JP2759917B2 - Air-fuel ratio control method for internal combustion engine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an air-fuel ratio control method for an internal combustion engine, and more particularly, to a method of mixing air supplied to an engine using an exhaust gas concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration. The present invention relates to an air-fuel ratio control method for feedback-controlling an air-fuel ratio of air to a target air-fuel ratio.

(Prior Art) Using an exhaust gas concentration sensor having an output characteristic that is substantially proportional to the exhaust gas concentration, the air that controls the air-fuel ratio of the air-fuel mixture supplied to the engine (hereinafter referred to as “supply air-fuel ratio”) to the target air-fuel ratio is feedback-controlled. In the fuel ratio control method, a proportional term (P term), an integral term (I term), and a differential term (D term) according to the deviation between the air-fuel ratio detected by the exhaust gas concentration sensor and the target air-fuel ratio.
Has been conventionally proposed in which the supply air-fuel ratio is feedback-controlled based on these PID terms (JP-A-62-251443).

(Problems to be Solved by the Invention) However, according to the method proposed above, immediately after the start of the acceleration operation of the engine or in the case where a misfire occurs, the deviation between the detected air-fuel ratio and the supply air-fuel ratio becomes large, and the PID There is a problem that the term becomes excessive and the supply air-fuel ratio causes hunting. That is, for example, when a misfire occurs, the detected air-fuel ratio greatly changes in the rich direction.
The D term is a value that greatly changes the supply air-fuel ratio in the lean direction. However, in actuality, the apparent supply air-fuel ratio only deviates greatly from the target air-fuel ratio due to misfire, so the PID term becomes a value that greatly changes the supply air-fuel ratio in the lean direction, and the actual supply air-fuel ratio becomes large. The result is that the fuel ratio deviates greatly from the target air-fuel ratio in the lean direction.

The present invention has been made in view of the above points, and the feedback control based on the PID term is appropriately performed, and even when the detected air-fuel ratio fluctuates greatly, such as at the beginning of acceleration or at the time of misfire, the supply air It is an object of the present invention to provide an air-fuel ratio control method capable of preventing deterioration of drivability or exhaust gas characteristics due to a large change in fuel ratio.

Means for Solving the Problems In order to achieve the above object, the present invention provides an air-fuel ratio detected using an exhaust concentration sensor provided in an exhaust system of an internal combustion engine and having an output characteristic substantially proportional to the exhaust gas concentration. An internal combustion engine that calculates a proportional term, an integral term, and a derivative term according to a deviation between the target air-fuel ratio and the target air-fuel ratio, and feedback-controls an air-fuel ratio of an air-fuel mixture supplied to the engine based on the calculated proportional term, integral term, and derivative term. In the air-fuel ratio control method, the feedback by the proportional term and the differential term is stopped when the deviation is larger than a predetermined value, and the integral term is held at the previous value.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is an overall configuration diagram of a control device to which the control method of the present invention is applied. A slot body 3 is provided in the middle of an intake pipe 2 of an engine 1, and a slot valve 3 'is disposed therein. Have been. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 ′, and outputs an electric signal corresponding to the opening of the throttle valve 3 and supplies it to an electronic control unit (hereinafter referred to as “ECU”) 5. I do.

The fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of the intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). Is electrically connected to the ECU together with the ECU
The valve opening time of the fuel injection is controlled by the signal from 5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 8 is provided immediately downstream of the throttle valve 3 via a pipe 7, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. Is done. Further, an intake air temperature (TA) sensor 9 is mounted downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies the electric signal to the ECU 5.

The engine water temperature (TW) sensor 10 mounted on the body of the engine 1 is composed of a thermistor or the like, detects the engine water temperature (cooling water temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5. The engine speed (NE) sensor 11 and the cylinder discrimination (CYL) sensor 12 are mounted around a camshaft or a crankshaft (not shown) of the engine 1. The engine speed sensor 11 outputs a pulse (hereinafter referred to as “TDC signal pulse”) at a predetermined crank angle position every time the crankshaft of the engine 1 rotates 180 degrees, and the cylinder discriminating sensor 12 outputs a predetermined crank angle of a specific cylinder. A signal pulse is output at the position, and each of these signal pulses is supplied to the ECU 5.

The three-way catalyst 14 is disposed in the exhaust pipe 13 of the engine 1 and purifies components such as HC, CO, and NOx in the exhaust gas. An oxygen concentration sensor (hereinafter, referred to as an “LAF sensor”) 15 as an exhaust concentration sensor is mounted on the exhaust pipe 13 on the upstream side of the three-way catalyst 14 and outputs an electric signal having a level substantially proportional to the oxygen concentration in the exhaust gas. Output and supply to ECU5.

