US20150143802A1

US20150143802A1 - System and method of controlling exhaust temperature

Info

Publication number: US20150143802A1
Application number: US14/091,668
Authority: US
Inventors: Spencer L. Huhn; Joshua C. Davis; Drew E. Heisel
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2013-11-27
Filing date: 2013-11-27
Publication date: 2015-05-28
Also published as: DE102014017456A1

Abstract

An internal combustion engine system is provided. The system includes an engine defining a combustion chamber, a clean emission module and an air supply unit. The clean emission module is fluidly connected to an exhaust conduit from the combustion chamber. The air supply unit is configured to supply air into the combustion chamber and includes a compressor and a flow control device provided between the compressor and the combustion chamber. The compressor is configured to receive air from an air inlet via a first passage and supply the compressed air into the combustion chamber via a second passage. The flow control device is configured to selectively reduce the supply of compressed air into the combustion chamber such that an air fuel ratio in the combustion chamber is maintained between 15:1 to 18:1 at 20-90% engine load during a regeneration period for the clean emissions module.

Description

TECHNICAL FIELD

The present disclosure relates to an engine and more particularly to a system and method of controlling exhaust temperature of the engine.

BACKGROUND

Turbochargers for diesel and gasoline engines are well known. However, turbochargers vent out pollutants such as Diesel Particulate Matter (DPM) or soot, and Nitrogen Oxides (NOx). Therefore, to regulate such emissions, Diesel particulate filtration devices (DPF) and Diesel Oxidation Catalysts (DOC) are generally used.
DPFs filter the particulate matter from the exhaust gases to prevent them from entering the atmosphere. However, after a period of time and/or operation, the collected particulate matter starts to clog the filter. Therefore, the filter either needs to be replaced or removed for cleaning. However, as removing or replacing the filter every time is not feasible, a method known as regeneration is used. Since DPM is made up primarily of carbon, and is therefore combustible, regeneration process is used where temperatures of the exhaust gases are high enough to combust the DPM within the filter.
U.S. Pat. No. 8,099,953 relates to an apparatus and method for cooling an exhaust gas flow of an internal combustion engine of a truck or other vehicle includes a conduit connected to an air source, for example, a turbo driven compressor to deliver air to the exhaust stack or tail pipe. The conduit is controlled by a valve, which is opened and closed by a controller responsive to a measured exhaust gas temperature above a threshold and a vehicle speed below a threshold. The invention is particularly advantageous for engine exhausts having after treatment devices that require high temperature regeneration.

SUMMARY

In one aspect of the present disclosure, an internal combustion engine system is provided. The system includes an engine defining a combustion chamber, a clean emission module and an air supply unit. The clean emission module is fluidly connected to an exhaust conduit from the combustion chamber. The air supply unit is configured to supply air into the combustion chamber and includes a compressor and a flow control device provided between the compressor and the combustion chamber. The compressor is configured to receive air from an air inlet via a first passage and supply the compressed air into the combustion chamber via a second passage. The flow control device is configured to selectively reduce the supply of compressed air into the combustion chamber such that an air fuel ratio in the combustion chamber is maintained in a range from about 15:1 to 18:1 at 20-90% engine load during a regeneration period for the clean emissions module.
In another aspect of the present disclosure, a method of modulating air supply to a combustion chamber of an internal combustion engine to regenerate a clean emission module at an engine load of 20-90% is provided. The method includes providing a flow of air from an air inlet to a compressor via a first passage. Further, the method includes providing a flow of compressed air from the compressor to the combustion chamber of the engine via a second passage. Furthermore, the method includes adjusting a flow control device provided between the compressor and the combustion chamber to selectively reduce an amount of compressed air provided to the combustion chamber such that the air to fuel ratio in the combustion chamber is in a range from about 15:1 and 18:1 during a regeneration period.
In a yet another aspect of the present disclosure, an internal combustion engine system is provided. The system includes an engine defining a combustion chamber, a clean emission module and an air supply unit. The clean emission module is fluidly connected to an exhaust conduit from the combustion chamber. The air supply unit is configured to supply air into the combustion chamber and includes a compressor, a flow control device provided between the compressor and the combustion chamber, and a controller. The compressor is configured to receive air from an air inlet via a first passage and supply the compressed air into the combustion chamber via a second passage. The flow control device is configured to selectively reduce the supply of compressed air into the combustion chamber. The controller is operatively coupled to the flow control device and configured to control the flow control device based on a predetermined engine parameter such that an air fuel ratio in the combustion chamber is maintained in a range from about 15:1 to 18:1 at 20-90% engine load during a regeneration period for the clean emissions module.
Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of an engine system having a flow control device positioned according to an embodiment of the present disclosure;

