Method of controlling fuel injection in an internal combustion engine

Abstract

A multi-cylinder internal combustion engine comprises fuel injectors arranged to fuel cylinders, a lambda sensor positioned to respond to exhaust gases generated by the cylinders and a control unit th

Description

Technical field The present invention generally relates to the control of gaseous fuel internal combustion engines, more specifically to a method of controlling injection in an internal combustion engine operating on gaseous fuel. Background Art On gasoline and hydrogen internal combustion engines, the combustion stability and the efficiency of after treatment systems (e.g. three-way catalyst) are highly sensitive to the air-fuel ratio. As a consequence, first of all, the injected fuel mass demand is continuously adapted to fit the estimated amount of air trapped in the cylinder. Secondly, a lambda control strategy adjusts the actual fuel mass injected in the cylinders using the feedback from a lambda sensor responsive of air fuel-ratio in the exhaust gases. This is done in closed loop and hence controls the average lambda, i.e. for the entire engine. Finally, a so-called Individual Cylinder Fuel Control, ICFC, strategy is designed to eliminate or reduce the imbalance in air-fuel ratio (lambda) from cylinder to cylinder. The ICFC function is configured to sample the lambda sensor signal twice per firing event at given angular positions, extract a raw stream of cylinder imbalance data and align each cylinder with the data. As on the one hand, the measurements are taken at fixed points in the engine cycle, but, on the other hand the cause / effect time delay depends on several parameters (exhaust flow, sensor lag dynamics...) the engine operating range is divided into cells (engine speed x engine load) and the data alignment function is calibrated on each of them. A known ICFC control strategy for gasoline engines is disclosed for example in US 6,382,198. General Description of the Invention The present invention aims to provide an improved ICFC strategy that is more reliable than the conventional approach described in US 6,382,198. The invention relates to a method of controlling fuel injection in a multi-cylinder internal combustion engine as claimed in claim 1. The engine comprises fuel injectors arranged to fuel engine cylinders and a lambda sensor positioned to respond to exhaust gases generated by the cylinders. The fuel injectors are operated to discharge into the engine cylinders fuel quantities corresponding to predetermined fuel commands. In general, a respective fuel injector is associated to each cylinder to discharge the fuel amount required for the respective combustion cycle. The fuel command is computed from a base fuel amount adjusted by a fuel correction factor determined by an imbalance control function. The imbalance control function (i.e. IFCF control) comprises the steps of: - sampling the lambda sensor signal at least twice per firing event; - filtering the sensor data to extract a series of cylinder imbalance data; - aligning the cylinder imbalance data with the corresponding cylinder and accordingly developing the respective fuel correction factor for each engine cylinder. It will be appreciated that the sampling of the lambda sensor signal is done at predetermined angular timings that are mapped in function of one or more engine parameters, said parameters including engine speed and load. The present invention is directed to an improved internal combustion engine fuel control wherein a lambda sensor responsive to the combined exhaust gas flow of a bank of engine cylinders is used to develop individual, i.e. cylinder specific correction factors. This can be achieved using a single lambda sensor arranged in the exhaust system. The lambda sensor is configured to present a sensor signal responsive to the air to fuel ratio in the exhaust gas. Any appropriate sensor technology may be used. Depending on the technology, the sensor signal can be a current or voltage that is representative of the air fuel ratio; it may be generated as an output signal or it may correspond to an internal operating signal of the sensor that can be sensed / measured. In summary, the inventive method the lambda sensor signal is sampled in synchronism with firing events and filtered to form a measure of the air fuel imbalance with respect to time. The cylinder imbalance data are then aligned with the corresponding cylinders and the respective fuel correction factor are developed, which are used to reduce the imbalance. A merit of the present invention is to have identified that the synchronized reading of the lambda sensor at fixed angular positions may lead to situations where the samples / measurements are less significant for an operating point or range, i.e. the imbalance is less observable. Furthermore, with fixed angular readings, the pairing between sample and cylinder changes depending on the operating point; and requires under certain conditions deactivating the ICFC function. By contrast, the use of variable sampling timings will allow calibrating the sampling timings to keep the cylinder correspondence across the engine operating range. This is beneficial as it permits improving the relevance of the signal sampling and hence results in a more reliable ICFC function with more accurate correction factors. Conventionally, firing event generally refers to a combustion event occurring in a respective cylinder. The firing event is part of the power stroke of the combustion cycle. As the cylinders in an engine bank are connected to the crankshaft, the firing event of each cylinder can be related to a given angular segment. For example, for an engine