Patent Publication Number: US-6659090-B2

Title: System for purging exhaust gases from exhaust gas recirculation system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system for purging exhaust gases from an exhaust gas recirculation (EGR) system for a compression-ignition internal combustion engine to minimize corrosion of EGR components caused by condensation of residual gases in the EGR system. 
     2. Background Art 
     Compression-ignition internal combustion engines may be equipped with EGR systems to reduce NOX emissions. EGR systems include an EGR circuit in which tubing interconnects an EGR cooler, EGR flowmeter, and EGR valve. 
     When an engine is operating, hot exhaust gases may be circulated through the EGR system. When the engine is shut down, the components of the EGR system cool causing condensation. The gases that condense in the EGR system after engine shut down are acidic and can cause corrosion of the components of the EGR circuit. As the exhaust gases cool in the EGR circuit, condensation forms on the interior surfaces of the components of the EGR circuit. 
     EGR systems for diesel engines equipped with a turbocharger that pressurizes intake air require a system for increasing pressure in the EGR system above the pressure of the intake. For example, with a variable geometry turbocharger, the vanes of the turbine can be partially closed to create back pressure to allow flow in the EGR system. 
     There is a need for a method and apparatus for purging gases from the EGR circuit when the engine is shut down to avoid or minimize condensation of the acidic EGR gases in the EGR circuit. There is also a need to purge EGR gases by flushing with fresh intake air to reduce the acidity of any condensate and prolong the life of the EGR circuit components by minimizing corrosion. 
     The above problems and needs are addressed by Applicant&#39;s invention as summarized below. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a system for providing exhaust gas recirculation in a multi-cylinder compression-ignition internal combustion engine having an intake side and an exhaust side is provided wherein intake air is ported through the EGR system prior to engine shut down. The system includes an EGR valve in communication with the exhaust side of the engine that selectively diverts a portion of exhaust gases from the internal combustion engine through an EGR circuit to the intake side of the engine. The EGR control can be used to provide higher intake manifold pressure than the exhaust manifold pressure prior to or as part of engine shut down. By creating higher pressure on the intake side while the EGR valve is held open, exhaust gases can be flushed from the EGR circuit. If a variable geometry turbocharger is provided as part of the engine, the turbine vanes may be adjusted to reduce pressure in the EGR circuit and thereby allow the intake manifold pressure to be higher than the exhaust manifold pressure. 
     According to another aspect of the invention, the EGR valve may be held open by an engine control module for a predetermined period of time after the engine reaches idle condition. The EGR valve may be held open for a predetermined period of time that is at least equal to the period of time required to fill the EGR system three times with air. 
     According to another aspect of the invention, a method of purging exhaust gases from an EGR system of a multi-cylinder compression-ignition internal combustion engine is provided. The method includes the step during engine shut down of setting the intake manifold pressure higher than the exhaust manifold pressure. The EGR valve is held open for a predetermined period of time so that air may be directed from the intake manifold to the EGR system and into the exhaust manifold. 
     According to another aspect of the invention, a method is provided for purging exhaust gases from an EGR system of a multi-cylinder compression internal combustion engine that powers a generator set. The engine has an intake side and an exhaust side that runs at light loads for a period of time before shut down while the engine is operating at light loads. The method includes setting the intake manifold pressure higher than the exhaust manifold pressure, opening the EGR valve for a predetermined period of time and directing air from the intake manifold to the exhaust manifold into the EGR system. 
     According to other aspects of the invention, the method of purging exhaust gases from the EGR system can be utilized on an engine flowing EGR at idle. When power to an ignition circuit is turned off, the method may also be carried out with an engine having a variable geometry turbocharger that may be set to hold the intake manifold pressure higher than the exhaust manifold pressure while the EGR valve is held open at engine shut down. 
     According to another aspect of the invention, if the intake manifold pressure is not maintained at a higher pressure than the exhaust manifold pressure, then the EGR valve may still be held open during engine spin down to allow exhaust gases (no combustion during engine spin down) to continue to enter the EGR system thereby allowing cleaner air to flush the EGR circuit. 
