Information
-
Patent Grant
-
6578564
-
Patent Number
6,578,564
-
Date Filed
Wednesday, September 19, 200122 years ago
-
Date Issued
Tuesday, June 17, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
-
International Classifications
-
Abstract
An improved method of operation for an electromechanical purge valve of a vehicle evaporative emission control system reduces the activation level of the purge valve below a nominal minimum level by a variable offset amount under specified operating conditions to lower purge flow. Specifically, the low flow control is permitted when the fuel in the purge vapor being drawn into the engine exceeds a calibrated percentage of the engine fuel requirement and the activation level of the purge valve has been reduced to the nominal minimum, provided that the system voltage level is at or above a specified value. When low flow control is permitted, the offset amount is incrementally increased so long as the engine fuel control is able to maintain the air/fuel ratio error at or below a calibrated amount, and incrementally decreased when the low flow control is no longer permitted or the air/fuel ratio becomes lean enough to potentially degrade combustion stability.
Description
TECHNICAL FIELD
The present invention is directed to a method of operation for the fuel vapor purge system of an internal combustion engine, and more particularly to a method of operation for an electromechanical purge valve that achieves a wide range of flow control.
BACKGROUND OF THE INVENTION
Effective control of evaporative emissions in a motor vehicle powered by an internal combustion engine requires a system for storing fuel tank vapor in a charcoal canister, and for activating an electromechanical purge valve to allow the stored fuel vapor to be drawn into the intake-manifold of the engine for combustion in the engine cylinders. Ordinarily, the purge valve activation level is calibrated as a function of engine operating parameters such as speed and load so that the purge vapor flow is a desired percentage of the engine airflow. The hydrocarbon concentration of the purge vapor may be estimated, and the fuel injection quantity correspondingly adjusted to maintain accurate control of the cylinder air/fuel ratio. See, for example, the co-pending U.S. patent application Ser. No. 09/264,524, filed on Mar. 8, 1999, and Ser. No. 09/950,283 filed on Sep. 10, 2001, both of which are assigned to the assignee of the present invention, and incorporated by reference herein.
Since the purge vapor flow for a given purge valve opening is limited by the intake manifold vacuum level, there are certain low-vacuum conditions under which the purge flow with a standard fully-open purge valve is too low to prevent saturation of the charcoal canister. This can occur, for example, in engines designed to operate at near-atmospheric intake manifold pressure, or in stratified combustion mode engines where the intake air flow is controlled to regulate the air/fuel ratio to a relatively high value (in this case, high throttle openings increase the intake manifold pressure). This is typically addressed by using a high-flow (i.e., large-opening) purge valve so that the desired purge flow can be achieved even at low intake manifold vacuum levels. However, using a high flow purge valve effectively raises the minimum purge flow for a given engine vacuum because the normal control range of an electromechanical valve does not include very low activation levels for which the activation level vs. valve opening relationship is highly nonlinear. As a result, the minimum flow position of a high-flow purge valve can allow higher than desired purge flow under high fuel vaporization conditions, such as when an engine is idled in a high temperature environment and/or with highly volatile fuel. Accordingly, what is needed is a control method for extending the low-flow capability of an electromechanical purge valve by utilizing its non-linear operating range.
SUMMARY OF THE INVENTION
The present invention is directed to an improved method of operation for an electromechanical purge valve of a vehicle evaporative emission control system, wherein the activation level of the purge valve is reduced below a nominal minimum level by a variable offset amount under specified operating conditions. Specifically, the low flow control is permitted when the percent of fuel from purge vapor exceeds a calibrated value and the activation level of the purge valve has been reduced to the nominal minimum, provided that the system voltage level is at or above a specified value. When low flow control is permitted, the offset amount is incrementally increased to lower the valve activation level so long as the engine fuel control is able to maintain the air/fuel ratio error at or below a calibrated amount, and incrementally decreased to raise the valve activation level when the low flow control is no longer permitted or the air/fuel ratio becomes lean enough to potentially degrade combustion stability.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a system diagram of an internal combustion engine and evaporative emission system including an electromechanical purge valve and a microprocessor-based control unit for activating the purge valve in accordance with this invention.