The ECU 5 shapes input signal waveforms from various sensors, corrects a voltage level to a predetermined level, and converts an analog signal value to a digital signal value. The input circuit 5a has a function of a central processing unit (hereinafter referred to as a “CPU”). 5b), a storage means 5c for storing various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, and the like.

Based on the various engine parameter signals described above, the CPU 5b determines various engine operation states such as a feedback control operation area and an open loop control operation area corresponding to the oxygen concentration in the exhaust gas, and determines the next according to the engine operation state. Based on the equation (1), a fuel injection time _TOUT of the fuel injection valve 6 synchronized with the TDC signal pulse is calculated.

T _OUT = Ti × KCMDM × KLAF × K ₁ + K ₂ (1) where Ti is the basic fuel amount, specifically the engine speed
This is a basic fuel injection time determined according to NE and the intake pipe absolute pressure PBA, and a Ti map for determining this Ti value is stored in the storage means 5c.

KCMDM is a corrected target air-fuel ratio coefficient, which is set according to the engine operating state, and calculated by multiplying a target air-fuel ratio coefficient KCMD representing the target air-fuel ratio by a fuel cooling correction coefficient KETV. The correction coefficient KETV is a coefficient for correcting the fuel injection amount in advance in consideration of the fact that the supply air-fuel ratio changes due to the cooling effect by actually injecting the fuel, and according to the value of the target air-fuel ratio coefficient KCMD. Is set. In addition, as is apparent from the above equation (1), the target air-fuel ratio coefficient
If KCMD increases, the fuel injection time T _OUT increases, so KC
The MD value and the KCMDM value are values proportional to the reciprocal of the so-called air-fuel ratio A / F.

KLAF is an air-fuel ratio correction coefficient calculated by a program shown in FIG. 2 described later. During the air-fuel ratio feedback control, KLAF is set so that the air-fuel ratio detected by the LAF sensor 15 matches the target air-fuel ratio. During control, it is set to a predetermined value according to the engine operating state.

K ₁ and K ₂ are other correction coefficients and correction variable computed according to various engine parameter signals, so that the fuel consumption characteristic according to engine operating conditions, the optimization of various properties such as the engine acceleration characteristics can be achieved Is set to an appropriate value.

CPU 5b, based on the result calculated as described above,
A signal for driving the fuel injection valve 6 is output via the output circuit 5d.

FIG. 2 is a flowchart of a program for calculating the air-fuel ratio correction coefficient KLAF. This program is executed in synchronism with the generation of each TDC signal.

In step S21, a delay TDC variable PTDC indicating the exhaust gas arrival delay by the number of occurrences of the TDC signal is read from the PTDC table set according to the intake pipe absolute pressure PBA. In the PTDC table, a predetermined number of times PTDC1 and PTDC2 are set for the predetermined pressures PBPTDC1 and PBPTDC2 as shown in FIG. 3, and the PTDC value is calculated by interpolation except for the set points. This PTDC table is set by paying attention to the fact that the delay until the air-fuel mixture supplied to the engine reaches the LAF sensor 15 changes according to the intake pipe pressure, and the higher the intake pipe absolute pressure PBA is, the higher the delay is. This indicates that the arrival delay is small.

In step S22, it is determined whether or not the flag FLAFFB, which is set to the value 1 during the execution of the air-fuel ratio feedback control, was 1 when the TDC signal was generated last time (when the program was last executed), and the answer is affirmative (YES). In step S24, the process immediately proceeds to step S24, and in the case of negative (NO), the previous calculation of the deviation between the equivalence ratio indicating the air-fuel ratio detected by the LAF sensor 15 (hereinafter referred to as "detected air-fuel ratio" for coal) and the target air-fuel ratio KCMD value
DKAF _(N-1) is set to the value 0, and the target air-fuel ratio coefficient KCMD _(NP) before P times is set to the current value KACT _(N) of the detected air-fuel ratio (step S23), and the process proceeds to step S24. Here, “P” is equal to the PTDC value calculated in step S21.

In step S24, the target air-fuel ratio coefficient KCMD _(NP) P times before
By subtracting the current value KACT _(N) of the detected air-fuel ratio from the above, the current value DKAF _(N) of the deviation is calculated. Step S23
When this step is reached via KCMD _(NP) = KAC
Since T _(N) , DKAF _(N) = 0.

