Abstract:
An exhaust aftertreatment system for an internal combustion engine is disclosed which mitigates deleterious poisoning of a catalytic converter or exhaust gas oxygen sensor by phosphorus containing species.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a system for minimizing the deleterious impact of oil-derived phosphorus containing compounds on automotive exhaust aftertreatment systems.  
           [0003]    2. Background of the Invention  
           [0004]    Automotive oils typically contain a zinc dialkyldithiophosphate (ZDDP) additive which forms an antiwear coating on engine components and acts as an antioxidant in the oil. Although engines are designed to minimize the quantity of engine oil exiting the engine via the combustion chamber and exhaust system, it is inevitable that a small fraction of engine oil is released by this mechanism. The ZDDP additive of engine oil deleteriously affects catalytic converters due to phosphorus from the ZDDP interfering with active sites within the catalyst. These phosphorus containing species deposit onto, or react with washcoat components, such as aluminum oxide and cerium oxide, and remain there indefinitely. This phenomenon is commonly referred to as phosphorus poisoning.  
           [0005]    Measures to eliminate or reduce ZDDP in engine oils have been investigated. Alternatives to ZDDP have been produced which have been shown to provide antioxidant and antiwear properties similar to ZDDP. However, they are cost prohibitive. Engine oils may be formulated with a lesser amount of ZDDP with the consequences that engine wear and oil oxidation increase, the former limiting engine life and the latter reducing useful oil life.  
           [0006]    The inventor of U.S. Pat. No. 5,857,326 has disclosed an exhaust poison trap which comprises a helical wall dividing the exhaust chamber into longitudinal helical passages for exhaust gas flow and porous means covering the interior of the peripheral wall. The inventor of &#39;326 teaches that exhaust gas is directed in a helical path causing particulate matter in the gas to be accelerated outwardly by centrifugal force and trapped in the porous means. The inventor of the present invention has recognized several limitations of the approach in &#39;326. The helical passages cause the exhaust gases to be rotated and particles that have a diameter less than a certain size follow the flow and avoid being trapped in the porous means near the walls of the tube and larger particles impact the porous means near the walls. The device disclosed in &#39;326 has the capability of causing only the largest particles to be removed. The figures in &#39;326 indicate that the helical wall causes the flow to rotate through at least two revolutions and as many as four revolutions. The length of the exhaust poison trap is approximately two to four pipe diameters long with the disadvantages of complicating the packaging of the exhaust poison trap and increasing the weight of the trap, the thermal mass of which interferes with the desire to bring the catalytic converter to its operating temperature as soon as possible after starting the engine to control cold start emissions.  
         SUMMARY OF INVENTION  
         [0007]    Disadvantages of prior art are overcome by an exhaust aftertreatment system for a spark-ignition, reciprocating internal combustion engine having a catalytic converter in an exhaust duct of the engine which receives an exhaust gas stream from the engine. The system comprises a trap in the exhaust duct located upstream of the catalytic converter. The trap is made of a porous ceramic or metallic material having an average pore size greater than about 80 micrometers. The porous material substantially fills the cross-section of the exhaust duct and has a volume of than 10% of a swept volume of the engine&#39;s cylinders coupled to the trap. Exhaust gases undergo multiple, random turns in traveling from an upstream side to a downstream side of the trap. The trap is located within 15 centimeters of the catalytic converter. An exhaust gas component sensor is placed downstream of the phosphorus trap.  
           [0008]    Also disclosed is an exhaust aftertreatment system for processing exhaust gases from a reciprocating internal combustion engine, which includes a catalytic converter disposed in an exhaust duct of the engine. The catalytic converter has channels for conducting exhaust gases from an upstream end to a downstream end. The channels are substantially parallel to each other and parallel to a direction of flow through the catalytic converter. The catalytic converter has a ceramic or metallic porous material disposed within the channels from the upstream end of the catalytic converter for a predetermined distance along the catalytic converter. The porous material has randomly oriented passageways causing the exhaust gases to undergo multiple turns in the course of being transmitted through the porous material.  