FIGS. 2-6
are flow diagrams depicting a software routine executed by the control unit of
FIG. 1
in carrying out the control of this invention.
FIG. 2
depicts a main flow diagram,
FIG. 3
details a portion of the main flow diagram concerning system voltage enable logic,
FIG. 4
details a portion of the main flow diagram concerning low purge flow enable logic,
FIG. 5
details a portion of the main flow diagram concerning purge valve control, and
FIG. 6
details a portion of the purge valve control concerning determination of the low flow offset amount.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The method of the present invention is disclosed in the context of a control system for an engine
10
in which fuel is injected directly into the engine cylinders, although it should be understood that the method equally applies to engines in which fuel is injected into intake runners upstream of the respective engine cylinders. A control system for engine
10
includes a fuel control system
12
and an evaporative emission control system (EECS)
14
, both of which are controlled by a microprocessor-based engine control module (ECM)
16
. In general, the EECS
14
manages evaporative emissions by storing fuel vapor and periodically releasing all or a portion of the stored vapor to engine
10
for combustion therein, and the fuel control system
12
injects a determined amount of fuel into engine
10
, taking into account any fuel vapor supplied by EECS
14
. In the illustrated embodiment, the fuel injection system
12
includes a mass airflow (MAF) sensor
20
, and idle air control valve
22
, a throttle position sensor
24
, a manifold absolute pressure (MAP) sensor
26
, a fuel sender
28
, an engine speed sensor
30
, a number of electrically activated fuel injectors
32
, and a wide-range air/fuel (WRAF) exhaust gas sensor
34
. The EECS
14
primarily includes a charcoal canister
36
, electrically operated canister vent and purge valves
38
,
40
, and fuel tank pressure and temperature sensors
42
,
44
.
The ECM
16
executes a number of software routines for regulating the operation of the EECS
14
and the fuel control system
12
, including functions such as fuel quantity calculations, fuel injection control, and fuel vapor purge control. Thus, ECM
16
receives output signals from the above-mentioned sensors
20
,
24
,
26
,
28
,
30
,
34
,
42
,
44
, and develops outputs signals for controlling idle air control valve
22
, fuel injector
32
, canister vent valve
38
and purge valve
40
.
The fuel injectors
32
inject fuel directly into respective engine cylinders
54
, as shown, and one or more intake valves
55
at each cylinder
54
open during an intake stroke to admit intake air and purged fuel vapor, if any. The intake air is ingested through a throttle valve
56
and an intake manifold
58
to which the various cylinders
54
are coupled by respective intake runners
60
. The idle air valve
22
provides a by-pass around throttle valve
56
, and its restriction is controlled by ECM
16
for purposes of regulating the engine idle speed. A piston
64
reciprocally disposed in each cylinder
54
and coupled to a rotary crankshaft
66
defines a combustion chamber
68
into which the fuel is injected. Following ignition of the air/fuel mixture by a spark plug (not shown), the products of combustion (that is, the exhaust gasses) exit the cylinder
54
through an exhaust valve
70
past WRAF sensor
34
to a catalytic converter and exhaust pipe (not shown). Operation of the engine
10
creates a sub-atmospheric pressure, or vacuum, in intake manifold
58
, and the vacuum draws stored fuel vapor from canister
36
into intake manifold
58
through purge valve
40
as fresh air is drawn into canister
36
via vent valve
38
. The fuel vapor stored in canister
36
originates in fuel tank
62
, and is supplied to canister
36
via a rollover valve
72
.
