Patent Publication Number: US-2010122523-A1

Title: Cold-start engine loading for accelerated warming of exhaust aftertreatment system

Description:
TECHNICAL FIELD 
     The present invention relates generally to exhaust aftertreatment systems. More particularly, the present invention is drawn to methods for accelerated warming of motor vehicle exhaust aftertreatment systems. 
     BACKGROUND OF THE INVENTION 
     Almost all conventional motorized vehicles, such as the modern-day automobile, include an exhaust aftertreatment system for mitigating the byproducts generated from operation of an internal combustion engine. Most exhaust aftertreatment systems include a catalytic converter for the reduction and oxidation of exhaust gas emissions, and a muffler assembly or similar device for attenuating noise generated by the exhaust emission process. The catalytic converter is normally placed between the engine exhaust manifold and the muffler of the automobile, but can also be integrated into the muffler assembly. 
     Catalytic converters normally include a monolith substrate, generally of the ceramic honeycomb or stainless steel foil honeycomb type. The monolith substrate is coated with a catalyst that contains a precious metal, such as platinum, palladium, or rhodium. The precious metal functions to convert noxious or otherwise environmentally unfriendly components of the exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO x ), into carbon dioxide (CO 2 ), water (H 2 O) and nitrogen (N). A “washcoat” is frequently employed to make catalytic converters more efficient. The washcoat, most often a mixture of silica and alumina, is added to the substrate, and forms a rough, irregular surface which has a far greater surface area than the flat core surfaces. The irregular surface gives the monolith substrate a larger overall surface area, and therefore more locations for active precious metal sites. 
     The NO x  emissions from an internal combustion engine, in particular a compression-ignited diesel engine, can also be lowered with the aid of Selective Catalytic Reduction (SCR). SCR is a means of converting NO x  emissions into diatomic nitrogen (N 2 ) and water (H 2 O) using an aqueous reducing agent introduced into the exhaust system, upstream of the hydrolysis catalytic converter. The reducing agent that is used for SCR is typically a gaseous ammonia (NH 3 ), ammonia in aqueous solution, or urea in aqueous solution. With regard to the latter, urea serves as an ammonia carrier and is injected into the exhaust system with the aid of a metering system. The urea is converted into ammonia by means of hydrolysis, and the ammonia in turn reduces the nitrogen oxides in the catalytic converter. 
     Some emission control devices, such as SCR systems, catalytic converters, and associated exhaust gas oxygen (EGO) and NO x  sensors, require a minimum operating temperature to function as desired. For example, one of the limitations to using an aqueous urea solution in SCR is that it is subject to freezing. If the urea solution freezes, it will not function in its desired manner as a reducing agent, nor will it freely flow to the reduction site. As such, line heaters are utilized to warm the aqueous urea. In addition, the catalyst coating inside of the catalytic converter requires a minimum “activation” temperature for efficient operation. As such, a considerable amount of overall tailpipe hydrocarbon emissions is generated during engine cold-start. During such time, the emissions-reducing catalysts are largely ineffective because they have not reached the temperature at which significant catalytic activity can be maintained, also known as catalytic “light-off”. 
     SUMMARY OF THE INVENTION 
     The methods of the present invention are adapted to adjust engine loading during catalyst warm up to accelerate heating of the exhaust aftertreatment system and thereby decrease catalyst light-off times. In so doing, overall tailpipe nitrogen oxide emissions generated during engine cold-start are significantly reduced. 
     According to one embodiment of the present invention, the method includes: monitoring the current temperature of the catalyst; determining if the current catalyst temperature is less than a predetermined minimum catalyst temperature; and, if the current catalyst temperature is less than the predetermined minimum catalyst temperature, increasing the current engine load. The current engine load is increased in accordance with the present invention by activating a reducing agent tank heating device, a reducing agent line heating device, or both. Adjusting the engine load during cold-start using, for example, the urea tank and line heaters will allow for precise calibration of the catalytic converter warm up cycle. 
     According to one aspect of this particular embodiment, the method also includes calculating the minimum engine load required to increase the current catalyst temperature to the predetermined minimum catalyst temperature. The current engine load is thus increased to equal the minimum engine load if the current catalyst temperature is less than the predetermined minimum catalyst temperature. 
     According to another aspect, the method also includes calculating the minimum alternator load necessary to induce the minimum engine load required to increase the current catalyst temperature to the predetermined minimum catalyst temperature. In this instance, the reducing agent tank heating device, reducing agent line heating device, or both, are commanded to generate the minimum alternator load. Ideally, the method will then also include calculating the requisite minimum electric draw of the reducing agent tank heating device and reducing agent line heating device to generate the minimum alternator load. 
     As part of another aspect of this embodiment, the method also includes determining whether the current engine load is less than the minimum engine load. To this regard, the current engine load is increased if both the current catalyst temperature is less than the predetermined minimum catalyst temperature and the current engine load is less than the minimum engine load. 