           [0009]    Also disclosed is an exhaust aftertreatment system for a reciprocating internal combustion engine comprising a phosphorus trap in an exhaust duct of the engine made of a porous material and substantially filling the cross-section of the exhaust duct. The porous material has an average pore size greater than a predetermined pore size and has randomly oriented passageways forcing exhaust gases passing through to undergo multiple turns. The system also has a catalytic converter disposed in the exhaust duct of the engine located downstream of the phosphorus trap and an electronic control unit operably connected to the engine. The electronic control unit provides an indication of an amount of phosphorous containing material trapped in the phosphorus trap and raises temperature in the phosphorous trap above a predetermined temperature when the amount of phosphorous containing material exceeds a predetermined quantity. The indication is based on time of operation or a value of an engine parameter since the predetermined temperature has been achieved.  
           [0010]    A primary advantage of the present invention is that phosphorus contamination of the exhaust aftertreatment system can be decreased by approximately 60% in the absence of taking other preventative measures, which are costly. Reduced phosphorus contamination, as provided by the present invention, allows the catalyst to operate at high conversion efficiency over the life of the vehicle.  
           [0011]    The inventors of the present invention have recognized that the effectiveness of the phosphorus trap is improved if it operates at a temperature close to the temperature of the catalytic converter. Thus, another advantage of the present invention is higher capture efficiency of deleterious phosphorus containing particles by placing the phosphorus trap in close proximity to the catalytic converter.  
           [0012]    Another advantage of the present invention is that it removes particles of smaller diameter than prior approaches and does so with a neglible pressure drop across the phosphorus trap.  
           [0013]    Yet another advantage of the present invention is that vehicles with unusual driving patterns may be operated in such a way to allow such vehicles to also benefit from the present invention.  
           [0014]    The present invention may also be used to advantage combined with quick warmup strategies such as cold start spark retard and exhaust port oxidation.  
           [0015]    The inventors of the present invention have recognized that the phosphorus trap may be much smaller than in prior approaches. The smaller size affects the warmup time of the exhaust system less than larger traps, a decided advantage in preventing cold start emissions.  
           [0016]    Another advantage of the present invention is that, if the phosphorus trap is placed upstream of an exhaust gas oxygen sensor or other exhaust component sensor, deterioration of the sensor is prevented or slowed.  
           [0017]    Without a phosphorus trap located upstream of a catalytic converter, the converter volume is chosen which provides sufficient conversion capacity over the targeted lifetime. An advantage of the present invention is that the volume can be reduced because the phosphorus trap protects the catalytic converter from phosphorus poisoning.  
           [0018]    Yet another advantage is that, if the phosphorus trap is coated with a washcoat, it can provide some additional conversion capability, particularly during cold start.  
           [0019]    The above advantages, other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]    The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:  
         [0021]    [0021]FIG. 1 is a schematic of an engine equipped with a phosphorous trap according to an aspect of the present invention;  
         [0022]    [0022]FIG. 2 shows a portion of a cross-section of a catalytic converter with a phosphorus trap integrated into the channels of the catalytic converter according to an aspect of the present invention;  
         [0023]    [0023]FIG. 3 shows a representative structure of a phosphorus trap;  
         [0024]    [0024]FIG. 4 is a flowchart of a method for operating an internal combustion engine according to an aspect of the present invention; and  
         [0025]    [0025]FIG. 5 is a flowchart of a method for operating an internal combustion engine according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]    An engine  10  equipped with a phosphorous trap  30  according to an aspect of the present invention is shown in FIG. 1. Engine  10  is supplied air through intake manifold  12  past throttle valve  14  and is supplied fuel through fuel injectors  16  spraying fuel into intake manifold  12 . The configuration shown in FIG. 1 is commonly referred to as port fuel injection. The present invention also applies to direct fuel injection, in which fuel injectors  16  are installed directly in cylinders  32 , central fuel injection, in which a single fuel injector  16  is placed in intake manifold  12  closer upstream of where intake manifold  12  separates into individual runners supplying individual cylinders  32 , carburetion, and other fuel supplying devices. Ignition is provided by spark plugs  18 . The exhaust gases are expelled through exhaust manifold  28 , into phosphorus trap  30 , into catalytic converter  26 , and exhausted to the atmosphere. Sensor  24  is an exhaust gas component sensor, preferably an exhaust gas oxygen sensor. Alternatively, sensor  24  is a NOx sensor, HC sensor, CO sensor, or other component sensor.  