The ECM
16
controls the purge and vent valves
38
,
40
so that the purge vapor flow is a desired percentage (PURGE_PCT_DES) of the engine airflow, where PURGE_PCT_DES is determined as a function of engine speed and load. When vapor purging is desired, the vent valve
38
is activated to a fully open state, and the purge valve
40
is variably activated by pulse-width-modulation (PWM) in which the modulation frequency is fixed, and the duty-cycle is scheduled open-loop for achieving PURGE_PCT_DES. Due to variations in engine operation and environmental conditions, the purge valve opening required to achieve PURGE_PCT_DES can vary over a relatively wide range. For example, the purge valve opening has to be large in engines designed for operation at near-atmospheric intake manifold pressure, and in direct injection engines operating in the stratified combustion mode. On the other hand, a small valve opening is required under high fuel vaporization conditions, such as when an engine is idled in a high temperature environment and/or with highly volatile fuel. This creates a problem because a purge valve designed to provide a large opening for low vacuum, high flow conditions when fully activated cannot be reliably controlled to a small enough opening under high fuel vaporization conditions. Theoretically, of course, the valve opening could be made smaller and smaller by simply reducing the activation level of the valve, but the valve opening for given activation level under such conditions varies widely from valve to valve, and with changes in environmental and other conditions, so that a given valve opening smaller than a certain size cannot be reliably achieved. For this reason, valve manufacturers typically specify a nominal minimum activation level which will reliably produce a desired valve opening within specified tolerance levels.
The present invention addresses the above-described problem with a control method that extends the low-flow capability of an electromechanical purge valve below the nominal minimum activation level. In this way, the maximum opening of the valve may be sized to provide sufficient purge flow under low vacuum conditions, and the control method operates the valve below its nominal minimum activation level to prevent excessive vapor purge flow under high fuel vaporization conditions. According to the invention, the low-flow control is enabled when the percentage of fuel from purge vapor exceeds a calibrated value and the activation level of the purge valve has been reduced to the nominal minimum, provided that the system voltage level is at or above a minimum energization voltage for reliably operating the valve at an activation level below the nominal activation level.
The hydrocarbon concentration of the purge vapor may be estimated based on the output of an exhaust gas oxygen sensor, as described in the aforementioned U.S. patent application Ser. Nos. 09/264,524 and 09/950,283, both of which are assigned to the assignee of the present invention, and incorporated herein by reference. In the illustrated embodiment, engine
10
may be operated in either homogeneous or stratified combustion modes, and different methods are used to estimate the hydrocarbon concentration of the purge vapor is depending on the combustion mode. In the homogeneous mode, fuel is injected so that the air/fuel mixture is evenly distributed throughout the cylinder
54
when the mixture is ignited during the ensuing combustion stroke, and a closed-loop fuel control adjusts base fuel injection quantity to maintain the air/fuel ratio at a desired value at or near the stoichiometric ratio. In this case, the hydrocarbon concentration of the purge vapor is estimated by an iterative process in which the estimate is incrementally increased or decreased if an integral of the measured air/fuel ratio error reaches respective rich or lean thresholds. When fuel vapor is not being purged, the integral of the measured air/fuel ratio error is used to update a closed-loop adaptive learning table. See the aforementioned U.S. patent application Ser. No. 09/264,524. In the stratified mode, the fuel is injected just prior to the ignition event, resulting in a rich air/fuel mixture in the vicinity of the spark plug at ignition; the injected fuel quantity is scheduled open-loop to achieve a commanded engine torque output, and the throttle valve
56
is adjusted to maintain the air/fuel ratio in a range significantly higher than the stoichiometric ratio. Under these conditions, there may be substantial error between the actual and desired air/fuel ratio even under steady-state operating conditions, and the air/fuel ratio error for purposes of estimating the purge vapor concentration is normalized for air/fuel ratio errors that exist under steady-state engine operation when the purge valve
40
is not activated. See the aforementioned U.S. patent application Ser. No. 950,283.
