Method and system for reducing vehicle tailpipe emissions when operating lean

1. Field of the Invention

The invention relates to methods and systems for controlling the operation of “lean-burn” internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy.

2. Background Art

The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituents, including hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO x ). The respective rates at which an engine generates these constituents are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (λ), engine speed and load, engine temperature, ambient humidity, ignition timing (“spark”), and percentage exhaust gas recirculation (“EGR”). The prior art often maps values for various of these “feedgas” constituents based, for example, on detected values for instantaneous engine speed and engine load.

In order to comply with modern restrictions regarding permissible levels of selected exhaust gas constituents, vehicle exhaust treatment systems often employ one or more three-way catalysts, referred to as an emission control device, disposed in an exhaust passage to store and release selected exhaust gas constituents, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NO x when the exhaust gas is lean, and releases previously-stored NO x when the exhaust gas is either stoichiometric or “rich” of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Significantly, a device's actual capacity to store a selected constituent gas, such as NO x , is often finite and, hence, in order to maintain low tailpipe NO x emissions, the device must be periodically cleansed or “purged” of stored NO x . The frequency or timing of each purge event must be controlled so that the device does not otherwise reach its actual NO x storage capacity, because engine-generated NO x would thereafter pass through the device and effect an increase in tailpipe NO x emissions. Further, the timing of each purge event is preferably controlled to avoid the purging of only partially-filled devices, due to the fuel penalty associated with the purge event's enriched air-fuel mixture and, particularly, the fuel penalty associated with the release of oxygen previously stored in any other upstream emission control device.

In response, U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153 teach use of NO x -estimating means which seeks to estimate the cumulative amount of NO x which has been generated by the engine and, presumptively, has been stored in the device during a given lean operating condition. The incremental amount of NO x believed to have been generated and stored in the device is obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). However, the disclosed NO x -estimating means fails to account for any instantaneous reduction in device efficiency, i.e., the device's ability to store an additional amount of feedgas NO x . The disclosed NO x -estimating means further fails to account for the device's initial storage of oxygen which likewise reduces the device's overall NO x -storing capacity.

The prior art has also recognized that the device's actual or maximum capacity to store selected exhaust gas constituents is often function of many variables, including device temperature, device history, sulfation level, and thermal damage, i.e., the extent of damage to the device's constituent-storing materials due to excessive heat. See, e.g., U.S. Pat. No. 5,437,153, which further teaches that, as the device approaches its maximum capacity, the incremental rate at which the device stores NO x may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NO x capacity which is significantly less than the actual NO x capacity of the device, to thereby theoretically provide the device with a perfect instantaneous NO x -storing efficiency, i.e., the device stores all engine-generated NO x , as long as stored NO x remains below the nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NO x reach the nominal device capacity. Unfortunately, however, the use of such a fixed nominal NO x capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NO x fill in order to ensure retention of engine-generated NO x .

When the engine is operated using a fuel containing sulfur, SO x accumulates in the device to cause a decrease in both the device's absolute capacity to store the selected exhaust gas constituent(s) and the device's instantaneous efficiency. When such device sulfation exceeds a critical level, the accumulated SO x must be “burned off” or released during a desulfation event, during which device temperatures are raised above perhaps about 650° C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650° C. by introducing a source of secondary air into the exhaust upstream of the NO x device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of stored SO x .

Therefore, the inventors herein have recognized a need for a method and system for controlling the filling and purging of an emission control device with a selected exhaust gas constituent which can more accurately regulate overall tailpipe emissions of the exhaust gas constituent than prior art methods and systems.

In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust treatment system including an emission control device that stores an exhaust gas constituent during lean engine operation and releases previously-stored exhaust gas constituent during engine operation at or rich of stoichiometry. Under the invention, during lean engine operation, the method includes determining a value representing an incremental amount, in grams per second, of a selected exhaust gas constituent, such as NO x , present in the engine feedgas as a function of current values for engine speed, engine load or torque, and the lean operating condition's air-fuel ratio. The method also includes determining a value representing the incremental amount of the exhaust gas constituent (e.g, NO x ) being instantaneously stored in the device, preferably, as a function of device temperature, the amount of the constituent that is already stored in the device, an amount of sulfur which has accumulated within the device, and a value representing device aging (the latter being caused by a permanent thermal aging of the device or the diffusion of sulfur into the core of the device material which cannot be purged).