         [0027]    ECU  40  is provided to control engine  10  as shown in FIG. 1. ECU  40  has a microprocessor  72 , called a central processing unit (CPU), in communication with memory management unit (MMU)  74 . MMU  74  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  72 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  76 , random-access memory (RAM)  80 , and keep-alive memory (KAM)  78 , for example. KAM  78  is used to store various operating variables while CPU  72  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU  72  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  72  communicates with various sensors and actuators via an input/output (I/O) interface  70 . Examples of items that are actuated under control by CPU  72 , through I/O interface  70 , are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve  14  position, spark plug  18  timing, and others. Sensors  42  communicating input through I/O interface  70  may be indicating engine rotational speed, vehicle speed, coolant temperature, intake manifold  12  pressure, pedal position, throttle valve  14  position, air temperature, exhaust temperature, and air flow. Some ECU  40  architectures do not contain MMU  74 . If no MMU  74  is employed, CPU  72  manages data and connects directly to ROM  76 , RAM  80 , and KAM  78 . The present invention could utilize more than one CPU  72  to provide engine control and ECU  40  may contain multiple ROM  76 , RAM  80 , and KAM  78  coupled to MMU  74  or CPU  74  depending upon the particular application.  
         [0028]    Catalytic converter  26  is commonly called a three-way catalyst which can process NOx, hydrocarbons, and CO, although the invention can potentially be used with a wide variety of catalyst systems including those for lean-burn engines, diesel engines, and various alternatively fueled vehicles among others. Although only one converter is shown in FIG. 1, it should be appreciated that most vehicles contain multiple catalyst elements, sometimes in the same converter housing and sometimes in separate converters. V engines often contain separate catalytic converters coupled to each engine bank of engine cylinders. In addition, typical converter systems consist of a catalyst mounted close to the engine (light-off converter) and one or more converters located downstream in either so-called to-board or underbody positions. In the present invention, the converter of greatest inters is the light-off converter because this is the one in which the majority of the poison species are captured.  
         [0029]    Typical three-way catalysts are comprised of extruded ceramic or metallic material forming a myriad of parallel passageways of about 1 millimeter in hydraulic diameter. The extruded substrate is treated to provide precious metals on the surface of the passageways through the substrate via high-surface-area washcoat components such as aluminum oxide, cerium oxide, and zirconium oxide. In particular, the cerium and zirconium oxide materials, and combinations of the two, constitute oxygen storing species which improve the efficacy of the conversion process. When these oxygen storage sites are occupied by phosphorus containing compounds, the number of oxygen storage sites that can be used for aiding in converting CO, NOx, and hydrocarbons is decreased. Alternatively, phosphorus species can react with aluminum oxide to form aluminum phosphate, thereby causing densification of the washcoat structure, pore blocking, and occlusion of active noble metals. Yet another mechanism by which phosphorus species can interfere with catalyst effectiveness is through the formation of an overlayer on the surface of the washcoat. This overlayer generally consists of various phosphate compounds of zinc, calcium, and magnesium, and can impede the diffusion of the reactive gases from the bulk gas stream to the active sites within the washcoat layer. If phosphorus contamination continues, in time, the effectiveness of catalytic converter  26  is seriously impaired.  
