Patent Publication Number: US-4543937-A

Title: Method and apparatus for controlling fuel injection rate in internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for controlling the rate of fuel injection in internal combustion engine. More particularly, the invention is concerned with a method and apparatus for controlling the rate of fuel injection in an internal combustion engine having an intake passage, of a comparatively large length and a fuel injector adapted to inject fuel into the intake passage, so that the fuel is mixed with the intake air thereby forming an air-fuel mixture which is then induced into the combustion chambers of the engine. 
     2. Description of the Prior Art 
     Modern internal combustion engines of the type described above incorporate electronic fuel injection controllers. The electronic fuel injection controller is adapted to compute the basic fuel injection time duration, i.e. the basic valve opening time, in accordance with data such as, for example, absolute pressure in the intake pipe and engine speed, and to make various correction computations in accordance with the condition of the engine including the warming-up of the engine, transient state and so forth to determine the final fuel injection time duration. In operation, the fuel injector is opened at each one of predetermined crank angles to achieve so-called synchronous injection. 
     The correction which is conducted in accordance with the state of warming-up of the engine is usually referred to as &#34;warm-up incremental correction&#34;. When the cooling water temperature of the engine is below 70° C. for example, the warm-up incremental correction is made by measuring the instant cooling water temperature by multiplying the basic injection time by a warm-up incremental coefficient which is beforehand determined in relation to the cooling water temperature in such a manner that the coefficient value becomes smaller as the engine cooling water temperature gets higher. 
     On the other hand, a fuel injection control referred to as &#34;warm-up acceleration incremental correction&#34; is conducted when the engine is accelerated during warming-up, in accordance with the following process. The amount of the acceleration is determined, for example, as the amount of change in the intake pressure. A first value correction coefficient is selected in accordance with the measured value of the amount of change in the intake pressure. Then, a second value correction coefficient is determined in accordance with the measured water temperature. The basic injection time duration is corrected using a warm-up acceleration incremental coefficient which is determined on the basis of the first and second value correction coefficients. 
     When the engine is accelerated quickly between the successive synchronous injections, the response of the engine will be impaired if any fuel injection is not made until the aforementioned synchronous injection is effected. Therefore, in some engines, the fuel injection is made regardless of the crank angle when a need for quick acceleration of the engine is detected. This injection is referred to as &#34;asynchronous injection&#34;. The rate of fuel injection during asynchronous injection is determined in accordance with the degree of acceleration of the engine and the cooling water temperature. For instance, asynchronous basic injection time duration, which takes a greater value as the degree of engine acceleration is large, is determined in accordance with the detected degree of engine acceleration and the value of the determined asynchronous injection basic time duration is corrected in view of the cooling water temperature, thereby determining the final asynchronous injection time duration. The correction in view of the cooling water temperature is intended to improve the transient response characteristics of the engine by increasing the fuel injection rate in the cold state of the engine in which the fuel can hardly be evaporated. 
     In the internal combustion engine of the kind described, the evaporation of fuel depends on the temperature of the wall defining the intake passage between the fuel injector and the combustion chamber. From this point of view, it is preferred that the temperature of the wall surface of the intake passage between the fuel injector and the combustion chamber provide more relevant information as to the basis for various correcting operations, such as the warm-up incremental correction for determining the increment of fuel injection in accordance with the state of warming up of the engine, the determination of the increment for acceleration during warming up by the use of the second coefficient mentioned before, and the temperature compensation in the asynchronous injection. Namely, in the cold state of the engine, the evaporation of the fuel takes place only at a small rate. The increase of the fuel injection in the cold state, therefore, is made to ensure a sufficient amount of fuel to be induced into the engine thereby stabilizing the engine operation. As a matter of fact, however, the evaporation rate of fuel is directly affected by the temperature of the wall surface of the intake passage between the fuel injector and the combustion chamber of the engine. This is the reason why the various correcting operations in relation to temperature should be made on the basis of the temperature of the wall surface of the intake passage. 
     The second correction coefficient also is incorporated in view of the smaller fuel evaporation rate in the cold state of the engine, than in the normal operating condition of the engine. In the correcting operation making use of the second coefficient, therefore, it is preferable to use the temperature of the intake passage wall as the basis for the correction. 