     The above advantages, and other advantages, objects, and features of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating one application of a system or method for providing EGR in a multi-cylinder compression ignition engine according to one embodiment of the present invention; and 
     FIG. 2 is a block diagram illustrating a representative EGR circuit for a compression ignition engine according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     FIG. 1 provides a schematic/block diagram illustrating operation of a system or method for providing EGR in a representative application according to one embodiment of the present invention. The system  10  includes a multi-cylinder compression ignition internal combustion engine, such as a diesel engine  12 , which may be installed in a vehicle  14  depending upon the particular application. In one embodiment, vehicle  14  includes a tractor  16  and semi-trailer  18 . Diesel engine  12  is installed in tractor  16  and interfaces with various sensors and actuators located on engine  12 , tractor  16 , and semi-trailer  18  via engine and vehicle wiring harnesses as described in further detail below. In other applications, engine  12  may be used to operate industrial and construction equipment, or in stationary applications for driving generators, compressors, and/or pumps and the like. 
     An electronic engine control module (ECM)  20  receives signals generated by engine sensors  22  and vehicle sensors  24  and processes the signals to control engine and/or vehicle actuators such as fuel injectors  26 . ECM  20  preferably includes computer-readable storage media, indicated generally by reference numeral  28  for storing data representing instructions executable by a computer to control engine  12 . Computer-readable storage media  28  may also include calibration information in addition to working variables, parameters, and the like. In one embodiment, computer-readable storage media  28  include a random access memory (RAM)  30  in addition to various non-volatile memory such as read-only memory (ROM)  32 , and keep-alive or non-volatile memory (KAM)  34 . Computer-readable storage media  28  communicate with a microprocessor  38  and input/output (I/O) circuitry  36  via a standard control/address bus. As will be appreciated by one of ordinary skill in the art, computer-readable storage media  28  may include various types of physical devices for temporary and/or persistent storage of data which includes solid state, magnetic, optical, and combination devices. For example, computer readable storage media  28  may be implemented using one or more physical devices such as DRAM, PROMS, EPROMS, EEPROMS, flash memory, and the like. Depending upon the particular application, computer-readable storage media  28  may also include floppy disks, CD ROM, and the like. 
     In a typical application, ECM  20  processes inputs from engine sensors  22 , and vehicle sensors/switches  24  by executing instructions stored in computer-readable storage media  28  to generate appropriate output signals for control of engine  12 . In one embodiment of the present invention, engine sensors  22  include a timing reference sensor (TRS)  40  which provides an indication of the crankshaft position and may be used to determine engine speed. An oil pressure sensor (OPS)  42  and oil temperature sensor (OTS)  44  are used to monitor the pressure and temperature of the engine oil, respectively. 
     An air temperature sensor (ATS)  46  is used to provide an indication of the current intake air temperature. A turbo boost sensor (TBS)  48  is used to provide an indication of the boost pressure of a turbocharger which is preferably a variable geometry or variable nozzle turbocharger as described in greater detail below. Coolant temperature sensor (CTS)  50  is used to provide an indication of the coolant temperature. Depending upon the particular engine configuration and application, various additional sensors may be included. For example, engines which utilize exhaust gas recirculation (EGR) according to the present invention preferably include an EGR temperature sensor (ETS)  51  and an EGR flow sensor (EFS)  53 . EFS  53  is preferably a hot wire anemometer type sensor which detects a differential temperature of two heated elements to determine the mass flow rate of EGR through the EGR circuit. The heated elements preferably provide pyrolitic cleaning by being heated to a temperature to reduce or prevent soot accumulation. Alternatively, a ΔP sensor may be used to determine the EGR flow rate as described in U.S. application Ser. No. 09/641,256 filed Aug. 16, 2000 and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference in its entirety. 