         [0030]    In FIG. 1, catalytic converter  26  is shown separated from phosphorus trap  30  and exhaust gas oxygen sensor  24  is placed downstream of phosphorus trap  30 . Like catalytic converter  26 , exhaust gas oxygen sensor  24  is treated with precious metals bonded onto its surface to catalyze the reaction of CO, NOx, and hydrocarbons. Exhaust gas oxygen sensor  24  is also harmed by contamination by phosphorus containing species. Thus, an advantage of the configuration shown in FIG. 1 is that exhaust gas oxygen sensor  24  is protected from deterioration by phosphorus species.  
         [0031]    Alternatively, phosphorus trap  30  is placed within the catalytic converter  26  housing at the upstream end of catalytic converter  26  (configuration not shown). In this configuration, exhaust gas oxygen sensor  24  is located upstream of both catalytic converter  26  and phosphorus trap  30  and is not protected from phosphorus contamination.  
         [0032]    Referring now to FIG. 2, another alternative configuration is shown. As described above, catalytic converter  26  contains many parallel passageways along its length, as shown in FIG. 2. According to an aspect of the present invention, the porous material, of which phosphorus trap  30  is comprised, is inserted into the upstream end of the passageways.  
         [0033]    The inventors of the present invention have recognized that unburned or partially oxidized engine oil containing ZDDP additive, condenses in the exhaust gas when the temperature is lower than about 200° C. Such condensable material is captured by the catalytic converter with high efficiency unlike more fully oxidized phosphorus containing species which exist in the vapor form and have greater likelihood of passing through the catalytic converter without being captured. The mechanism, by which catalytic converter  26  is harmed, is that the unburned or partially oxidized phosphorus containing species condense on the surfaces of catalytic converter  26 . Catalytic converter  26  contains high-surface-area components such as aluminum oxide and cerium oxide, on the surface. Unoxidized and partially oxidized phosphorus species (arising from ZDDP additive in the oil) adsorb onto these components. It is believed that the phosphorus species and the washcoat components form chemical bonds. Based on the present day state of the art, no in situ, cost effective measure of breaking those chemical bonds has been determined. Thus, oxygen storage sites that have been contaminated by phosphorus compounds are essentially unrecoverable, i.e., they are no longer able to participate in catalytic reactions.  
         [0034]    The inventors of the present invention have performed laboratory experiments showing contamination or capture efficiency of the partially oxidized or unoxidized phosphorus species of at least 50% and possibly up to nearly 100% in catalytic converter  26 .  
         [0035]    When exhaust temperature exceeds about 400-500° C., partially oxidized or unoxidized phosphorus species largely react to form fully oxidized phosphates or species, which are more oxidized, such as phosphoric acid, phosphorus pentoxide, and a dimer of phosphorus pentoxide. These species are vapor phase, even at temperatures below 200-250° C. These species do not normally condense until the exhaust gas temperature falls to levels below 80-100° C. where condensation occurs along with condensation of water from the exhaust gases. The inventors of the present invention have found that the capture efficiency of these vapor phase phosphorus compounds and the phosphate related particulates (eg., zinc phosphate) is less than about 20% in catalytic converter  26  and the phosphates are largely benign. Thus, the inventors of the present invention have recognized that if harmful condensable phosphorus species can be prevented from entering the catalytic converter when the temperature is less than 200-250° C., when the exhaust system subsequently achieves a temperature exceeding 400° C., harmful condensable phosphorus species react into the less harmful vapor species or to the nearly harmless phosphates. The probability of the phosphorus materials poisoning catalyst  26  reduces from 50% to 20% if reacted to the vapor species or to much less than 20% if reacted to phosphates. The poisoning risk to catalyst  26  is reduced by more than 60%. The oxidation temperature for the phosphorus species is in the range of 200-250° C. Below, the temperature 225° C. is used to indicate this range.  