     During the warming up of the engine at an extremely low ambient air temperature, the rise of the temperature of the intake passage wall downstream from the fuel injector lags behind the rise of the water temperature for a long period of time, so that the fuel evaporation rate is kept small for a considerably long time. Under such a condition, even when the asynchronous injection is made to cope with a demand for quick engine acceleration, the engine cannot respond to this demand because only a small amount of fuel is induced into the combustion chamber. 
     Thus, in the known electronic fuel injection controller in which the warm-up incremental correction, warm-up acceleration incremental correction, by the use of the second coefficient, and the asynchronous fuel injection are made on the basis of the cooling water temperature, it is impossible to optimize the rate of fuel supply to the combustion chamber. As a result, the driveability of the engine is possibly impaired, because the temperature rise of the intake passage wall downstream from the fuel injector lags behind the rise of the cooling water temperature for a long period of time, particularly during the warming up of the engine at extremely low ambient air temperature. 
     To obviate this problem, it has been proposed to circulate the heated cooling water through a riser formed on the outer wall surface of the intake passage to heat up the intake passage wall and, hence, the fuel thereby promoting the evaporation of the fuel. This proposal, however, cannot perfectly eliminate the above-stated problem, particularly when the ambient air temperature is very low. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary object of the invention to improve the driveability of the engine during warming up, particularly when the ambient air temperature is extremely low, thereby overcoming the above-described problem in the prior art. 
     It is a second object of the invention to improve the acceleration performance of the engine during warming up to overcome the above-described problem in the prior art. 
     The present inventors have confirmed through experiments that the engine temperature at the time of start up and, more precisely, the temperature of the intake air are the factors which materially determine the time length required for heating the intake passage wall surface, between the fuel injector and the combustion chamber, up to a predetermined temperature. The present invention has been accomplished on the basis of this discovery. 
     More specifically, according to one aspect of the invention, there is provided a method of controlling the rate of fuel injection in an internal combustion engine, the internal combustion engine having a fuel injector adapted to inject a fuel into an intake passage so as to be mixed with the intake air, thereby forming an air-fuel mixture which is then induced into a combustion chamber of the engine over a comparatively long distance along the intake passage. The method comprises the steps of: computing, in accordance with the engine speed and the load on the engine, a basic injection time duration for injecting the fuel in synchronism with the crank rotation angle; and correcting the basic injection time duration, i.e. the rate of synchronous fuel injection, during warming up of the engine by using, at least, a start temperature correction value which is selected in accordance with a first engine temperature detected at the time of start up of the engine and attenuated thereafter in accordance with the time elapsed after the start up of the engine, and a warm-up correction coefficient which is selected in accordance with a second engine temperature detected during the operation of the engine. 
     According to a second aspect of the invention, there is provided a method of controlling the fuel injection rate in an internal combustion engine, the internal combustion engine having a fuel injector adapted to inject a fuel into an intake passage so as to be mixed with the intake air, thereby forming an air-fuel mixture which is then induced into a combustion chamber of the engine over a comparatively long distance along the intake passage, the method comprising the steps of: computing, in accordance with the engine speed and the load on the engine, a basic injection time duration for injecting the fuel in synchronism with the crank rotation angle; and correcting the basic injection time duration, i.e. the rate of synchronous fuel injection, during acceleration of the engine while the same is being warmed up, by using, at least, a start temperature correction value which is selected in acordance with the engine temperature at the time of or immediately after the start up of the engine and attenuated thereafter in accordance with the time elapsed after the start up of the engine, a first warm-up acceleration correction coefficient selected in accordance with the degree of acceleration of the engine, and a second warm-up correction coefficient selected in accordance with the engine temperature during the operation of the engine. 
