Patent Publication Number: US-7913672-B2

Title: Hydrocarbon retaining and purging system

Description:
The present application claims priority to provisional application 60/987,350, filed Nov. 12, 2007, the entire contents of which are incorporated herein by reference for all purposes. 
    
    
     FIELD 
     The present description relates generally to an exhaust treatment system for a combustion engine. 
     BACKGROUND/SUMMARY 
     Engine exhaust systems have utilized hydrocarbon retaining devices, such as hydrocarbon traps, to retain cold start emissions for later reaction, or to recirculate into the engine intake system. 
     However, the inventors herein have recognized that the porosity of the hydrocarbon retaining device may be too small in some examples. Specifically large molecule hydrocarbons can become irremovably lodged in the devices. On the other hand, porosity of the of the hydrocarbon retaining device may be too large in some examples where depending on the fuel combusted, a significantly large number of small molecule hydrocarbons can be produced and pass through to the exhaust. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic depiction of an engine, exhaust system, and a hydrocarbon (HC) retaining system. 
         FIG. 2  shows a schematic depiction of an internal combustion engine. 
         FIGS. 3 ,  4 ,  5 ,  6 , and  7  show various embodiments of the HC Retaining System. 
         FIGS. 8 ,  9 ,  10 ,  11 ,  12 ,  13 , and  14  show high level flow charts illustrating operation of the engine, exhaust system, as well as the HC Retaining System. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic depiction of a vehicle system  6 . The vehicle system  6  includes an engine system  8  coupled to a hydrocarbon (HC) retaining system  22  and a fuel system  18 . 
     The engine system  8  may include an engine  10  having a plurality of cylinders  30 . The engine  10  includes an intake  23  and an exhaust  25 . The intake  23  includes a throttle  62  fluidly coupled to the engine intake manifold  44  via an intake passage  42 . The exhaust  25  includes an exhaust manifold  48  leading to an exhaust passage  35  that routes exhaust gas to the atmosphere. The exhaust  25  may include one or more emission control devices  70 , which may be mounted in a close-coupled position in the exhaust. One or more emission control devices may include a three-way catalyst, lean NOx trap, diesel particulate filter, oxidation catalyst, etc. It can be appreciated that other components may be included in the engine such as a variety of valves and sensors, shown in the example engine of  FIG. 2 . 
     The exhaust  25  may also be operatively coupled to hydrocarbon retaining system  22  via conduit  26  and valve  24 . In one example, exhaust gases may be routed to the hydrocarbon retaining system  22  during engine cold start operation. Then, once the emission control device  70  has reached its operating temperature, the hydrocarbons retained in system  22  may be purged to the engine via intake  23 , as described below herein. 
     Returning to  FIG. 1 , fuel system  18  may include a fuel tank  20  coupled to a fuel pump system  21 . The fuel pump system  21  may include one or more pumps for pressurizing fuel delivered to the injectors of engine  10 , such as the example injector  66  shown. While only a single injector  66  is shown, additional injectors are provided for each cylinder. It can be appreciated that fuel system  18  may be a returnless fuel system, a return fuel system, or various other types of fuel system. Vapors generated in the fuel system  18  may be routed to hydrocarbon retaining system  22 , described further below, via conduit  31 , before being purged to the engine intake  23 . 
     The fuel tank  20  may hold a plurality of fuel blends, including fuel with a range of alcohol concentrations, such as various gasoline-ethanol blends, including E10, E85, gasoline, etc., and combinations thereof. 
     Hydrocarbon retaining system  22  may include one or more hydrocarbon retaining devices, such as a hydrocarbon trap configured to temporarily trap hydrocarbons from entering gases. Hydrocarbon retaining system  22  may further include a vent  27  which may route gases out of the retaining system  22  to atmosphere when storing, or trapping, hydrocarbons from the exhaust  25  and/or fuel system  18 . Vent  27  may also allow fresh air to be drawn into hydrocarbon retaining system  22  when purging stored hydrocarbons from the exhaust  25  and/or fuel system  18  to the intake  23  via purge conduit  28  and purge valve  29 . While this example shows vent  27  communicated with fresh, unheated air, various modifications may also be used. For example, heated intake air from an air cleaner box may be used. Further, heated exhaust gas may be used under selected conditions. 
     Various system configurations of hydrocarbon retaining system  22  in system  6  may be used, with various combinations of valves, sensors, and the like. For example, various system configurations are described herein with regard to  FIGS. 3-7 , discussed below. And while the various configurations of  FIGS. 3-7  show various alternative features in particular combinations, the various features from among  FIGS. 3-7  may also be combined together to form still further example configurations. Further various additional components may be included in the intake, exhaust, and fuel system, such as a muffler downstream of valve  24 . 
     The vehicle system  6  may further include control system  14 . Control system  14  is shown receiving information from a plurality of sensors  16  (various examples of which are described herein) and sending control signals to a plurality of actuators  81  (various examples of which are described herein). As one example, sensors  16  may include exhaust gas sensor  126  located upstream of the emission control device, temperature sensor  128 , and pressure sensor  129 . Other sensors such as pressure, temperature, and composition sensors may be coupled to various locations in the vehicle system  6 , as discussed in more detail herein. As another example, the actuators may include fuel injector  66 , valve  28 , valve  24 , and throttle  62 . The control system  14  may include a controller  12 . The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. Example control routines are described herein with regard to  FIGS. 8-12 . 
     Hydrocarbon retaining system  22  can operate to store hydrocarbons from a plurality of sources, including the engine exhaust  25  and the fuel system  18 , concurrently or during different operating conditions. Under some operating conditions, such as during engine starts when the emission control device is not yet to its light-off temperature (e.g., a temperature at which the device reaches a selected, sufficiently high, conversion efficiency for a particular exhaust constituent), exhaust gas may be routed from the exhaust  25  to the hydrocarbon retaining system  22 , and then to the atmosphere through vent  27 . In this way, an increase amount of cold start hydrocarbon emissions may stored in hydrocarbon retaining system  22  while the exhaust gases heat emission control device  70 . Then, once device  70  reaches sufficient operating temperature, exhaust gases are then routed to atmosphere through conduit  35  and the hydrocarbon retaining system  22  is substantially isolated from the engine exhaust gas. Additionally, fuel vapors generated in the fuel tank  20  may be routed to hydrocarbon retaining system  22  for storage before being delivered to the engine intake  25  and combusted in the engine  10 . These different storing modes (from the exhaust  25 , and from fuel system  18 ) may be carried out concurrently, separately, or combinations thereof. 
     After being isolated from the exhaust gas, the hydrocarbon retaining system  22  may be coupled to the engine intake to draw fresh air through vent  27  and purge stored hydrocarbons into the engine intake to be combusted in the engine. Such purging operation may occur during selected engine operating conditions as described herein. 
     Additional details of purging and storing operation are described herein. 
