ture input of ECU 20 via signal path 24. Temperature sensor 23 is operable to produce a temperature signal on signal path 24 indicative of the temperature of exhaust gas flowing through the The system further includes a pressure drop sensor 25 disposed in fluid communication with exhaust gas flowing through main-catalyzing unit 13 and electrically connected to a pressure drop input of ECU 20 via signal path 26. Pressure drop sensor 25 is operable to produce a pressure drop signal on signal path 26 indicative of the pressure drop of exhaust gas flowing through the main-catalyzing unit 14.

Engine control unit 20 further includes a number of outputs for controlling engine functions associated with the exhaust system.

For instance, the system includes an EGR valve actuator 27 electrically connected to an EGR valve control output of ECU 20 via signal path 28. ECU 20 is operable to produce an EGR valve 15 control signal on signal path 28 with the EGR valve actuator 27 being responsive to the EGR valve control signal to control the position of the EGR valve 15. Accordingly, ECU 20 is operable to control the EGR valve 15 to selectively provide a flow of recir- culated exhaust gas from exhaust manifold 10 to intake manifold 2.

The system further includes a variable geometry turbocharger (VGT) actuator 30 electrically connected to a VGT control output of ECU 20 via signal path 29. The VGT actuator 30 may be embodied as any combination of mechanical or electromechanical mechanisms to selectively control the swallowing capacity (efficiency) of the turbocharger 4, for instance in order to modify the effective geometry of the turbocharger turbine 8. Accordingly, ECU 20 is operable to produce a turbocharger actuator control signal on signal path 29 with the turbocharger actuator 30 being responsive to the turbocharger actuator control signal to selectively The system further includes a throttle valve 32 disposed in-line with intake conduit 5 between intercooler 7 and intake manifold 2 for regulating the amount of ambient air flowing towards the intake manifold 2. Throttle valve 32 is electrically connected to a throttle valve control output of ECU 20 via signal path 33. ECU 20 is operable to produce a throttle valve control signal on signal path 33 with the throttle valve 32 being responsive to the throttle valve control signal to control the position of the throttle valve 32. Accordingly, ECU 20 is operable to control the throttle valve 32 to selectively provide a flow of ambient air to intake manifold 2.

ECU 20 further includes a combustion engine control output electrically connected via signal path 31 to combustion engine 2 for control of various operations of combustion engine 2. Accordingly, ECU 20 is operable to produce combustion engine control signals on signal path 31 with combustion engine actuators (not shown in FIG. 1.) such as injection valve actuators being responsive to the combustion engine control signals in a known manner. Accordingly, ECU 20 is operable to selectively control operation of the combustion engine 1.

ECU 20 is further operable to control EGR valve 15 to determine flow rate of the exhaust gas such that NO_x emission is below a selectable threshold value.

To this end, before using EGR cooler 17 in the EGR system, for a plurality of differently aged EGR coolers including a fully-aged EGR cooler that is defined to be one that has been run on an engine with a representative calibration out to the maximum mileage requirement as specified by government a concentration of NO_x is below a given threshold value as specified in the government regulations. Typically, starting with fully-aged EGR cooler, the EGR system is calibrated to a flow rate level which gives the desired NO_x value below the emission requirement specified in the government regulations including some safety margins. Then, another EGR cooler with lower aging is installed in the EGR system and the EGR exhaust gas flow rate (calibration value) is adjusted (decreased) to achieve the same NO_x as with the fully-aged EGR cooler. This step is repeated several times using EGR coolers having ever lower agings till a fresh (new) EGR cooler is installed into the EGR system, and, for each of these EGR coolers, the EGR exhaust gas flow rate (calibration value) is adjusted (decreased) to achieve the same NO_x as with the fully-aged EGR cooler.

The measured calibration values then are used to populate a calibration table to establish a relationship between various EGR cooler ages and exhaust gas flow rates.

After that, a fresh EGR cooler is installed in EGR system, and, for that same EGR cooler, the age of the EGR cooler is determined and the exhaust gas flow rate is modified based on the established relationship in response to the determined age of the EGR cooler.

Reference is now made to FIG. 4 which is a graph (calibration diagram) establishing a relationship between EGR exhaust gas flow modifier (multiplication factor) and combustion engine running time. The multiplication factor is obtained dividing the determined exhaust gas flow rate of any younger EGR cooler by the determined exhaust gas flow rate of the fully-aged EGR multiplying the current flow rate with the multiplication factor belonging to the actual aging of the EGR cooler.

For finding the calibration table, engine running hours were used as the estimator for EGR cooler aging. Alternatively, vehicle mileage could also be used for the abscisse values as this is normally a stored value in the engine control unit.

According to own experiments, engine running time was found to be a better estimator of EGR cooler aging than vehicle mileage.

Such calibration diagram represents the simplest implementation of the method of the invention. It is a single global modifier on the entire base EGR flow calibration map, which is normally a 3-dimensional table. More complex schemes could be used, such as a second 3-dimensional table which would modify the EGR flow rate differently in different areas of the base map. For even better accuracy, a temperature sensor could be installed to measure the EGR cooler outlet gas temperature. By comparing this value at different engine operating conditions with a table of temperature values obtained from a fresh EGR cooler, an accurate estimation of the EGR cooler aging could obtained and used as a better input to the EGR flow modifier table (s) instead of vehicle mileage or engine running hours.