     According to a third aspect of the invention, there is provided a method of controlling the fuel injection rate in an internal combustion engine, the internal combustion engine having a fuel injector adapted to inject a fuel into an intake passage so as to be mixed with the intake air, thereby forming an air-fuel mixture which is then induced into a combustion chamber of the engine over a comparatively long distance along the intake passage, the method comprising the steps of: computing, in accordance with the engine speed and the load on the engine, a basic injection time duration; determining a start temperature correction value which is selected on the basis of the engine temperature at the time of or immediately after the start up of the engine and attenuated in accordance with the time elapsed after the start up of the engine, such that, the lower the engine temperature is at the time of start up, the greater the start temperature correction value is, and controlling the rate of asynchronous fuel injection conducted asynchronously with the crank rotation angle, in accordance with both the start temperature correction value and the condition of acceleration of the engine. 
     The invention as summarized above can produce a remarkable effect in that the acceleration characteristics of the engine are remarkably improved, particularly when the ambient air temperature is very low, without necessitating the detection of the temperature of the intake passage wall surface between the fuel injector and the combustion chamber and without being accompanied by problems such as an addition of a sensor, wiring or increasing the number of terminals of the control circuit. 
     These and other objects, features and advantages of the invention will become clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of an automotive internal combustion engine to which the present invention is applied; 
     FIG. 2 is a detailed block diagram of an example of the control circuit; 
     FIG. 3 is a flow chart of an example of the process for injecting the fuel; 
     FIG. 4 is a diagram showing an example of a map from which the basic injection time TP is read from the engine speed Ne and the intake pressure PM; 
     FIG. 5 is a flow chart showing an example of the process for determining the corrected fuel injection time duration; 
     FIG. 6 is a flow chart showing an example of the process for determining start temperature correction value ADD; 
     FIG. 7 is a graph showing the relationship between start intake air temperature THA and the start temperature correction value ADD; 
     FIG. 8 is a graph showing the atenuation of the start temperature correction value ADD in relation to time; 
     FIG. 9 is a flow chart showing an example of the process for processing of the intake pressure PM; 
     FIG. 10 is a diagram for explaining the steps of the process shown in FIG. 9; 
     FIG. 11 is a flow chart showing an example of the process for computing the warm-up incremental coefficient FWL; 
     FIG. 12 is a graph showing the relationship between the cooling water temperature THW and the warm-up correction coefficient FWLO; 
     FIG. 13 is a graph showing the relationship beween the engine speed Ne and the warm-up correction coefficient KWL; 
     FIG. 14 is a flow chart showing an example of the process for computing feedback correction coefficient FAF; 
     FIG. 15 is a time chart showing how the air-fuel ratio signal S7 and the correction coefficient FAF are changed in relation to time; 
     FIG. 16 is a flow chart showing an example of the computation of the warm-up acceleration incremental coefficient FTC; 
     FIG. 17 is a graph showing the amount DPM of change in the intake pressure and the warm-up acceleration correction coefficient FTCO; 
     FIG. 18 is a graph showing the relationship between the cooling water temperature THW and the warm-up acceleration correction coefficient KTC; 
     FIG. 19 is a time chart showing how the intake pressure PM, amount DPM of change of the intake pressure and correction coefficient FTCO are changed in relation to time; 
     FIG. 20 is a flow chart showing an example of the computation of the final injection time duration Fτ; 
     FIG. 21 is a graph showing the relationship between the battery voltage BV and voltage correction coefficient τV; 
     FIG. 22 is a flow chart showing an example of the computation of asynchronous injection; 
     FIG. 23 is a graph showing the relationship between the amount DDPM of change in the intake pressure and the asynchronous injection time duration TP ASY  ; and 
     FIG. 24 is a flow chart showing an example of the computation of the final injection time duration Fτ ASY . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be described hereinunder with reference to the accompanying drawings. 
     FIG. 1 shows the construction of an automotive internal combustion engine incorporating an electronic fuel injection controller in accordance with the invention. Referrning to this Figure, an air filter 1 is connected to the throttle body 5 through an inlet pipe 3. The throttle body 5 is provided at its upstream side with a fuel injector 7. An intake throttle valve 9 disposed at the downstream side of the fuel injector 7 is operatively connected to an acceleration pedal (not shown) so as to control the intake air flow rate in accordance with the position of the accelerator pedal (not shown). An absolute intake pressure sensor 11 disposed at the downstream side of the intake throttle valve 9 is adapted to sense the absolute pressure of the intake air at that portion. The intake throttle valve 9 is associated with various other parts such as the valve open position sensor for measuring the opening degree of the intake throttle valve 9, an idle switch 4 which takes on position only when the intake throttle valve 9 is fully closed, and a power switch 6 which is kept in on state when the opening degree of the intake throttle valve 9 exceeds a predetermined value such as, for example, 40°. 