       FIG. 2  is a schematic diagram showing one cylinder of multi-cylinder engine  10 . As described with regard to  FIG. 1 , engine  10  may be controlled at least partially by a control system including controller  12 , as well as by input from a vehicle operator  132  via an input device  130 . In this example, input device  130  includes an accelerator pedal and a pedal position sensor  134  for generating a proportional pedal position signal PP. Combustion chamber, or cylinder,  30  of engine  10  may include combustion chamber walls  32  with piston  36  positioned therein. Piston  36  may be coupled to crankshaft  40  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft  40  may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to crankshaft  40  via a flywheel to enable a starting operation of engine  10 . 
     Combustion chamber  30  may receive intake air from intake manifold  44  via intake passage  42  and may exhaust combustion gases via exhaust manifold  48 . Intake manifold  44  and exhaust manifold  48  can selectively communicate with combustion chamber  30  via respective intake valve  52  and exhaust valve  54 . In some embodiments, combustion chamber  30  may include two or more intake valves and/or two or more exhaust valves. The intake manifold may include a purge path fluidly coupled to the exhaust stream allowing purge vapors to be delivered to the engine intake manifold  44 . While this example shows the purge path downstream of the throttle to enable manifold vacuum to draw in the gases, various other configurations may also be used. For example, in the case of a turbocharged engine, the purge line(s) may lead to upstream of the turbocharger compressor inlet. 
     In this example, intake valve  52  and exhaust valves  54  may be controlled by cam actuation via respective cam actuation systems  51  and  53 . Cam actuation systems  51  and  53  may each include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems that may be operated by controller  12  to vary valve operation. The position of intake valve  52  and exhaust valve  54  may be determined by position sensors  55  and  57 , respectively. In alternative embodiments, intake valve  52  and/or exhaust valve  54  may be controlled by electric valve actuation. For example, cylinder  30  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT systems. 
     Fuel injector  66  is shown coupled directly to combustion chamber  30  for injecting fuel directly therein in proportion to the pulse width of signal FPW received from controller  12  via electronic driver  68 . In this manner, fuel injector  66  provides what is known as direct injection of fuel into combustion chamber  30 . The fuel injector may be mounted in the side of the combustion chamber or in the top of the combustion chamber, for example. Fuel may be delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail. In some embodiments, combustion chamber  30  may alternatively or additionally include a fuel injector arranged in intake passage  44  in a configuration that provides what is known as port injection of fuel into the intake port upstream of combustion chamber  30 . 
     Intake passage  42  may include throttle  62  having a throttle plate  64 . In this particular example, the position of throttle plate  64  may be varied by controller  12  via a signal provided to an electric motor or actuator included with throttle  62 , a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, throttle  62  may be operated to vary the intake air provided to combustion chamber  30  among other engine cylinders. The position of throttle plate  64  may be provided to controller  12  by throttle position signal TP. Intake passage  42  may include a mass air flow sensor  120  and a manifold air pressure sensor  122  for providing respective signals MAF and MAP to controller  12 . 
     Ignition system  88  can provide an ignition spark to combustion chamber  30  via spark plug  92  in response to spark advance signal SA from controller  12 , under select operating modes. Though spark ignition components are shown, in some embodiments, combustion chamber  30  or one or more other combustion chambers of engine  10  may be operated in a compression ignition mode, with or without an ignition spark. 
     Exhaust gas sensor  126  is shown coupled to exhaust passage  48  upstream of emission control device  70 . Sensor  126  may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NOx, HC, or CO sensor. Emission control device  70  is shown arranged along exhaust passage  48  downstream of exhaust gas sensor  126 . Device  70  may be a three-way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof. In some embodiments, during operation of engine  10 , emission control device  70  may be periodically reset by operating at least one cylinder of the engine within a particular air/fuel ratio. 
     Controller  12  is shown in  FIG. 1  as a microcomputer, including microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as read only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a data bus. Storage medium read-only memory  106  can be programmed with computer readable data representing instructions executable by processor  102  for performing the methods described below as well as other variants that are anticipated but not specifically listed. Controller  12  may receive various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF); engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a profile ignition pickup signal (PIP) from Hall effect sensor  118  (or other type) coupled to crankshaft  40 ; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal (MAP) from sensor  122 . Engine speed signal, RPM, may be generated by controller  12  from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. 
     As described above,  FIG. 2  shows only one cylinder of a multi-cylinder engine  10 , and that each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc. 
       FIG. 3  shows a first embodiment  300  of the hydrocarbon retaining system including hydrocarbon retaining devices, such as HC trap  310  arranged in parallel with exhaust conduit  35 , downstream of the emission control device  70 , and canister  322 . An exhaust conduit valve  24  is located in the exhaust conduit for blocking exhaust flow during a first mode, and allowing exhaust flow to pass during a second mode. A bypass conduit  312  is coupled upstream and downstream of the exhaust conduit valve  24 . Further, the bypass conduit  312  includes a first bypass valve  314 , an HC trap  310 , and a second bypass valve  318 . In this example the HC trap may include a brick  320 . In other examples the HC trap may include a plurality of bricks or other structures, such as activated charcoal. Further, HC trap  310  may also be a canister using micropore (0.5 nm pore size range) activated carbon or zeolites. The bypass conduit  312  leads to the atmosphere through vent  27 , and optionally through valve  318  and conduit  35 . 
     Furthermore, the HC trap  310  may be fluidly coupled to a purge canister  322  via passage  324 . The purge canister  322  may be configured to collect vapor from the fuel system, such as via a conduit  328  leading to the fuel tank  20 . The passage  324  may include a valve  326 . The purge canister  322  may also be fluidly coupled to the engine intake via a conduit  330  and valve  332 . Similar to the HC trap  310 , the canister  322  may include activated charcoal, although it may have a different, e.g., high, porosity. 
     While this example shows two hydrocarbon retaining devices (e.g., HC trap  310  and purge canister  322 ), various alternatives may be provided. For example, a single hydrocarbon retaining device, such as a canister, may be used, where the canister operates to store both exhaust hydrocarbons as well as fuel tank vapors. Further, the two hydrocarbon retaining devices may each be canisters, or may each be HC traps. Irrespective of the particular configuration of the HC retaining system, the hydrocarbon retaining devices can receive routed cold start hydrocarbons and retain the hydrocarbons. 
     In one example, the hydrocarbon retaining devices may comprise granular activated carbon contained in a plastic housing, since the exhaust gas temperatures may be substantially lower than the full range of exhaust temperature experienced in the exhaust system. 
     The hydrocarbon retaining system  22  of  FIG. 3  may be operated by a controller in a plurality of modes by selective adjustment of the various valves. For example, the following operating modes may be performed: 
     MODE A: Exhaust Hydrocarbon Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may close valves  24  and  326 , and open valves  314  and optionally  318 . Additionally, valve  332  is closed. Example operating conditions include cold engine starting operation before the emission control device has reached a light-off temperature. In this mode, exhaust from engine  10  is routed through HC trap  310  before exiting to the atmosphere via vent  27 , while the purge canister  322  is isolated from the exhaust gas and hydrocarbon retaining system. 
