Soot burning method for particulate filters

Methods and systems are provided for regenerating a particulate filter in an engine exhaust, where burning of soot is initiated by introducing additional oxygen into the exhaust gas upstream of the particulate filter where an exhaust temperature exceeds a threshold, a soot burn rate controlled by adjusting pulsing of the additional oxygen. Further, the pulsing of the additional oxygen is introduced via a high-pressure EGR passage during boosted engine conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to European Patent Application No. 12175874.2, filed on Jul. 11, 2012, the entire contents of which are hereby incorporated by reference for all purposes.

FIELD

The present invention relates to a method of burning soot in an exhaust particulate filter of a combustion engine to regenerate the particulate filter and to a vehicle, adapted to perform this method.

BACKGROUND AND SUMMARY

Engines may utilize particulate filters in an exhaust system for reducing the amount of soot emissions by trapping soot particles. During the operation of an engine, particulate filters may be regenerated in order to decrease the amount of trapped particulate matter within the filter. However, during engine operation, exhaust gas temperatures may increase and cause damage to the particulate filter.

U.S. Pat. No. 7,640,729 describes an approach with a secondary air flow passage located upstream of a particulate filter. To regenerate the particulate filter, the disclosed method delivers a secondary air flow upstream from the particulate filter based on the particulate filter temperature and particulate matter burn off rate. Further, a temperature sensor is located downstream from the particulate filter in the exhaust passage in order to measure the exhaust gas temperature.

The Inventors have recognized a problem with streaming additional oxygen into the exhaust passage for regeneration of a particulate filter. The flow of additional oxygen into the exhaust passage via a secondary passage may occur at non-optimal times during engine operation. Further, having a temperature sensor downstream from the particulate filter may cause a delay in the shut-off of the additional oxygen flow. Further, the delay may cause an inaccurate reading of the particulate filter temperature resulting in damage to the filter.

In one example, some of the above issues may be addressed by a method comprising, regenerating a particulate filter in an engine exhaust, where burning of soot is initiated by introducing additional oxygen into the exhaust gas upstream of the particulate filter when an exhaust temperature exceeds a threshold, and a soot burn rate is controlled by adjusting pulsing of the additional oxygen introduced based on engine operation. For example, when intake boost pressure is greater than exhaust pressure, pulsed air is introduced via a high-pressure EGR passage. As such, the frequency and/or pulse-width of the additional oxygen pulses may be responsive to operating conditions by adjusting pulsing of the high-pressure EGR valve positioned in the passage. Thus, introduction of additional oxygen via pulses may allow for the soot burning process to be controlled more precisely.

In another example, a temperature sensor may be positioned inside the particulate filter of an engine exhaust passage. In this way, the temperature sensor may be able to make a more precise measurement of the exhaust gas temperature. For example, termination of additional oxygen pulses may occur once the temperature inside the particulate filter reaches a maximal threshold. Therefore, a temperature sensor positioned inside the particulate filter may allow for improved regeneration of a particulate filter.

DETAILED DESCRIPTION

The following description relates to systems and methods for a regeneration system for particulate filters (FIG. 1). Particulate filter regeneration may include monitoring temperature and the level of oxygen present in exhaust gasses in order to initiate a soot burn. However, the exhaust gases may need additional oxygen in order to initiate a soot burn. Therefore, additional oxygen (e.g., in addition to excess oxygen from lean combustion in the cylinders) may be delivered to the exhaust gas passage via pulses by controlling a valve in the HP-EGR system (FIGS. 2-3). Soot burn rates are determined by temperature of the exhaust gas (FIG. 4), the type of soot (e.g. diesel soot vs. commercial soot) (FIG. 5), and the amount of oxygen in the exhaust gas (FIG. 6).

Referring now toFIG. 1, a typical regeneration system22for particulate filters is shown. A particulate filter11is accommodated in an aftertreatment system of a combustion engine12. Engine12may be controlled at least partially by a control system including controller13and by input from a vehicle operator132via an input device130. In this example, input device130includes an accelerator pedal and a pedal position sensor134for generating a proportional pedal position signal PP. Combustion chamber (i.e., cylinder)30of engine12may include combustion chamber walls136with piston138positioned therein. In some embodiments, the face of piston138inside cylinder30may have a bowl. Piston138may be coupled to crankshaft140so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft140may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to crankshaft140via a flywheel to enable a starting operation of engine12.

Combustion chamber30may receive intake air from intake manifold146via intake passage19and may exhaust combustion gases via exhaust passage18. Intake manifold146and exhaust passage18can selectively communicate with combustion chamber30via respective intake valve156and exhaust valve150. In some embodiments, combustion chamber30may include two or more intake valves and/or two or more exhaust valves.