         [0036]    In engines equipped with three-way, oxygen-storing catalytic converters exhaust temperatures are below 225° C. only during cold start and extended idle periods. Thus, if condensable phosphorus species are collected in trap  30  prior to entering catalyst  26 , these condensable phosphorus species convert to less harmful species when the temperature in trap  30  rises above 225° C. Thus, the trap regenerates spontaneously when exhaust temperature achieves normal operating temperature. The inventors of the present invention have recognized that only a small amount of the condensable phase phosphorus species is generated by the engine during any such operating interval with low exhaust temperatures, except for unusual operating patterns and thus, the desired volume of trap  30  capable of capturing the emitted material is small. On an exceptional basis, exhaust temperatures may remain low under unusual operating cycles, which is discussed in more detail below. The small size and mass of trap  30 , according to the present invention, overcomes the disadvantage of traps with large thermal inertia of prior approaches. Trap  30  is constructed of ceramic or metallic foam of pore size roughly 100 micrometers and a minimum pore size of 20 micrometers. Alternatively, trap  30  may be constructed of other porous materials, which provide pore sizes as mentioned above, random passages there through, and can withstand the temperatures encountered in the exhaust duct. Unlike catalytic converter  26 , which has parallel passageways through which the exhaust gases pass, trap  30  has random passageways causing the exhaust gases to twist and turn to pass through trap  30 . It is the inability of the droplets and aerosol particles to negotiate turns in trap  30  that causes them to impact onto the foam material itself. The inventors of the present invention have recognized that the volume of trap  30  is less than about 10% of the swept volume (or displacement) of engine  10 . Swept volume is found by multiplying the cross-sectional area of a piston times the travel distance of the piston during a single stroke times the number of cylinders in engine  10 .  
         [0037]    Trap  30  can be coated with a washcoat similar to that used in a three-way catalyst. Trap  30  would be beneficial in reducing tailpipe emissions during cold start, i.e., prior to when catalyst  26  has reached operating temperature. The washcoat of trap  30  would become poisoned over time and its ability to provide conversion hampered. Nevertheless, during the time that trap  30  is fresh, cold start emissions would be reduced.  
         [0038]    If engine  10  is a multi-bank engine, eg. V-8, in which catalysts are disposed in exhaust ducts coming from each bank of the engine, preferably a phosphorus trap  30  is placed in each exhaust duct upstream of the catalyst. In this case, the volume of phosphorus trap  30  is related to the displaced volume of the cylinders to which it is coupled. Each trap  30  is comprised, preferably, of a single, integral structure requiring little external support, except for being held in place at the periphery. This is in contrast to a pellet-type trap comprised of numerous pellets which must be retained within a container.  
         [0039]    It is known in the art to use a diesel particulate filter (DPF) to trap carbonaceous particles exhausted from a diesel engine. DPFs are designed such that they collect greater than 90% of all particles. To be able to collect the smallest particles (as small as several nanometers), the average pore size of a DPF is typically about 20 micrometers and a DPF has a volume roughly equal to 1-3 times the engine&#39;s displacement volume. Because of the DPF&#39;s small pore size and large volume, a DPF provides considerable resistance to exhaust gas flow, roughly 25 kPa when the trap is empty and roughly 50 kPa when the trap is full (these pressure drops occur at an engine condition generating peak engine power, i.e., when flow through the exhaust system is at highest). Typical DPFs are constructed of parallel channels along the direction of flow through the DPF. Every other channel is blocked on the upstream end. On the downstream end of the DPF those channels, which are unblocked on the upstream end, are blocked on the downstream end. This forces exhaust gases to traverse through channel walls. This has been found to allow high collection efficiency over a wide range of particle sizes.  
         [0040]    In contrast, the desire is for phosphorus trap  30  to collect only particles greater than about several micrometers in diameter and allow the passage of smaller particles. The inventors of the present invention have recognized that it is preferable to allow smaller particles to travel through trap  30  without being trapped because smaller particles permanently lodged in trap  30  ultimately occludes the trap, causing a significant pressure drop. According to the present invention, phosphorus trap  30  has an average pore size of at least 50 micrometers with a minimum pore size of greater than about 20 micrometers. For the purposes of the present invention, trap  30  need only be about 10% of engine displaced volume, if the trap is made of metallic foam, and about 15% of engine displacement volume, if the trap is made of ceramic foam. Because of the relatively large pore size and small volume of trap  30 , the pressure drop across trap  30  is negligible, less than about 1 kPa.  
