Fuel Control Method and System for Engine With Variable Cam Timing

This invention relates generally to fuel injection systems for internal combustion engines and more particularly to a method of accurately predicting cylinder air charge value in engines with variable cam timing in order to more accurately control the air/fuel mixture introduced into the engine.

Fuel injection systems for internal combustion engines typically include a mass air flow sensor for sensing air flow into the engine intake manifold. Data from the sensor is used to calculate a cylinder air charge from which the fuel injector pulse width needed to achieve a desired air/fuel ratio is determined. One prior art approach is disclosed in Follmer et al U.S. Pat. No. 5,159,914, assigned to the assignee of the present invention, and incorporated herein by reference. In Follmer et al a hot wire mass airflow sensor provides raw airflow rate data to the computer. The computer corrects the raw data to compensate for airflow leakage in the intake system. The compensated airflow data is used in a manifold filling model that modifies the data to account for the effects of engine load on volumetric efficiency of the engine. The computer calculates the cylinder air charge utilizing the modified data from the manifold filing model. After cylinder air charge is determined, the fuel injector pulse width necessary to achieve a desired air/fuel ratio is determined. Another method of predicting cylinder air charge is disclosed in U.S. Ser. No. 08/016,322 filed Feb. 10, 1993, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.

One measure taken to improve fuel efficiency and reduce harmful emissions in internal combustion engine is to vary the timing or relative phase angle between the camshaft and crankshaft to achieve more optimum engine operation at various speed/load conditions. One approach to phaseshift control is disclosed in Maurer et al U.S. Pat. No. 5,209,202 assigned to the assignee of the present invention and incorporated herein by reference. That phase shift control uses a hydraulic device which rotationally couples a camshaft drive sprocket to a camshaft flange. The engine control computer in the aforementioned patent responds to continuously varying engine operating conditions to control the hydraulic device and set the desired relative phase angle for the condition existing at any point in time. A suitable hydraulic device is disclosed in Schechter et al U.S. Pat. No. 5,117,784 assigned to the assignee of the present invention and incorporated herein by reference.

Volumetric efficiency (Veff) describes how effective an engine is at filling a cylinder and is expressed as a percentage of total cylinder capacity. Because the cylinder does not totally fill up with air and fuel, Veff typically varies from 50% to 85% of the total cylinder capacity. Veff is influenced by such factors as cylinder displacement, piston stroke, air intake geometry, and valve timing. The intake system, in particular the intake valve, restricts the amount of air which an engine can induct. Varying the cam timing in response to continuously varying engine operating conditions as discussed above affects the time when the intake valve opens and closes and thus changes the Veff of the engine. In order to maintain or improve fuel economy gains and the reductions in harmful emissions obtained with variable cam timing, it is desirable that the strategy for calculating cylinder air charge, and thus fuel pulse width, account for the changes in Veff which occur when cam timing is varied.

In accordance with the present invention the calculation of cylinder air charge, and thus fuel injector pulse width, is dependent on the instantaneous phase angle of the camshaft relative to the crankshaft. In particular, the calculation of the coefficient of manifold filling to be used in a manifold filling model is a function of calibration values which reflect the effect of cam phase angle and engine speed on Veff. The manifold filling model uses mass air flow sensor data which has been compensated for airflow leakage in the intake system. The manifold filling model modifies the compensated air flow data in accordance with the coefficient of manifold filling and thus reflects the relationship between Veff and the existing cam phase angle. The air charge computed by the manifold filling model is used in predicting a cylinder air charge to be used in calculating an injector pulse width to achieve a desired air/fuel ratio.

A more complete understanding of the present invention may be had from the following detailed description in which:

FIG. 1 is a block diagram of the apparatus of the invention; and

FIG. 2 is a flow chart of the software used to control the computer in FIG. 1.

