Abstract:
An engine controller system for a direct injection spark ignited internal combustion engine that is capable of operating a stratified mode where fuel is injected during a compression stroke of the engine and a homogeneous mode where fuels is injected during an intake stroke of the engine. The engine controller monitors adjusts the flow rate of the evaporated fuel vapors as a function of the catalyst temperature and the fuel level in the evaporated fuel vapors. The engine controller determines the fuel level in the evaporated fuel vapors as a function of the exhaust gas oxygen senor output.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/211,085, filed Jun. 13, 2000, titled “Measurement of Canister Purge Fuel Content in a Stratified Direct Injection Gasoline Engine.” 
    
    
     BACKGROUND 
     This invention relates generally to the field of gasoline-based internal combustion engines and more specifically to direct injection engines. 
     Direct injection engines, also called “in-cylinder injection engines,” inject fuel directly into the cylinders. Diesel engines commonly use direct injection. Gasoline engines, also called spark ignition engines, normally are port fuel injection engines that inject fuel via a carburetor or a fuel injector into the intake manifold or over the intake values into the cylinders. Recently, direct injection spark ignited (“DISI”) engines have been proposed. A DISI engine requires the fuel to be injected at relatively high pressure. Such a DISI engine also requires that the air-fuel ratio be leaner than convention gasoline engines and the air-fuel ratio must remain within tighter margins. 
     DISI engines operate in a stratified mode or a homogenous mode. When a DISI engine is in the stratified mode, the combustion chambers contain stratified layers with different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture, that is a slightly richer mixture, and subsequent strata contain progressively leaner mixtures. When the engine is in the homogeneous mode, a homogeneous mixture of air and fuel is injected into the combustion chamber. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry. 
     When a DISI engine operates in the stratified mode, the fuel is injected late in the compression cycle, usually during a compression stroke. Because of the late injection, a very short time is available for mixing of the air and fuel in the cylinder. Stable combustion is obtained because the rich zone air/fuel mixture near the spark plug is within the combustion limits while the overall air/fuel mixture in the cylinder is leaner than the air/fuel mixture normally used when the engine is in the homogeneous mode. When the engine is in the homogeneous mode, fuel is injected during an intake stroke of the engine. More mixing time is available in the homogenous mode. The stratified mode is more fuel efficient than the homogenous mode. 
     Direct injection engines are commonly coupled to three-way catalytic converters to reduce CO, HC, and NOx emissions. When operating at air/fuel mixtures lean of stoichiometry, an NOx trap or an NOx catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx emissions. The stratified mode may be used for light to medium loads and the homogeneous mode may be used for medium to heavy loads. 
     Gasoline engines typically collect evaporated fuel vapors from the gasoline tank in a carbon canister, also called a charcoal canister or an evaporative canister. The evaporated fuel vapors are purged from the carbon canister into the intake manifold and burned in the combustion chambers, that is in the cylinders, along with the fuel that is injected via the fuel injectors. Since the air-fuel ratio of the evaporated fuel vapors is not known, the net air-fuel ratio in the cylinder or in a strata in the cylinder is not known. Thus, it is desirable to measure the air-fuel ratio of the evaporated fuel vapors and control the air-fuel ratio injected into the combustion chambers accordingly. Traditionally, the fuel level of the evaporated fuel vapors is measured with an exhaust gas oxygen sensor, such as a switching HEGO sensor or a linear UEGO sensor. When the engine is running lean, the exhaust gas oxygen sensor may not be able to accurately detect the fuel level of the evaporated fuel vapors. 
     As gasoline engines become more efficient, that is they are run leaner, the exhaust gas sensors become less accurate or stop working. For example, a DISI engine operating in the stratified mode uses a much leaner mixture than when it is operating in the homogenous mode. The evaporated fuel vapors, which need to be burned, contain a homogenous mixture of air and fuel. However, the fuel level in the evaporated fuel vapors varies over time. Thus, traditional engines often shut off the flow of evaporated fuel vapors when the engine is running lean, for example in the stratified mode. Shut off of the flow of the evaporated fuel vapors limits the efficiency of the engine by limiting the time the traditional engine can be operated in the stratified mode. 
     Further, when a traditional DISI engine running in the stratified mode purges evaporated fuel vapors into the combustion chamber, the strata outside the rich stratified zone may be too lean to support combustion. This may result in large quantities of unburned fuel in the exhaust gas, which may result in an undesirable large exothermic reaction in the catalyst as the catalyst oxidizes the unburned fuel. If the evaporated fuel vapors contain too much fuel, making the mixture too rich, the torque produced by the engine undesirably increases. 
     SUMMARY 
     An engine controller for a direct injection spark ignited internal combustion engine can operate in a stratified mode where fuel is injected during a compression stroke of the engine and a homogeneous mode where fuels is injected during an intake stroke of the engine. The engine controller adjusts the flow rate of evaporated fuel vapors as a function of the catalyst temperature and the fuel level in the evaporated fuel vapors. The engine controller determines the fuel level in the evaporated fuel vapors as a function of the exhaust gas oxygen senor output. 