     Applications utilizing a common rail fuel system may include a corresponding fuel pressure sensor (CFPS)  52 . Similarly, an intercooler coolant pressure sensor (ICPS)  54  and temperature sensor (ICTS)  56  may be provided to sense the pressure and temperature of the intercooler coolant. Engine  12  also preferably includes a fuel temperature sensor (FTS)  58  and a synchronous reference sensor (SRS)  60 . SRS  60  provides an indication of a specific cylinder in the firing order for engine  12 . This sensor may be used to coordinate or synchronize control of a multiple-engine configuration such as used in some stationary generator applications. An EGR cooler and corresponding temperature sensor may also be provided to cool recirculated exhaust gas prior to introduction to the engine intake. 
     Engine  12  may also include an oil level sensor (OLS)  62  to provide various engine protection features related to a low oil level. A fuel restriction sensor (FRS)  64  may be used to monitor a fuel filter and provide a warning for preventative maintenance purposes. A fuel pressure sensor (FPS)  68  provides an indication of fuel pressure to warn of impending power loss and engine fueling. Similarly, a crankcase pressure sensor (CPS)  66  provides an indication of crankcase pressure which may be used for various engine protection features by detecting a sudden increase in crankcase pressure indicative of an engine malfunction. 
     System  10  preferably includes various vehicle sensors/switches  24  to monitor vehicle operating parameters and driver input used in controlling vehicle  14  and engine  12 . For example, vehicle sensors/switches  24  may include a vehicle speed sensor (VSS) which provides an indication of the current vehicle speed. A coolant level sensor (CLS)  72  monitors the level of engine coolant in a vehicle radiator. Switches used to select an engine operating mode or otherwise control operation of engine  12  or vehicle  14  may include an engine braking selection switch  74  which preferably provides for low, medium, high, and off selections, cruise control switches  76 ,  78 , and  80 , a diagnostic switch  82 , and various optional, digital, and/or analog switches  84 . ECM  20  also receives signals associated with an accelerator or foot pedal  86 , a clutch  88 , and a brake  90 . ECM  20  may also monitor position of a key switch  92  and a system voltage provided by a vehicle battery  94 . 
     ECM  20  may communicate with various vehicle output devices such as status indicators/lights  96 , analog displays  98 , digital displays  100 , and various analog/digital gauges  102 . In one embodiment of the present invention, ECM  20  utilizes an industry standard data link  104  to broadcast various status and/or control messages which may include engine speed, accelerator pedal position, vehicle speed, and the like. Preferably, data link  104  conforms to SAE J1939 and SAE J1587 to provide various service, diagnostic, and control information to other engine systems, subsystems, and connected devices such as display  100 . Preferably, ECM  20  includes control logic to determine EGR flow and temperature and to selectively divert at least a portion of the EGR flow around the EGR cooler to reduce or eliminate condensation of the recirculated exhaust gas. 
     A service tool  106  may be periodically connected via data link  104  to program selected parameters stored in ECM  20  and/or receive diagnostic information from ECM  20 . Likewise, a computer  108  may be connected with the appropriate software and hardware via data link  104  to transfer information to ECM  20  and receive various information relative to operation of engine  12 , and/or vehicle  14 . 
     FIG. 2 is a block diagram illustrating a representative EGR system. Engine  120  includes an intake manifold  122 , an exhaust manifold  124 , and an exhaust gas recirculation (EGR) system indicated generally by reference numeral  126 . An engine control module (ECM)  128  includes stored data representing instructions and calibration information for controlling engine  120 . ECM  128  communicates with various sensors and actuators including EGR sensors such as EGR flow sensor  130  and EGR temperature sensor  132 . As described above, EGR flow sensor  130  is preferably an anemometer-type sensor. ECM  128  controls EGR system  126  via actuators such as an EGR valve  134 . In addition, ECM  128  preferably controls a variable nozzle or variable geometry turbocharger (VGT)  138  may monitor an associated turbo speed sensor  140  and turbo boost sensor  48  as described with reference to FIG.  1 . 
     EGR system  126  preferably includes an EGR cooler  142  which is connected to the engine coolant circuit indicated generally by reference numeral  144 . EGR cooler  142  is preferably a full-flow cooler connected in-line with the engine coolant system. EGR cooler  142  may be directly coupled to a corresponding water or coolant pump  146 , or may be placed at a different location in the engine cooling circuit depending upon the particular application. 