     MODE B: Fuel Vapor Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may open valve  24  and  326 , and close valves  314  and  318 . Valve  332  may be closed, in which case fuel tank vapors are merely stored, and not purged. Alternatively, valve  332  may be open to a controlled amount, such that some fuel tank vapors are stored and some are delivered to the engine for combustion. In this mode, at least some fuel tank vapors are routed through and retained in both purge canister  322  and HC trap  310  before being vented through vent  27 . As such, the fuel tank may be communicated to the hydrocarbon retaining system. 
     MODE C: HC Trap and/or Canister Purging 
     During select engine and/or vehicle operating conditions, the controller  12  may open valve  24  and  326 , and close valves  314  and  318 . Additionally, valve  332  may be open to a controlled amount, such that fresh air is drawn through vent  27  to purge HC trap  310  and purge canister  322  through  330  and valve  332  to the engine intake  25 . In this mode, the purged fuel vapors from the canister and/or HC trap are combusted in the engine, while exhaust gas is routed through conduit  35  to the atmosphere after treatment by the emission control device. 
     In an alternative embodiment, rather than drawing in fresh air via vent  27  to purging stored hydrocarbons, the system may additionally or alternatively route exhaust gases to the vent to enable heated exhaust to heat the hydrocarbon retaining system and better purge stored hydrocarbons. Such operation may be used during specific conditions, such as cold ambient temperatures. Further, under some conditions, the system may intentionally route exhaust gases during the storing mode for a longer duration to heat the hydrocarbon retaining system to a higher temperature, thereby improving the subsequent purging operation. In one example, before a purging operation, exhaust gases may be routed to the hydrocarbon retaining system (even during non-starting conditions) to increase temperature and improve the subsequent purging efficiency. This operation may be used when temperature of the hydrocarbon retaining system falls below a threshold value, or when reduce purging opportunities are present. 
     Note the configuration of  FIG. 3  enables the gas flowing through the HC trap  310  to be routed back to the exhaust  35  and out the tailpipe. Thus, by using the valve  318  (which can be a lower temperature, lower cost valve compared to other exhaust valves in the system) that allows the exhaust flow to return to the exhaust pipe (or be routed to a tailpipe location), various advantages may be achieved. For example, the configuration allows for sufficient flow during the cold start trapping operation without sending exhaust gas to another location other than the tailpipe. Further, by providing a separate port for the purge flow inlet, it is possible to draw purge air from a location that is less susceptible to water ingestion. 
     Additionally, when the exhaust gas is routed back to the exhaust system as shown in  FIG. 3 , the HC trap can be positioned upstream of the muffler, thus reducing effects of muffler leakage on the emission system performance. 
       FIG. 4  shows a second embodiment  400  of the hydrocarbon retaining system  22 . In this configuration, a valve  24  includes a diverter valve that either routes exhaust gas through conduit  35  to the atmosphere, or to conduit  412 .  FIG. 4  shows HC trap  410  arranged in parallel with exhaust conduit  35 , downstream of the emission control device  70 . Conduit  412  is coupled upstream of the valve  24  and includes HC trap  410 . 
     Furthermore, the HC trap  410  may be fluidly coupled to a purge canister  422  via passage  424  and valve  426 . The purge canister  422  may be configured to collect vapor from the fuel system, such as via a conduit  428  leading to the fuel tank  20 ). The purge canister  422  may also be fluidly coupled to the engine intake via a conduit  430  and valve  432 . The hydrocarbon retaining system  22  may also include a first pressure sensor  436  and a second pressure sensor  438  coupled to conduit  424 . Additionally, an HC trap temperature sensor  440  may be coupled directly to the HC trap. 
     As shown schematically in  FIG. 4 , vent  27  is thermally coupled to at least a portion of conduit  35  so that heat is transferred therebetween. In one example, the vent  27  may be spatially adjacent to at least a portion of conduit  35 . 
     The hydrocarbon retaining system of  FIG. 4  may be operated by a controller in a plurality of modes by selective adjustment of the various valves. For example, the following operating modes may be performed: 
     MODE A: Exhaust Hydrocarbon Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may adjust valve  24  to divert exhaust gas to conduit  412 , as well as close valves  432  and  426 . Example operating conditions include cold engine starting operation before the emission control device has reached a light-off temperature. In this mode, exhaust from engine  10  is routed through HC trap  410  before exiting to the atmosphere via vent  27 , while the purge canister  422  is isolated from the exhaust gas and hydrocarbon retaining system. 
     MODE B: Fuel Vapor Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may adjust valve  24  to direct exhaust gas through conduit  35 , and isolate the hydrocarbon trap  410  from exhaust gas. Additionally, the control may open valve  326 , and close valve  432 . In this mode, at least some fuel tank vapors are routed through and retained in both purge canister  422  and HC trap  410  before being vented through vent  27 . 
     MODE C: HC Trap and/or Canister Purging 
     During select engine and/or vehicle operating conditions, the controller  12  may adjust valve  24  to direct exhaust gas through conduit  35 , and isolate the hydrocarbon trap  410  from exhaust gas. Additionally, the control may open valve  326 , and controllably open valve  432  such that fresh air is drawn through vent  27  to purge HC trap  410  and purge canister  422  through  430  and valve  432  to the engine intake  25 . In this mode, the purged fuel vapors from the canister and/or HC trap are combusted in the engine, while exhaust gas is routed through conduit  35  to the atmosphere after treatment by the emission control device. As noted above, heat may be transferred to the fresh air inducted to vent  27 . In some examples heat may be transferred through conduction, convection, forced convection, combinations thereof, or alternative forms of thermal communication. In this way, when the system is warming up to operating temperature, the warmed air may improve release of hydrocarbons stored in HC trap  410  and/or purge canister  422 . 
       FIG. 5  shows a schematic depiction of a third embodiment of the hydrocarbon retaining system  22 . This embodiment is similar to that of  FIG. 3 , except that the HC trap  510  includes two bricks  520 , and is configured to pass exhaust gasses serially through the bricks, but purge them in parallel. Specifically, the system of  FIG. 5  includes HC trap  510  arranged in parallel with exhaust conduit  35 , downstream of the emission control device  70 . An exhaust conduit valve  24 , which in this example may be a throttle, is located in the exhaust conduit for blocking exhaust flow during a first mode, and allowing exhaust flow to pass during a second mode. A bypass conduit  512  is coupled upstream and downstream of the exhaust conduit valve  24 . Further, the bypass conduit  512  includes a first bypass valve  514 , the HC trap  510 , and a second bypass valve  518 . The bypass conduit  512  is also coupled to vent  27  via two parallel passages  534  and valve  538 . 
     Furthermore, the HC trap  510  is fluidly coupled to a purge canister  522  via passage  524 . The purge canister  522  may be configured to collect vapor from the fuel system, such as via a conduit  528  leading to the fuel tank  20  (not shown). The purge canister  522  may also be fluidly coupled to the engine intake via a conduit  530  and valve  532 . 
     The hydrocarbon retaining system of  FIG. 5  may be operated by a controller in a plurality of modes by selective adjustment of the various valves. For example, the following operating modes may be performed: 
     MODE A: Exhaust Hydrocarbon Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may close valves  24  and  538 , and open valves  514  and  518 . Additionally, valve  532  is closed. Example operating conditions include cold engine starting operation before the emission control device has reached a light-off temperature. In this mode, exhaust from engine  10  is routed through HC trap  510  (sequentially through bricks  520   a  and then  520   b ) before exiting to the atmosphere via conduit  35 , while the purge canister  522  is effectively isolated from the exhaust gas as the flow is deadheaded against valve  526 . 