FIG. 2 illustrates a schematic view of the engine system having the flow control device positioned according to an alternate embodiment of the present disclosure;

FIG. 3 illustrates a schematic view of an engine system having the flow control device positioned according to a yet another embodiment of the present disclosure; and

FIG. 4 illustrates a flowchart of a method of controlling exhaust temperature.

DETAILED DESCRIPTION

The present disclosure relates to a system and method of controlling exhaust temperature in an internal combustion engine. The present disclosure will now be described in detail with reference being made to accompanying figures. FIG. 1 illustrates a schematic representation of an engine system 100 of a machine (not shown), according to an embodiment of the present disclosure. Various embodiments described herein have been explained for a diesel engine. However, it may be contemplated that the described embodiments may be implemented with any type of spark-ignited engine such as a gasoline engine, a natural gas engine, a hybrid fuel engine, or an engine using gaseous fuels like propane, or methane. The engine system 100 includes an engine 102 having one or more cylinders 104 made of metallic alloys such as steel, aluminum based alloys, etc. Each of the cylinders 104 may define a combustion chamber 106 and includes a piston (not shown) adapted to reciprocate therein. Further, the engine 102 may include fuel injectors 108 to supply fuel into the respective combustion chambers 106. It may be understood by a person having ordinary skill in the art, that the number of combustion chambers shown in FIG. 1 is merely exemplary and may be varied without limiting the scope of the claimed subject matter.
The engine system 100 may include an air supply unit 110 to supply air into the combustion chamber 106. The air supply unit 110 includes a turbocharger 112 to provide compressed air to the combustion chamber 106. Although only one turbocharger 112 is shown in FIG. 1, it may be contemplated that more than one turbochargers may be included and disposed in parallel or in series relationship. The turbocharger 112 includes a compressor 114 configured to receive ambient air from an air inlet 115 via a first passage 116 and supply the compressed air to the combustion chamber 106 via second passage 118. In an embodiment, the first passage 116 and the second passage 118 may be in the form of intake manifolds. The second passage 118 may be configured to supply fluid to the combustion chamber 106. The fluid may be air, mixture of air and engine exhaust, etc. The compressor 114 may include a fixed geometry type compressor, a variable geometry type compressor or any other type of compressor known in the art.
Further, the compressed air from the compressor 114 is supplied to the combustion chamber 106 via a heat exchanger 120. The heat exchanger 120 may be configured to extract heat from the compressed air provided by the compressor 114 to lower the temperature of the compressed air in the second passage 118 and increase air density of the air provided to the combustion chamber 106. The heat exchanger 120 is fluidly connected to the combustion chamber 106 via a third passage 122. As will be understood by a person ordinarily skilled in the art that the third passage 122 may also be in the form of an intake manifold. In an exemplary embodiment of the present disclosure, the heat exchanger 120 may be an air-to-air aftercooler fluidly connected to the compressor 114. In various embodiments, the heat exchanger 120 may use a liquid coolant, such as water, or other cooling techniques known in the art.
The engine system 100 includes an exhaust system 124 configured to receive the exhaust from the engine 102 via an exhaust conduit 126. The exhaust system 124 further includes a turbine 128 of the turbocharger 112 configured to receive exhaust from the combustion chamber 106 via the exhaust conduit 126. For example, the exhaust conduit 126 may be in the form of exhaust manifolds. The turbine 128 may be a fixed geometry type turbine, a variable geometry type turbine or any other type of turbine known in the art. The turbine 128 is drivably connected to the compressor 114 by a turbocharger shaft 129 and configured to drive the compressor 114 to compress the ambient air.
In an aspect of the present disclosure, the exhaust system 124 may include a clean emission module 130 fluidly connected to the exhaust conduit 126 from the combustion chamber 106. The clean emission module 130 is configured to reduce an amount of diesel particulate matter (DPM) and other gaseous constituents in the exhaust from the combustion chamber 106. For example, the clean emission module 130 may include a diesel particulate filtration (DPF) device 132 and a Diesel Oxidation Catalyst (DOC) 134 configured to reduce the particulate content in the exhaust in the exhaust conduit 126.