Intake valve156may be controlled by controller13via electric valve actuator (EVA)151. Similarly, exhaust valve150may be controlled by controller13via EVA153. Alternatively, the variable valve actuator may be electro hydraulic or any other conceivable mechanism to enable valve actuation. During some conditions, controller13may vary the signals provided to actuators151and153to control the opening and closing of the respective intake and exhaust valves. The position of intake valve156and exhaust valve150may be determined by valve position sensors155and157, respectively. In alternative embodiments, one or more of the intake and exhaust valves may be actuated by 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 to vary valve operation. For example, cylinder30may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT.

Fuel injector166is shown coupled directly to combustion chamber30for injecting fuel directly therein in proportion to the pulse-width of signal FPW received from controller13via electronic driver168. In this manner, fuel injector166provides what is known as direct injection of fuel into combustion chamber30. 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 injector166by a fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail.

Ignition system190can provide an ignition spark to combustion chamber30via spark plug92in response to spark advance signal SA from controller13, under select operating modes. Though spark ignition components are shown, in some embodiments, combustion chamber30or one or more other combustion chambers of engine12may be operated in a compression ignition mode, with or without an ignition spark.

Intake passage19may include throttle162and a throttle plate164. In this particular example, the positions of throttle plate164may be varied by controller13via signals provided to an electric motor or actuator included with throttle162, a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, throttle162may be operated to vary the intake air provided to combustion chamber30among other engine cylinders. The positions of throttle plate164may be provided to controller13by throttle position signals TP. Pressure, temperature, and mass air flow may be measured at various points along intake passage19and intake manifold146. For example, intake passage19may include a mass air flow sensor122for measuring clean air mass flow. The clean air mass flow may be communicated to controller13via the MAF signal.

Engine12may further include a compression device such as a turbocharger or supercharger including at least a compressor174arranged upstream of intake manifold146. For a turbocharger, compressor174may be at least partially driven by a turbine176(e.g., via a shaft) arranged along exhaust passage18. Thus, the amount of compression provided to one or more cylinders of the engine via a turbocharger may be varied by controller13.

Downstream of the engine12in the exhaust gas passage18, a catalytic converter15is arranged. A controller13is connected to the engine12via communication lines17and to sensors16A and16B at least upstream of and downstream of the particulate filter11. In one embodiment, sensor16B may be positioned inside particulate filter11, as shown inFIG. 1. The exhaust gases emitted by the combustion engine12are treated firstly in the catalytic converter15and subsequently in the particulate filter11. The exhaust gases leave the regeneration system22through the outlet21. The particulate filter11collects particulate matter from the exhaust gases. After a certain time, the particulate filter11has to be regenerated. The soot inside the particulate filter11has to be converted into carbon dioxide. In order to meet specific requirements a predefined temperature and predefined oxygen level have to be achieved.

Further, oxygen containing gas enters the regeneration system22through an inlet19. Preferably, the oxygen containing gas is fresh air. The fresh air is supplied to the combustion engine12and to a high pressure (HP)-EGR passage20. For example, the fresh air is routed through HP-EGR passage20from upstream of turbine176to downstream of compressor174. In another example, the particulate filter is positioned in an exhaust passage downstream from the HP-EGR passage20. The controller13is also connected to HP-EGR valve14. The HP-EGR passage20supplies part of the fresh air directly to the intake of the particulate filter11. Further, a controller13contains instructions stored in non-transitory memory to pulse the HP-EGR valve14at a selected frequency and pulse-width to pulse air into the exhaust upstream of the particulate filter11during boosted engine operating conditions responsive to filter regeneration.

Under some conditions, the EGR system may be used to regulate the temperature of the air and fuel mixture within combustion chamber30. Thus, it may be desirable to measure or estimate the EGR mass flow. EGR sensors may be arranged within EGR passages and may provide an indication of one or more of mass flow, pressure, temperature, concentration of O2, and concentration of the exhaust gas. Further, followed by air cooler145, an HP-EGR sensor119may be arranged within the HP-EGR passage and may provide an indication of one or more pressure, temperature, and concentration of the exhaust gas.

Exhaust gas sensor16A is shown coupled to exhaust passage18downstream of turbine176and upstream of particulate filter11. Sensor16A may be any suitable sensor for providing an indication of temperature and/or 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. Further, sensor16B, positioned in particulate filter11, may be any suitable sensor for providing an indication of temperature of the particulate filter.