In step S25, it is determined whether or not the thinning-out TDC variable NITDC has a value of 0. If the answer is negative (NO), NITD
C is decremented by 1 (step S26), and this program ends. The thinning-out TDC variable NITDC is a variable for updating the air-fuel ratio coefficient KLAF every time the TDC signal is generated by the thinning-out number NI set according to the engine operating state, and the answer in step S25 is affirmative (YES), That is, when NITDC = 0, the process proceeds to step S27 and the subsequent steps to update the KLAF value.

In step S27, a proportional term (P term) coefficient KP, an integral term (I term) coefficient KI, a differential term (D term) coefficient KD, and the thinning number NI are calculated. KP, KI, KD and NI are engine speeds
The value is set to a predetermined value for each of a plurality of engine operation regions determined by NE, the intake pipe absolute pressure PBA, and the like, and a value corresponding to the detected engine operation state is read.

In step S28, it is determined whether or not the absolute value of the current value DKAF _(N) of the deviation calculated in step S24 is larger than a predetermined value DKPID, and the answer is negative (NO), that is, | DKAF _(N) | ≦ DKPID. When the answer is affirmative (YES), that is, when | DKAF _(N) |> DKPID, both the previous value DKAF _(N-1) and the current value DKAF _(N) of the deviation are set to the values. The value is set to 0 (step S29), and the process proceeds to step S30. In step S30, the P term KLAFP, the I term KLAFI, and the D term KLAFD are calculated by the following equations (2) to (4).

KLAFP = DKAF _(N) x KP ... (2) KLAFI = KLAFI + DKAF _(N) x KI ... (3) KLAFD = (DKAF _(N) -DKAF _(N-1) ) x KD ... (4) If the answer in step S28 is affirmative (YES) and | DKAF _(N) |> DKPID holds, DKAF _(N) and DKAF _(N-1) are both set to the value 0, so that KLAFP = KLAF
D = 0 and KLAFI = KLAFI. That is, the feedback control by the P term and the D term is stopped, and the I term is maintained at the previous value.

As a result, when the detected air-fuel ratio KACT fluctuates greatly, such as at the beginning of acceleration or when a misfire occurs, | DKAF _(N) |> D
It becomes KPID, the feedback control by the P term and the D term is stopped, and the I term is held at the previous value.
It is possible to prevent the drivability or the exhaust gas characteristics from being deteriorated due to a large change in the supplied air-fuel ratio.

In steps S31 to S34, a limit check of the I term KLAFI is performed. That is, the magnitude relationship between the KLAFI value and the predetermined upper and lower limit values LAFIH, LAFIL is compared (steps S31, S32), and as a result, the KLAFI
When the term exceeds the upper limit value LAFIH, the upper limit value is set (step S33), and when the term is smaller than the lower limit value LAFI, the lower limit value is set (step S34).

In step S35, the air-fuel ratio correction coefficient KLAF is calculated by adding the PID terms KLAFP, KLAFI, and KLAFD, and the current calculated value DKAF _(N) of the deviation is set to the previous value DKAF _(N-1) (step S36). Further, the thinning variable NITDC is set to the thinning number NI calculated in step S27 (step S37), and the program ends.

(Effect of the Invention) As described above in detail, according to the present invention, when the deviation between the detected air-fuel ratio and the target air-fuel ratio is larger than a predetermined value, the feedback by the proportional term and the differential term is stopped, and the integral term Is maintained at the previous value, so that even when the detected air-fuel ratio fluctuates greatly, such as at the beginning of acceleration or when a misfire occurs, the supply air-fuel ratio fluctuates greatly and the drivability or exhaust gas characteristics deteriorate. Can be prevented.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a fuel supply control device to which the control method of the present invention is applied, FIG. 2 is a flowchart of a program for calculating an air-fuel ratio correction coefficient KLAF, and FIG.
FIG. 4 is a diagram showing a table for calculating a C variable (PTDC). 1 ... internal combustion engine, 5 ... electronic control unit (ECU), 6 ... fuel injection valve, 15 ... exhaust gas concentration sensor (oxygen concentration sensor).

Claims (1)

(57) [Claims] 1. A proportional term according to a deviation between an air-fuel ratio and a target air-fuel ratio detected using an exhaust concentration sensor provided in an exhaust system of an internal combustion engine and having an output characteristic substantially proportional to the exhaust gas concentration. Term and derivative term, and the calculated proportional term,
In an air-fuel ratio control method for an internal combustion engine, in which an air-fuel ratio of an air-fuel mixture supplied to the engine is feedback-controlled by an integral term and a derivative term, when the deviation is larger than a predetermined value, the feedback by the proportional term and the derivative term is stopped. An air-fuel ratio control method for an internal combustion engine, wherein the integral term is held at a previous value.