Abstract:
An engine system includes an engine with a secondary air source, a fuel system that communicates with the engine, and a controller that communicates with the fuel system. The controller controls a first quantity of liquid fuel to the engine at a first A/F ratio and a second quantity of vapor fuel to the engine at a second A/F ratio. The liquid and vapor fuel mixture has a third A/F ratio. A secondary air source provides additional air to the third A/F ratio. The controller determines an available A/F ratio of vapor fuel within the fuel tank and performs a comparison with a target A/F ratio range. The second quantity is set to zero if the A/F ratio of the vapor fuel is outside of the target A/F ratio range. The controller adjusts the first and second quantities based on a comparison between an exhaust A/F ratio and a target exhaust A/F ratio.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to engine control systems, and more particularly to engine control systems that provide vapor enrichment of fuel flowing to an engine during cold start conditions.  
       BACKGROUND OF THE INVENTION  
       [0002]     During combustion, an internal combustion engine oxidizes gasoline and combines hydrogen (H 2 ) and carbon (C) with air. Combustion creates chemical compounds such as carbon dioxide (CO 2 ), water (H 2 O), carbon monoxide (CO), nitrogen oxides (NO x ), unburned hydrocarbons (HC), sulfur oxides (SO x ), and other compounds. During an initial startup period after a long soak, the engine is still “cold” and combustion of the gasoline is incomplete. A catalytic converter treats exhaust gases from the engine. During the startup period, the catalytic converter is also “cold” and does not operate optimally.  
         [0003]     In one conventional approach, an engine controller commands a lean air/fuel (A/F) ratio thereby reducing the mass of liquid fuel to the engine. More air is available relative to the mass of liquid fuel to sufficiently oxidize the exhaust CO and HC. The resulting exhaust products then heat-up the catalytic converter. However, operating the cold engine at the lean condition can reduce engine stability, adversely impact vehicle drivability, and limits the catalyst warm-up rate.  
         [0004]     In another conventional approach, the engine controller commands an excessively fuel-rich mixture which provides stable combustion, good vehicle drivability and excess CO and HC for generating exhaust heat. A secondary air injection system is added to inject air into the rich combustion exhaust products and provide an overall lean exhaust A/F ratio. The additional injected air oxidizes the excess CO and HC in the exhaust, and this releases heat for fast warm-up of the catalytic converter. The warmed catalytic converter then further oxidizes CO and HC in the oxidizing environment provided by the air injection system. Once the catalyst reaches operating temperature, the excess air from the air injection system must be eliminated to provide a stoichiometric exhaust A/F ratio for the catalyst to simultaneously reduce HC, CO and NOx emissions.  
         [0005]     To ensure consistent CO and HC exhaust combustion, secondary air injection systems typically require much richer A/F ratios (20-30% more) than would be required for stable engine operation. However, injecting excess liquid fuel into a cold engine to generate a very fuel-rich exhaust mixture leads to excessive fuel wetting of the engine intake and cylinder surfaces. This results in high, uncontrolled HC emissions, oil contamination, spark ignition problems and increased fuel consumption.  
       SUMMARY OF THE INVENTION  
       [0006]     An engine system according to the present invention includes an engine including a secondary air source, a fuel system that communicates with the engine, and a controller that communicates with the fuel system. The controller controls a first quantity of liquid fuel to the engine at a first A/F ratio and a second quantity of vapor fuel to the engine at a second A/F ratio when the secondary air source is operational. The liquid and vapor fuel provide a fuel mixture having a third A/F ratio.  
         [0007]     In other features, the controller controls an initial A/F ratio of liquid fuel supplied to the engine and estimates the third A/F ratio based thereon.  
         [0008]     In yet other features, the controller determines an available A/F ratio of vapor fuel within the fuel tank and performs a comparison with a target A/F ratio range. The second quantity is set to zero if the A/F ratio of the vapor fuel is outside of the target A/F ratio range.  
         [0009]     In still other features, the controller receives an exhaust A/F ratio from an exhaust A/F ratio sensor and compares the exhaust A/F ratio to a target A/F ratio range. The controller adjusts the first and second quantities if the exhaust A/F ratio is outside of the target A/F ratio range.  