     In operation, ECM  128  controls EGR system  126  and VGT  138  based on current operating conditions and calibration information to mix recirculated exhaust gas with charge air via mixer  150  which is preferably a pipe union. The combined charge air and recirculated exhaust gas is then provided to engine  120  through intake manifold  122 . In the illustrated embodiment, engine  120  is a 6-cylinder compression-ignition internal combustion engine. ECM  128  includes control logic to monitor current engine control parameters and operating conditions to control EGR system  126 . During operation of engine  120 , intake air passes through compressor portion  152  of VGT  138  which is powered by turbine portion  154  via hot exhaust gasses. Compressed air travels through charge air cooler  156  which is preferably an air-to-air cooler cooled by ram air  158 . Charge air passes through cooler  156  to mixer  150  where it is combined with recirculated exhaust gas. 
     The exhaust gas recirculation system  126  of the engine  120  receives exhaust gases from the exhaust manifold  124  of the engine  120  through the EGR valve  134 . A portion of the exhaust gases are directed to the variable geometry turbocharger  138  and another portion of the exhaust gases are ported through the EGR cooler  142 . To pressurize the EGR, the vanes of the turbine  154  may be partially closed during normal engine operation. Exhaust gases are then directed through an EGR flow sensor  130  and EGR temperature sensor  132 . The exhaust gases then are directed to a mixer  150  that mixes the exhaust gas with charge air. The mixture of exhaust gas and charge air is directed to the intake manifold  122  of the engine  120 . 
     The EGR system  126  may be flushed with fresh air by maintaining the intake manifold  122  pressure higher than the exhaust manifold  124  pressure so that charge air received from the charge air cooler  156  flushes the EGR system  126 . The variable geometry turbocharger  138  may be used to maintain the intake manifold pressure at a higher level than the exhaust manifold pressure. 
     According to another approach, the EGR valve  134  may be held open by the engine control module  128  for a predetermined period of time after the engine begins its shut down procedure. Holding the EGR valve  134  open while the engine shuts down allows air to flow from the charge air cooler  158  and continues to fill the engine  120  as combustion is terminated. Preferably, a predetermined period of time is provided such that the EGR system may be filled at least three times with air. During engine spin down the EGR valve  134  could be held open even if the exhaust manifold  124  pressure is greater than the intake manifold  122  pressure since the fuel is turned off and the exhaust air does not contain combustion gases. This clean exhaust air can be used to purge the EGR system  126 . 
     According to the method of the present invention, exhaust gases may be purged from the EGR system  126  of the engine  120 . The engine  120  has an intake side  122  and an exhaust side  124 . The engine  120  normally is run at idle for a period of time before shut down. As the engine  120  runs at idle prior to shut down, the intake manifold pressure is set higher than the exhaust manifold pressure. The next step is to open the EGR valve  134  for a predetermined period of time. Air is directed from the intake manifold  122  to the EGR system  126  and into the exhaust manifold  124  to flush the EGR system  126  with intake or charge air. Air is directed from the intake manifold  122  through the EGR system  126  for a period of time sufficient to allow the EGR system to be flushed. 
     According to another aspect of the invention, exhaust gases are purged from an EGR system  126  and an engine  120  that powers the generator set. As described above, the engine has an intake side and an exhaust side and in the generator set application the engine runs at light loads for a period of time before shut down. As the engine runs at light load, the intake manifold  122  pressure is set higher than the exhaust manifold  124  pressure. The EGR valve is held open for a period of time so that air may be directed from the intake manifold  122  to the EGR system  126  and into the exhaust manifold  124 . If the engine flows EGR at idle, power to an ignition circuit may be turned off and the variable geometry turbocharger  138  may be set to hold the intake manifold pressure higher than the exhaust manifold pressure while the EGR valve  134  is held open. If the intake manifold pressure can not be maintained higher than the exhaust manifold pressure, then the EGR valve  134  may be held closed during engine spin down to prevent exhaust gases from continuing to enter the EGR system  126 . 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.