Abstract:
An engine system includes an engine, 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 during a predetermined period after start-up. The liquid and vapor fuel mixture has a 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 
     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 
     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” after starting 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. 
     In one conventional approach, an engine controller commands a lean air/fuel (A/F) ratio and supplies a reduced mass of liquid fuel to the engine to provide compensation. More air is available relative to the mass of liquid fuel to sufficiently oxidize the CO and HC. However, the lean condition reduces engine stability and adversely impacts vehicle drivability. 
     In another conventional approach, the engine controller commands a fuel-rich mixture for stable combustion and good vehicle drivability. A secondary air injection system provides an overall lean exhaust A/F ratio. The secondary air injector injects air into the exhaust stream during the initial start-up period. The additional injected air heats the catalytic converter by oxidizing the excess CO and HC. The warmed catalytic converter oxidizes CO and HC and reduces NO x  to lower emissions levels. However, the secondary air injection system increases cost and complexity of the engine control system and is only used during a short initial cold start period. 
     SUMMARY OF THE INVENTION 
     An engine system according to the present invention includes an engine, 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 during a predetermined period after start-up. The liquid and vapor fuel provide a fuel mixture having a third A/F ratio. 
     In other features, the controller adjusts the first and second quantities based on a temperature of the engine. The second quantity is zero if the engine temperature is outside of a specified temperature range. The controller controls an initial A/F ratio of liquid fuel supplied to the engine during start-up and estimates the third A/F ratio based thereon. 
     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. 
     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. 
     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 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an engine control system and a fuel system; 
         FIG. 2  is a graph illustrating a liquid fuel A/F ratio and a vapor fuel A/F ratio according to the present invention; 
         FIG. 3  is a flowchart showing steps of a cold start fuel vapor enrichment control method according to the present invention; and 
         FIG. 4  is a flowchart showing steps of the cold start fuel vapor enrichment control method including determining an A/F ratio offset. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment 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. 
     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. 
     The engine system  10  includes an engine  16 , an intake manifold  18 , and an exhaust  20 . Air and fuel are drawn into the engine  16  and combusted therein. Exhaust gases flow through the exhaust  20  and are treated in a catalytic converter  22 . First and second O 2  sensors  24  and  26  communicate with the controller  14  and provide exhaust A/F ratio signals to the controller  14 . 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 A/F ratio supplied to the engine  16 . An intake manifold temperature sensor  29  generates an intake air temperature signal that is sent to the controller  14 . 
     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 . 
     The fuel pump  38  pumps liquid fuel through the liquid fuel line  40  to the engine  16 . 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 . 
     Referring to  FIGS. 2 and 3 , a cold start 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 cold start of the engine  16 . 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 . 
     The 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 18 or 20, to achieve a target exhaust A/F ratio of about 15.5. 
     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 intake manifold temperature (T IM ) is measured and compared to a predetermined temperature range. If T IM  falls outside of the temperature range, the controller  14  operates the engine using only liquid fuel in step  108 . If T IM  falls within the temperature range, the controller  14  initiates a vapor enrichment mode. In one embodiment, the predetermined temperature range is between approximately 30° F. and 85° F., although other temperature values may be used. 
     Alternatively, in step  106 , intake valve temperature is estimated and compared to a threshold value. The intake valve temperature is estimated based on engine coolant temperature, engine speed, manifold absolute pressure (MAP), and an equivalence ratio. The equivalence ratio is defined as the stoichiometric A/F ratio divided by the actual A/F ratio. A predictive model for intake valve temperature is provided in “Intake-Valve Temperature and the Factors Affecting It”, Alkidas, A. C., SAE Paper 971729, 1997, expressly incorporated herein by reference. If the intake valve temperature is greater than the threshold value, the controller  14  operates the engine using only liquid fuel in step  108 . If the intake valve temperature is less than the threshold value, the controller  14  initiates the vapor enrichment mode. The threshold temperature is provided as 120° C., however, it is appreciated that the specific value of the threshold temperature may vary. 
     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 . If the vapor fuel A/F ratio is insufficient (i.e., greater than the target vapor fuel A/F ratio), control continues with step  108 . If the vapor A/F ratio is sufficient (i.e., less than the target vapor fuel A/F ratio), 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. 
     In step  122 , the controller  14  determines whether the first O 2  sensor is ready to provide an exhaust A/F ratio measurement. If the first O 2  sensor is not ready, control loops back to step  106 . If the first O 2  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 . 
     Control continuously loops through the vapor enrichment mode until T IM  achieves a temperature outside of the specified range. An end of the start-up period occurs when T IM  is a sufficiently high temperature and control loops to step  108  to initiate normal operation of the engine. 
     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. 
     The cold start fuel vapor enrichment control method of the present invention significantly reduces the liquid fuel required during cold start and warm up. Further, HC emissions are reduced and the engine is able to operate slightly lean of the stoichiometric A/F ratio to enable quick O 2  catalyst warm-up. Additionally, the control strategy of the present invention can be readily implemented in a traditional engine system with minimal hardware modification. 
     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.