When low flow control is permitted, the activation level of the purge valve is reduced by an offset amount that is incrementally increased so long as the engine fuel control is able to maintain the air/fuel ratio error at or below a calibrated amount, and incrementally decreased when the low flow control is no longer permitted or the air/fuel ratio becomes lean enough to degrade combustion stability. In other words, activating the purge valve at a level below the nominal minimum level will likely produce purge flow error due to valve nonlinearities as discussed above, and if the error is not too large, the fuel control will be able to adjust the fuel injection amount as required to maintain the air/fuel ratio error reasonably close to the desired value. In this way, the activation level of the purge valve is reduced below the nominal activation level to approach the desired purge percentage under high fuel vaporization conditions, so long as reasonably accurate air/fuel ratio control is maintained and the system voltage is sufficient to ensure reliable valve operation at the reduced activation level. The activation level is not allowed under any circumstance to be less than an absolute minimum level for reliable operation at the worst case voltage level. The amount by which the activation level may be reduced below the nominal minimum level will vary depending on valve and environmental conditions, but the dynamic range of the valve will be increased in any event.
The flow diagrams of
FIGS. 2-6
depict a software routine periodically executed by ECM
16
for carrying out the control method of this invention.
FIG. 2
depicts a main flow diagram, while
FIGS. 3-6
detail various portions of the routine referenced in FIG.
2
.
Referring to
FIG. 2
, the main flow diagram involves periodically executing the blocks
80
-
88
. Block
80
involves comparing the system voltage to a minimum energization voltage for reliably operating purge valve
40
, and setting the status of the VOLT_ENABLE flag accordingly; see FIG.
3
. Block
82
involves determining whether the various low flow entry conditions have been met and setting the status of the LOW_FLOW flag accordingly; see FIG.
4
. Block
84
involves determining the desired purge concentration and a PWM duty cycle (PURGE_DC) for purge valve
40
; see
FIGS. 5-6
. Block
86
adjusts the fuel injection quantity to take into account the fuel obtained due to vapor purging, and block
88
updates the hydrocarbon concentration estimate (PURGE_CONC) of the purge vapor, as described above.
Referring to the system voltage enable logic of
FIG. 3
, the blocks
90
and
92
compare the system voltage SYS_VOLT to upper and lower thresholds THRlow, THRhigh defining a minimum energization voltage for reliably operating purge valve
40
. If SYS_VOLT is above THRhigh, the block
94
sets the VOLT_ENABLE flag to TRUE, while if SYS_VOLT falls below THRlow, the block
96
sets the VOLT_ENABLE flag to FALSE.
Referring to the low-flow enable logic of
FIG. 4
, the blocks
98
-
100
determine if the percent of fuel from purge vapor, PURGE_PCT, is greater than a calibration value CAL_PCT_FUEL, the blocks
106
and
108
are executed to determine if PURGE_DC is at the nominal minimum activation level NOM_MIN, and if the VOLT_ENABLE flag is TRUE. If all three conditions are met (that is, if blocks
98
,
106
and
108
are answered in the affirmative), the block
110
sets the LOW_FLOW flag to TRUE. If blocks
106
or
108
is answered in the negative, or if block
100
determines that PURGE_PCT falls below the quantity (CAL_PCT_FUEL-Khys), the block
102
is executed to set the LOW_FLOW flag to FALSE. If PURGE_PCT is between CAL_PCT_FUEL and (CAL_PCT_FUEL-Khys), the block
104
is executed to determine if the VOLT_ENABLE flag is TRUE. If so, the status of the LOW_FLOW flag is unchanged; if not, the block
102
is executed to set the LOW_FLOW flag to FALSE.
Referring to
FIG. 5
, determining PURGE_DC involves determining a desired percentage of purge vapor (PURGE_PCT_DES) as indicated at block
114
, determining a purge rate factor PRF based on the deviation of the current purge vapor percent PURGE_PCT from PURGE_PCT_DES, updating the minimum duty cycle MIN_DC based on the low-flow offset LF_OFFSET, and then determining PURGE_DC based on PRF and the minimum duty cycle MIN_DC. The determination of LF_OFFSET is described below in reference to FIG.
6
.