Engine control unit 20 is operable (programmed) to adjust exhaust gas flow rate in response to the determined age of the EGR cooler, which can be done periodically, controlling the opening degree of the EGR valve 15.

The inventors also found that the constituents in the exhaust higher hydrocarbons and soot in the exhaust caused increased soot buildup rate inside the cooler and thus resulted in a faster aging rate. Those of skill in the art will recognize that increasing EGR flow rate reduces NO_x, however increases hydrocarbons and soot, therefore increasing the soot buildup rate in the EGR cooler. Therefore, the proposed method which reduces the EGR flow rate initially will actually result in a slower soot buildup rate in the EGR cooler and thus result in reduced EGR cooler aging. An additional benefit is reduced soot-loading of the Diesel Particulate Filter.

Obviously many modifications and variations of the present invention are possible in light of the above description. It is therefore to be understood, that within the scope of appended claims, the invention may be practiced otherwise than as specifically devised.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one of skill in the art that various changes and modifications can be made therein without departing from the scope thereof. Accordingly, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

List of reference signs

1 Combustion engine

2 Intake manifold

3 Compressor

4 Turbocharger

5 Intake conduit

6 Ambient air intake conduit

7 Intercooler

8 Turbine

9 Drive shaft

10 Exhaust manifold

11 Exhaust conduit

12 Exhaust conduit

13 Pre-catalyzing unit

14 Main-catalyzing unit

15 EGR valve

16 EGR conduit

17 EGR cooler

18 EGR conduit

19 EGR conduit

20 Engine control unit

21 Air flow sensor

22 Signal path

23 Temperature sensor

24 Signal path

25 Pressure drop sensor

26 Signal path

27 EGR valve actuator

28 Signal path

29 Signal path

30 VGT actuator

31 Signal path

32 Throttle valve

33 Signal path

Claims

1. A method of operating an exhaust gas recirculation (EGR) system of a combustion engine (1) of a vehicle, said EGR system comprising: said combustion engine (1) having an intake manifold (2) and an exhaust manifold (10), an EGR conduit (18, 16, 19) fluidly coupled between said intake and exhaust manifolds, an EGR cooler (17) for cooling of exhaust gas disposed in-line with said EGR conduit such that exhaust gas flowing through said EGR conduit also flows through said EGR cooler, and a flow rate control valve (15) for determining a flow rate of exhaust gas through said EGR conduit, said flow rate control valve being controlled by an electronic control unit

(20), said electronic control unit (20) being provided with a relationship between different EGR cooler ages and critical exhaust gas flow rates such that a concentration of a selected exhaust gas component is below a selected threshold value, said method comprising the steps of: determining of the age of said EGR cooler, and modifying a flow rate of exhaust gas flowing through said EGR conduit to be a critical exhaust gas flow rate based on said relationship and in response to the determined age of the EGR cooler.

2. The method of claim 1, wherein the age of the EGR cooler is determined based on a determination of operation time of the combustion engine.

3. The method of claim 1, wherein the age of the EGR cooler is determined based on a determination of vehicle mileage.

4. The method according to one of the preceding claims, wherein, for establishing said relationship between EGR cooler ages and critical exhaust gas flow rates, for each of a plurality of differently aged EGR coolers, a critical exhaust gas flow rate such that a concentration of a selected exhaust gas component is below a selected threshold value is determined.

5. The method according to one of the preceding claims, wherein, in response to the determined age of the EGR cooler, a critical exhaust gas flow rate is modified by multiplying the critical gas flow rate of a selectably maximum aged EGR cooler with an EGR exhaust gas flow multiplication factor, said EGR exhaust gas flow multiplication factor being obtained by dividing critical exhaust gas flow rates of EGR coolers being younger than said selectably maximum aged EGR cooler by the critical exhaust gas flow rate of said selectably maximum aged EGR cooler.

6. The method of claim 1, wherein the age of the EGR cooler is determined based on a determination of the EGR cooler outlet gas temperature.

7. The method of claim 6, wherein said electronic control unit is provided with a relationship of different EGR cooler outlet gas temperatures and critical exhaust gas flow rates with said critical exhaust gas flow rate of a same EGR cooler being modified in response to the determined EGR cooler outlet gas temperature.

8. The method of claim I₁ wherein, in response to the determined EGR cooler outlet gas temperature, a critical exhaust gas flow rate is modified by multiplying the critical gas flow rate of a selectably maximum aged EGR cooler with an EGR exhaust gas flow multiplication factor, said EGR exhaust gas flow multiplication factor being obtained by dividing critical exhaust gas flow rates of EGR coolers being younger than said selectably maximum aged EGR cooler by the critical exhaust gas flow rate of said selectably maximum aged EGR cooler.

9. The method according to any one of the preceding claims, wherein said exhaust gas component is selected to be oxides of nitrogen (NO_x) .

10. Electronic control unit for controlling the flow rate control valve of an EGR system installed to run a computer program that includes logic instructions that cause the control unit to perform the method according to anyone of the preceding claims 1 to 8.

11. Exhaust gas recirculation system comprising an electronic control unit according to claim 10.

12. Computer program for an electronic control unit for controlling the flow rate control valve of an EGR system that includes logic instructions that cause the control unit to perform the method according to anyone of the preceding claims 1 to 8.

13. Computer program product storing a computer program according to claim 12.