     In accordance with another aspect, the minimum engine load and predetermined minimum catalyst temperature parameters are each based, at least in part, upon the current engine load and speed. 
     According to yet another aspect, the method adjusts activation of the reducing agent tank heating device and/or reducing agent line heating device in response to variations in vehicle operating conditions (e.g., changes in vehicle speed, tractive demands, electric system demands, etc.). Adjusting activation of the reducing agent tank heating device and/or reducing agent line heating device in this manner allows the system to shift engine loading into an optimal zone for catalyst warm-up and light-off. 
     According to even yet another aspect, the method also includes adjusting engine fuel command to compensate for the increase in engine load generated via activation of the reducing agent tank heating device/reducing agent line heating device. 
     In accordance with yet another facet of this embodiment, the method also includes: monitoring the current temperature of the exhaust gas; determining whether the current exhaust temperature is less than a predetermined minimum exhaust temperature; and increasing the current engine load if both the current catalyst temperature is less than the predetermined minimum catalyst temperature and the current exhaust temperature is less than the predetermined minimum exhaust temperature. 
     The above features and advantages, and other features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiments and best modes for carrying out the present invention when taken in connection with the accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram or flow chart illustrating a method according to a preferred embodiment of the present invention; 
         FIG. 2  is a graphical illustration of conversion efficiency as a function of catalyst temperature at various exhaust mass flow rates; and 
         FIG. 3  is a graphical illustration of catalyst temperature as a function of engine load at various engine speeds. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings,  FIG. 1  illustrates a control algorithm for regulating the temperature of an exhaust gas aftertreatment system in a motorized vehicle (not shown). Specifically, an improved method for accelerated warming of motor vehicle exhaust aftertreatment systems is shown in  FIG. 1  in accordance with a preferred embodiment of the present invention, designated generally as  100 . The method  100  preferably includes at least those steps shown in FIG.  1 —i.e., steps  101 - 115 . However, it is within the scope and spirit of the present invention to omit steps, include additional steps, and/or modify the order presented in  FIG. 1 . It should be further noted that the method  100  represents a single operation. As such, it is contemplated that the method  100  be applied in a systematic and repetitive manner, run in real-time to continuously adjust engine loading and optimize operation of the exhaust aftertreatment system. 
     The control algorithm  100  preferably resides in an engine control module (ECM, not shown). In other words, the series of blocks shown in  FIG. 1  may represent individual steps performed by the ECM. The ECM is a constituent part of the vehicle&#39;s powertrain system, which includes an internal combustion engine (ICE)—e.g., a 4-stroke compression-ignited diesel engine or a 4-stroke spark-ignited gasoline engine (neither of which are explicitly depicted herein). The vehicle will also include many other standard components and systems, such as suspension, drive train, brake system, steering and body components, that are also well known in the art. Thus, these structures will not be individually illustrated or explicitly discussed in detail herein. 
     The vehicle will also include an exhaust aftertreatment system utilized to mitigate the byproducts generated from operation of the ICE, and route the exhaust gasses away from the engine for subsequent expulsion into the ambient atmosphere. The exhaust system includes a number of exhaust pipes or conduits that fluidly couple a catalytic converter device of conventional architecture to an exhaust manifold of the ICE. Other exhaust aftertreatment devices may also be included. For example, a muffler or silencer that is fluidly communicated with a resonator may be placed downstream from the catalytic converter device via a second intermediate exhaust pipe. 
     The exhaust system also includes a Selective Catalytic Reduction (SCR) assembly. The reducing agent used in this exemplary embodiment is an aqueous urea solution, which is stored in a reducing agent storage vessel (also referred to herein as “urea tank”). A metering control apparatus, which is assigned to the urea tank, has an electrically actuated pump for delivering the reducing agent to a delivery site (which may be upstream from or directly at the catalytic converter device) via a feed line. The metering control apparatus controls an electromagnetic metering valve which regulates the distribution of urea solution. An electrical heater device operates to selectively heat the urea tank, for example, during cold-start operation. An electrical line heater may also be employed to heat the reducing agent as it exits the tank. While the methods of the present invention may be used in any vehicle having a reducing agent reservoir and corresponding heating device, the present invention is particularly well suited for use with a vehicle having a compression-ignited diesel-fueled internal combustion engine (ICE) assembly. 