The method further includes calculating a value representing instantaneous tailpipe emissions of the exhaust gas constituent (e.g., NO x ) based on the difference between the feedgas value and the incremental constituent-storage value; comparing the instantaneous tailpipe constituent emissions value to a predetermined threshold value; and discontinuing the lean engine operating condition when the instantaneous tailpipe constituent emissions value exceeds the predetermined threshold level, either instantaneously or as averaged over the course of a device purge-fill cycle, whose duration is determined by a timer which is nominally reset to zero upon commencement of an immediately prior rich engine operating condition.

In accordance with another feature of the invention, in a preferred embodiment, the method further includes generating a value representative of the cumulative number of miles that the vehicle has traveled during a given device purge-fill cycle; and determining a value representing average tailpipe constituent emissions in grams per mile using the instantaneous tailpipe constituent emissions value and the accumulated mileage value.

In accordance with another feature of the invention, an exemplary method further includes determining a need for releasing previously-stored exhaust gas constituent from the device; and deselecting the device-filling lean engine operation in response to the determined need. More specifically, under the invention, determining the need for releasing previously-stored exhaust gas constituent includes calculating a value representing the cumulative amount of the constituent that has been stored in the device during a given lean operation condition, based on the incremental constituent-storage value; determining a value representing an instantaneous constituent-storage capacity for the device; and comparing the cumulative constituent-storage value to the instantaneous constituent-storage capacity value. In a preferred embodiment, the step of determining the instantaneous constituent-storage capacity value includes estimating an amount of sulfur which has accumulated within the device.

Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.

The Drawing is a schematic of an exemplary system for practicing the invention.

Referring to the Drawing, an exemplary control system 10 for a four-cylinder, gasoline-powered engine 12 for a motor vehicle includes an electronic engine controller 14 having ROM, RAM and a processor (“CPU”) as indicated, as well as an engine-off timer that provides a value for the elapsed time since the engine 12 was last turned off as a variable, “soak time.” The controller 14 controls the operation of each of a set of fuel injectors 16 . The fuel injectors 16 , which are of conventional design, are each positioned to inject fuel into a respective cylinder 18 of the engine 12 in precise quantities as determined by the controller 14 . The controller 14 similarly controls the individual operation, i.e., timing, of the current directed through each of a set of spark plugs 20 in a known manner.

The controller 14 also controls an electronic throttle 22 that regulates the mass flow of air into the engine 12 . An air mass flow sensor 24 , positioned at the air intake of engine's intake manifold 26 , provides a signal regarding the air mass flow resulting from positioning of the engine's throttle 22 . The air flow signal from the air mass flow sensor 24 is utilized by the controller 14 to calculate an air mass value AM which is indicative of a mass of air flowing per unit time into the engine's induction system.

A first oxygen sensor 28 coupled to the engine's exhaust manifold detects the oxygen content of the exhaust gas generated by the engine 12 and transmits a representative output signal to the controller 14 . The first oxygen sensor 28 provides feedback to the controller 14 for improved control of the air-fuel ratio of the air-fuel mixture supplied to the engine 12 , particularly during operation of the engine 12 at or near the stoichiometric air-fuel ratio (λ=1.00). A plurality of other sensors, including an engine speed sensor and an engine load sensor, indicated generally at 30 , also generate additional signals in a known manner for use by the controller 14 .

An exhaust system 32 transports exhaust gas produced from combustion of an air-fuel mixture in each cylinder 18 through a pair of emission control device 34 , 36 , each of which functions in a known manner to reduce the amount of a selected constituent of the engine-generated exhaust gas, such as NO x , exiting the vehicle tailpipe 38 during lean engine operation. A second oxygen sensor 40 , which may also be a switching-type HEGO sensor, is positioned in the exhaust system 32 between the two emission control devices 34 , 36 . A third oxygen sensor 42 , which likewise is a switching-type HEGO sensor, is positioned downstream of the device 36 . In accordance with another feature of the invention, a temperature sensor 43 generates a signal representing the instantaneous temperature T of the device 36 , also useful in optimizing device performance as described more fully below.

