Method for Controlling Fuel Injection for Automotive Engines

The present invention relates to a method for controlling fuel injection for an automotive engine, and more particularly for a fuel injection system employed with an intake-air pressure detecting method. In an ordinary fuel injection system employed with an intake-air quantity detecting method, an airflow meter is provided in an intake passage at a position upstream of a throttle valve to detect the quantity of intake air (Q). Basic fuel injection time of period Tp is determined by a calculation of Tp =Q/N (N is engine speed).

In the fuel injection control system dependent on intake-air pressure (Japanese Patent Laid Open No.58-107825), the quantity of intake-air is indirectly detected by the value of the pressure of intake-air. The intake-air pressure increases with decrease of engine speed and decreases with increase of engine speed. However, the variation of the intake-air pressure delays because of the volume of the intake system.

FIG. 5 shows variations of engine speed N, intake-air pressure P and engine torque T at idling of an engine.

If engine speed at idling varies as the curve N because of external disturbance, intake-air pressure should vary as a curve P' theoretically. However, the intake-air pressure varies actually as the curve P with a delay because of the volume of the air intake system. Since the basic fuel injection time Tp is determined by the pressure P, the engine torque varies as the curve T with a phase delay of about 270.degree. with respect to the variation of the engine speed N. Namely, at a point A where the engine speed is at a maximum deceleration, the engine torque is minimum, and at a point B where the engine speed is at a maximum acceleration, the torque is maximum.

Accordingly, the variation of the idling engine speed is maintained without damping.

The object of the present invention is to provide a method which decreases variation of idling speed of an engine to stabilize the speed.

The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings .

FIG. 1 is a schematic diagram showing a system to which the present invention is applied;

FIG. 2 is a block diagram showing a control system;

FIGS. 3a and 3b are schematic views of an air intake system;

FIGS. 4a, 4b and 5 are graphs showing variations of engine speeds.

Referring to FIG. 1, in an intake passage 2 of an engine, a throttle chamber 3 is provided downstream of a throttle valve 4 so as to absorb the pulsation of intake-air. Fuel injectors 11 are provided in the intake passage 2 at adjacent intake valves 5 of the engine so as to supply fuel to each cylinder 1 of the engine. A pressure sensor 10 is provided for detecting the pressure of intake-air in the chamber 3 and for producing an intake-air pressure signal. A detected signal is supplied to a control unit 6 consisting of a microcomputer. A distributor 9 produces an engine speed signal for the control unit 6. The control unit 6 determines a basic quantity of fuel injected from the injectors 11 dependent on the intake-air pressure signal and the engine speed signal, and supplies a signal to the injectors 11. In order to detect the idling state of the engine, a throttle position switch 12 is provided adjacent the throttle valve 4. The throttle position switch 12 is turned on when the throttle valve 4 completely closes. An ON-signal from the switch 12 is applied to the control unit 6 to correct the basic fuel injection time Tp.

Referring to FIG. 2, the control unit 6 comprises a central processor unit CPU 14 having an arithmetic and logic circuit ALU 19, a read only memory ROM 20, and a random access memory RAM 21. The control unit 6 operates in accordance with a program stored in the ROM 20. A waveform shaping circuit 15 is supplied with the engine speed signal from the distributor 9 for shaping waveforms of the signal. An A/D converter 16 (A/DC) is supplied with an analog voltage signal from the pressure sensor 10 to convert the analog voltage signal into a digital signal. An input interface 17 is provided for determining an ON or OFF-signal from the throttle position switch 12 and an output signal of the interface 17 is supplied to CPU 14. A driver 18 produces a pulse signal for driving the injectors 11 in responsive to an output signal of the CPU 14.

The engine speed signal from the waveform shaping circuit 15 and intake-air pressure signal from the A/D converter 16 are stored in the RAM 21 through the ALU 19. The basic fuel injection time Tp is calculated based on the stored data in the RAM and a map stored in the ROM, for driving the injectors 11.

The relationship between the intake-air pressure detecting method and the intake-air quantity detecting method is described hereinafter.

Referring to FIGS. 3a and 3b, the air intake system schematically illustrated in FIG. 3a approximately equals to the electric circuit of FIG. 3b. Namely, the pressure P in the intake passage downstream of the throttle valve 4 corresponds to the voltage P', the quantity of intake-air passing through an airflow meter 7 in the intake passage at upstream of the throttle 4 corresponds to current Q', the airflow meter 7 corresponds to a current meter I and the pressure sensor 10 corresponds to a voltage meter V in FIG. 3b. P.sub.A represents pressure at upstream of the throttle valve 4 and Qe represents actual quantity Qe of air induced in the cylinder 1.

In FIG. 3b R.sub..theta. is a resistance at the throttle valve 4, C is a capacity for the chamber 3 and Re is an intake resistance of the cylinder 1. The relationship between the current Q' detected by the current meter I and the voltage P' detected by the voltage meter V is expressed as follows. Q'=Qe'+C(dP'/dt) namely

Basic fuel injection time Tp is

where K is a constant for an injector.

A basic fuel injection time Tp obtained by the intake-air pressure P is

The Tp is obtained from a lookup table in ROM 20 in accordance with pressure P and engine speed N.

Substituting the Q of the equation (2) with the equation (1),

Since the first term of equation (4) is the equation (3),

The second term of the equation (5) can be regarded as a correcting quantity for the basic injection time at idling of the engine. The fuel injection time dependent on the equation (5) is equivalent to the fuel injection time obtained by the intake-air quantity. Accordingly, the variation of engine speed at idling and reduction of the engine speed at throttle rapid closing can be prevented by supplying the fuel dependent on the equation (5).

FIG. 4a shows variation of idling speed without a correcting quantity where the deviation from the desired idling speed is 42.4 rpm (mean value), causing hunting. FIG. 4b shows variation of idling speed by corrected fuel injection time where the deviation is 18.0 rpm and hunting is prevented.

The second term of the equation (5) can be simplified, as described hereinafter.

The second term (TPIDL) of the equation is

If the intake-air pressure at time t is P and the intake-air pressure at time t.sub.o is P.sub.o ##EQU1##

If one rotating time of the crankshaft of the engine is .DELTA.T,

From the equation (7), (8), the equation (6) is ##EQU2##

If times t and t.sub.o are determined so as to have t-t.sub.o =.DELTA.T, t-t.sub.o becomes sufficiently small. Accordingly, the equation (9) is

In other words, the equation (10) means the calculation based on the intake-air pressure at every one rotation of the crankshaft. Since the equation (10) does not include a differentiation (dP/dt), the calculation is simplified and the capacity of ROM 20 can be reduced.

While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.