Abstract:
A system and method are disclosed for operating an internal combustion to provide a temperature rise in an exhaust aftertreatment device. An exhaust valve is actuated during a compression stroke to release unburned fuel and air. The fuel oxidizes with the air in the exhaust aftertreatment device causing an exotherm. The opening and closing time of the exhaust valve are determined to cause the desired amount of fuel and air to be released into the exhaust aftertreatment device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system and method to operate an internal combustion engine to provide unburned fuel and air into the exhaust aftertreatment device to improve the conversion efficiency of the exhaust aftertreatment device. 
     2. Background of the Invention 
     A factor in achieving low exhaust emissions from internal combustion engines, at the current state of the art, is to bring the exhaust aftertreatment device to its operating temperature as rapidly as possible after initiating engine operation. It has long been recognized that by introducing some unburned fuel and air into the exhaust duct of the engine at a location upstream of the exhaust aftertreatment device that the fuel and air react in the exhaust aftertreatment device creating an exotherm, thereby quickly raising the temperature of the exhaust aftertreatment device. 
     A common method to provide fuel and air to the exhaust aftertreatment device is described in U.S. Pat. No. 5,410,872, which is assigned to the assignee of the present invention. The air and fuel delivered to the combustion chamber of the engine is rich, i.e., contains excess fuel. Consequently, the engine exhaust contains unburned and partially burned fuel. Secondary air is introduced into the exhaust duct by an air pump. The incompletely burned fuel and secondary air mix and are introduced into the exhaust aftertreatment device in which oxidation occurs, creating an exotherm. 
     A problem with prior art approaches is in metering the fuel, primary air, and secondary air to provide the desired mixture and flow rates to the combustion chamber and to the exhaust aftertreatment device. The constraints are: providing a stoichiometric mixture (air to fuel ratio such that if reacted to completion, fuel and oxygen are completely consumed) to the exhaust aftertreatment device, providing sufficient unburned mixture to the exhaust aftertreatment device to provide the desired temperature rise in the exhaust aftertreatment device, and providing a rich, but combustible mixture, to the combustion chamber. This combination of constraints presents a complicated control task. 
     Another difficulty with prior art is the hardware required to accomplish the task: air pump, air lines, switches, metering devices, and others, which add weight, cost, additional plumbing, etc. 
     The inventor of the present invention has recognized a method to provide unburned fuel and air to the exhaust aftertreatment device overcoming metering problems and relying on existing hardware. 
     SUMMARY OF INVENTION 
     The above disadvantages are overcome by a method for operating a multi-cylinder internal combustion engine by providing an amount of air and fuel to a cylinder of the engine. The air and fuel are compressed. During compression, an exhaust valve of the cylinder is opened releasing a portion of the air and fuel into the exhaust aftertreatment device. In this way, the temperature of the exhaust aftertreatment device is increased. 
     Preferably, a system is disclosed for providing fuel and air to an exhaust aftertreatment device of a reciprocating multi-cylinder internal combustion engine which includes an exhaust valve coupled to a cylinder of the engine, capable of being actuated during a compression stroke of the engine, and an engine controller connected to the engine and the exhaust valve to actuate the exhaust valve during the compression stroke to release a portion of the contents of the cylinder into the exhaust aftertreatment device. 
     An advantage of the present invention utilized with an engine having exhaust valves that can be opened during the compression stroke, is that fuel and air from the combustion chamber can be released into the exhaust system without a separate air pump, additional plumbing, and valves of prior art approaches. 
     An advantage of the present invention is that the fuel and air are metered by existing hardware and well known strategies. That is, no control strategy need be developed to meter the fuel, primary air, and secondary air, as is the case in prior art. Specifically in the present invention, the air-fuel ratio may be controlled open loop based on a measure of airflow to the engine and controlling fuel pulse width or closed loop based on a signal from an exhaust gas oxygen sensor according to the present invention. 
     A further advantage of the present invention is that the air and fuel delivered to the combustion chamber may be in stoichiometric proportion thereby overcoming the potential of a rich misfire in the combustion chamber as may occur in the prior art. 
     Another advantage is that the present invention may be used in a diesel engine, or other engine with low exhaust temperatures such as homogeneous charge compression ignition engines, to maintain a high enough exhaust temperature in an exhaust aftertreatment device for high conversion efficiency. 