Controller13is shown inFIG. 1as a microcomputer, including microprocessor unit106, input/output ports108, an electronic storage medium for executable programs and calibration values shown as read only memory chip110in this particular example, random access memory112, keep alive memory114, and a data bus. Controller13may receive various signals from sensors coupled to engine12, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor122; engine coolant temperature (ECT) from temperature sensor116coupled to cooling sleeve118; a profile ignition pickup signal (PIP) from Hall effect sensor120(or other type) coupled to crankshaft140; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal, MAP, from sensor124. Engine speed signal, RPM, may be generated by controller13from 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. Note that various combinations of the above sensors may be used, such as a MAF sensor without a MAP sensor, or vice versa. During stoichiometric operation, the MAP sensor can give an indication of engine torque. Further, this sensor, along with the detected engine speed, can provide an estimate of charge (including air) inducted into the cylinder. In one example, sensor120, which is also used as an engine speed sensor, may produce a predetermined number of equally spaced pulses every revolution of the crankshaft.

Storage medium read-only memory110can be programmed with computer readable data representing instructions executable by processor106for performing the methods described below as well as other variants that are anticipated but not specifically listed.

As described above, the system comprises an internal combustion engine having a turbocharger, an EGR system with an EGR valve, a particulate filter positioned in an exhaust passage downstream from the EGR system, and a controller. A controller may include instructions stored in non-transitory memory to pulse the valve at a selected frequency and pulse-width to pulse air into the exhaust upstream during boosted engine operating conditions responsive to filter regeneration. In one example, the EGR system is a high pressure EGR system. In another example, the exhaust passage includes a temperature sensor upstream from the particulate filter. A temperature sensor may also be an oxygen sensor. In yet another example, the particulate filter includes a temperature sensor.

Referring toFIG. 2, a method of regenerating a particulate filter in an engine exhaust is shown. The burning of soot is initiated by introducing additional oxygen into the exhaust gas upstream of the particulate filter when an exhaust temperature exceeds a threshold (e.g. above 250° C.). The soot burn rate is controlled by adjusting the pulsing of the additional oxygen introduced into the exhaust passage. Thus, controlled introduction of oxygen containing gas is used to control the amount of soot load and soot combustion, thereby protecting the particulate filter against reaching high temperatures.

The regeneration method may include three steps including, a first step at204, monitoring exhaust gas temperatures. A second step of the method may also include monitoring the oxygen level of the exhaust gas, at206. A third step of the method, at218, may include monitoring the temperature of the particulate filter. In one example, these three steps are performed by sensors in the exhaust passage upstream and/or downstream of the particulate filter.

The method continues at210where it is determined if the exhaust gas temperature is greater than a threshold (e.g. the temperature is greater than 250° C.). If the exhaust gas is greater than a threshold, at214, it is determined if the exhaust gas oxygen level is lower than a threshold. Therefore, when exhaust gas temperature is high and the oxygen level is low, at216, the HP-EGR valve is adjusted to generate additional oxygen pulses based on the soot burn rate. Further, the additional oxygen is compressed before it is introduced into the exhaust gas. For example, when the exhaust gas temperature reaches or exceeds a threshold, for instance 250° C., the additional oxygen may be introduced into the exhaust gas by adjusting the HP-EGR valve in order to deliver the oxygen in pulses, thereby initializing a soot burn. In addition, the frequency and/or pulse-width of the pulses may be controlled by the HP-EGR valve in accordance with the desired soot burn rate and/or based on operating conditions (see alsoFIG. 3). In one example, during boosted particulate filter regeneration conditions, an exhaust gas recirculation valve, responsive to temperature and excess oxygen in the exhaust, may be pulsed in order to deliver intake gasses to the particulate filter. In one example, the pulsed air may be introduced via a high pressure (HP)-EGR passage only when intake boost pressure is greater than exhaust pressure. In addition, the oxygen containing gas may be compressed by a compression unit before introduction into the exhaust passage.

Based on the known soot burn rate at a respective temperature of the exhaust gas, the known oxygen concentration of the inlet gas, and a predicted amount of the soot load, an additionally required amount of oxygen containing gas may be determined by the controller. Therefore, depending on the identified needs of oxygen, the controller initiates a soot burn by either opening the valve (216) and/or regulating the operation of the engine (218). For example, the frequency of the HP-EGR valve pulsing occurs at lower exhaust gas flow rates. In addition, the adjusting of the frequency of HP-EGR valve pulsing may be proportional to engine speed.

Returning to210, if the exhaust gas temperature is below the threshold, at210, the HP-EGR is adjusted (e.g. closed) so that no pulsed air is introduced during regeneration or when the filter is not burning soot. Further, the pulsed air introduction is terminated if the exhaust temperature rises above an upper threshold (e.g. 800° C.) based on operating conditions. Additionally, at214, if the exhaust gas oxygen level is greater than threshold, the HP-EGR is closed and the air introduction is terminated in order to maintain a threshold oxygen level based on operating conditions (e.g. engine load).