having N cylinders in a bank that are sequentially fired, each firing event spreads over an angular segment of 720° / N. According to the invention, the sampling is done at least twice per firing event at variable timings, i.e. the method adapts the angle of the synchronous measurements in the engine cycle. The sampling timings are defined by calibration maps in function of engine speed and load. In particular, the maps of the sampling timings are developed based on extensive calibration, in order to improve detection of imbalance. This capability allows better operation of the lambda sensor, in particular in case of wide range oxygen sensors. The higher reliability of the ICFC function is desirable to meet emission standards, to improve combustion control. Although beneficial on all combustion engines, a stable combustion is even more critical on hydrogen combustion engines where deviation from nominal parameters may rapidly increase NOx emissions (too rich) or cause misfires (too lean). The engine speed, i.e. the crankshaft revolution speed, can be measured by any appropriate means. It may be conveniently done by means of a toothed wheel attached to the crankshaft and associated magnetic sensor. The term load is to be generally understood in its convention meaning, representing the demand place on the engine at any given moment. In particular, the load can be defined by one or more parameters related to air management, for example intake manifold air pressure, mass air flow, throttle position, charge air mass, or charge air ratio (i.e. ratio of inducted air mass over the corresponding air mass at standard pressure and temperature). In modern ECUs, a calculated load value may be available, being calculated from a combination of sensors and inputs. Hence, the term load is to be understood as a parameter corresponding to one of these parameters, or calculated based on one or more of these parameters. Advantageously, the lambda sensor is also used to control the overall or average air / fuel ratio. Accordingly, the fuel command is further computed from a global fuel correction amount Cg that is determined based on the lambda sensor signal for the entire engine. The lambda sensor may in particular be an oxygen sensor. In embodiments, the oxygen sensor may be of the so-called binary type, type having an output signal that switches or toggles between first and second states corresponding to lean and rich conditions of the sensed exhaust gas, relative to a stoichiometric air / fuel ratio. Alternatively, the oxygen sensor may be a so-called wide range air fuel (WRAF) sensor, or wide band oxygen sensor, where the relevant signal is the pumping current, which is proportional to the oxygen concentration in the exhaust and hence representative of lambda. The pumping current is thus the sensor signal in the context of the invention. The present invention is applicable to any internal combustion engine, operating on liquid fuel (gasoline, diesel, biofuels, synthetic fuels, tec.) or gaseous fuel (CNG, hydrogen...). These and other features of the invention are also recited in the appended dependent claims. According to another aspect, the invention relates to a multi-cylinder internal combustion engine comprising fuel injectors arranged to fuel cylinders and a lambda sensor positioned to respond to exhaust gases generated by the cylinders, and a control unit configured to implement the method according to any of the preceding claims. In particular the engine may comprise a single lambda sensor in the exhaust system. The features, embodiments and techniques disclosed in relation to the inventive method also apply to the inventive engine. Brief Description of the Drawings Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein: Fig. 1 is a functional diagram implementing an embodiment of the present method; and Fig. 2A and Fig.2B are plots of lambda sensor signal vs. crank angle without and with imbalance. Description of Preferred Embodiments The present invention has been developed for application to gasoline internal combustion engines to improve cylinder imbalance detection and correction. The diagram of Fig.1 shows an implementation of the present method. The functions and equipment shown in Fig.1 are generally known and will only be briefly explained. However, as already discussed herein, a merit of the present invention is to use a variable timing for the sampling of the lambda sensor signal. The present invention relates to fuel control strategies in internal combustion engines operating with liquid or gaseous fuel. The fuel delivery system typically comprises a fuel tank, e.g. containing gasoline, a fuel rail and a plurality of fuel injectors for selective fuel injection into the engine. The fuel injectors may be coupled directly to the fuel rail via so-called sockets, or indirectly via tubes. Conventionally, one fuel injector is provided per cylinder, either in direct injection (DI) configuration (fuel is introduced directly into the cylinder I combustion chamber) or in PFI configuration (the injector is arranged to discharge fuel upstream of the intake valve(s)). As is known in the art, the engine comprises an engine block with a plurality of cylinders with associated reciprocating pistons mechanically coupled to a crankshaft. At least one fuel injector is provided per cylinder -according to the mentioned configuration PFI or DI- to inject fuel to be combusted in the respective combustion chamber and generate torque. Each cylinder comprises at least one intake valve for admitting fresh air and at least one exhaust valve for discharging combustion gases. Introduction of fuel in a given cylinder, i.e. cylinder fueling, is performed during an injection