     MODE B: Fuel Vapor Storage 
     During select engine and/or vehicle operating conditions, the controller  12  may open valve  24 ,  526 , and  538 , and close valves  514  and  518 . Valve  532  may also be closed. The exhaust gases are thus isolated from the canister  522  and HC trap  510 . In this mode, at least some fuel tank vapors are routed through and retained in both purge canister  522  and HC trap  510  before being vented through valve  538  and vent  27 . In particular, the fuel vapors are first routed through canister  522  and then routed, in parallel through bricks  520   a  and  520   b,  respectively, before being vented through  27 . In this way, fuel tank vapors may be stored in different concentrations at different locations of the traps, due to the different direction of flow of exhaust gas and fuel tank vapors, at least from brick  510   a.    
     MODE C: HC Trap and/or Canister Purging 
     During select engine and/or vehicle operating conditions, the controller  12  may open valve  24  and  538 , and close valves  514  and  518 . Additionally, valve  532  may be open to a controlled amount, such that fresh air is drawn through vent  27  to purge HC trap  510  and purge canister  522  through  530  and valve  532  to the engine intake  25 . In this mode, the purged fuel vapors from the canister and/or HC trap are combusted in the engine, while exhaust gas is routed through conduit  35  to the atmosphere after treatment by the emission control device. Again, in flow movement opposite to that of MODE B of  FIG. 5 , fresh air is routed in parallel to bricks  520   a  and  520   b,  before being combined and routed through purge canister  522 . In this way, purging of vapors occurs in a direction opposite to that of fuel tank vapor storing in both bricks, and opposite to the direction of exhaust gas hydrocarbon storing in at least one brick ( 520   a ). 
     Such differential storing and purging directions, making use of both serial storing and parallel purging, can enable improved storage and release, thus increasing efficiency of the engine and emission system. 
       FIG. 6  shows additional details of an example of the third embodiment of  FIG. 5 . Similar components are labeled accordingly. In this example, valves  24 ,  518 , and  514  are vacuum actuated throttle valves. The HC trap includes two bricks  520   a,b  having activated carbon pellets  612 . A vacuum regulator  614  is coupled to the valves  24 ,  514 , and  518 , and may be configured to closed valve  24  whenever valves  518  and  514  are open, and vice versa. The vacuum regulator  614  may be electronically actuated by controller  12  in one example. 
     Continuing with  FIG. 6 , a first pressure sensor  540  and a second pressure sensor  542  are coupled to the intake  616  and outlet  618  of the HC trap. On Board Diagnostic (OBD) sensor  620  may be coupled between the first and the second bricks,  520   a  and  520   b,  respectively. The OBD sensor may measure the gas pressure, composition, or a combination thereof in the HC trap. 
     In this example, HC trap  510  may be located after an underbody catalyst can but before a muffler. Further, the length of the conduits from exhaust  35  can be &gt;12 inches to reduce the exhaust gas temperature from 800° C. to 100° C. and to permit plastic components. Further, carbon may further operate to reduce noise when bypassing a muffler in the exhaust. 
     Continuing with one example, the exhaust flaps at the canister inlet and outlet ( 616 ,  618 ) can be plastic and can be integrated into a plastic housing of the trap/canister. The device may include a rubber gasket/o-ring to help seal around the housing where the flap would close or around the plastic flap due to reduced temperature. The housing of device  510  may comprise 1.5″ ID entry/exit tubes, molded bosses in the spring section for a sensor and poppet valve, molded ports on the cones for air intake, screens on both sides of the carbon bed to allow flow through the canister and actuated exhaust flaps. The inlet valve coupled to  524  may be located between the packed beds. At that spot, OBD sensor  620  can observe loading and unloading of the beds. If the plastic exhaust flaps seal, the poppet valve will let in air from the air cleaner box, in one example. The fresh air inlet at each cone is split from the inlet air source downstream of the vent valve. The outlet may lead to the fuel tank vapor purge system, either in series into the canister  522 , for example, or in parallel thereto. 
       FIG. 7  shows a fourth embodiment of a hydrocarbon retaining system  22 . This example is similar to that of  FIG. 4 , although different valve and conduit configurations are used, along with two HC traps, in bypass  712 . Specifically, in this example, valve  24  operates as a diverter valve to either allow exhaust gas flow through conduit  35  to atmosphere, or to direct the exhaust gas flow to conduit  714  and valve  716 . In this example, valve  716  is another diverter valve which either allows communication between conduit  714  and conduit  740 , or conduit  714  and conduit  742 . Bypass  712  further includes a first and second HC trap,  726  and  728 , respectively. Further still, a valve  726  is shown for controlling flow leading to, or coming from, the purge canister (not shown), which then leads to the fuel tank and intake manifold, similar to the other example configurations described above. As such, fuel vapors from the fuel tank may be routed to the first and second traps,  726  and  728 , in that order, and purged in the opposite order. 
     In  FIG. 7 , the two traps  726  and  728  are configured with different HC trapping characteristics. For example, trap  726  may be of a larger porosity activated carbon to capture larger HC molecules as compared to trap  728 . In this way, during storage, the gases may be routed first through  726  and then through  728 , such that HC molecule size in the gas is progressively reduced. Additionally, since stored HC molecules may be difficult to remove from the smaller pore activated carbon, the smaller pore activated carbon may be buffered by the larger pore size activated carbon to inhibit HC molecules from irreversibly absorbing into the smaller pores. Furthermore, the adsorption of smaller HC species into trap  728  may occur at higher pressures. As such, the inlet piping diameter from the diverter valve  716  to the traps may be sized sufficiently to provide a desired pressure during storage and purge operation. 
     The configuration of  FIG. 7  also provides for opposite flow direction for storage and purging operation, for both traps  726  and  728 . Specifically, during storage of HC in the exhaust gas, the diverter valves  24  and  716  route the exhaust gas through  714  and then to trap  726 , and then  728 , before being vented via  27 . During purging of the traps, the valves are adjusted to route fresh air through vent  27 , trap  728 , and then trap  726 , before being delivered through  726  and then to the intake. In this way, improved storage and release of HC can be achieved, especially in light of the differential characteristics (e.g., porosity) of the traps. The configuration of  FIG. 7  can thus operate in any of MODES A, B, and C. 
     While  726  and  728  may each be HC traps in one example, two charcoal canisters may also be used. Alternatively, as noted, zeolites can be included. For example, canister  726  may be configured to adsorb cold-start propylene and higher mass hydrocarbons rather than a separate device. Micro-pore activated carbons in  728  adsorb lower sized hydrocarbons, but can be more protected than  726  in series. Additionally, purging of the traps with preheated air from an air cleaner box or hot vehicle exhaust, in counter-current flow relative to the adsorption flow direction, can further provide improved performance. 
