Apparatus for injecting fuel into internal combustion engine

In a fuel injection system for an internal combustion engine, the voltage of a vehicle-mounted battery is monitored to see if the voltage is below a predetermined voltage at which a microcomputer used in an electronic control unit is disabled or malfunctions on engine start where large current is consumed by a starter motor. During engine start, asynchronous fuel injection is performed using the result of the voltage monitoring in place of normal or main fuel injection. The amount of fuel to be injected by the asynchronous fuel injection may be limited and/or the number of times of asynchronous fuel injection may be limited so as to prevent excessive fuel supply. In monitoring the battery voltage, hysteresis characteristic may be given to a reference voltage so as to avoid undesirable chattering.

BACKGROUND OF THE INVENTION 
This invention relates generally to apparatus for injecting fuel into 
internal combustion engines, and more particularly to an improvement in 
fuel supply on engine start with an electronic fuel injection apparatus. 
Internal combustion engines mounted on motor vehicles or the like are 
widely controlled electronically in recent days so that the quantity of 
fuel to be supplied to an engine is computed by a microcomputer in 
accordance with operating conditions of the engine. The so called 
electronic fuel injection (EFI) control unit which controls the opening 
duration of fuel injection valve(s) is becoming popular. In such an 
electronic fuel injection control apparatus, the operation of a 
microcomputer used for comuting the fuel injecting duration has to be 
normal, but the microcomputer is apt to suffer from undesirable influence 
caused from voltage fluctuation of the power source. 
Especially when an engine starter is operated, the voltage of the power 
source, i.e. a battery mounted on a motor vehicle, drops to a considerable 
extent since a large current flows into the starter motor. Therefore, in 
the case that the battery is deteriorated or in poor condition of low 
ambient temperature, the voltage of the battery sometimes drops below a 
value where the operation of the microcomputer cannot be ensured on engine 
start. 
To ensure accurate operation of the microcomputer on engine start 
irrespective of the dropping of the battery voltage, various measures have 
hitherto been devised. According to one conventional electronic fuel 
injection control apparatus, an additional fuel injection valve, which is 
called start injector, is provided so that fuel is supplied to the engine 
even if the battery voltage is low. This start injector is provided to an 
intake pipe and is arranged to be responsive to a time switch using a 
bimetallic element so that fuel is supplied to the engine for a given 
period of time on engine start. According to another conventional 
electronic fuel injection apparatus disclosed in Japanese Patent 
Provisional Publication No. 58-217737, a backup memory is used for 
prestoring fuel injection duration suitable for engine start, and when the 
battery voltage drops below a given voltage, the fuel injection duration 
from the backup memory is used in place of the results of operation 
performed by the microcomputer. 
However, these conventional techniques suffer from the following problems: 
(A) When the above-mentioned start injector is used for supplying fuel on 
engine start, a separate electrical system and a fuel supply system are 
necessary in addition to the normal fuel injection system, and thereby the 
structure of the entire fuel supply system becomes complex. As a result, 
the reliability of the entire system is apt to be lowered, while the 
number of manufacturing processes increases resulting in a cost increase. 
Furthermore, the amount of fuel to be injected is unequivocally determined 
by the time switch, and therefore, precise control in accordance with 
starting condition of the engine, such as engine coolant temperature or 
the number of times of fuel injections, cannot be performed. This also 
applies to the other method of fuel injection using the backup memory. 
(B) Since the starter motor receives a maximum load when one of the 
cylinders of the engine is in the last part of its compression stroke, the 
battery voltage drastically fluctuates in correspondence with load 
variation. Therefore, the battery voltage may fluctuate between a voltage 
with which the microcomputer can normally operate and another voltage with 
which the microcomputer cannot normally operate. As a result, the entire 
microcomputer is reset to an initial state each time the battery voltage 
drops below a given voltage, and therefore, the microcomputer always 
starts operating from its initial state whenever the battery voltage is 
restored. Accordingly, when the battery voltage drops below the given 
voltage to reset the microcomputer before or in the middle of necessary 
computation of fuel amount to be injected, accurate fuel amount required 
for engine start cannot be obtained. 
SUMMARY OF THE INVENTION 
The present invention has been developed in order to remove the 
above-described drawbacks inherent to the conventional fuel injection 
systems. 
It is, therefore, an object of the present invention to provide a new and 
useful fuel injection system with which fuel is securely supplied to 
engine cylinders on engine start even if the battery voltage fluctuates, 
without requiring particular start injectors or an additional fuel supply 
system. 
According to a feature of the present invention the voltage of a 
vehicle-mounted battery is monitored to see if the voltage is below a 
predetermined value, and an electronic control unit is arranged so as to 
carry out fuel injection on engine start apart from normal or main fuel 
injection using the result of monitoring. 