     With reference again to  FIG. 1 , the method starts at step  101  with monitoring the current temperature of the catalyst inside of the catalytic converter, which can be accomplished, for example, using a precious metal resistor-precise thermo couple. In step  103 , the method then determines whether the current catalyst temperature is below a target minimum catalyst temperature. The target minimum catalyst temperature may be predefined as a single optimal temperature for all operating conditions, or determined contemporaneously with step  103  using a map of temperatures as a function of the current engine speed and load. For example,  FIG. 2  illustrates the relationship between catalyst temperature, in degrees Celsius (° C.), and the conversion efficiency of the catalyst (i.e., ratio of NO x  entering catalytic converter versus NO x  leaving the catalytic converter) at several exhaust mass flow rates, provided in kilograms per hour (kg/hr). As can be seen in  FIG. 2 , a 250° C. catalyst temperature produces approximately an 85% efficiency or better, regardless of mass flow rate. As such, the target minimum catalyst temperature may be predefined at 250° C. for this particular catalytic converter configuration. Alternatively, if a 90% or better efficiency is required, the target minimum catalyst temperature may be varied depending upon the exhaust mass flow rate, engine speed, and/or engine load to achieve a 90% efficiency. 
     If, at step  103 , the current catalyst temperature is greater than (i.e., hotter) or equal to the target minimum catalyst temperature, the control algorithm  100  returns to step  101 . If, at step  103 , the current catalyst temperature is less than (i.e., cooler) the target minimum catalyst temperature, the method  100  proceeds to step  105 . In step  105 , the control algorithm  100  detects the current engine speed, preferably in revolutions per minute (rpm), and engine load, preferably in Newton-meters (Nm). According to preferred practice, the engine speed and engine load are monitored continuously throughout execution of method  100 . 
     Contemporaneous with step  105 , the minimum engine load required to increase the current catalyst temperature to the predetermined minimum catalyst temperature is calculated in step  107 . The minimum engine load parameter is based, at least in part, upon the current engine load and speed.  FIG. 3  of the drawings illustrates the relationship between catalyst temperature, in degrees Celsius (° C.), and engine load, preferably in Newton-meters (Nm), at various engine speeds, provided in revolutions per minute (rpm). By way of example, if the target minimum catalyst temperature is 250° C. and the engine is idling during vehicle startup at 800 rpm, the engine load will have to be increased to approximately 152 Nm to achieve the desired catalyst temperature. If, however, the engine is running at 1000 rpm, the minimum engine load parameter would be set to approximately 112 Nm to achieve the desired 250° C. catalyst temperature. 
     Prior to, contemporaneous with, or immediately after steps  105  and  107 , the current engine load is adjusted to equal or exceed the minimum engine load established above. The current engine load is increased in accordance with the present invention by activating the urea tank heater and line heater, either individually or in concert, at step  111 . Exhaust temperature generally rises as engine load increases, whereas exhaust temperature generally falls as engine load decreases. To ensure that the urea tank heater and/or line heater generate sufficient additional load on the engine during activation, the method also includes, in step  109 , calculating the minimum alternator load necessary to induce the minimum engine load. This may also require calculating the minimum electric draw of the urea tank heater and/or line heaters necessary to generate the minimum alternator load. In this instance, the method  100  commands the reducing agent tank heater, reducing agent line heating device, or both, to generate the minimum alternator load. 
     Adjusting the engine load, for example, during cold-start using the urea tank heater and line heaters will accelerate heating of the exhaust aftertreatment system and thereby decrease catalyst light-off times. The present invention also allows for precise calibration of the catalytic converter warm up cycle. In addition, regulating engine load in accordance with the present invention is effectively seamless to the vehicle operator, as turning on the urea tank and corresponding heating elements is an entirely invisible process to an end user. 
     Prior to step  111 , it is desirable that the method  100  determine whether the engine is already operating at or above the minimum engine load. If the current engine load is already equal to or greater than the minimum engine load required to achieve the target minimum catalyst temperature, the method  100  returns to step  101 . If not, the method  100  will proceed, as described above, to step  111 . 
     With continuing reference to  FIG. 1 , step  113  of method  100  provides for adjusting the urea tank and line heater activity in response to variations in vehicle operating conditions. Such operating conditions may include, but certainly are not limited to, changes in vehicle speed, tractive demands, electric system demands, etc. Adjusting activation of the reducing agent tank and/or reducing agent heating device in this manner allows the system to shift engine loading into an optimal zone for catalyst warm up and light-off. Due to the additional loading on the engine, the fuel command may need to be adjusted to offset the additional demand. Accordingly, in step  115 , the method  100  also includes adjusting engine fuel command to compensate for the increase in engine load generated via activation of the reducing agent tank and/or reducing agent heating device. 
     Prior to completing the control algorithm, it may be desirable to monitor the current temperature of the exhaust gas, which may be accomplished, for example, using an electrical exhaust gas temperature (EGT) gauge. Thereafter, the method  100  will determine whether the current exhaust temperature is less than a predetermined minimum exhaust temperature. In this instance, the current engine load is increased if both the current catalyst temperature is less than the predetermined minimum catalyst temperature and the current exhaust temperature is less than the predetermined minimum exhaust temperature. 
     While the best modes for carrying out the present invention have been described in detail herein, those familiar with the art to which this invention pertains will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.