Further, excess oxygen present in the air-fuel mixture results in generation of Nitrogen Oxides (NOx), which may be a pollutant. The clean emission module 130 of the exhaust system 124 may further include a Selective Catalytic Reduction (SCR) system 136 configured to reduce the NOx emissions in the exhaust conduit 126. Generally, the SCR system 136 uses catalytic solutions to convert the NOx into elemental nitrogen N₂and water H₂O. Example of such catalytic solutions used in the SCR system 136 includes Diesel Exhaust Fluids (DEF). SCR systems are sensitive to chemical impurities, therefore the DEF solution uses demineralized water, which makes the DEF solution clear, non-toxic and safe to use. In various other embodiments, the DEF may include urea, or aqueous ammonia. However, after a period of time, the DEF and/or the DPM may get accumulated within the clean emission module 130 such as within the SCR system 136 and/or the DPF 132.
In an embodiment of the present disclosure, the air supply unit 110 of the engine system 100 includes a flow control device 138 provided between the second passage 118 and the first passage 116. The flow control device 138 is in fluid communication with the second passage 118 to selectively divert the compressed air in the second passage 118 through an intermediate passage 117 and back into the first passage 116. Therefore, the flow control device 138 selectively reduces the supply of compressed air into the combustion chamber 106. In an exemplary embodiment of the present disclosure, the flow control device 138 may be an ON/OFF valve, or a variable position valve that is capable of modulation to any number of positions between the ON and OFF position.
It may be contemplated that during the regeneration process, a temperature of the exhaust gases, hereinafter interchangeably referred to as the exhaust temperature, exiting the engine 102 via the exhaust conduit 126 is elevated such that the DEF and the DPM in the clean emission module 130, the SCR system 136 and the DPF filter 132 are combusted to clean the clean emission module 130. The temperature of the exhaust gases exiting the engine 102 may be at least and/or partly dependent on the air-to-fuel ratio of the mixture combusted in the combustion chamber 106. Therefore, the flow control device 138 may reduce the amount of air provided in the combustion chamber 106, resulting in a reduced ratio of air with respect to the fuel in the air-fuel mixture provided in the combustion chamber 106, which may increase the temperature of the exhaust. In an exemplary embodiment of the present disclosure, for a load of about 20-90% on the engine 102 during the regeneration period for the clean emission module 130, the air-fuel ratio in the combustion chamber 106 is maintained in a range from about 15:1 to 18:1. This means that for every 15 or 18 units of air respectively, only 1 unit of fuel is used in the air-fuel mixture. In an alternative embodiment, the air fuel ratio may also be maintained accordingly for a load of upto 100% on the engine during the regeneration period.
In an embodiment of the present disclosure, a controller 140 is provided to control the operation of the flow control device 138 based on a desired engine parameter and/or when regeneration is required. As will be understood by a person ordinarily skilled in the art that the controller 140 may be a machine engine control module (ECM). The air flow from the second passage 118 to the first passage 116 may be regulated by using the flow control device 138, such as to limit the diverting to those situations when it is desirable to elevate the temperature of the exhaust gases. In an exemplary embodiment, the controller 140 may communicate with pressure sensors associated with the clean emission module 130 and/or the DPF filter 132 to monitor a pressure change across the clean emission module 130, to determine when the regeneration is required.
In an exemplary embodiment, the engine parameter may be a desired temperature of the exhaust, based on which, the controller 140 may selectively control the operations of the flow control device 138 to correspondingly regulate the amount of air supplied to the combustion chamber 106. In other words, the controller 140 may control the operations of the flow control device 138 to control the exhaust temperature in the exhaust conduit 126. Further, by diverting the compressed air in the second passage 118 back into the first passage 116, a temperature of inlet air provided to the compressor 114 may also be increased, which may further result in an increased exhaust temperature.
In an exemplary aspect of the present disclosure, the controller 140 may communicate with a sensor assembly 142 associated with the clean emission module 130 of the exhaust system 124 to determine the exhaust temperature. For example, the sensor assembly 142 may include temperature sensors (not shown) associated with each of the DPF filter 132, the DOC 134, the SCR system 136. The sensor assembly 142 may be configured to monitor the temperature of the exhaust gases in the exhaust conduit 126 and/or the clean emission module 130 to provide output signals indicative of the monitored temperature to the controller 140.