     The throttle body 5 is connected to an intake manifold 13 having branch pipes leading to respective cylinders of the engine. The intake manifold 13 is provided with an intake air temperature sensor 15 adapted to sense the temperature of the intake air in the intake manifold 13. The intake manifold 13 is provided, on the bottom wall 13a at the upstream side of the branching point, with a riser portion 17 through which heated cooling water is circulated to heat the air-fuel mixture through the wall of the intake manifold. 
     A reference numeral 19 designates the body of the engine which is known per se. The engine is provided with a plurality of clinders 23, pistons 21 and cylinder heads 25 which in combination define combustion chambers 27 (only one of which is shown). Each cylinder is provided with an intake valve 29 through which the air-fuel mixture is introduced into the combustion chamber 27. The mixture is then ignited by a spark plug 31. During operation, the cylinder 23 and other associated parts are cooled by cooling water which is circulated through a water jacket 33 formed around the cylinder 23. The temperature of the cooling water in the water jacket 33 is sensed by a cooling water temperature sensor 37 atached to the outer wall of the clinder block 35. 
     Branch pipes of an exhaust manifold 39 are connected to the exhaust ports (not shown) formed in the cylinder heads 25 of respective cylinders 23. The exhaust manifold 39 is provided at its downstream end portions with O 2  sensors 41 adapted to sense the residual oxygen content in the exhaust gas. The exhaust manifold 39 is connected to an exhaust pipe 45 through a ternary catalyst 43. 
     The speed of the automobile is sensed by a vehicle speed sensor 49 which is attached to the final output shaft of a transmission 47 coupled to the body 19 of the engine. Reference numerals 51, 53 and 55 denote, respectively, a key switch, igniter and a distributer. The distributor 55 is provided with an Ne sensor 57 adapted to produce an on-off signal for each angle θ1 of crank rotation. It is possible to detect the engine speed and desired angular position of the crank from the output of the Ne sensor 57. A G sensor 59 which also is provided in the distributor 55 produces an on-off signal for each angle θ2 of crank rotation greater than the above-mentioned angle θ1. The discrimination or identification of the cylinders and detection of the top dead centers are made by processing the output signal from the G sensor 59. A reference numeral 60 designates a battery. 
     A control circuit 61 is connected to various sensors such as the valve position sensor 2, idle switch 4, power switch 6, intake pressure sensor 11, intake air temperature sensor 15, cooling water temperature sensor 37, O 2  sensor 41, vehicle speed sensor 49, key switch 51, Ne sensor 57, G sensor 59 and the battery 60. Thus, the control circuit 61 receives from these sensors various signals such as a throttle valve opening degree signal S1, idle signal S2, power signal S3, intake pressure signal S4, intake air temperature signal S5, water temperature signal S6, air-fuel ratio signal S7, vehicle speed signal S8, start signal S9, engine speed signal S10, cylinder identification signal S11 and the battery voltage signal S14. The control circuit 61 is connected also to the fuel injector 7 and the igniter 53 so that it can produce a fuel injection signal S12 and an ignition signal S13. 
     As shown in FIG. 2, the control circuit 61 has the following parts or constituents: a central processing unit (CPU) 61a for controlling various devices; read only memory (ROM) 61b in which are written various numerical values and programs; a random access memory (RAM) 61c having regions in which are written numerical values obtained in the course of computation, as well as flags; an A/D converter (ADC) 61d for converting analog input signal into digital signals; an input.output interface (I/O) 61e through which various digital signals are inputted into and outputted from the control circuit; a backup memory (BU-RAM) 61f adapted to be supplied with electric power from an auxiliary power source when the engine is not operating thereby holding the contents of the memory; and a BUS line 61g through which these constituents are connected to one another. Programs which will be described in detail later are written in the ROM 61b. 