         [0010]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]      FIG. 1  is a functional block diagram of an engine control system including a secondary air source and a fuel system;  
         [0013]      FIG. 2  is a graph illustrating a liquid fuel A/F ratio and a vapor fuel A/F ratio according to the present invention;  
         [0014]      FIG. 3  is a flowchart showing steps of a fuel vapor enrichment control method according to the present invention; and  
         [0015]      FIG. 4  is a flowchart showing steps of the fuel vapor enrichment control method including determining an A/F ratio offset.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.  
         [0017]     Referring to  FIG. 1 , an engine system  10  and a fuel system  12  are shown. One or more controllers  14  communicate with the engine and fuel systems  10 ,  12 . The fuel system  12  selectively supplies liquid and/or vapor fuel to the engine system  10 , as will be described in further detail below. [ 0018 ] The engine system  10  includes an engine  16 , an intake manifold  18 , an exhaust manifold  20 , and a secondary air source  21 . Air and fuel are drawn into the engine  16  and combusted therein. The secondary air source  21  supplies additional air to the rich exhaust gas flowing through the exhaust manifold  20 . The combined gases are treated in a catalytic converter  22 . First and second exhaust air-fuel sensors  24  and  26  communicate with the controller  14  and provide exhaust A/F ratio signals to the controller  14 . The exhaust air-fuel fuel sensors can be stoichiometric switching O 2  or wide-range air-fuel sensors. In practice, the catalyst outlet sensor  26  is typically a switching O2 sensor. A mass air flow (MAF) sensor  28  is located within an air inlet and provides a mass air flow (MAF) signal based on the mass of air flowing into the intake manifold  18 . The controller  14  uses the MAF signal to determine the amount of fuel required by the engine  16 . An intake manifold temperature sensor  29  generates an intake air temperature signal that is sent to the controller  14 .  
         [0018]     The fuel system  12  includes a fuel tank  30  that contains liquid fuel and fuel vapors. A fuel inlet  32  extends from the fuel tank  30  to allow fuel filling. A fuel cap  34  closes the fuel inlet  32  and may include a bleed hole (not shown). A modular reservoir assembly (MRA)  36  is disposed within the fuel tank  30  and includes a fuel pump  38 . The MRA  36  includes a liquid fuel line  40  and a vapor fuel line  42 .  
         [0019]     The fuel pump  38  pumps liquid fuel through the liquid fuel line  40  to the engine  16 . The fuel is delivered to the intake manifold  18  by fuel injectors. Vapor fuel flows through the vapor fuel line  42  into an on-board refueling vapor recovery (ORVR) canister  44 . A vapor fuel line  48  connects a purge solenoid valve  46  to the ORVR canister  44 . The controller  14  modulates the purge solenoid valve  46  to selectively enable vapor fuel flow to the engine  16 . The controller  14  modulates a canister vent solenoid valve  50  to selectively enable air flow from atmosphere into the ORVR canister  44 .  
         [0020]     Referring to  FIGS. 2 and 3 , a fuel vapor enrichment control method will be described in further detail. In general, vapor fuel is used to supplement and enrich the A/F mixture during engine  16  and secondary air source  21  operation. The vapor fuel within the fuel tank  30  retains a predictable A/F ratio between engine cold starts. The A/F ratio of the fuel can be estimated based on temperature and a Reid vapor pressure (RVP) rating of the fuel. In an exemplary manner, the RVP value of the fuel is estimated during closed loop, steady-state engine operation based on a hydrocarbon purge flow and the temperature of the fuel tank  30 .  
         [0021]     The tank vapor fuel is typically very rich. Therefore, a relatively small amount of vapor fuel is able to provide a significant portion of the fuel required to compensate the engine  16 . Vapor fuel is present within the fuel tank  30  at atmospheric pressure. A sufficient amount of vapor fuel is usually available to handle throttle crowds and step-in maneuvers. As shown graphically in  FIG. 2 , fuel vapor having an A/F ratio within the designated range of approximately 2 to approximately 3, can be supplied in conjunction with liquid fuel having an A/F ratio of up to 14.7 to achieve a rich target exhaust A/F ratio of less than  12 . Additional air supplied by the secondary air source  21  allows the extra fuel to be combusted in the exhaust  20  and the catalytic converter  22 . This method of adding exhaust energy is used to heat a catalytic converter  22  located further downstream from the engine  16 .  