     The improved system and method of controlling a direct injection spark ignition engine is described. The improved system and method may use sensors that are found on existing DISI engines to indirectly measure the fuel level in the evaporated fuel vapors. The improved system and method allows the evaporated fuel vapors to be purged into the intake manifold when the engine is run lean. Thus, the engine operates longer in the more efficient stratified mode resulting in a lower fuel consumption. 
     The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
     FIG. 1 illustrates a direct injection engine with an improved evaporated fuel vapors control system; and 
     FIG. 2 illustrates a flow diagram of a method of controlling the direct injection engine of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     A. Definitions 
     AFR—Air to Fuel Ratio. The air to fuel ratio may be the mass of Air divided by the mass of fuel. 
     CFD—Computational Fluid Dynamics 
     CTS—Catalyst Temperature Sensor 
     DI—Direct Injection 
     DISI—Direct Injection Spark Ignition 
     EFC—Electronic Flow Control 
     EG—Exhaust Gas 
     EGR—Exhaust Gas Re-circulation 
     EGO—Exhaust Gas Oxygen 
     EMVA—Electro-Mechanical Valve Actuation 
     ETC—Electronic Temperature Control 
     HEGO—Heated Exhaust Gas Oxygen 
     MAP—Manifold Absolute Pressure 
     OHC—Overhead Cam 
     PCV—Positive Crankcase Ventilation 
     PFI—Port Fuel Injection 
     SI—Spark Ignited 
     SIDI—Spark Ignition Direct-injection 
     SOHC—Single Overhead Cam 
     Stoichiometric combustion—An ideal combustion in which the fuel is completely burned. In a stoichiometric combustion, all the Carbon is converted to CO2, all hydrogen is converted to H2O, and all the sulfur is converted to SO2. 
     TWC—Three-Way Catalyst or Three-Way Catalytic converter 
     UEGO—Universal Exhaust Gas Oxygen 
     B. Introduction 
     The improved system and method of controlling a direct injection spark ignition engine is described. The improved system and method may use sensors that are found on existing DISI engines to indirectly measure the fuel level in the evaporated fuel vapors. The improved system and method allows the evaporated fuel vapors to be purged into the intake manifold when the engine is run lean. Thus, the engine operates longer in the more efficient stratified mode resulting in a lower fuel consumption. 
     The following description of the preferred embodiments of the invention is not intended to limit the scope of the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use the invention. 
     C. Direct Injection System 
     FIG. 1 illustrates an embodiment of a direct injection spark ignition (“DISI”) engine  100 . The DISI engine  100  may include such components as an accelerator pedal and sensor assembly  102 , a crank angle sensor  104 , a CAM position sensor  106 , an engine coolant sensor  108  or cylinder head temperature sensor  108 , a powertrain control module  110 , an engine control module  112 , a throttle control module  114 , an injector driver module  116 , a carbon canister  118 , a temperature/mass air flow meter  120 , a MAP sensor  122 , a vapor management valve  124 , an ignition control  126 , a fuel pump  128 , a fuel injector  130 , a fuel tank  132 , a spark plug  134 , an exhaust gas re-circulation (“EGR”) valve  136 , a high compression engine  138 , a universal exhaust gas oxygen (“UEGO”) sensor  140 , a three-way catalytic converter (“TWC”)  142 , a NOx trap/catalyst  144 , a catalyst temperature sensor (“CTS”)  146 , a heated exhaust gas oxygen (“HEGO”) sensor  148 , a throttle  150 , an intake manifold  152 , and an exhaust manifold  154 . 
     The evaporative fuel vapor system captures evaporative fuel vapors from the gasoline tank  132 , stores the evaporative fuel vapor in the canister  118 , and releases the vapors into the intake manifold  152  via the vapor management valve  124 . The vapor management valve  124  may be controlled by the engine controller  112 . The evaporative fuel vapors tend to be a homogeneous mixture of fuel vapors and air. The fuel level in the evaporative fuel vapors varies over time. The vapor management valve  124  may release the evaporative fuel vapors into the intake manifold  152 . Then the evaporative fuel vapors enter the combustion chamber via an intake valve. The fuel injector  130  injects fuel directly into the combustion chamber. The injected fuel and the evaporative fuel vapors mix before combustion. The exhaust gases resulting from the combustion are exhausted via an exhaust valve into the exhaust manifold  154 . The CTS  146  is located near the lean NOx trap/catalyst  144  and measures the temperature of the lean NOx trap/catalyst  144  during the cleansing of the wash coat of SOx to control poisoning. 
     The engine  100  is controlled by various engine controllers including the power train control module  110 . The power train control module  110  may in include such controllers as an engine control  112 , a throttle control  114 , and an injector driver  116 . For example, the power train control module  110  may include a canister purge valve controller for a direct injection spark ignited internal combustion engine that includes a temperature sensor interface that receives a temperature of a catalyst in the engine&#39;s exhaust system, an engine mode controller that switches the engine between a stratified mode and a homogeneous mode, and a canister purge valve interface that controls a flow rate of evaporated fuel vapors into the engine&#39;s intake manifold via a canister purge valve. The canister purge valve controller may control the flow rate of evaporated fuel vapors as a function of the temperature received by the temperature sensor interface and the engine&#39;s mode. 