Referring now to the drawings and initially to FIG. 1, a block diagram of the control system of the present invention is shown. Variable cam timing (VCT) solenoids 10 and 12 are employed to position respective pistons in a suitable hydraulic device to set the phase angle of the left and right camshafts respectively, relative to the crankshaft, of an internal combustion engine generally designated 14. The solenoid 10 is energized from a d-c source, not shown, under the control of a constant frequency variable duty cycle pulse width modulated (PWM) output signal VCT1 from an engine control computer generally designated 16. The computer 16 includes the usual input/output (I/O) section, read only memory (ROM), random access memory (RAM), and central processor unit (CPU). The solenoid 12 is controlled from a PWM output signal VCT2. Position sensors 18 and 20, such as disclosed in the aforementioned patent application, provide closed loop feedback control signals indicative of actual camshaft phase angle, to inputs LCAM and RCAM of the computer 16. One of the sensors, for example 18 may provide a cylinder identification signal at input CID of computer 16. A profile ignition pickup (PIP) signal provides crankshaft position, and thus engine speed, information to the computer 16 at input PIP and also provides the timing/interrupt signals to initiate airflow and fuel control calculations. The PIP signal, as discussed in the aforementioned patent application, may be generated in response to the output of a crankshaft sensor of conventional design. The computer 16 uses the CID, PIP, LCAM, and RCAM data as well as other engine operating parameter to compute the optimum VCT1 and VCT2 outputs in order to improve engine idle, increase engine torque, and aid in the reduction of harmful emissions. A PID control routine is performed in the computer 16 on the difference between the desired camshaft phase angle commanded and the actual camshaft phase angle measured in order reduce any error.

The computer 16 provides a fuel pulse width output (FUELPW) to the engine fuel injection system as will be described below in connection with various software modules which control the operation of the computer 16. Conventional sensors (not shown) for monitoring engine coolant temperature (ECT), air charge temperature (ACT), throttle position (TP), engine speed (RPM), and mass air flow (MAF) provide inputs to the computer 16 for use in determining the output FUELPW. The various analog inputs are periodically read under program control and converted to digital words by an A/D converter and stored in RAM for use by the CPU in calculating a digital word which defines FUELPW for each cylinder. FUELPW is converted to a variable duty cycle pulse width modulated control signal and supplied to a conventional fuel injection system including a plurality of fuel injectors.

Referring now to FIG. 2, a flowchart of the air and fuel calculation is shown. The output of the MAF sensor is converted to a digital value, and stored in RAM of computer 16. A Current Air Charge Calculation module 30 integrates the MAF data over two sample periods to obtain a current integrated air mass value AM.sub.-- INT[t]. An air mass amount, AIR.sub.-- CHG.sub.-- COMP, represents the sum of the air leakage in the intake manifold i.e. air flow not measured by the MAF sensor, and airmeter drift components. AIR.sub.-- CHG.sub.-- COMP is calculated and added to the value AM.sub.-- INT[t] to obtain a current air charge, AIR.sub.-- CHG.sub.-- CUR[t] where [t] is used to identify the current integrated air charge value. The previous integrated air charge calculation, is hereinafter referred to as AIR.sub.-- CHG.sub.-- CUR[t-1].

A Cam Angle Sensing and Selection module 32 reads the LCAM and RCAM inputs representing the sensed phase angle of the left and right engine banks and selects the input having the greater angle i.e. the one which is most retarded relative to the crankshaft. This selection results in the slowest time constant for the manifold filling model. In other words, using the most retarded phase angle produces a smaller manifold filling coefficient and therefore a slower response to changes in air flow. The digital word corresponding to the angle so selected is designated AIR.sub.-- PHASE and is stored in the RAM of the computer 16.