     While this example shows two traps in the configuration of  FIG. 7 , more or less may be used. 
     In one particular example, device  726  can include a carbon canister (of larger porosity) to capture medium to large sized HC molecules, followed by a smaller pore activated carbon canister  728 . During storage, gases first pass through  726 , and then to  728 . Removal is performed in the opposite direction. In this way, the larger pores better protect the smaller pores and reduce irreversible adsorption of larger HC molecules into the smaller pores. The adsorption of small HC species into the small pore medium may better respond to pressure, such as approximately 5 MP, which can be provided by sizing of the inlet piping diameter from the diverter valve to the canisters. Similarly, removal of the HC species may benefit from increased temperature, such as through heat transfer from exhaust gas, or by using at least some exhaust gas, such as during idle conditions. 
     It should be appreciated that with regard to  FIGS. 3-7 , MODE A may not only operate to store HC from the exhaust in the hydrocarbon retaining system, but it may also substantially heat the system. This may be advantageously used to thus prepare the hydrocarbon retaining system  22  for subsequent purging operation, as increased temperature can act to more easily release stored hydrocarbons. 
     Further, while the examples of  FIGS. 3-7  show combined purging of the fuel tank vapors and stored exhaust hydrocarbons, in alternative configurations these may be purged independently. For example, the example HC traps may be purged via a purge line in parallel with a purge line of the example canisters, to thereby enable separate and/or independent purging of the different hydrocarbon storage systems. For example, one hydrocarbon retaining device may be purged without purging a second hydrocarbon retaining device. Further still, in some example configurations of  FIGS. 3-7 , the fuel tank vapors in the purge canisters can be purged sufficiently faster than the hydrocarbons in the HC traps. As such, during initial purging, the learned vapor amount/concentration can be used to determine the amount of fuel vapors stored in the canister, and then, during a later portion of the purging operation (when the canister is sufficiently purged/empty), the learned vapor amount/concentration can be used to estimate a loading state of the HC traps (that was generated during the most recent storing operation, such as the most recent cold start event). 
     Further still, while the examples of  FIGS. 3-7  show non-concurrent storing of exhaust gas hydrocarbons and purging of the retaining system  22 , in alternative embodiments, some stored hydrocarbons may be purged during storage of other hydrocarbons. 
     The following method of  FIGS. 8-14  may be implemented using systems, components, and devices described herein, but alternatively may be implemented using other suitable systems, components, and devices. 
     Referring now to  FIG. 8 , a routine  800  is shown for controlling engine operation, fuel vapor management, and exhaust emission management. Routine  800  may operate during a warm-up condition, such as while the engine or exhaust warms from ambient temperatures to it normal operating temperature range. 
     First, at  810 , the routine determines whether an engine start is present. For example, the routine may determine whether the engine is being started from rest. Additionally, or alternatively, the routine may determine whether the engine has been started via engine cranking operation. When an engine start is present, the routine continues to  812 , where the routine estimates or measures emission control device temperature based on various parameters. In one example, the routine may determine emission control device temperature based on an engine off time (soak time), ambient temperature, engine coolant temperature, and intake air charge temperature. Alternatively, or in addition, the routine may determine emission control device temperature based on one or more exhaust gas temperature sensors located in exhaust passage  35  or exhaust manifold  48 . Further still, the routine may determine temperature of an emission control device based on a temperature sensor mounted to the emission control device. 
     In  814 , the routine determines a temperature threshold value based on operating conditions. One example approach for determining the temperature threshold is described herein with regard to  FIG. 9 . Alternatively, a fixed temperature threshold may be used. In one example, the temperature threshold correlates to a catalyst light-off temperature. Next, the routine continues to  816  to determine whether storage of exhaust generated hydrocarbons in the retaining system  22  is enabled, and if not, to end such routing. Such operation may be adjusted based on various conditions, including whether a capacity of hydrocarbon storage of the retaining system  22  is greater than a threshold value. For example, the routine may enable storage in the retaining system  22  when the storage system  22  has been purged during previous engine operation. Additionally, the routine may enable storage of hydrocarbons in retaining system  22  when temperature of retaining system  22  is less than a maximum storage temperature. Further, the routine may enable storage of exhaust hydrocarbons in retaining system  22  as based on a fuel property of the fuel combusted in the engine, such as an alcohol amount in the fuel, where the alcohol amount in the fuel may be learned during previous engine operation. In this way, the ending of the routing of exhaust gas to the hydrocarbon retaining system  22  may be adjusted to take advantage of the different storage characteristics of different fuels. For example routing may continue up to higher storage amounts, or for a longer duration, when combusting higher alcohol content fuels as compared to lower alcohol content fuels. Further still, the routine may enable storage of hydrocarbons in retaining system  22  when an engine soak time (e.g., an engine/vehicle off-duration) is greater than a threshold amount. In this way, during hot re-starts, for example, the engine start may direct exhaust gases through exhaust system  35  and bypass the retaining system  22 . 
     When the answer to  816  is yes, the routine continues to  818  to determine whether temperature of emission control device  70  is less than the threshold temperature determined in  814 . If so, the routine continues to  820  to operate one or more of the exhaust valves (e.g.,  24 ) to direct exhaust gas through the hydrocarbon retaining system  22 , and in particular through one or more hydrocarbon traps. For example, depending on the system configuration, control system  14  may adjust one or more exhaust valves to route exhaust gas from downstream of emission control device  70  to and through the hydrocarbon retaining system  22  before being vented or routed to the atmosphere. In one example, the routine operates the system in MODE A, described above herein. Further, the system may operate with spark timing substantially at a peak torque timing, or may operate with an amount of spark retard, depending on operating conditions as described herein with regard to  FIG. 10 , for example. 
     Next, in  822 , the routine estimates an amount of hydrocarbon stored in the hydrocarbon retaining system  22 , such as the amount of hydrocarbon stored in HC trap  310 . The routine may estimate the amount of stored hydrocarbons based on exhaust gas flow rate, exhaust temperature, temperature of hydrocarbon retaining system  22 , engine speed, and various other parameters. 
     When the answer to either  816  is no or  818  is yes, the routine continues to  824  to operate the exhaust valves to direct exhaust gas from the emission control device  70  to bypass the hydrocarbon retaining system  22  and be delivered to the atmosphere through conduit  35 . Also, as noted herein, the exhaust gas may be routed through various additional emission control devices and/or mufflers before being delivered to the atmosphere. In one example during the operation of  824 , the routine operates the engine to increase exhaust gas temperature thus retarding ignition timing sufficiently past MBT timing as well as performing combustion at a slightly lean air-fuel ratio in the combustion chamber. Further, the operation of  824  may thus result in the system not operating in MODE A, although the system may or may not operate in MODES B and C, as described further herein with regard to  FIG. 8 . 