event, by applying a drive signal to the fuel injector to activate an electromechanical actuator, typically a solenoid actuator, to cause the injector to open during a predetermined time period. Much simplified, injection control strategies use mappings (known as calibrated flow curves) that relate the fuel quantity to the injector actuation time that is referred to as pulse width, PW. To perform an injection event, a drive pulse is applied during a time period PW to discharge a corresponding fuel amount. Conventionally, injection control strategies are programmed in the Engine Control Unit, ECU, that receives various signals indicating the state of the engine from various sensors, and is, inter alia, configured to determine a fuel quantity to be injected and a corresponding timing of injection. More specifically, the ECU is configured to determine a desired fuel quantity to be injected to achieve a given torque demand, and subsequently determines the injector control signal (injector actuation PW) corresponding to the desired fuel quantity. It may be noted that most of the functionalities and techniques (and their implementation) used in the inventive method are known in the art and will not be explained in detail. In general, the method according to the present disclosure can be implemented by hardware and / or software. In practice, it may conveniently be implemented by a control unit comprising a processor and a memory, such as e.g. an Engine Control Unit. In such case the memory may contain instructions which, when executed by the ECU / processor, cause the latter to carry out the present method. Turning now to Fig.1, engine components are represented in box 10 and comprise: - injectors 12 with associated drive unit, configured to selectively inject / discharge predetermined fuel quantities into the associated cylinders. The drive unit is conventionally designed to apply the control signals to the injector to discharge a fuel quantity corresponding to the respective fuel command for each cylinder combustion cycle; - an engine 14 with cylinders defining the combustion chamber; - an exhaust system 16 with an exhaust manifold and exhaust pipe, which collects and directs exhaust gases from the engine cylinders out of the vehicle, aiding in efficient engine performance and emission control; - a lambda sensor 18 responsive to the air-fuel ratio in exhaust gasses in the exhaust system 16. Specifically, the engine comprises a single lambda sensor 18 arranged in the exhaust system. The lambda sensor 18 may conventionally be an oxygen sensor, namely a wide-range oxygen sensor (also wide range or UEGO sensor) where the sensor signal (i.e. signal representing the oxygen concentration) corresponds to the pumping current. For the sake of exemplification, Fig.2A and Fig.2B are plots of the lambda sensor signal vs. crank angle. The vertical axis represents the pumping current of the wide range type oxygen sensor, which varies in amplitude in relation to the oxygen concentration, and thus representative of lambda. Hence, it is not required to actually determine the lambda values, but one can readily use the sensor signal values. In accordance with the present invention, the sensor signal is sampled in synchronism with the engine firing events. In this embodiment, the sensor signal is sampled twice per firing event, at predetermined timings Ts,i to Ts,s. In the present method, the timings Ts,i are variable -as indicated by the arrows- and mapped in function of engine speed and load, as will be explained in detail below. As it will appear to those skilled in the art, the plot of Fig.2A corresponds to a globally stable and flat sensor signal with no substantial imbalance, whereas the plot of Fig.2B illustrates a manifest imbalance situation. Back to Fig.1, reference sign 20 indicates a fuel adjusting function, which outputs the fuel command Qc to the injectors drive unit 12. The drive unit 12 operates each injector by applying control signals thereto in such a way as to discharge the fuel amount Qc through one or more injection events for a given combustion cycle. In case cylinder fueling is done in one injection event, then Qc is converted into a drive signal having a duration PW using the above-mentioned flow-curves. The fuel command Qc is computed by fuel adjusting function 20 based on a base fuel amount Qb, on a global correcting factor Cg and on a cylinder-specific correcting factor Ci. The fuel determination is thus cylinder specific and can be written as: Qc = Qb x Cg x Ci [Eq.1] The base fuel amount Qb may typically originate from a conventional fuel determination structure, not shown here. Typically, a driver torque function receives torque demands from various components, for example direct torque demand from the driver (accelerator pedal) or indirect torque demand via cruise control, torque demands from the transmission system, from driving dynamics, from the gearbox or torque demands related to specific components (e.g. accessory torque). The driver torque demand function coordinates these various demands and generates a global torque demand Td. This global torque demand Td may be limited / capped by a maximum torque Tmax that may depend on various factors. This function outputs a gross indicated torque demand Togross. A desired fuel mass Qd (or fuel demand) is then determined to meet the torque request Togross, typically by calculation based on IMEP (Indicated Mean Effective Pressure), cylinder volume and combustion efficiency coefficients. This is typically based on calibrated mappings. In parallel, a lambda set point is determined, referred to as desired lambda Ld, in function of the current operating point (engine speed, load...). As