     The above advantages and other advantages, objects, 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 
     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: 
     FIG. 1 is a schematic of a multi-cylinder engine; 
     FIG. 2 is a schematic of the valvetrain of a single cylinder of the multi-cylinder engine according to an aspect of the present invention; and 
     FIG. 3 is a flowchart of a method by which the present invention may be used to advantage. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1 a four-cylinder engine  10  is shown. Intake manifold  26  delivers air to engine  10  and exhaust manifold  28  receives combusted gases from engine  10 . Spark plugs  12  are installed in the combustion chambers of engine  10 . The present invention also applies to engines, such as diesel and homogenous charge compression ignition, which do not rely on ignition devices. Engine  10  may be equipped with a throttle valve  24 , which is used to control air delivered to engine  10 . In this particular example shown in FIG. 1, a mass air flow meter  48  is installed in the intake line of the engine. Alternatively, mass air flow rate can be obtained by what is known by those skilled in the art as a speed-density system which computes air flow based on engine speed and intake manifold  26  pressure. Engine  10  is be equipped with a temperature sensor to measure the temperature of the coolant in engine  10 , thereby providing an indication of the operating temperature of engine  10 . Exhaust aftertreatment device  30  processes exhaust gases from engine  10 . An exhaust gas sensor  44  is be installed in the exhaust duct, upstream of exhaust aftertreatment device  30  (as shown) or downstream of exhaust aftertreatment device  30  (alternative not shown). Exhaust gas sensor  44  may be an exhaust gas oxygen sensor, a hydrocarbon sensor, or other exhaust gas component sensor. 
     In FIG. 1, a secondary air pump  32 , as known in the prior art, is shown coupled to engine  10 . Air is drawn into pump  32  from the intake duct downstream of air mass flow sensor  48 . Alternatively, air is drawn from the atmosphere (not shown). Secondary air flows through duct  34  from the intake duct to the exhaust manifold. Valve  35  is closed when pump  32  is not being operated. To protect pump  32  from possible ill effects by exposure with exhaust gases, valve  35  is closed when pump  32  is not being operated. The present invention does not include elements  32 ,  34 , and  35 ; instead these indicate hardware used in prior art approaches. 
     Referring now to FIG. 2, a single cylinder of engine  10  is shown. Engine  10  receives air via intake port  27  through intake valve  14 . Intake port  27  couples to intake manifold  26  (intake manifold  26  not shown in FIG.  2 ). Intake valve  14  is actuated by camshaft  20  through tappet  18 . Fuel is provided to engine  10  by a fuel injector  25  installed in intake port  27 . Alternative fuel metering devices which could be used instead of port fuel injectors  25  are carburetion or central fuel injection. Also, the fuel could be a liquid or gaseous fuel. The fuel and air inducted into the cylinder are ignited by spark plug  12  in a spark ignited engine; alternatively, the fuel and air spontaneously ignite in a compression ignited engine. The products of combustion leave the combustion chamber via exhaust port  29  through exhaust valve  16 . Exhaust port  29  couples to exhaust manifold  28  (exhaust manifold  28  not shown in FIG.  2 ). Exhaust valve  16  is electromagnetically actuated by element  22 . The present invention applies to any type of valve configuration for intake valve  14 , including, but not limited to, rotary valves, valves actuated by multiple cams (cam switching devices), electromagnetically actuated valves, and electrohydraulically actuated valves. In regards to exhaust valve  16 , the present invention applies to valves which can be actuated twice or more for each combustion cycle, which includes, but is not limited to, electromagnetically actuated valves, electrohydraulically actuated valves, and valves actuated by multiple cams. In the latter situation, a cam with multiple lobes would be selected during a period when double pulsing of exhaust valve  16  is desired during warmup of the engine and a cam with a single lobe would be selected otherwise. 
     The present invention applies to spark-ignited or compression-ignited engines in which the air and fuel are substantially premixed prior to induction into the cylinder, such as port injected, central injected, and carbureted engines providing liquid or gaseous fuels, as mentioned above. The present invention may also be practiced in engines in which the fuel is added directly to the cylinder, such as diesel or direct injection gasoline engines. If the fuel injection hardware, in these direct injection engines, allows multiple fuel pulses to be injected during each engine cycle, fuel can be injected into the cylinder during an expansion stroke of the engine to provide unburned fuel to the exhaust aftertreatment device, as known in the prior art. However, the present invention may be preferred over injecting during the exhaust stroke. For example, some direct injection fuel injection systems do not allow multiple injections per engine cycle, thus not allowing injection during an expansion stroke. Even if expansion stroke injection were allowed by the fuel injection hardware, known problems with in the prior art are wetting the cylinder walls with fuel, which washes off the cylinder&#39;s oil layer making the walls susceptible to wear, and fuel dilution of the fuel, thereby diminishing the oil&#39;s ability to provide lubrication. These problems could be mitigated by the present invention in which fuel and air are released without relying on an extra injection event during expansion. The present invention may present an additional advantage by releasing fuel and air during the compression stroke because the distribution of fuel in the cylinder might be more desirable than that which exists during an expansion stroke injection thereby providing the desired fuel and air to the exhaust aftertreatment device. 
     In FIG. 2, a piston  30  is shown disposed in engine  10 . Piston  30  reciprocates in a cylinder of engine  10 . In four-stroke operation, the processes are: an intake stroke during which piston  30  moves down (away from valves  14  and  16 ), a compression stroke as piston  30  moves up, an expansion (or power) stroke as piston  30  moves down, and an exhaust stroke as piston  30  moves up. Combustion is typically initiated toward the end of the compression stroke with the majority of combustion occurring during the expansion stroke. Intake and exhaust valves ( 14  and  16 ) are closed during most of the compression stroke. In the present invention, exhaust valve  16  opens for a portion of the compression stroke releasing some of the gases (uncombusted fuel and air) from the combustion chamber. The compression process is interrupted during the time that exhaust valve  16  is open and resumes when exhaust valve  16  is closed. 