     From  824  or  822 , the routine continues to  826  to determine whether purging of stored hydrocarbons from the hydrocarbon retaining system  22  should be carried out (for example, whether the system can operate in MODE C). In one example, the routine of  FIG. 11  shows one example of determining whether to purge the hydrocarbon retaining system  22 . Initiating the purging of the hydrocarbon retaining system  22  may be based on various engine and vehicle operating parameters, including the amount of hydrocarbon stored in the retaining system  22  (such as the amount of hydrocarbons stored in HC trap  310 ), the temperature of the emission control device  70 , temperature of the hydrocarbon retaining system, fuel temperature, the number of starts since the last purge, fuel properties (such as the alcohol amount in the combusted fuel), and various others. In one example at  826 , the routine determines whether exhaust gases were routed to the hydrocarbon retaining system  22  during the current engine start. In one particular example, the initiating of the purging may be adjusted to be initiated at higher temperatures as the alcohol amount in the fuel increases. 
     When the answer to  826  is yes, the routine continues to  828  to operate the exhaust valves to draw fresh air through hydrocarbon retaining system  22  and into intake passage  42  of engine  10 , which may include operating the system in MODE C. Engine  10  then operates to induct the vapors along with injected fuel and intake air, and perform combustion thereof. Next, in  830 , the routine estimates the amount of purged hydrocarbons from the hydrocarbon retaining system  22  (which may be attributed to one or both of an HC trap and purge canister) based on various parameters including the amount of stored hydrocarbons, as well as feedback information from exhaust air-fuel ratio sensors. In one example, feedback from an exhaust gas oxygen sensor may be used to learn or update the estimated amount of hydrocarbons stored in the HC trap to determine degradation of the HC trap based on such an estimate. Further, the routine may also adjust throttling of the engine to adjust an amount of purge gases drawn through a purge canister, HC trap, or both. 
     When the answer to  826  is no, or from  830 , the routine continues to  832 . At  832 , the routine determines whether to purge hydrocarbon vapors generated from the fuel tank, in the event that such vapors were not purged at  828 , or were not sufficiently purged at  828 . One example routine for determining whether to purge fuel tank generated vapors is described herein with regard to  FIG. 12 . When the answer to  832  is yes, the routine continues to  834  to operate the exhaust valves to purge fuel tank generated vapors from the hydrocarbon retaining system  22 , which may include operating the system in MODE C. Otherwise when the answer to  832  is no, or from  834 , the routine continues to the end. 
     While not illustrated specifically in  FIG. 8 , the control system may also operate the system in MODE B whenever the system is not operating in MODE A or C. Alternatively, MODE B may be selectively enabled based on various operating conditions, including ambient temperature, fuel temperature, and others. 
     Additionally, by appropriate temperature considerations, it is possible to further provide control of routing of exhaust gas to the hydrocarbon retaining system, and purging of the hydrocarbon retaining system to the engine intake, to account for warm engine restart conditions. For example, the system may consider engine coolant temperature and catalyst temperature in the enabling of the routine and initiation of the purging such that if coolant temperature is above a warm re-start threshold and/or catalyst temperature is above a warm re-start threshold, the routing of exhaust gas to the hydrocarbon retaining system is disabled and purging is initiated once engine crank/run-up is completed. 
     Referring now to  FIG. 9 , routine  900  is described for determining a temperature threshold below which exhaust gases may be routed to hydrocarbon retaining system  22 . First, at  910 , the routine determines a fuel composition. In one example, the routine may determine an alcohol concentration and/or fuel blend of the fuel stored in tank  20 . The alcohol concentration may be learned based on feedback from exhaust gas oxygen sensors indicating a shift in the stoichiometric air-fuel ratio. Additionally, or alternatively, the routine may determine an alcohol concentration of the fuel based on a fuel concentration sensor. Note that the routine may determine a relative alcohol amount in a gasoline fuel, or various other fuel compositions. 
     Next, at  912 , the routine determines a fuel temperature. In one example, the routine may measure fuel temperature from a fuel temperature sensor coupled to fuel tank  20 . Alternatively, the routine may estimate fuel temperature based on various parameters, including engine coolant temperature, ambient air temperature, and/or various others. 
     Next, at  914 , the routine determines a temperature threshold based on the fuel temperature and composition/makeup of  912  and  910 , respectively. Additionally, the routine may determine the threshold based on additional operating conditions, including aging of catalytic material of the emission control device  70 . In one example, the routine may increase the temperature threshold as the amount of alcohol in the fuel blend increases. In this way, it is possible to adjust the temperature range at which exhaust gas is routed to a hydrocarbon trap in the hydrocarbon retaining system  22 . Specifically, such operation can take advantage of the different storage properties of the trap as the amount of alcohol in the fuel varies. For example, fuels may have different adsorption and/or desorption characteristics with varying alcohol content. Also, water in the exhaust may affect temperature at which storage/release of hydrocarbon species occurs. As the alcohol content of the fuel varies, different HC chains may be produced in the exhaust, thus generating the different storage/release characteristics. In one particular example, HCs may be trapped up to higher temperatures with increase alcohol content in the fuel, and therefore the termination or reduction of operation in MODE A may be delayed to higher temperature with increase alcohol content in the fuel. 
     Referring now to  FIG. 10 , it shows an example routine  1000  for controlling combustion air-fuel ratio during engine starting based on the mode of exhaust gas routing, and/or hydrocarbon retaining system purging. 
     First, at  1010 , the routine determines whether the engine is starting, such as described with regard to  810 . If so, the routine continues to  1012  to determine whether the system is directing gas to hydrocarbon retaining system  22  (e.g., whether MODE A operation is present). If so, the routing continues to  1014  to select combustion air-fuel ratio based on engine coolant temperature, as well as various other parameters. Next, at  1016 , the routine operates at the selected air to fuel ratio with decreased or no spark retard, such as by operating at MBT timing, potentially even during conditions where temperature of the emission control device (e.g., catalytic converter in the exhaust) is substantially below its light-off temperature. In one example, the air-fuel ratio selected at  1014  may initially, during the start, be slightly lean, and after cranking and run up of engine speed, the selected air-fuel ratio may be approximately stoichiometry, or slightly rich. Alternatively, the air-fuel ratio may be selected based on the amount of hydrocarbons stored in hydrocarbon retaining system  22  (such as the amount of hydrocarbons stored in HC trap  310 ). For example, as the amount of hydrocarbon stored in the hydrocarbon retaining system  22  increases, the combustion air-fuel ratio may be correspondingly adjusted higher, (e.g., less rich, or more lean). 
     When the answer to  1012  is no, the routine continues to  1018  to determine whether temperature of the emission control device  70  is less than a light off temperature. When the answer to  1018  is no, the routine continues to operate the engine to combust approximately stoichiometric air-fuel ratio with selective adjustment of spark retard (e.g., substantially no spark retard, unless needed for knock abatement, torque reduction, etc.). Alternatively, when the answer to  1018  is yes, the routine continues to  1022  to operate the engine to combust a near stoichiometric or slightly lean air-fuel ratio with ignition timing retarded more than operation at  1016  and/or  1020 . Otherwise, at  1020 , the routine operates the engine to combust a stoichiometric air-fuel ratio with selective adjustment of spark retard, with less retard than at  1022 . For example, the ignition timing may be retarded an amount based on catalyst temperature. 