Based on the temperature of the exhaust gases in the exhaust conduit 126 received from the sensor assembly 142, the controller 140 may determine the amount of compressed air to be provided into the combustion chamber 106. For example, if the temperature of the exhaust gases is high enough to combust the accumulated DEF and the DPM in the DPF filter 132, then the controller 140 may completely close the flow control device 138 to allow compressed air to be provided to the combustion chamber 106. It may be contemplated that during high operational loads on the engine 102, the exhaust temperature is high enough to regenerate without assistance.
However, when the exhaust temperature is low, such as during light or highly cyclic loads, or when the ambient temperatures are low, the controller 140 may be configured to regulate the amount of compressed air provided to the combustion chamber 106 by selectively controlling the flow control device 138 to further elevate the exhaust temperature accordingly.
Further, the high temperature exhaust gases in the exhaust conduit 126 may be supplied to the clean emission module 130 to facilitate regeneration process to combust the accumulated DEFs and the DPMs. Regeneration is typically run for a predetermined amount of time. The controller 140 may include a timer function to determine when the regeneration is complete. Alternatively, the controller 140 may communicate with pressure sensors associated with the clean emission module 130 to monitor a pressure change across the clean emission module 130 to determine when the regeneration is completed.
In an exemplary aspect of the present disclosure, the engine system 100 may also include a NOx reduction Strategy (NRS) cooler 144 configured to cool a portion of the exhaust gases exiting from the engine 102 which is mixed with clean air and fed back to the combustion chamber 106.
FIG. 2 illustrates a schematic representation of the engine system 100 having the flow control device 138 positioned at another position, according to an alternate embodiment of the present disclosure. As shown in FIG. 2, the flow control device 138 may be fluidly connected to the second passage 118 and configured to selectively divert the compressed air from the compressor 114 in the second passage 118 to an exhaust conduit 202 exiting from the clean emission module 130 of the exhaust system 124 via a passage 204 or, alternatively, from the second passage 118 through the intermediate passage 117 and back into the first passage 116. Therefore, the amount of compressed air provided to the combustion chamber 106 may be decreased resulting in the increased exhaust temperature. Further, when the regeneration is completed, the exhaust gas exiting the clean emission module 130 via the exhaust conduit 202 may be diluted and/or cooled down by the diverted air from the second passage 118.
Furthermore, when the regeneration is completed, the clean emission module 130 may need to be cooled down. Therefore, the controller 140 may be configured to monitor a temperature at the exit of the clean emission module 130 and speed of the engine 102 to accordingly regulate the flow control device 138 to divert the compressed air from the second passage 118 into the exhaust conduit 202 exiting the clean emission module 130. For example, if the speed of the engine 102 is less that a predetermined value, to indicate that the machine is at an idle state or is travelling at a low speed, then the controller 140 may open the flow control device 138 to divert the compressed air to dilute and cool the exhaust gases exiting the clean emission module 130, and the machine.
FIG. 3 illustrates a schematic representation of the engine system 100 having the flow control device 138 positioned at another position, according to a yet another embodiment of the present disclosure. As shown in FIG. 3, the flow control device 138 may be fluidly connected to the third passage 122 between the heat exchanger 120 and the combustion chamber 106 and/or the second passage 118 between the compressor 114 and the combustion chamber 106. In an embodiment of the present disclosure, the flow control device 138 is configured to selectively divert the compressed air in the second passage 118 from the compressor 114 to an external atmosphere outside the engine system 100. Therefore, the amount of compressed air provided to the combustion chamber 106 may be reduced, thereby increasing the exhaust temperature. In an alternative aspect of the present disclosure, the compressed air may be exited from the engine system 100 and the machine via a silencer and/or a muffler (not shown). The muffler may be installed within the exhaust system 124 and the diverted air may be routed to the external atmosphere via the muffler in the exhaust system 124.