Upon commencing lean engine operation, the controller 14 adjusts the output of the fuel injectors 16 to thereby achieve a lean air-fuel mixture for combustion within each cylinder 18 having an air-fuel ratio greater than about 1.3 times the stoichiometric air-fuel ratio. In accordance with the invention, for each subsequent background loop of the controller 14 during lean engine operation, the controller 14 determines a value representing the instantaneous rate FG_NOX_RATE at which NO x is being generated by the engine 12 as a function of instantaneous engine operating conditions, which may include, without limitation, engine speed, engine load, air-fuel ratio, EGR, and spark.

By way of example only, in a preferred embodiment, the controller 14 retrieves a stored estimate FG_NOX_RATE for the instantaneous NO x -generation rate from a lookup table stored in ROM based upon sensed values for engine speed N and engine load LOAD, wherein the stored estimates FG_NOX_RATE are originally obtained from engine mapping data.

During a first engine operating condition, characterized by combustion in the engine 12 of a lean air-fuel mixture (e.g., λ>1.3), the controller 14 determines incremental or delta feedgas emissions from the engine, in grams/hr, generated since the last time through this loop, and preferably expressed by the following relationship:

FG_NOX_RATE=FNXXX1(N,LOAD)*FNXXA(λ)*FNXXB(EGRACT)*FNXXC(SPK_DELTA)*FMXXD(ECT-200)

where:

FNXXX 1 (N,LOAD) is a lookup table containing NO x emission rate values in gram/hr for current engine speed N and engine load LOAD;

FNXXA(λ) is a lookup table for adjusting the FG_NOX_RATE value for air-fuel which inherently adjusts the FG_NOX_RATE value for barometric pressure;

FNXXB(EGRACT) is a lookup table for adjusting the FG_NOX_RATE value for actual exhaust gas recirculation percentage;

FNXXC(SPK_DELTA) is a lookup table for adjusting the FG_NOX_RATE value for the effect of knock sensor or hot open-loop induced spark retard, with NO x production being reduced with greater spark retard; and

FMXXD(ECT-200) is a lookup table for adjusting the FG_NOX_RATE value for the effect of engine coolant temperature above 200° F.

Preferably, the determined feedgas NO x rate FG_NOX_RATE is further modified to reflect any reduction in feedgas NO x concentration upon passage of the exhaust gas through the upstream emission control device 34 , as through use of a ROM-based lookup table of three-way catalyst efficiency in reducing NO x as a function of the current air-fuel ratio λ, to obtain an adjusted instantaneous feedgas NO x rate ADJ_FG_NOX_RATE. The adjusted feedgas NO x rate is accumulated over the length of time t i,j that the engine 12 is operated within a given engine speed/load cell for which the feedgas NO x generation rate R i,j applies, which is typically assumed to be the duration of the control process's nominal background loop, to obtain a value representing an instantaneous amount ADJ_FG_NOX of feedgas NO x entering the device during the background loop.

Also during the lean operating condition, the controller 14 calculates an instantaneous value INCREMENTAL_NOX representing the incremental amount of NO x stored in the device 36 during each background loop executed by the controller 14 during a given lean operating condition, in accordance with the following formula:

INCREMENTAL_NOX=ADJ_FG_NOX_RATE*t i,j *μ,

where:

μ represents a set of adjustment factors for instantaneous device temperature T, open-loop accumulation of SO x in the device 36 (which, in a preferred embodiment, is itself generated as a function of fuel flow and device temperature T), desired device utilization percentage, and a current estimate of the cumulative amount of NO x which has already been stored in the device 36 during the given lean operating condition. The controller 14 thereafter calculates a value INST_TP_NOX based on the difference between the adjusted instantaneous feedgas NO x value ADJ_FG_NOX and the instantaneous value INCREMENTAL_NOX representing the incremental amount of NO x stored in the downstream emission control device 36 . The controller 14 then compares the value INST_TP_NOX to a predetermined threshold level MAX_TP_NOX. If the controller 14 determines that the value INST_TP_NOX exceeds the predetermined threshold level MAX_TP_NOX, the controller 14 immediately discontinues the on-going lean engine operating condition in favor of either near-stoichiometric engine operating condition or a device-purging rich engine operating condition.