     As still another example, the system may thus operate in a plurality of ignition timing retard modes based on whether exhaust gasses are routed to the hydrocarbon retaining device. For example, the modes may include a first mode where the routine routes exhaust gases to the hydrocarbon retaining system and operates during at least a portion of first mode with spark at a first timing, and with a first engine/exhaust air-fuel ratio; and a second mode where the routine bypasses the hydrocarbon retaining system and operates during at least a portion of the second mode with spark at a second timing more retarded than first timing or a second engine/exhaust air-fuel ratio leaner than the first exhaust air-fuel ratio. In one particular example, the second mode is performed after the first mode, both during a common engine start, and where the first mode is performed at a lower temperature of the emission control device than the second mode. Further, the first air-fuel ratio may be lean during a first portion, and stoichiometric or rich during a second portion of the first mode. Alternatively, the first mode may be performed during a first engine start, and the second mode may be performed during a second engine start, where during the second start, the hydrocarbon retaining system may have a greater amount of hydrocarbons stored than during the first start. 
     Next, at  1024 , from  1020 , the routine determines whether purging of the retaining system  22  is present, including purging of one or both of stored fuel tank vapors and stored exhaust gas hydrocarbons. If so, the routine continues to  1026  where the control system adjusts the fuel injection amount responsive to feedback from one or more exhaust gas oxygen sensors to maintain stoichiometric combustion and learning an amount of hydrocarbons being purged from and/or an amount of hydrocarbons stored in, retaining system  22 . Additionally, the routine may adjust the throttle position responsive to an amount of fresh air drawing through the hydrocarbon retaining system. For example, the routine may close the throttle when initiating the purging operation to compensate for additional flow. 
     In one example, the engine may provide a desired air-fuel ratio by operating some cylinders leaner than the desired air-fuel ratio (e.g., stoichiometric), and some cylinders richer than the desired air-fuel ratio. Such operation may be advantageous in that it can generate additional exothermic exhaust heat, while still providing a stoichiometric exhaust air-fuel ratio. In one example, such split air-fuel ratio operation may be provided during  1016 , for example, to enable increased heating of the emission control device. Further, by utilizing an HC trap in the retaining system  22 , for example, any remaining hydrocarbons exiting the device  70  can be trapped to maintain desired emission levels. 
     Referring now to  FIG. 11 , a routine is described for determining whether to purge the exhaust generated hydrocarbons from retaining system  22 . In one example, the routine of  FIG. 11  determines whether to end fuel tank vapor storage (e.g., MODE B) and transition to purging (e.g., MODE C). 
     First, in  1110 , the routine determines whether a HC retaining device, such as an HC trap or canister, of system  22 , was operated to store hydrocarbons during the engine start. If so, the routine continues to  1112  to determine whether the system is purging fuel tank vapors. If not, the routine continues to  1114  to determine whether an exhaust temperature, such as the temperature of the emission control device, is greater than a threshold temperature, such as the threshold of  818 . For example, the threshold may be adjusted based on the alcohol content of fuel in the fuel tank and injected into the engine. 
     If the answer to  1114  is yes, the routine continues to  1116  to determine whether the amount of stored hydrocarbons in the retaining system  22  (for example the amount of hydrocarbons stored in HC trap  310 ) is greater than a threshold value. If so, the routine continues to  118  to determine whether the air-fuel ratio of combustion in engine  10  is approximately the stoichiometric ratio. If so, the routine continue to  1120  to purge the retaining system, such as to purge an HC trap and/or canister (e.g., operate in MODE C). 
     In one example, the operation of  FIG. 11  may be carried out after engine starting, or during starting conditions, such as during hot re-start conditions. 
     Referring now to  FIG. 12 , a routine is described for determining whether to purge the fuel tank generated hydrocarbons from retaining system  22 . In one example, the routine of  FIG. 12  determines whether to end fuel tank vapor storage (e.g., MODE B) and transition to purging (e.g., MODE C). In one example, the routines of  FIGS. 11 and 12  may be coordinated together, such as where fuel tank vapor purging and exhaust gas generated hydrocarbon purging occur concurrently. In another example, the routines of  FIGS. 11 and 12  are carried out independently, such as where fuel tank vapor purging and exhaust gas generated hydrocarbon purging occur independently. 
     First, in  1210 , the routine determines whether an HC retaining device, such as an HC trap or canister, of system  22 , is being purged. If so, the routine ends. Otherwise, the routine continues to  1212  to determine whether an engine start is present, such as described herein. If not, the routine continues to  1214  to coordinate fuel vapor purging with adaptive learning of fuel injector errors, MAF sensor errors, etc., and then the routine continues to  1216  to carry out purging of fuel tank vapors to the intake, as coordinated in  1214 . 
     When the answer to  1212  is yes, the routine continues to  1218  to determine whether the combustion air-fuel ratio is approximately the stoichiometric air-fuel ratio (see  FIG. 10 ). If not, the routine does not enable purge, such as when HEGO sensors are used, since it may be difficult to control air-fuel ratio in the presence of purging operation. If so, the routine continues to  1220  to determine whether the system is currently operating to direct exhaust gases through the retaining system  22 . If so, the routine ends (in the case where purging and storage occur non-concurrently). Otherwise, the routine continues to  1216  to purge the retaining system to the intake. 
     Referring now to  FIG. 13 , a routine is described for determining whether to route exhaust gases to the hydrocarbon retaining system  22  during conditions other than engine starting operation, such as other engine operating conditions where the engine conditions to spin and/or the vehicle is traveling. The routine of  FIG. 13  may be used after engine starting/warm-up operations, such as those described above herein. 
     For example, the operation described with regard to  FIG. 13  may be advantageous during cylinder valve deactivation conditions of one or more cylinders (e.g., variable displacement engine (VDE) operation) where temperature of an emission control device may drop below an activation or light-off temperature. Under such conditions, the remaining active cylinders may operate stoichiometric or slightly rich, and the exhaust gas routed to retaining system  22  for storage of excess hydrocarbons exiting the cooled emission control device. Further, it may be advantageous during deceleration fuel shut-off (DFSO) operation or after reactivation of DFSO cylinders, where a significant amount of fresh, oxygen rich gas is stored in the emission control device, thereby reducing its catalytic reaction of emissions from the engine, as well as its temperature. This can be especially advantageous when reactivating combustion after DFSO, where combustion air-fuel ratio during reactivation is substantially rich to reduce the stored oxygen and re-establish the catalyst&#39;s desired oxygen storage state. The operation of  FIG. 13  may also be used during catalyst over-temperature conditions (such as when catalyst temperature is above its maximum allowed temperature for the current conditions) where the engine is operated rich to reduce the catalyst temperature. 
     In still another example, the routine of exhaust gases to the hydrocarbon retaining system  22  during conditions other than engine starting operation may be used to heat the hydrocarbon retaining system  22  for subsequent purging. For example, once the hydrocarbon retaining system  22  is sufficiently heated, purging operation may be enabled. This can be advantageous when the hydrocarbon retaining system  22  is below a threshold temperature, and would otherwise result in slow and/or inefficient hydrocarbon release and/or desorption. 