As indicated at block
114
, the PURGE_PCT_DES is determined primarily as a function of engine speed (ES) and load (LOAD) for the current combustion mode of engine
10
. The percent of fuel from purge vapor, PURGE_PCT, is determined at block
116
as a function of the air/fuel ratio (AFR), the purge vapor mass flow rate (MFRpurge), the intake mass flow rate (MFRintake) and PURGE_CONC, as follows:
PURGE
—
PCT
=(PURGE
—
CONC*MFR
purge*
AFR
)/
MFR
intake (1)
The quantities MFRpurge and MFRintake may be measured or estimated based n various factors, as disclosed for example, in the U.S. Pat. No. 5,845,627, issued on Dec. 8, 1998, and incorporated herein by reference. If PURGE_PCT is less than or equal to PURGE_PCT_DES, as determined at block
118
, the block
120
sets PURGE_DC to a value based on PURGE_PCT_DES, the air/fuel ratio error AFR_ERROR, and the measured mass air flow MAF. If AFR_ERROR is reasonably low, PURGE_DC is adjusted to achieve PURGE_PCT_DES; however, PURGE_DC is controlled to achieve a value less than PURGE_PCT_DES if AFR_ERROR indicates that there is significant fueling error. If PURGE_PCT is greater than PURGE_PCT_DES, the blocks
122
and
124
are executed to determine a ramp factor PRF for controlling the rate of change of PURGE_DC. The value of PRF computed at block
122
according to the expression:
PRF
=(
K
fast_rate*PURGE
—
PCT
—
LMT
/PURGE
—
PCT
)+[(1−
K
fast_rate)*
K
slow_rate] (2)
where Kfast_rate and Kslow_rate are calibrated values corresponding to the predetermined changes per unit time in the value of PURGE_DC. For example, Kfast_rate may be 0.60, corresponding to a 40% reduction of PURGE_DC each time PRF is applied to PURGE_DC, and Kslow_rate may be 0.95, corresponding to a 5% reduction of PURGE_DC each time PRF is applied to PURGE_DC. Thus, if PURGE_PCT is only slightly higher than PURGE_PCT_LMT, as may occur in normal purge control, PRF will be approximately equal to Kslow_rate. On the other hand, if PURGE_PCT is significantly higher than PURGE_PCT_LMT, as may occur when the combustion mode switches from homogeneous to stratified, the product [Kfast_rate*(PURGE_PCT_LMT/PURGE_PCT)] becomes smaller, resulting in a smaller value of PRF and a faster reduction of PURGE_-DC. The block
124
sets the purge rate factor PRF equal to the lower of the PRF value computed at block
122
and Kslow_rate. The block
126
updates the low flow offset LF_OFFSET, as described below in reference to
FIG. 6
, and the block
128
updates PURGE_DC by applying LF_OFFSET to the nominal minimum duty cycle MIN_DC_NOM, and then computing PURGE_DC according to:
PURGE
—
DC
=[(100−
MIN
—
DC
)*
PRF]+MIN
—
DC
(3)
Referring to
FIG. 6
, determining LF_OFFSET initially involves executing block
130
to determine if the LOW_FLOW flag is TRUE. In general, if the LOW_FLOW flag is TRUE, LF_OFFSET is incrementally increased toward a limit value to correspondingly reduce MIN_DC if block
130
is answered in the affirmative, the air/fuel ratio error AFR_ERROR is relatively low, and PURGE_PCT is greater than a calibrated value. Referring to the flow diagram, the blocks
142
-
148
are executed to increase LF_OFFSET if block
132
is answered in the negative and blocks
136
,
138
and
140
are answered in the affirmative. Block
132
determines if AFR_ERROR is greater than a calibrated threshold CAL_LEAN indicative of an excessive uncorrected air/fuel ratio error in the lean direction. Block
136
determines if PURGE_PCT is greater than a calibrated value such as 20%, and block
138
determines if the magnitude of AFR_ERROR is less than a calibrated value such as 5%. Finally, block
140
determines if incrementing LF_OFFSET by the step increment CAL_STEP would reduce PURGE_DC below an absolute minimum level ABS_MIN. If the conditions for incrementing LF_OFFSET are satisfied, the block
142
increments a TIMER, and blocks
144
-
146
increase LF_OFFSET by CAL_STEP when TIMER reaches a calibrated threshold CAL_TIME. The block
148
resets TIMER to zero each time LF_OFFSET is increased. If at any time during low-flow control block
132
is answered in the affirmative, the block
134
is executed to immediately decrease LF_OFFSET by CAL_STEP, thereby immediately increasing MIN_DC to increase PURGE_DC; this serves to prevent degraded driveability due to exceeding the lean combustion limit in situations where the purge vapor fuel is a significant percentage of the engine fuel requirement. If block
132
continues to be answered in the negative, but blocks
136
,
138
or
140
are answered in the negative, the block
148
is executed to reset the TIMER to zero, thereby postponing further increases in LF_OFFSET.