is known, the Lambda number determines the mass ratio of air and fuel in the combustion chamber, in regards to the stoichiometric air-fuel ratio. The lambda setpoint may be determined to optimize combustion efficiency, combustion stability and pollutant emissions (NOx). A desired air mass Md (representing the air mass desired in the cylinder) is computed based on the desired fuel mass Qd and taking into account the desired lambda Ld. The throttle and turbocharger gate positions are adjusted on the basis of the desired air Md. It may be noted that since the air loop is typically much slower than the fuel loop, the calculation of the fuel mass to be injected is done on the basis of lambda desired Ld as well as, advantageously, on the basis of the air mass Mf (or fresh air flow) actually entering the cylinder (hence not on the Md). Air mass Mf can be estimated based on the intake manifold pressure and temperature and volumetric efficiency. This logic privileges the respect of the air / fuel ratio and is advantageous during transitory regimes where the inducted air mass can significantly vary from the computed air mass Md. Finally, the base fuel amount Qb is determined as the fuel quantity to be injected in the next scheduled injection event I combustion. The base fuel amount Qb is advantageously determined taking into account the lambda setpoint (desired lambda Ld) and on the basis of the fresh air flow Mf. In Eq.1, the global correcting factor Cg is a factor determined from an average closed loop lambda control module 19 based on the sensor signal. This is a conventional function where the lambda sensor signal is read asynchronously, e.g. every 10 ms. The global correcting factor Cg is determined by a proportional integral controller function based on the deviation from the lambda setpoint. The cylinder-specific correcting factor Ci is determined by the ICFC function detailed below. Box 40 relates to the ICFC function and comprises: - a data extraction function 42; - a filtering function 44: - a data-cylinder alignment function 46; and - a controller function 48 that develops the cylinder specific fuel correction factors Ci. The data extraction function 42 is configured to receive the signal from the lambda sensor 18 and sample it in synchronism with the engine firing events. According to the invention, the sampling is not done at fixed timings. Instead, the sampling is done at predetermined (angular) timings Ts.i in the engine cycle (twice or more), which are mapped in function of engine speed and load. This is shown in Fig. 1, where it can be seen that the sampling timings Ts,i are determined from a mapping 43 in function of RPM and load. In an example, two samples are taken per firing event. Accordingly, for a 4-cylinder engine, a collection of 8 samples is obtained per engine cycle. The output of function 42 thus forms a time-varying raw imbalance signal (or stream). The filtering function 44 is configured as a high-pass filter to provide signal conditioning. As is known in the art, such filtering allows removing low-frequency drift or design lambda variations induced by engine control strategies . The filtering function 44 may comprise a second-order Butterworth filter. Filtering function 44 receives the raw imbalance signal and hence outputs a filtered imbalance signal (or stream). The filtered imbalance signal (which comprises eight samples per engine cycle) is applied as input to the alignment function 46, which is designed to perform cylinder alignment. That is, alignment function 46 assigns the filtered samples to the respective cylinders. In the present variant, only one sample per cylinder is kept per cylinder. In the alignment function 46, the filtered signal is parsed into an array of imbalance values. The imbalance values, in turn, are applied as inputs to a so-called Cause-Effect Alignment sub-function, which associates the imbalance values with individual engine cylinders based on MAP vs. RPM engine operating point, producing cylinder-specific imbalance values. Next, the imbalance values are applied as input to the ICFC Controller function 48, which is configured to develop the cylinder-specific correction factors C1 ... C4 for the respective engine cylinders so as to reduce the imbalance in each cylinder while preserving the overall or average air / fuel ratio established by the closed-loop control. When it appears that the correction factors Ci have settled, the correction factors Ci are stored in an adaptive learn table 52 for use by ICFC Controller 48. As indicated, the correction factors Ci are stored in Table 52 based on Load vs. RPM engine operating point. In general, the controller 48 may be designed as a PI controller including feed-forward, proportional feedback and integral feedback. The controller 48 is thus designed to develop the correction factors Ci using PI functions, based on the determined cylinder imbalance data and reference correction factors (i.e. the correction factors Ci determined previously and stored in table 52). A mode controller 50, dependent on engine speed and load, determines enabling conditions for the controller 48. For example, the operation of controller 48, i.e. the learning of correction factors Ci, may be disabled when the load is too low and / or engine speed is too high, or before the lambda sensor is functional (reached operating temperature). The filtering 44, alignment 46 and controller 48 functions have only been briefly discussed as they are not the focus of the invention and any appropriate alignment / control approach may be used with the inventive method. However, such functions are described in detail in US 6,382,198, and can be applied in the context of the invention.