An Air Charge Background Calculation module 34 performs two table look-up functions using the values of AIR.sub.-- PHASE and RPM stored in RAM. The values contained in the tables relate to the slope, SL, and offset to be used in calculating an inferred manifold absolute pressure value, INF.sub.-- MAP, for the cam angle and engine speed of interest. The value of the constants stored in the look-up tables are obtained from tests performed on the vehicle of interest, to determine the effect of variation in cam phase angle over a range of engine speeds on the manifold absolute pressure and thus volumetric efficiency. An interpolation routine will normally be performed on the values obtained from each look-up table to obtain the value for the speed and angle of interest. The offset obtained from the look-up table is preferably corrected for barometric pressure, BP, by a multiplier obtained from a third look-up table using BP as the input. The corrected offset is hereinafter referred to as OFF. Using these look-up table values, MAP may be inferred as follows:

Veff may be expressed as actual air charge at a given MAP corrected for air charge temperature (ACT) and engine coolant temperature (ECT) divided by the theoretical air charge if the cylinder were filled with air at the given MAP or:

where

Thus: ##EQU1## where ENGDIS refers to the engine displacement,

ENGCYL refers to the number of cylinder in the engine, and

FN(ACT,ECT) is a calibration constant obtained from a look-up table using as inputs the value of ACT and ECT.

A Manifold Filling module 36 calculates a manifold filling coefficient F[t] which is used as a multiplier to modify the AIR.sub.-- CHG.sub.-- CUR[t] value obtained from module 30, to obtain a corrected air charge value AIR.sub.-- CHG[t], to account for the change in volumetric efficiency that results from a change in cam phase angle. The background calculations performed by the module 34 are synchronous to the foreground calculations performed by the module 36 and subsequent module in the flow chart. The values calculated by module 34 are stored in RAM and are available for use by module 36 at the time AIR.sub.-- CHG.sub.-- CUR[t] is available from module 30. The module 34 stores the two values SL and OFF in RAM for use by the module 36, as well as a third value designated AIR.sub.-- NUM.sub.-- MULT which represents the constant 11781.34/(ENGDIS/ENGCYL) as modified by the multiplier FN(ACT,ECT). The module 36 uses these values to calculate: ##EQU2## where AIR.sub.-- CHG[t-1] is the previously calculated and stored value of AIR.sub.-- CHG[t]. The value of F[t] is stored as F[t-1] for use during the next calculation so that the volumetric efficiency changes from one event to the next are compensated. If F[t] is not equal to zero, then the module 36 calculates an air charge value corrected to reflect the existing cam phase angle in accordance with the following equation: ##EQU3## If F[t] is equal to zero, as during initialization, then:

The value AIR.sub.-- CHG[t] is used by an Anticipation Module 38 which predicts the air charge two events in the future in order to account for fuel scheduling delays. The calculations performed in module 38 are based on the assumption of linear changes in AIR.sub.-- CHG.sub.-- CUR[t] for the next event [t+1] and that no change occurs from that level for the following [t+2] event. The calculations performed in module 38 are also based on the assumption that the manifold filling coefficient F[t] remains the same for the next two events. Thus:

The anticipated air charge value AIR.sub.-- CHG[t+2] is stored in RAM and used by a Final Aircharge module 40 in determining a CYL.sub.-- AIR.sub.-- CHG. CYL.sub.-- AIR.sub.-- CHG is used by a Fuel Calculation module 42 in determining the cylinder fuel charge and the corresponding injector pulse width FUELPW, needed to achieve a desired air/fuel ratio. The anticipation performed by module 38, may produce an AIR.sub.-- CHG[t+2] value which is greater than the maximum possible engine air flow and therefore should not be used in calculating fuel flow. The module 40 compares AIR.sub.-- CHG[t+2] with a maximum anticipated air flow threshold value, AIR.sub.-- WOT.sub.-- ANT, calculated by module 34 using the engine standard air charge corrected for ACT, ECT, RPM, relative throttle position and barometric pressure. If AIR.sub.-- CHG[t+2] is greater than AIR.sub.-- WOT.sub.-- ANT, then CYL.sub.-- AIR.sub.-- CHG is set equal to the output of the manifold filling model, AIR.sub.-- CHG[t]. A second threshold value, AIR.sub.-- CHG.sub.-- WOT, which is the maximum air charge for the engine of interest, is used to limit CYL.sub.-- AIR.sub.-- CHG to the maximum engine air charge.

While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.