         [0022]     As detailed in  FIG. 3 , after a key-on event occurs in step  100 , the controller  14  determines the amount of liquid fuel required during engine crank (i.e. initial ignition). Currently available parameters including engine coolant temperature (T COOL ), ambient air temperature (T AMB ), and fuel temperature (T FUEL ) are measured in step  102 . In step  104 , the engine is cranked and initially runs and burns the liquid fuel having an initial A/F ratio. In step  106 , the secondary air source is checked for operation. In one embodiment, the secondary air source will operate when T COOL  and T AMB  are within a temperature range, although other operational conditions can be used. The temperature range for T COOL  is approximately 4.4° C. to approximately 50° C. and for T AMB  is approximately 4.4° C. to approximately 60° C. If the air system is operational, the controller initiates a vapor enrichment mode. If the air system is not operational, the controller operates the engine in a non-air operation mode.  
         [0023]     Depending on the application, the non-air operating mode can use a liquid fuel only calibration, typically 10-15% rich of stoichiometry, or it can use a Vapor Assist calibration. The non-air operation mode with Vapor Assist is described in “Cold Start Fuel Vapor Enrichment,” (U.S. Ser. No. 10/383,783), expressly incorporated herein by reference in its entirety.  
         [0024]     In the vapor enrichment mode, the A/F ratio of the vapor fuel within the fuel tank is estimated in step  112 . In step  114 , the present liquid fuel A/F ratio is determined and the target vapor fuel A/F ratio is calculated. The vapor fuel A/F ratio is compared to the target vapor fuel A/F ratio in step  116 . The vapor fuel rate is then calculated. If the vapor fuel rate is insufficient (i.e. less than the target vapor fuel rate), control continues with step  110 . In step  110 , the controller operates the engine using liquid fuel. If the vapor fuel rate is sufficient (i.e. greater than the target vapor fuel rate), control continues with step  118 . In step  118 , a duty-cycle for the purge solenoid valve  46  is calculated to achieve the appropriate flow of vapor fuel into the engine  16 . In step  120 , the controller  14  operates the vapor control valve at the calculated duty-cycle.  
         [0025]     In step  122 , the controller  14  determines whether the first exhaust air-fuel sensor is ready to provide an exhaust A/F ratio measurement. If the first exhaust air-fuel sensor is not ready, control loops back to step  106 . If the first exhaust air-fuel sensor is ready, the controller  14  continues with step  124  where a measured exhaust A/F ratio is compared to the target exhaust A/F ratio. If the exhaust A/F ratio is equal to the target exhaust A/F ratio, control loops back to step  106 . However, if the exhaust A/F ratio is not equal to the target exhaust A/F ratio, control continues with step  126 . In step  126 , the vapor fuel supply is adjusted using the purge solenoid valve duty cycle in step  118 .  
         [0026]     When the air system is operational and there exists sufficient fuel tank vapor, control continuously loops through the vapor enrichment mode. Normal engine operation will commence upon sufficient heating of the catalytic converter.  
         [0027]     With reference to  FIG. 4 , the fuel tank vapor A/F ratio calculated in step  112  can be trimmed or corrected. In step  123 , an offset is calculated as the difference between the exhaust A/F ratio and the target exhaust A/F ratio. The offset is updated in memory in step  125  as control loops through the vapor enrichment mode. Upon the next cold-start of the vehicle, calculation of the fuel tank vapor A/F ratio in step  112  takes into account the offset value stored in memory. This enables more accurate control of the A/F ratios. The offset value can be compared with the RVP estimate to further improve the vapor A/F ratio estimate.  
         [0028]     The cold start fuel vapor enrichment control method of the present invention reduces the amount of liquid fuel that is required during engine cold start and warm up. This reduces the HC emissions in the engine exhaust before the catalyst is active. Further, the engine is able to operate with a rich A/F ratio and the excess exhaust CO and HC can be combusted with secondary air injection to generate additional heat to enable quick catalyst warm-up.  
         [0029]     Using fuel vapor to provide the exhaust combustibles also offers the potential for reducing the secondary air injection flow requirement compared to the liquid fuel only case. Because less liquid fuel reaches the combustion chamber, and the excess fuel vapor is already highly volatile, the resulting rich A/F ratio in the combustion will burn quicker and at a higher temperature. This would allow running lower levels of exhaust CO and HC and still generating the necessary exhaust temperatures needed for catalyst warm-up. By using fuel vapor to replace the excess liquid fuel needed for the excessively rich exhaust A/F ratio, lower flow, less costly air injection systems, such as pulsed air are also viable.  
         [0030]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.