The method continues at220where it is determined if the temperature of the particulate filter is greater than a threshold temperature. For example, in case the temperature inside the particulate filter exceeds an upper limit, for instance 900° C., the supply of oxygen containing gas is terminated. In one embodiment, for protecting the particulate filter against degeneration due to very high temperatures, the supply of oxygen containing gas may be terminated by closing the HP-EGR valve, a fuel cut may be avoided, or a rich mixture during the time when temperatures are very high may be created. Therefore, if the particulate filter has a temperature greater than 900° C., at224, the HP-EGR valve may be closed in order to maintain a low temperature in the particulate filter (see alsoFIG. 3). In one example, the temperature of the particulate filter is measured directly by a sensor inside the filter.

Returning to220, if the temperature of the particulate filter is less than a threshold, at222, the HP-EGR valve is adjusted in order to maintain a threshold temperature (e.g. HP-EGR valve is opened).

In this way, a method comprises, during boosted particulate filter regeneration conditions, pulsing an exhaust gas recirculation valve to deliver pulsed intake gasses to the particulate filter responsive to temperature and excess oxygen in the exhaust. In one example, an increase in the frequency of pulsing occurs at lower exhaust gas flow rates. In addition, the frequency of pulsing is proportional to engine speed. In another example, the exhaust gas recirculation valve is positioned in a high-pressure exhaust gas recirculation passage distinct from a low-pressure exhaust gas recirculation passage of the engine. Further, during non-boosted conditions, the exhaust gas recirculation valve is adjusted to control an amount of exhaust gas recirculation delivered to the engine intake in response to a desired amount of exhaust gas recirculation. In yet another example, the pulsing includes oscillating the valve from full open to full closed a selected frequency and a selected pulse-width. The pulse-width may be adjusted based on responsive to particulate filter regeneration rate, particulate filter temperature, and an amount of stored particulate in the particulate filter.

Referring toFIG. 3, a graphical example of HP-EGR valve operation for regeneration of a particulate filter is shown. Plot301shows a boosted engine is operating where the boost conditions may include conditions during which the compressor is in operation. For example, the boost condition may include a high engine load condition. During boosted conditions, at302, a HP-EGR valve may be opened in order to allow excess oxygen to flow into the exhaust passage. For example, at t1, a soot burn may be initiated based on exhaust gas temperatures being above a threshold (303) (e.g. greater than 400° C.) and exhaust gas oxygen levels are low (304). Therefore, at302, the EGR valve is opened, excess oxygen is pulsed for a set duration of time (d1), at set intervals (d3) and the pluses continue for a duration of time (d2) based on a predetermined soot burn rate. However, at t2, when the oxygen level of exhaust gas starts to increase, at305, the EGR valve is adjusted in order to reduce the amount of excess oxygen being delivered to the exhaust passage. For example, if the oxygen levels of exhaust gas are high (e.g. engine load increases), the duration of the pulses of oxygen, d4, from the HP-EGR valve is shorter as compared to pulse duration d1. Further, the duration of the pulse intervals, d5, may be longer as compared to pulse interval duration d3based on the decreased need of excess oxygen to the exhaust passage from the HP-EGR system.

Plot306shows a temperature range (r1) with an upper threshold307and a lower threshold308. When the temperature of the particulate filter, at309, is within a normal temperature range (e.g. below 900° C., above 400° C.) the EGR valve is opened, excess oxygen is pulsed for a set duration of time (d6), with a set interval between pulses (d7), and the pluses continue for a set duration of time (d8). However, at310, when the temperature of the particulate filter reaches an upper limit temperature range (e.g. above 900° C.), the HP-EGR valve is closed at t3and excess oxygen is no longer introduced into the exhaust passage.

Referring toFIG. 4, a graphical example of soot oxidation rate vs. soot conversion at different temperatures is shown. It was determined that soot oxidation rates increase with increasing temperature.

Referring toFIG. 5, a graphical example of soot oxidation rates vs. soot conversion for FW2, diesel soot and gasoline soot is shown. FW2 is a type of commercial soot. It was determined that soot oxidation rates decrease with decreasing amounts of soot in the filter.

Referring toFIG. 6, a graphical example of soot oxidation rates vs. O2concentration at Temperature 460° C. and at 50% soot conversion is shown. It was determined that with increasing concentration of oxygen, the soot oxidation rates increase.

When creating the diagrams, as shown inFIGS. 4,5, and6, the concentration of O2was 2-20 vol. %, and H2O 10%. Nitrogen was used as a balance. Oxygen was introduced in continuous flow or in pulses. The total flows were set to obtain SV 7.5 k h−1. The temperatures of the inlet gases remained constant between 440 to 500° C. for a time period of 4650 s, followed by increasing the temperature up to 800° C. with a temperature ramp of 10° C./min for burning all remaining soot in the particulate filter.

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 actions, operations, and/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 actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system.