INDUSTRIAL APPLICABILITY

The industrial applicability of the controller 140 in the engine system 100 for controlling the exhaust temperature described herein will be readily appreciated from the foregoing discussion.
Turbochargers for diesel and gasoline engines are well known. However, turbochargers vent out pollutants such as Diesel Particulate Matter (DPM) or soot, and Nitrogen Oxides (NOx). Therefore, to regulate such emissions, Diesel particulate filtration devices (DPF) and Diesel Oxidation Catalysts (DOC) are generally used.
DPFs filter the particulate matter from the exhaust gases to prevent them from exiting the engine. However, after a period of time and/or operation, the collected particulate matter starts to clog the filter. Therefore, the filter either needs to be replaced or removed for cleaning. Since removing or replacing the filter every time may not be feasible, regeneration process is used to combust the accumulated DEF and/or the DPM within the filter.
The present disclosure discloses the controller 140 operably connected to the flow control device 138 for controlling the exhaust temperature during regeneration process. The controller 140 may determine when the regeneration is required and/or when the regeneration is completed by using a number of sensors associated with the clean emission module 130. Further, the controller 140 may selectively control the operation of the flow control device 138 to regulate an amount of air supplied to the combustion chamber 106 during the regeneration process. In an embodiment of the present disclosure, the amount of compressed air supplied to the combustion chamber 106 is reduced by controlling the flow control device 138. Consequently, the ratio of air with respect to fuel may be reduced, resulting in more fuel burned. More fuel burned results in increased exhaust temperature exiting the engine 102, which may be further utilized by the clean emission module 130 for the regeneration process. In an exemplary embodiment of the present disclosure, the flow control device 138 may be controlled such that the air fuel ratio in the combustion chamber 106 is maintained in a range from about 15:1 to 18:1 for a load of about 20-90% on the engine 102 during the regeneration process. The controller 140 and the flow control device 138 together provide a compact solution for regeneration without requiring an additional external regeneration system as done conventionally. It is also price efficient and easy to implement without any extra additional components.
FIG. 4 illustrates a flowchart of an exemplary method 400 of modulating the amount of compressed air supplied to the combustion chamber 106 to control the exhaust temperature. At step 402, a flow of air is provided from an air inlet 115 into the compressor 114 via the first passage 116. In an exemplary embodiment of the present disclosure, the first passage 116 may be in the form of an intake manifold. Further, the ambient air is compressed by the compressor 114.
At step 404, the compressed air is provided to the combustion chamber 106 via the second passage 118. In an exemplary embodiment, the second passage 118 may also be in the form of an intake manifold. Further, the compressed air from the compressor 114 is provided to the combustion chamber 106 via the heat exchanger 120 to lower the fluid temperature in the second passage 118 and increase air density of the compressed air provided to the combustion chamber 106.
Further, at step 406, the flow control device 138 is adjusted to selectively reduce the amount of compressed air provided to the combustion chamber 106, such that the air-to-fuel ratio in the combustion chamber 106 is in a range of about 15:1 and 18:1 during the regeneration process. In an exemplary aspect of the present disclosure, the flow control device 138 is provided between the compressor 114 and the combustion chamber 106. In an embodiment of the present disclosure, the flow control device 138 is in fluid communication with the second passage 118 such that the compressed air from the compressor 114 is selectively diverted back into the first passage 116 via the intermediate passage 117. As explained previously, by diverting the compressed air in the second passage 118 back into the first passage 116, the temperature of the inlet air is also raised which further increases the temperature of the exhaust exiting the engine 102.
In an alternate embodiment, the flow control device 138 may be provided in fluid connection with the second passage 118 such that the compressed air in the second passage 118 is selectively diverted to the exhaust conduit 202 exiting the clean emission module 130 via the passage 204 after the regeneration process is completed or through the intermediate passage 117 to the first passage 116. Therefore, the amount of compressed air provided to the combustion chamber 106 is reduced to increase the exhaust temperature. As explained previously, by diverting the compressed air into the exhaust conduit 202, the temperature of the exhaust gases in the exhaust conduit 202 is reduced before exiting the engine system 100 and the machine.
In a yet another embodiment, the flow control device 138 may be provided in fluid communication with the second passage 118 such that the compressed air is diverted into the external environment outside the engine system 100. Therefore, the amount of air provided to the combustion chamber 106 may be reduced, resulting in an increased temperature of the exhaust gases in the exhaust conduit 126.
While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof.