In accordance with another feature of the invention, an exemplary method includes generating a value representing a cumulative number of miles that the vehicle has traveled during a given device purge-fill cycle, i.e., since the commencement of an immediately prior device-purging rich engine operating condition; and determining a value representing average tailpipe NO x . emissions in grams per mile using the third value and the accumulated mileage value. More specifically, when the system 10 is initially operated with a lean engine operating condition, the efficiency of the downstream device 36 is very high, and the tailpipe NO x . emissions are correlatively very low. As the downstream device 36 fills, the efficiency of the downstream device 36 begins to fall, and the tailpipe NO x emissions value INST_TP_NOX will slowly rise up towards the threshold value MAX_TP_NOX. However, since the initial portion of the lean engine operating condition was characterized by very low tailpipe NO x emissions, the lean engine operating condition can be maintained for some time after the instantaneous value INST_TP_NOX exceeds the threshold value MAX_TP_NOX before average tailpipe NO x emissions exceed the threshold value MAX_TP_NOX. Moreover, since a purge event is likewise characterized by very low instantaneous tailpipe NO x emissions, average tailpipe NO x . emissions are preferably calculated using a time period which is reset at the beginning of the immediately prior purge event.

To the extent that the calculated tailpipe NO x . emissions does not exceed the predetermined threshold level, the controller 14 continues to track device fill time, as follows: the controller 14 iteratively updates a stored value TOTAL_NOX representing the cumulative amount of NO x which has been stored in the downstream device 44 during the given lean operating condition, in accordance with the following formula:

The controller 14 further determines a suitable value NOX_CAP representing the instantaneous NO x -storage capacity estimate for the device 36 . By way of example only, in a preferred embodiment, the value NOX_CAP varies as a function of device temperature T, as further modified by an adaption factor K i periodically updated during fill-time optimization to reflect the impact of both temporary and permanent sulfur poisoning, device aging, and other device-deterioration effects.

The controller 14 then compares the updated value TOTAL_NOX representing the cumulative amount of NO x stored in the downstream device 36 with the determined value NOX_CAP representing the downstream device's instantaneous NO x -storage capacity. The controller 14 discontinues the given lean operating condition and schedules a purge event when the updated value TOTAL_NOX exceeds the determined value NOX_CAP.

For example, in a preferred embodiment, if the controller 14 determines that the value INST_TP_NOX exceeds the predetermined threshold level MAX_TP_NOX, the controller 14 immediately schedules a purge event using an open-loop purge time based on the current value TOTAL_NOX representing the cumulative amount of NO x which has been stored in the device 44 during the preceding lean operating condition. In this regard, it is noted that the instantaneous device temperature T, along with the air-fuel ratio and air mass flow rate employed during the purge event, are preferably taken into account in determining a suitable open-loop purge time, i.e., a purge time that is sufficient to release substantially all of the NO x and oxygen previously stored in the downstream device 36 .

As noted above, a temperature sensor directly measures the temperature T of the downstream device 36 ; however, it will be appreciated that device temperature may be inferred, for example, in the manner disclosed in U.S. Pat. No. 5,894,725 and U.S. Pat. No. 5,414,994, which disclosures are incorporated herein by reference.

If, at the end of the purge event, the controller 14 determines that the value INST_TP_NOX continues to exceed the predetermined threshold level MAX_TP_NOX, the controller 14 either selects a near-stoichiometric engine operating condition, or schedules another open-loop purge event.

Preferably, in accordance with another feature of the invention, the controller 14 initializes certain temperature and sulfur-accumulation variables in a manner to account for instances where an engine may be turned off for short periods of time in which the downstream device 36 may not have cooled to ambient temperature. More specifically, rather than resetting these variable to zero upon commencing lean engine operation, the controller 14 estimates these variables upon engine ignition as a function of respective values for the variables immediately preceding engine shutoff, ambient temperature, ambient humidity, and at least one respective calibratable time constant representing an amount of time for the variable to deteriorate to a value corresponding to the passage of a relatively large amount of time. Thus, for example, an initialization routine for a device temperature variable TEMP_INIT after a soak time SOAKTIME is preferably expressed as follows:

where:

TEMP_PREVIOUS is a value for device temperature T during the immediately preceding engine operating condition;

AMBIENT is a measured or inferred value representing current ambient temperature;

FNEXP is a lookup table value that approximates an exponential function;

SOAKTIME is the time elapsed since the engine was shut down, in seconds; and

TEMP_TIME_CONST is an empirically derived time constant associated with the cooling-off of the exhaust gas at an identified location on the downstream device 36 , in seconds.

While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.