     In the operation of the engine described above, fuel is injected in accordance with the flow chart shown in FIG. 3. More specifically, in a step P1, the engine speed Ne is read in the form of the engine speed signal S1 which is the reference position signal. At the same time, the intake pressure PM is read in the form of an intake pressure signal S4. In a step P2, the basic injection time duration TP is read from the map shown in FIG. 4 using the read values of the engine speed Ne and the intake pressure PM. In a step P3, a corrected injection time duration τ is determined through a computation which is conducted in accordance with the operating condition of the engine. 
     A detailed description will be made hereinunder as to the process for computing the corrected injection time duration τ in the step P3. 
     The injection time duration τ is generally obtainable from the folowing formula. 
     
         τ=TP×FWL×FAF×(1+FTC)×FTHA      (1) 
    
     where, 
     Tp: basic injection time duration 
     FWL: warm-up incremental coefficient 
     FAF: air-fuel ratio feedback coefficient 
     FTC: transient air-fuel ratio correction coefficient 
     FTHA: intake air temperature correction coefficient. 
     These coefficients are calculated in accordance with the τ operation routine shown in FIG. 5 and the injection time duration τ is determined using these coefficients. Namely, in a step P11, a calculation is made to determine the warm-up incremetal coefficient FWL, whereas, in a step P12, a calculation is made to determine the air-fuel ratio feedback correction coefficient FAF. In the next step P13, a calculation is made to determine the air-fuel ratio correction coefficient FTC in the transient period. Subsequently, a calculation of (THA+k) is made to determine the correction coefficient FTHA in a step P14. Finally, the calculation of the above-mentioned formula (1) is made in a step P15 to determine the injection time duration τ. 
     Before turning to the detailed explanation of calculation made in each of the steps P11 to P13, a description will be made as to an example of the routine for computing the start temperature correction value ADD and as to an example of the routine for processing the intake pressure PM, which are essential features of the first aspect of the present invention. 
     (Computation of Start Temperature Correction Value ADD) 
     FIG. 6 shows the routine for computing the correction value ADD. As this routine is started at a predetermined timing, a judgement is made in a step P21 as to whether the engine is being started, making use of the engine speed signal S10. If the answer is affirmative, i.e. if the engine is being started, the start intake air temperature THA is read as the engine start temperature, on the basis of the intake air temperature, signal S5. In the next step P23, the correction value ADD is read in accordance with the read value of the start intake air temperature THA, from the map written in the ROM 61b. As will be seen from FIG. 7, this map shows the relationship between the correction value ADD and the intake air temperature THA. Then, in a step P24, a judgement is made as to whether a predetermined period necessary for the attenuation of the correction value ADD by a pedetermined amount α has passed. If the answer is affirmative, the process proceeds to the next step P25. In this step P25, a value (ADD-α) is made and the result is stored as a new correction value ADD in a predetermined storage region. A judgement is made in the next step P26 as to whether the correction value ADD is smaller than zero or not. If the answer is affirmative, the correction value is nullified, i.e. set at zero, in a step P27 and then the routine for the determination of the correction value ADD is completed. If the answer to the question in the step P26 is negative, the ADD computation routine skips over the step P27. If this routine is started after the starting up of the engine, a negative answer is made in response to the inquiry made in the step P21 and the process jumps directly to the step P24. If the answer in this step is affirmative, the steps P25 to P27 are taken as explained above. If the answer is negative, the process skips over the steps P25, P26 and P27 and the series of operation is completed. 
     As will be seen from the foregoing description, as well as from FIG. 8, the start temperature correction value ADD read on the basis of the intake air temperature THA at the time of starting up of the engine is attenuated at a constant rate α at a predetermined period. 