If the LOW_FLOW flag is FALSE and LF_OFFSET is non-zero, as determined at blocks
130
and
150
, the blocks
151
-
158
are executed to decrease LF_OFFSET for exiting the low-flow control mode. If the VOLT_ENABLE flag is TRUE, as determined at block
151
, the block
152
increments a TIMER, and blocks
154
-
156
decrease LF_OFFSET by CAL_STEP when TIMER reaches a calibrated threshold CAL_TIME. If block
151
determines that the VOLT_ENABLE flag is FALSE, however, the blocks
152
-
154
are skipped, and the block
156
is immediately executed to decrease LF_OFFSET by CAL_STEP. In either event, the block
158
resets TIMER to zero each time LF_OFFSET is decreased.
In summary, the control of the present invention allows purge valve
40
to be operated below its nominal minimum level by a variable offset amount under specified conditions to effectively expand the dynamic range of purge flow control. When low flow control is permitted, the activation level of the valve is incrementally decreased so long as the engine fuel control is able to maintain the air/fuel ratio error at or below a calibrated amount, and incrementally increased when the low flow control is no longer permitted or the air/fuel ratio has become lean enough to potentially degrade combustion stability.
While the present invention has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. Thus, it will be understood that methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.
Claims
- 1. A method of operation for an internal combustion engine having a fuel control for maintaining an air/fuel ratio of said engine at a desired value, and a fuel vapor purge system including a purge valve that is electrically activated at a variable level to define an effective opening corresponding to such activation level through which stored fuel vapor is purged into said engine, said purge valve having a nominal minimum activation level for reliably defining a corresponding minimum effective opening, the method comprising the steps of:estimating a percentage of engine fuel supplied by said purged fuel vapor; initiating a low flow control of said purge valve when the estimated percentage exceeds a calibrated value and the activation level of the purge valve has been reduced to said nominal minimum level; and when said low flow control is initiated, progressively reducing said activation level below said nominal minimum level to define effective openings of said valve that are smaller than said minimum effective opening so long as the air/fuel ratio of said engine is within a calibrated amount of said desired value.
- 2. The method of operation of claim 1, including the step of:interrupting the progressive reduction of said activation level when said activation level reaches a calibrated minimum activation level which is lower than said nominal minimum activation level.
- 3. The method of operation of claim 2, wherein said calibrated minimum activation level corresponds to an activation level for obtaining reliable operation of said valve when a system voltage used to activate said valve is at a specified minimum value.
- 4. The method of operation of claim 3, including the step of:preventing initiation of said low flow control when the system voltage is below said specified minimum value.
- 5. The method of operation of claim 3, including the step of:terminating said low flow control by increasing said activation level to said nominal minimum activation level when the system voltage falls below said specified minimum voltage.
- 6. The method of operation of claim 1, including the step of:terminating said low flow control by progressively increasing said activation level when said estimated percentage falls below said calibrated value.
- 7. The method of operation of claim 1, wherein said low flow control includes the step of:progressively increasing said activation level if said air/fuel ratio becomes lean enough to potentially degrade combustion stability in said engine.
US Referenced Citations (10)