         [0041]    Typically, DPFs have a porosity of about 50%; whereas, the phosphorus trap  30 , of the present invention, has a porosity greater than about 90%. Because of the high porosity and small volume of phosphorus trap  30 , for a typical automotive engine, the mass of the porous material in phosphorus trap  30  is roughly 50 to 200 g depending on the material of trap  30 . It is expected that the mass of phosphorus trap  30  be related to displacement of the cylinders to which trap  30  is coupled, eg., mass (in grams) of trap  30  is less than roughly engine displacement (in cubic centimeters) divided by 25. The length of trap  30  is about one-third of the diameter of the exhaust duct in which it is contained.  
         [0042]    In the preceding discussion, collection characteristics of a DPF and a phosphorus trap  30  are compared. It was stated that phosphorus trap  30  collects particles above one micrometer in diameter. It is known, however, to those skilled in the art, that filters of the types discussed do not have sharp cutoffs in the size of particles collected. Thus, phosphorus trap  30 , even though designed to collect particles greater in diameter than one micrometer, collects particles smaller than one micrometer, but at low efficiency. Furthermore, the collection efficiency, as a function of particle diameter, is affected by the velocity of the gases at the face of the filter or trap. Thus, the numbers given above are representative, but not limiting. Also, the specific characteristics of phosphorus trap  30  are given by way of example and are not intended to be limiting.  
         [0043]    An example of the structure of phosphorus trap  30  is shown in FIG. 3. FIG. 3 is a drawing based on a photomicrograph of the face of a metallic foam suitable for use as phosphorus trap  30 . The magnification in the drawing is roughly 100×. The smaller pores, in FIG. 3, that are significantly smaller than the expected 100 micrometers are due to them being pores which are slightly below the surface, and thus partially occluded from view by portions of upper pores. It can be seen in FIG. 3 that the material is irregular causing gases to twist and turn randomly in passing through phosphorus trap  30 .  
         [0044]    As mentioned above, some vehicles with unusual operating patterns may operate for extended periods with the exhaust temperature less thaOLE_LINK2n 225° OLE_LINK2C. As example is a taxicab, which may idle for extended intervals. The inventors of the present invention have recognized that phosphorus trap  30  is purged of condensable phase phosphorus species if the temperature is raised above 225° C. for a short period. According to the present invention, it is determined when trap  30  can no longer retain more droplets containing condensable phase phosphorus species. When that determination is made, engine  10  operation is changed to cause the temperature in the exhaust to exceed 225° C.  
         [0045]    The method of purging phosphorus trap  30 , according to an aspect of the present invention, is shown in FIG. 4. A phosphorus trap purge routine begins in step  100  when engine  10  is started. In step  102 , RAM  80  memory location, t, is filled with the contents of a KAM  78  memory location, t resid , which is the operating time elapsed since the last purge of trap  30 . As engine  10  has just been started, the value of t resid  is based on a prior operating interval of engine  10 . In step  104 , it is determined whether the temperature in trap  30 , T Ptrap , greater than a threshold temperature, T thresh . T thresh  is the temperature at which condensable phase phosphorus compounds oxidize to form less harmful phosphorus species. If step  104  yields a positive result, control passes to step  116 , where t is reset to 0, which means that trap  30  is purged of condensable phase phosphorus species. In normal, warmed up operation, the routine of FIG. 4 cycles between steps  116  and  104 . However, during unusual operating patterns and until engine  10  is warmed up, a negative result from step  104  occurs. Control then passes to step  106  in which memory location t is incremented by the time elapsed since the last time t was updated, Δt. In step  108 , t is compared to t thresh , which is a threshold time for which trap  30  has been operating long enough since the last purge to be substantially full. If a negative result from step  108 , control passes back to step  104 . If a positive result in step  108 , control passes to step  110  where operating conditions of engine  10  are altered to cause T Ptrap  to exceed T thresh . Control passes to step  112  where a check whether trap  30  has had sufficient time to oxidize the condensable phase phosphorus species. If a negative result, engine  10  is maintained at the operating condition to keep T Ptrap  above T thresh . When a positive results from step  112 , control passes to step  114  in which engine  10  is returned to normal operating conditions. Control passes to step  116  in which t is reset to 0. Also shown in FIG. 4 is an interrupt, step  120 , which is when engine  10  is shut off. The routine of steps  100 - 116  is diverted to step  120 , when the interrupt is received. Control passes to step  122 , in which the current value of t is stored in t resid , the latter of which is in KAM  78 . The routine is ended in step  124 .  