     (Computation of Intake Pressure PM) 
     The process for computing the intake air pressure PM shown in FIG. 9 is conducted repeatedly at a predetermined period as will be seen from FIG. 10. In a step P31, the absolute intake pressure signal S4 is converted into a digital signal. In the next step P32, the digital values PMi(i being an integer) are successively stored in regions Ro to R3 at a predetermined period. Then, the following computation is conducted in the following step P33. For instance, the intake pressure PM-4 which was stored in the register R1 at an instant (t-4) is subtracted from the intake pressure PM-2 stored in the register R1 at an instant (t-2). The result DPM 2  of this operation is stored in a register DR 2 . Then, the process proceeds to the next step P34. In this step, at an instant t 0  for example, the value DPM 1  stored in the register DR 1  is subtracted from the value DPM 0  stored in the register DR 0 , and the result DDPM of this calculation is stored in a register DDR as a second-order differentiation value. In the next step P35, the second-order differentiation value DDPM of the intake pressure stored in the register DDR is compared with a reference value REF 1. If the condition DDPM≧REF 1 is met, the process jumps to an asynchronous injection routine which will be explained later with reference to FIG. 22. On the other hand, this process is completed if the condition of DDPM&lt;REF 1 is met. 
     Thus, the intake pressures PM stored in respective registers at every moment are used in the computation of the basic injection time duration TP. On the other hand, the first-order differentiation value DPM of the intake pressure PM is used in the computation of the synchronous acceleration incremental correction, while the second-order differentiation value DDPM is used in the computation for the asynchronous acceleration incremental correction. 
     An explanation will be made hereinunder as to the operations for determining the coefficients in respective steps of the process explained before in connection with FIG. 5. 
     (1) Computation of Warm-Up Incremental Coefficient FWL 
     An example of the process for computing the warm-up incremental coefficient will be explained hereinunder with reference to FIG. 11. In a step P41, the cooling water temperature THW is read in the form of the water temperature signal S6. At the same time, the engine speed Ne is read on the basis of the engine speed signal S10. Furthermore, the correction value ADD computed in the routine shown in FIG. 6 is also read in this step. In a step P42, the correction coefficient FWLO is determined on the basis of the newest water temperature THW from a map (see FIG. 12) which shows the relationship between the correction coefficient FWLO and the cooling water temperature. In the subsequent step P43, the correction coefficient KWL is read on the basis of the newest engine speed Ne from a map (see FIG. 13) which shows the relationship between the engine speed Ne and the correction coefficient KWL. In a step P44, the following computation is executed to determine the warm-up incremental coefficient FWL to complete a series of operation. 
     
         FWL=(correction coefficient FWLO+correction value ADD)×correction coefficient KWL+1.0 
    
     (2) Computation of Feedback Correction Coefficient FAF 
     An example of the process for computing the feedback correction coefficient FAF is shown in FIG. 14. 
     As the routine for computing the air-fuel ratio feedback correction coefficient FAF is started, a judgement is made in a step P51 to judge whether the feedback condition has been established. The condition for the feedback is established when all of the following requirements are met: engine is not being stated; engine is not in the fuel incremental condition after start up, cooling temperature is not lower than 40° C.; engine is not in the power incremental phase; and engine is not under lean control. If the condition for the feedback has not been established, the feedback correction coefficient FAF is set at 1.0 in the step P52 to prohibit feedback control, thereby completing this process. On the other hand, if the condition for the feedback has been established, the process proceeds to a step P53. 
     The air-fuel ratio signal S7 is read in the step P53. In a step P54, the voltage value of this air-fuel ratio signal is compared with a reference value REF2. When the level of the signal S7 exceeds or equals the reference value REF2, it is judged that the air-fuel ratio is too small, i.e. the mixture is too rich, and the process is started to increase the air-fuel ratio, i.e. to make the mixture more lean. Namely, after setting the flag CAFL at zero in a step P55, the process proceeds to a step P56 in which a judgement is made as to whether the flag CAFR is zero or not. The state of the flag CAFR is zero if the process has been shifted to the too rich side for the first time, so that the process proceeds to a step P58 in which a predetermined value α1 is subtracted from the correction coefficient FAF stored in the RAM 61C and the result of this calculation is used as new correction coefficient FAF. In the step P59, the flag CAFR is set to be 1. Therefore, if the air-fuel mixture is judged to be too rich in two successive judging cycles in the step P54, negative judgement is made without fail in the step P56 in the second cycle and the following judging cycles, so that the process proceeds to a step P57 in which a predetermined value β1 is subtracted from the correction coefficient FAF. The result of this calculation is then determined as the new correction coefficient FAF, thus completing the FAF operation. 