Claims

What is claimed is:

1. An internal combustion engine system comprising:

an engine defining a combustion chamber;

a clean emission module fluidly connected to an exhaust conduit from the combustion chamber; and

an air supply unit configured to supply air into the combustion chamber, the air supply unit including:

a compressor configured to receive air from an air inlet via a first passage, the compressor further configured to supply compressed air into the combustion chamber via a second passage; and

a flow control device provided between the compressor and the combustion chamber of the engine, the flow control device being configured to selectively reduce the supply of compressed air into the combustion chamber such that an air fuel ratio in the combustion chamber is maintained in a range from about 15:1 to 18:1 at 20-90% engine load during a regeneration period for the clean emission module.

2. The system of claim 1, wherein the air supply unit further includes a controller operatively coupled to the flow control device and configured to control the flow control device based on a predetermined engine parameter.

3. The system of claim 2, wherein the predetermined engine parameter is a desired temperature associated with an exhaust of the engine.

4. The system of claim 3 further includes one or more sensors associated with the exhaust of the engine, wherein the sensors are configured to provide output signals indicative of the exhaust temperature to the controller.

5. The system of claim 1, wherein the flow control device is configured to selectively divert an amount of air from the second passage to the first passage.

6. The system of claim 1, wherein the flow control device is configured to selectively divert an amount of compressed air from the second passage to an external atmosphere outside the engine system.

7. The system of claim 1, wherein the flow control device is configured to selectively divert an amount of compressed air from the second passage to an exhaust passage exiting from the clean emission module.

8. The system of claim 1, wherein the flow control device includes a variable position valve.

9. A method of modulating air supply to a combustion chamber of an internal combustion engine to regenerate a clean emission module at an engine load of 20-90%, the method comprising:

providing a flow of air from an air inlet to a compressor via a first passage;

providing a flow of compressed air from the compressor to the combustion chamber of the engine via a second passage; and

adjusting a flow control device provided between the compressor and the combustion chamber to selectively reduce an amount of compressed air provided to the combustion chamber such that the air to fuel ratio in the combustion chamber is between 15:1 and 18:1 during a regeneration period.

10. The method of claim 9, wherein adjusting the flow control device further includes adjusting the flow control device based on a predetermined temperature associated with an exhaust of the engine.

11. The method of claim 9 further comprises selectively diverting an amount of compressed air from the second passage to the first passage.

12. The method of claim 9 further comprises selectively diverting an amount of compressed air from the second passage to an external atmosphere outside the engine system.

13. The method of claim 9 further comprises selectively diverting an amount of compressed air from the second passage to an exhaust passage exiting from the clean emission module.

14. The method of claim 9, wherein the flow control device includes a variable position valve.

15. An internal combustion engine system comprising:

an engine defining a combustion chamber

a clean emission module fluidly connected to an exhaust conduit from combustion chamber; and

a compressor configured to receive air from an air inlet via a first passage, the compressor further configured to supply compressed air into the combustion chamber via a second passage;

a flow control device provided between the compressor and the combustion chamber of the engine, the flow control device being configured to selectively reduce the supply of compressed air into the combustion chamber; and

a controller operatively coupled to the flow control device and configured to control the flow control device based on a predetermined engine parameter such that an air to fuel ratio in the combustion chamber is maintained between 15:1 to 18:1 at 20-90% engine load during a regeneration period for the clean emissions module.

16. The system of claim 15, wherein the predetermined engine parameter is a desired temperature associated with an exhaust of the engine.

17. The system of claim 15, wherein the flow control device includes a variable position valve.

18. The system of claim 15, wherein the flow control device is configured to selectively divert an amount of air from the second passage to the first passage.

19. The system of claim 15, wherein the flow control device is configured to selectively divert an amount of compressed air from the second passage to an external atmosphere outside the engine system.

20. The system of claim 15, wherein the flow control device is configured to selectively divert an amount of compressed air from the second passage to an exhaust passage exiting from the clean emission module.