     The components of the DISl engine  100  may include various variations of the devices described above. The fuel pump  128  may include a high pressure fuel pump, an electronically controlled pressure regulator, and a pressure sensor assembly. The fuel injector  130  may include a high pressure fuel rail assembly. The fuel tank  132  may include a feed pump. The EGR valve  136  may include an electric EGR valve. The engine  138  may include multiple cylinders, each with a direct injection fuel injector. The TWC  142  may include a quick light-off TWC. The NOx trap/catalyst  144  may include a lean NOx trap, a lean NOx catalyst, or other device to reduce NOx in the exhaust gas. The throttle  150  may include an electronic controlled throttle. Other variations in the components of the DISI engine  100  will also be apparent and within the scope of the invention. 
     FIG. 2 illustrates a flow diagram of a method of controlling the DISI engine  100  of FIG.  1 . 
     In block  202 , the CTS  146  (FIG. 1) is tested to determine if it is operating within calibration limits without evaporated fuel vapors being introduced into the combustion chamber. The engine  100  (FIG. 1) may be in the homogeneous mode or the stratified mode during the calibration. The calibration includes comparing the catalyst temperature as measured by the CTS to an expected range. If the engine  100  is outside the calibration range, the engine  100  may be operated in the stratified mode with the evaporative fuel vapors turned off and in the homogeneous mode with the evaporative fuel vapors on or off. The activities in block  204  to  220  are not executed when the engine  100  is operating outside the calibration range. 
     In block  204 , if the engine  100  is in the homogeneous mode, the engine  100  is switched to the stratified mode. The engine control  112  (FIG. 1) may control the mode of operation of the engine  100 . 
     In block  206 , the evaporated fuel vapors from the carbon canister  118  (FIG. 1) are allowed to enter the intake manifold  152  (FIG.  1 ). The flow rate of the evaporated fuel vapors may be controlled by the vapor management valve  124  (FIG. 1) and the engine control  112 . 
     In block  208 , the temperature of the catalyst is measured when the evaporated fuel vapors are being introduced into the combustion chamber. The temperature of the catalyst may be measured by the CTS  146 . The catalyst temperature should rise after the evaporated fuel vapors are introduced due to the oxidation of the unburned fuel in the exhaust gas. The catalyst temperature change can be determined as the temperature difference between the temperature measured in block  208  and the temperature measured in block  202  or as the temperature difference between the temperature measured in block  208  and an expected temperature based on a temperature model. 
     In block  210 , if the catalyst temperature change from block  208  is within an acceptable range, the engine can remain in the stratified mode with the evaporated fuel vapors being introduced, and the fuel level of the evaporated fuel vapors can be determined as a function of the temperature change measured in block  208 . The acceptable range may have different upper and lower thresholds depending on various factors. 
     In block  214 , if the catalyst temperature change from block  208  is above the acceptable range, the evaporated fuel vapors contain too much fuel. The flow rate of the evaporated fuel vapors may be adjusted, normally reduced, to allow some of the evaporated fuel vapors to be purged while maintaining the engine  100  in the stratified mode. The flow rate can be repeatedly reduced in block  214  until the catalyst temperature is within an acceptable range. Alternatively, the engine  100  could be switched to the homogeneous mode with the evaporated fuel vapors being purged at the same rate until the fuel level in the evaporated fuel vapors is reduced to an acceptable level for operation in the stratified mode. 
     In block  212 , if the catalyst temperature change from block  208  is below the acceptable range, the fuel level in the evaporated fuel vapors is either low or very high. If the fuel level in the evaporated fuel vapors is very high, the entire stratified charge becomes combustible. To determine whether the fuel level in the evaporated fuel vapors is low or very high, the engine  100  is switched to the homogeneous mode, then block  218  is executed. 
     In block  218 , after the engine  100  is switched to the homogeneous mode in block  212 , the fuel level of the evaporated fuel vapors can be accurately measured via an EGO sensor, for example an UEGO. Then either block  216  or  218  is executed based on whether the EGO sensor indicates a high or low level of fuel in the evaporative fuel vapors. 
     In block  216 , if the fuel level of the evaporated fuel vapors measured in block  218  is below a fuel level threshold, the engine is switched back to the stratified mode. Then, the purge fuel level is determined as a function of the catalyst temperature in block  210 . 
     In block  220 , if the fuel level of the evaporated fuel vapors measured in block  218  is above the fuel level threshold, the engine is switched back to the stratified mode. Then, the flow rate of the evaporated fuel vapors is reduced in block  214 . Alternatively, the engine  100  could be operated in the homogeneous mode with the evaporated fuel vapors being purged at the same rate until the fuel level in the evaporated fuel vapors is reduced to an acceptable level for operation in the stratified mode. 
     The process or portions of the process described in blocks  202  to  220  may be repeated a number of times. Further, the process described in blocks  202  to  220  may be a component of a larger process that cycles the engine between the stratified mode and the homogeneous mode based on other factors, such as engine load, acceleration, and other factors. 
     As a person skilled in the art will recognize from the previous description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of the invention defined in the following claims.