     On the other hand, if the judgement in the step P54 proves the level of the signal S7 to be smaller than the reference value REF2, it is judged that the air-fuel ratio is too large, i.e. the mixture is too lean, so that a process is taken to decrease the air-fuel ratio, i.e. to make the mixture richer. More specifically, the process proceeds to a step P91 after setting the flag CAFR at zero in a step P90. In the step P91, a judgement is made as to whether the state of the flag CAFL is zero or not. If the process has been shifted to the too lean side for the first time, the process proceeds to a step P92 because the state of the flag CAFL is zero. In the step P92, a predetermined value α2 is added to the correction coefficient FAF and the result of this addition is used as the new FAF. In a step P93, the state of the flag CAFL is set to be 1. Therefore, if the mixture is judged to be too lean in two successive judging cycles, in the step P54, a negative judgement is made without fail in the second cycle and the following judging cycles in the step P91. Then, the process proceeds to a step P94 in which a predetermined value β2 is added to the correction coefficient FAF and the result of this addition is determined as the new FAF, thus completing the FAF operation. The values α1, α2, β1 and β2 used in the steps P57, P58, P92 and P94 are the values which have been determined beforehand. 
     The feedback correction coefficient FAF determined through this operation is shown in FIG. 15 together with the air-fuel ratio signal S7. The following will be noted from this Figure. Namely, when the signal S7 rises above the reference value REF2 or drops below the same, the correction coefficient FAF is skipped by an amount α1 or α2. Thereafter, when the signal S7 exceeds the reference value, the predetermined value β1 is subtracted successively, whereas, if the signal S7 is below the reference value, the predetermined value β2 is added successively. 
     (3) Computation of Air-Fuel Ratio Correction Coefficient in Transient Period 
     An explanation will be made hereinunder with specific reference to FIG. 16 as an example of the process for computing the air-fuel ratio correction coefficient FTC in the transient period. This process constitutes an essential feature of the second aspect of the invention. The amount DPM K  of change of the intake pressure PM obtained through the routine shown in FIG. 9 is read in a step P61. Then, in a step P62, a warm-up acceleration correction coefficient ΔFTCO is determined using a map shown in FIG. 17. As will be seen from FIG. 17, this map shows the relationship between the amount DPM K  of change in the intake pressure and the warm-up acceleration correction coefficient ΔFTCO. Then, in a step P63, the correction coefficient FTCO which has been determined beforehand is added to the correction coefficient ΔFTCO which is determined in the step P62. Using the result of this addition calculation as the new correction coefficient FTCO, the process proceeds to a step P64. In the step P64, a judgement is made as to whether a predetermined period for attenuation of the thus obtained correction coefficient FTCO by a predetermined amount α has elapsed. If the answer is affirmative, the process proceeds to a step P65. In the step P65, (FTCO-γ) is calculated and the result of this calculation is stored in a predetermined storage region as a new correction coefficient FTCO. In the next step P66, a judgement is made as to whether the correction coefficient FTCO is smaller than or equal to zero. If the answer is affirmative, the process proceeds to a step P68 after setting the correction coefficient FTCO at zero in a step P67. The process jumps to the step P68 also when a negative answer is obtained in the step P64 or the step P66. 
     In the step P68, the cooling water temperature THW is read on the basis of the water temperature signal S6. In a next step P69, the warm-up acceleration correction coefficient KTC is read from a map shown in FIG. 18, using the read value of the cooling water temperature THW. As will be seen from FIG. 18, this map shows the relationship between the cooling water temperature THW and the warm-up acceleration correction coefficient KTC. In a next step P70, the start temperature correction value ADD determined by the routine shown in FIG. 6 is read. The process then proceeds to a step P71 in which the following calculation is made to determine the warm-up acceleration correction coefficient FTC, using the correction coefficients FTCO, KTC and ADD which have been obtained as explained hereinbefore: 
     
         FTC=FTCO×(KTC+ADD+1.0) 
    
     The correction coefficient FTC obtained through the steps P61 to P65 is shown in FIG. 19 together with the intake pressure PM and the amount DPM of change in the intake pressure. The following will be noted from this Figure. Namely, in successive moments, a predetermined value ΔFTCO is added to FTCO at each time the amount DPM of change in intake pressure exceeds the reference value REF1. At the same time, in the period between successive moments, a value γ is subtracted from the correction coefficient FTCO at a predetermined period. 