         [0046]    The routine discussed in regard to FIG. 4 is based on a time of operation since the last purge. An alternative is to model the amount of condensable phase phosphorus material that is released. The model could be based on engine speed, other engine operating parameters, which are known in ECU  40 , or a combination of such parameters. Such a routine is shown in FIG. 5. The routine of FIG. 5 is identical to the routine of FIG. 4, except that rather than basing the purge on a time of operation, the purge is based on a modeled mass of condensable phase phosphorus species, m, in trap  30 .  
         [0047]    The method of purging phosphorus trap  30 , according to an aspect of the present invention, is shown in FIG. 5. A phosphorus trap purge routine begins in step  200  when engine  10  is started. In step  202 , RAM  80  memory location, m, is filled with the contents of a KAM  78  memory location, m resid , which is the time elapsed since the last purge of trap  30 . As engine  10  has just been started, the value of m resid  is based on a prior operating interval of engine  10 . In step  204 , it is determined whether the temperature in trap  30 , T Ptrap , is greater than a threshold temperature, T thresh . T thresh  is the temperature at which condensable phase phosphorus compounds oxidize to form less harmful phosphorus species. If step  204  yields a positive result, control passes to step  216 , where m is reset to 0, which means that trap  30  is purged of condensable phase phosphorus species. In normal, warmed up operation, the routine of FIG. 4 cycles between steps  216  and  204 . However, during unusual operating patterns and until engine  10  is warmed up, a negative result from step  204  occurs. Control then passes to step  206  in which memory location t is incremented by the time elapsed since the last time t was updated, Δt. In step  208 , t is compared to t thresh , which is a threshold time for which trap  30  has been operating long enough since the last purge to be substantially full. If a negative result from step  208 , control passes back to step  204 . If a positive result in step  208 , control passes to step  210  where operating conditions of engine  10  are altered to cause T Ptrap  to exceed T thresh . Control passes to step  212  where a check whether trap  30  has had sufficient time to oxidize the condensable phase phosphorus species. If a negative result, engine  10  is maintained at the operating condition to keep T Ptrap  above T thresh . When a positive results from step  212 , control passes to step  214  in which engine  10  is returned to normal operating conditions. Control passes to step  216  in which m is reset to 0. Also shown in FIG. 4 is an interrupt, step  220 , which is when engine  10  is shut off. The routine of steps  200 - 216  is diverted to step  220 , when the interrupt is received. Control passes to step  222 , in which the current value of m is stored in m resid , the latter of which is stored in KAM  78 . The routine is ended in step  224 .  
         [0048]    To cause the temperature of the exhaust to rise, as discussed in regards to step  110  of FIG. 4 and step  210  of FIG. 5, one or more of the following measures may be undertaken: retarding the spark timing for some or all cylinders, providing air and fuel to the exhaust such as by operating some cylinders rich and others lean or by introducing secondary air into the exhaust, loading engine  10  by causing the alternator to generate electricity for storage in the battery, loading engine  10  with a power consuming accessory such as air conditioning, reducing cooling water flow rate to engine  10 , turning off an engine cooling fan, raising engine speed, reducing exhaust gas recirculation, changing valve timing in engines equipped with variable valve timing mechanisms, and raising the temperature of the intake air.  
         [0049]    While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.