In accordance with the present invention there is provided an 
electronically-controlled fuel injection system for an internal combustion 
engine, comprising: means for detecting operating conditions of said 
engine; means for injecting fuel into said engine when activated; means 
supplied with a supply voltage for controlling said injecting means, said 
controlling means initiating, during normal operation of said engine, 
activation of said injecting means in relation to rotational position of 
said engine and maintaining activation of said injecting means during a 
time period calculated in accordance with the operating conditions of said 
engine detected by said detecting means; and means for monitoring the 
supply voltage and producing first and second outputs indicating that the 
monitored supply voltage is below and above a predetermined level 
corresponding to a lowest possible voltage for the operation of said 
controlling means; said controlling means initiating, during cranking of 
said engine, activation of said injecting means each time output condition 
of said monitoring means changes from the first to second output and 
maintaining activation of said injecting means during a predetermined time 
period.

The same or corresponding elements and parts are designated at like 
reference numerals throughout the drawings. 
DETAILED DESCRIPTION OF THE INVENTION 
Prior to describing preferred embodiments of the present invention, the 
general concept of the present invention will be described with reference 
to FIG. 1. In FIG. 1, an internal combustion engine to be controlled is 
designated at the reference M1, and a fuel injection control means by the 
reference M4. The reference M3 is fuel injection means responsive to the 
fuel injection control means M4, and the reference M2 indicates operating 
condition detecting means which detects the operating condition of the 
engine M1. Power supply voltage monitoring means M5 is provided for 
detecting and monitoring the voltage of an unshown vehicle-mounted battery 
to supply the result of monitoring to the fuel injection control means M4. 
The operating condition detecting means M2 is used for detecting various 
operating conditions of the engine M1, such as the rotational speed Ne, 
coolant temperature Thw, intake air quantity Q, intake air temperature Ta 
or the like. Some of these parameters which are necessary for controlling 
the internal combustion engine M2 may be used. 
The fuel injection control means M4 comprises a well known microcomputer 
having one or several IC chips. More specifically, the microcomputer 
includes a central processing unit (CPU), memories with a RAM and a ROM, 
analog and digital input output ports, a timer, a couter and so on. The 
fuel injection control means M4 computes amount of fuel to be injected on 
the basis of operating condition(s) of the engine M1 detected by the 
operating condition detecting means M2 so as to control the amount of fuel 
supplied with electromagnetic fuel injection valve(s) being opened and 
closed. 
The power source voltage monitoring means M5 is used for watching the power 
source voltage fed to the fuel injection control means M4, and is arranged 
to detect a given voltage which is higher than a voltage at which the fuel 
injection control means M4 is disabled, with which given voltage the 
operation of the fuel injection control means M4 is ensured, and another 
given voltage with which the resumption of the operation of the 
microcomputer is ensured. 
In the electronic fuel injection control apparatus according to the present 
invention, the power source voltage monitoring means M5 detects whether 
the power source voltage is above the above-mentioned given voltage where 
the fuel injection control means M4 is capable of performing normal 
operation, and each time the battery voltage, rises beyond the given 
voltage asynchronous injection of a given amount of fuel is performed by 
the fuel injection control means M4. 
The term "asynchronous fuel injection" throughout this specification refers 
to fuel injection which is not necessarily synchronized with engine 
rotation. On the other hand, normal fuel injection, performed during usual 
operation of the engine, is referred to as normal or main fuel injection. 
In the above-mentioned control, it is preferable that a given hysterisis 
characteristic is given to the given voltage where the normal operation of 
the microcomputer is ensured since undesirable chattering around the given 
voltage can be effectively prevented. Similarly, it is also preferable 
that the asynchronous fuel injection is performed from an instant where a 
predetermined delay time has elapsed from the instant of the power source 
voltage restoration. In addition, it is also preferable that the amount of 
fuel to be injected by a single asynchronous injection is determined on 
the basis of the coolant temperature, since engine start characteristic 
can be enhanced in this way. 
Referring now to FIG. 2 a first embodiment of the present invention will be 
described. In FIG. 2, the reference 1 is an internal combustion engine 
corresponding to M1 of FIG. 1, with a four-cycle four-cylinder engine 1 
being illustrated as an example. The reference 2 is an electronic control 
unit corresponding to the fuel injection control means M4. The reference 3 
is a vehicle-mounted battery used for supplying electrical power to 
various electrical and electronic equipment of a motor vehicle (not 
shown). To an intake pipe of the engine 1 are provided air cleaner 5, 
airflow meter 7, intake air temperature sensor 9, throttle valve 11, idle 
switch 12 in a direction from the upstream portion toward the downstream 
portion so that intake air is sucked into unshown engine cylinders as 
air-fuel mixture after being mixed with fuel which is injected through 
electromagnetic fuel injection valves 17 provided to an intake manifold 15 
An oxygen sensor 21 is provided to an exhaust pipe 19 of the engine 1 for 
detecting the concentration of oxygen contained in exhaust gasses. 