     The coefficients FWL, FAF and FTC used in the steps P11 to P13 of the process shown in FIG. 5 are determined in the manner described hereinbefore. Then, in a step P15, an operation is made in accordance with the following formula to determine the corrected injection time duration τ: t=TP×FWL×FAF×(1+FTC)×FTHA. The process is then returned to the step P4 shown in FIG. 3. 
     In FIG. 3 there is shown a computation for voltage compensation which is conducted in a step P4 using a voltage compensation computing routine as shown in FIG. 20. In a step P81, the battery voltage BV is read in accordance with the batery voltage signal S14. In a step P82, the voltage correction coefficient τV is read from the map shown in FIG. 21 using the thus read battery voltage BV. As will be seen from FIG. 21, this map shows the relationship between the battery voltage BV and the voltage correction coefficient τV. In a step P83, a computation of (τ+τV) is executed to determine the final injection time duration F2. The process then returns to the step P5 shown in FIG. 3. If the instant moment coincides with the injection timing, an injection signal S12 is issued from the control circuit 61 to the injector 7, thereby driving the latter. 
     In the process shown in FIG. 5, the intake air temperature correction FTHA in the step P14 is conducted to compensate for the variation of the density of the intake air due to a change in the air temperature. 
     An explanation will be made hereinunder as to the asynchronous injection computing routine which constitutes an essential feature of the third aspect of the invention. 
     The routine shown in FIG. 22 is started by a jump from the step P36 shown in FIG. 9. In a step P100, the amount of the change in the pressure, which is stored in a register DDR, is read and the process proceeds to a step P102. In the step P102, an asynchronous injection time duration TP asy  is read from a map shown in FIG. 23, making use of the thus read pressure changing amount DDPM. As will be seen from FIG. 23, this map shows the relationship between the changing amount DDPM of the intake pressure and the asynchronous injection time duration TP ASY . Then, after reading the newest start temperature correction value ADD calculated through the routine shown in FIG. 6, the process proceeds to a step P103. In the step P103, a computation of (TP ASY  ×(ADD+1.0)) is executed to store the result in a predetermined storage region. On the other hand, in a step 104, a correction processing in accordance with the battery voltage is executed to determine the final asynchronous injection time duration Fτ ASY . 
     FIG. 24 shows an example of the routine for computing the asynchronous injection time duration Fτ ASY . First of all, in a step P110, the battery voltage BV is read in terms of the battery voltage signal S14. Then, in the next step P111, a voltage correction coefficient τV is read from a map shown in FIG. 21, using the thus read battery voltage BV. As will be seen from FIG. 21, this map shows the relationship between the battery voltage BV and the voltage correction coefficient τV. The process then proceeds to a step P112 in which a computation of (τ ASY  +τV) is made to determine the final asynchronous injection time duration Fτ ASY . After storing this value in a predetermined storage region, the process is returned to a step 105 shown in FIG. 22. 
     In the step P105, an injection signal S12 is delivered to the injector 7 in accordance with the thus determined final asynchronous injection time duration Fτ ASY , thereby conducting the asynchronous injection. 
     In the embodiments described hereinbefore, the intake pressure is used as the index of the degree of the engine acceleration. However, it is possible to use the amount of change of the opening degree of the intake throttle valve or amount of change of the intake air per revolution of the engine shaft as the index of degree of the engine acceleration. The selection of the start temperature compensation value ADD can be made in accordance with the temperature THW of the cooling water, engine oil or the cylinder block at the time of start up of the engine, although in the described embodiments the same is conducted in accordance with the intake air temperature at the time of start up of the engine. 
     In the described embodiment, the basic injection time duration TP is determined in accordance with the engine speed and the intake pressure. This, however, is not exclusive and the basic injection time duration can be determined in accordance with the engine speed and the flow rate of intake air. Furthermore, in the described embodiment, the engine speed is taken into account in the determination of the warm-up incremental coefficient FWL. This, however, is not exclusive and the warm-up incremental coefficient FWL can be determined without taking the engine speed into account.