     Referring now specifically to  FIG. 13 , at  1310  the routine determines at  1310  whether an engine start is present, such as described herein. If so, the routine continues to  1312  to determine whether the hydrocarbon retaining system  22  has been purged since the engine start of the present engine operation and/or whether the amount of hydrocarbons and fuel vapors estimated to be stored in the system  22  is less than a threshold value. For example, the routine may determine whether the amount of hydrocarbons stored in the HC trap  310  is less than a threshold value. 
     If so, the routine continues to  1314  to determine whether select conditions are present. In one example, the select conditions include when an increased potential for hydrocarbon breakthrough from emission control device  70  is greater than a threshold. In another example, the select conditions include deceleration fuel shut-off of one or more cylinders, and/or reactivation from such conditions. In still another example, the select conditions include when the exhaust air-fuel ratio from the engine is sufficiently rich of stoichiometry (e.g., richer than a threshold). In yet another example, the select conditions include when one or more cylinders is deactivated, such as when intake and exhaust valves are held closed throughout the cycle 4-stroke cycle. If so, the routine continues to  1316  to operate the exhaust valves to route exhaust gas through the hydrocarbon retaining system  22  (e.g., operate in MODE A). 
     In this way, improved emission may be achieved, even during non-engine starting conditions. Further, it will be appreciated that additional purging, after the storing operation of  1316 , may be carried out during subsequent engine operation once the operation at  1316  is ended. 
     In one particular example, during conditions other than an engine start, where the catalytic emission control device is above its light-off temperature, the routine may route exhaust gas to the hydrocarbon retaining system when temperature of the hydrocarbon retaining system is below a threshold value, the routing continuing until the hydrocarbon retaining system reaches a threshold purging temperature, and then route exhaust gases to atmosphere, bypassing the hydrocarbon retaining system, and purge the hydrocarbon retaining system to the engine intake 
     As noted in  FIG. 14 , the control system may enable determination of degradation of the retaining system  22 , such as to determine and differentiate degradation between a purge canister and an HC trap. In some examples, the purge hydrocarbon concentration learned from exhaust gas oxygen sensor feedback and fuel injection adjustment may provide independent estimates of the hydrocarbon storage, such as when a fuel tank canister (e.g.,  422 ) is at a different temperature than an HC trap (e.g.,  310 ). Alternatively, the learned hydrocarbon amount during purging may be used to determine performance of an HC trap that was loaded and then purged during condition with little to no fuel tank vapor generation, and before which the fuel tank vapors were sufficiently purged. In another particular example, the HC trap may be diagnosed based on detecting change in a temperature condition of gas during purging of gases through the trap. Various examples, as well as additional diagnostic approaches, are described below herein. 
     In the following examples, the diagnostics are described with respect to the example of a carbon based hydrocarbon trap coupled in retaining system  22 . Further, the operation is described in the example where, following a start up event, the exhaust flow is diverted through the HC trap until the emission control device is sufficiently heated and reached a desired conversion efficiency. Further, following the storage, the trapped hydrocarbons are then purged by flowing air through the HC trap back into the engine for combustion. The purge flow may be in series or in parallel with the fuel tank generated evaporative emissions carbon canister purge. However, the following diagnostics may also be applied to various other system configurations and operating modes. 
     The following diagnostics may be applied as either or both of a functional or threshold monitor. The functional monitor may determine if a device is connected and functioning as expected while a threshold monitor may determine the effectiveness of the emission control device. 
     One potential degradation mode may include vibration and mechanical damage to carbon in the HC trap—e.g., carbon granules can be fractured and worn down. As such, the HC trapping capability of the trap may be reduced if the carbon granules are broken down and potentially lost out the tailpipe. If enough carbon is lost or compacted and the volume of the trap is no longer filled, some of the exhaust gas may bypass the carbon bed and the tailpipe emissions can increase. Thus, one approach uses a monitor that detects a loss of carbon volume as a threshold monitor. 
     In a first approach, the control system may measure temperature at the outlet of the HC trap during trapping and purging. Following a cold start, it is expected that the temperature at the outlet will increase as the exhaust system warms up. This indicates that the system is connected to the exhaust system and the valves used to divert the flow to the trap are functioning. After the flow is allowed to bypass the trap, the temperature should stabilize, indicating that the valves have cycled properly. A continued increase in the temperature would indicate that the trap is still being exposed to exhaust gas. During the purge cycle, the temperature should drop as fresh air is inducted into the trap. 
     In a second approach, pressure differential across the carbon bed of the HC trap may be measured during the trapping or purging as an indication of the presence of a flow restriction due to the carbon. A loss of carbon would lead to a reduction in the pressure difference, for a given flow rate. One or more pressure sensors may be mounted upstream and or downstream of the HC trap. 
     In a third approach, a fuel vapor/hydrocarbon sensor may be used. For example, an HC sensor in the trap outlet may directly measure the effectiveness of the trap during trapping since any degradation in the trap would result in an increase in the HC breakthrough at vent  27 . Some modeling of the concentration and composition of the exhaust gas as a function of speed/load/spark retard/temperatures and/or total fuel consumed since start along with the expected reduction in pre-trap HC due to catalyst light off may be used to improve the capability of a threshold monitor. Alternately, an empirical model of the expected HC out of the trap could be used. 
     In a fourth approach, temperature sensors placed before and after the HC trap can provide an indication of a temperature difference during the trapping (e.g., due to adsorption) and purging (e.g., desorption) phases. The temperature difference may also be affected by the percentage of the flow that is adsorbed or desorbed on the carbon bed (primarily water and HC). In other examples, temperature deviations from expected or estimated values may be used. In the examples using an HC trap in the exhaust system with a temperature sensor located downstream, the system may utilize an inferred exhaust temperature at the trap outlet to generate a temperature difference between the inferred and actual temperature at the trap outlet. The system can then diagnose adsorption function based on this temperature difference. Further, due to potential errors in temperature estimates, identification of a temperature shift may be based on the initial temperature detected by the temperature sensor, and then considering a measured temperature rise in comparison to an estimated temperature rise during select conditions, such as storing or purging. Note also that in some examples, exhaust gas air-fuel ratio sensors may be operated as a temperature sensor under select conditions. 
     In a fifth approach, a position measurement of the HC trap can be used. Specifically, to reduce vibration from damaging the carbon in the trap and/or canister, the devices may include a floating end plate and one or more springs to maintain some level of pressure on the carbon. If the carbon deteriorates in spite of these measures, the reduction in volume can lead to a change in the position of the end plate(s) and a reduction in the force applied by the spring. Various methods to detect the movement may be used to identify the loss of carbon. In one example, these may include a position sensor, a thin wire that would break when the spring is extended, contacts that would open if the plate moves, transducers to measure the spring force, or strain gauge like devices built into the spring. 
     In a sixth approach, probes that measure the conductivity of the carbon bed may be used to provide an impedance measurement that would increase with the loss of spring pressure or a reduction in the carbon volume. The resistance between two contact surfaces applied to the top of the canister may show a larger increase if the bed settled enough to provide a bypass path for the exhaust gasses. In one example, the conductivity may be measured during select conditions, such as during warm-up, or after warm-up. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.