Claims

1. A method of controlling fuel injection in a multi-cylinder internal combustion engine comprising fuel injectors arranged to fuel cylinders and a lambda sensor positioned to respond to exhaust gases generated by the cylinders,wherein the fuel injectors are operated to discharge into engine cylinders fuel quantities corresponding to predetermined fuel commands (Qc);wherein the fuel command (Qc) is computed from a base fuel amount (Qb) adjusted by a fuel correction factor (Ci) determined by an imbalance control function;wherein the imbalance control function (40) comprises the steps of:sampling (42) the lambda sensor signal at least twice per firing event;filtering (44) the sensor data to extract a series of cylinder imbalance data;aligning (46) the cylinder imbalance data with the corresponding cylinder and accordingly developing (48) the respective fuel correction factor (Ci) for each engine cylinder;characterized in thatsaid sampling of the lambda sensor signal is done at predetermined angular timings (Ts,i) that are mapped (43) in function of one or more engine parameters, said parameters including engine speed and load.

2. The method according to claim 1, wherein said fuel command is further computed from a global fuel correction amount (Cg) that is determined based on the lambda sensor signal for the entire engine.

3. The method according to claim 1 or 2, wherein the step of aligning comprises processing the cylinder imbalance data based on predetermined models in function of engine speed and load to determine cylinder-specific imbalance values.

4. The method according to claim 3, wherein the fuel correction factors (Ci) are developed based on the the cylinder-specific imbalance values and on previously learned fuel correction factors.

5. The method according to claim 4, wherein the fuel correction factors (Ci) are developed to reduce the imbalance in each respective cylinder while preserving the air-fuel ratio established by the determination of the base fuel amount.

6. The method according to any of the preceding claims, wherein the base fuel amount (Qb) is determined based on torque demand, fresh air mass and lambda setpoint.

7. The method according to any of the preceding claims, wherein said filtering comprises filtering by a high-pass filter.

8. The method according to claim 7, wherein filtering is dependent on engine speed.

9. The method according to any of the preceding claims, wherein the lambda sensor is an oxygen sensor.

10. The method according to any of the preceding claims, wherein said engine load is based on one or more of an intake manifold air pressure, a mass air flow, a throttle position, a charge air mass, a charge air ratio, a calculated load value. .

11. A multi-cylinder internal combustion engine comprising fuel injectors arranged to fuel cylinders and a lambda sensor positioned to respond to exhaust gases generated by the cylinders, and a control unit configured to implement the method according to any of the preceding claims.

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