     Referring to FIGS. 1 and 2, an electronic control unit (ECU)  40  is provided to control the hybrid camless engine. ECU  40  has a microprocessor  50 , called a central processing unit (CPU), in communication with memory management unit (MMU)  60 . MMU  60  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  50 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  58 , random-access memory (RAM)  56 , and keep-alive memory (KAM)  54 , for example. KAM  54  may be used to store various operating variables while CPU  50  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  50  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  50  communicates with various sensors and actuators via an input/output (I/O) interface  52 . Examples of items that are actuated under control by CPU  50 , through I/O interface  70 , are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve  24  position, timing of spark plug  12  firing, actuation of valve element  22 , and others. Sensors  42  communicating input through I/O interface  52  may indicate engine speed, vehicle speed, coolant temperature, manifold pressure, pedal position, throttle valve  24  position, air temperature, exhaust temperature, and mass air flow rate  48 . Some ECU  40  architectures do not contain MMU  60 . If no MMU  60  is employed, CPU  50  manages data and connects directly to ROM  58 , RAM  56 , and KAM  54 . Of course, the present invention could utilize more than one CPU  50  to provide engine control and ECU  40  may contain multiple ROM  58 , RAM  56 , and KAM  54  coupled to MMU  60  or CPU  60  depending upon the particular application. 
     The present invention may be used to rapidly raise temperature in an exhaust aftertreatment device  30  after engine operation has been initiated. It could also be used in situations in which exhaust temperatures are often too low to maintain the operating temperature of exhaust aftertreatment device  30 . One such example is in a diesel engine, where due to very lean air-fuel ratios and high thermal efficiencies at which diesels operate, the exhaust temperatures, at many operating conditions, are not high enough to keep the catalyst warm. During such operating conditions with low exhaust temperatures, the present invention may be employed to advantage to provide an exotherm in exhaust aftertreatment device  30 . In the case of a diesel engine, the contents in the cylinder are only air and burned gases from prior events until fuel is injected, which typically occurs during the compression stroke. The present invention relates to exhausting both fuel and air from the cylinder to provide the desired exotherm in the aftertreatment device. To provide both fuel and air from a diesel engine, exhaust valve  16  opening may occur after the start of fuel injection. If the fuel injection hardware allows multiple injections, an amount of fuel may be injected in advance of the primary injection event, termed pilot injection by those skilled in the art. Exhausted fuel and air may be used in a diesel engine to provide an exotherm, as described in reference to a spark ignited engine above. 
     The present invention may also be used in the situation of exhaust aftertreatment devices which provide higher conversion efficiencies in the presence of a reducing agent. In this case, the exhausted fuel, as supplied to the exhaust aftertreatment device according to an aspect of the present invention, may be used to provide fuel as a reducing agent to exhaust aftertreatment devices such as a lean NOx catalyst. 
     A starting sequence by which the present invention can be used to advantage is shown in FIG.  3 . The engine starts in step  70 . Normal valve settings are used for the first fire in each cylinder in step  72 . In step  74 , ΔT is computed which is the difference between engine temperature, T eng , and a threshold temperature, T thresh . T thresh  indicates that temperature is high enough in the exhaust aftertreatment so that there is no need to provide an exotherm. An example of such an occurrence is a restart of the engine before it has cooled down from previous operation. T eng  may be an engine coolant temperature, an engine metal temperature, an exhaust temperature, an exhaust aftertreatment temperature, a combination of the above temperatures, or a model in ECU  40  of temperature which may be based on any of the above temperatures. In block  76 , if ΔT is greater than 0, control passes to step  88  and normal valve timings are used. Control then passes to step  90  in which the starting sequence is ended. If in step  76 , ΔT is less than 0, control passes to step  78  in which the fuel and air is delivered to the cylinders. The amount of fuel to deliver to the cylinder, m f , has two components: m f,P , the amount of fuel to provide the desired torque, and Δm a , the amount of fuel which will be released into the exhaust system to provide an exotherm in the exhaust aftertreatment device  30 . The amount of air to deliver to the combustion chamber, m a , can be computed similarly as the computation for fuel. Or, as shown in step  78 , m a  may be computed based on the desired air-fuel ratio, AF, and m f  already computed. Control passes to step  82  in which opening and closing times of exhaust valve  16  are determined and exhaust valve  16  are actuated such that the Δm f  and Δm a  are released. Control passes to step  84  in which combustion is initiated. The spark timing may be retarded to further assist heating exhaust aftertreatment device  30 . Control passes to step  86  in which it is determined whether the temperature of the exhaust aftertreatment device, T EAD , is greater than the lightoff temperature of the catalyst, T lightoff . If step  86  yields a positive result, control passes to step  88  in which normal valve timings are adopted. Otherwise, control passes to step  78  in which the method of the present invention is continued until a negative result occurs in step  86 . As described above, control continues to step  88  and finishes in step  90 . 
     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.