The reference 23 is an igniter, and the reference 25 is a distributer which 
distributes high voltage generated by the igniter 23 to unshown respective 
spark plugs in synchronizm with the rotation of engine crankshaft 27. The 
distributer 25 is arranged to generate cylinder-determination signal G1 
and engine speed signal Ne. The reference 29 is an ignition switch which 
connects the battery 3 to the electronic control unit 2, the reference 31 
being a starter switch gang-controlled by the ignition switch 29 for 
turning on a starter motor 32, and the reference 33 being a coolant 
temperature sensor detecting the temperature of engine coolant. 
The electronic control unit 2 comprises, as shown in FIG. 3, a 
microcomputer 50 as its core member, an A/D converter 53, an analog input 
circuit 52, a digital input circuit 54, a backup circuit 56, 
signal-changeover circuit 58, a power circuit 60, and output signal 
buffers 62 and 63. The analog input circuit 52 of the electronic control 
unit 2 receives an output signal from the airflow meter 7 indicative of 
intake air quantity Us, an output signal from the coolant temperature 
sensor 33 indicative of the coolant temperature Thw, and an output signal 
from the intake air temperature sensor 9 indicative of the intake air 
temperature Ta. These signals are then fed to the A/D converter 53 to be 
converted into digital signals. A voltage +B from the battery 3 is also 
fed via the ignition switch 29 to the A/D converter 53 to be converted 
into a digital signal. Digital signals obtained by the A/D converter 53 
are fed to the microcomputer 50 in accordance with appropriate 
instructions from the microcomputer 50 as will be described in detail 
hereinlater. 
The digital input circuit 54 receives the above-mentioned 
cylinder-determination signal G1 and the engine speed signal Ne from the 
distributer 25, a lean-rich signal Ox from the oxygen sensor 21, an output 
signal Idle from the idle switch 12 indicative of fully open state of the 
throttle valve 11, and an output signal STA from the starter switch 31 
indicative of the state thereof. These digital signals are then fed to the 
microcomputer 50 while the cylinder-determination signal G1 and the engine 
speed signal Ne are also fed to the backup circuit 56. 
The power circuit 60 is coupled with the vehicle-mounted battery 3 in two 
ways, one being direct connection for receiving backup voltage Batt and 
the other being a connection via the ignition switch 29 for receiving the 
battery voltage +B. Upon receiving Batt and +B, the power circuit 60 
generates a constant voltage Vsub to be fed to the microcomputer 50 and 
another constant voltage Vc to be fed to remaining circuits. Furthermore, 
the power circuit 60 produces a signal Wi by monitoring the constant 
voltage Vsub, and also produces an initial signal init on the basis of a 
watch-dog clear signal wdc from the microcomputer 50 indicative of the 
normal operation thereof. 
The microcomputer 50 used in the electronic control unit 2 of FIG. 3 may be 
a one-chip IC including well known CPU or microprocessor 70, a ROM 71, a 
RAM 73, an input port 74, an output port 76, a clock generator 78, a 
common bus 76 and so on as shown in FIG. 4. In the illustrated embodiment, 
a wi signal detecting circuit 86 is also built in where the wi signal 
detecting circuit 86 comprises a decoder 81, RS flip-flop 82, an inverter 
83, and a bus driver 84 with a gate. The clock generator 78 is arranged to 
generate a basic clock signal with an external crystal 88 being connected. 
The CPU 70 reads various engine operating conditions via the input port 74 
so as to compute ignition timing, the amount of fuel to be injected and 
fuel injection timing respectively. More specifically, the CPU 70 outputs 
via the output port 76 various signals including a control signal of the 
A/D converter 53, an ignition timing control signal to be fed to the 
backup circuit 56, fuel injection control signals .tau.1 and .tau.2 fed to 
the signal changeover circuit 58, and the above-mentioned watch-dog clear 
signal wdc fed to the power circuit 60. In the above, the fuel injection 
control signal .tau.1 is a control signal for normal or main fuel 
injection performed in synchronizm with engine rotation while the other 
fuel injection control signal .tau.2 is a control signal for asynchronous 
fuel injection on engine start taking place according to the present 
inventionl. This asynchronous fuel injection control signal .tau.2 will be 
described in detail with reference to a flowchart hereinlater. 
Turning back to FIG. 3, the backup circuit 56 is provided as a fail-safe 
circuit so that the operation of the microcomputer 50 is supplemented when 
the microcomputer 50 is put in an abnormal state. During the operation of 
the engine 1, the microcomputer 50 outputs, under the control of the CPU 
70, the ignition timing control signal ig at an interval which is 
determined by the rotational speed Ne of the engine 1, no matter whether 
it is in an engine starting period or not. Therefore, when the ignition 
timing control signal ig is not outputted at a given interval, it is 
determined that the microcomputer 50 is in an abnormal state, and then an 
ignition signal IGt determined by the cylinder-determination signal G1 and 
the rotational speed signal Ne is outputted via the buffer 62 to the 
igniter 23. Simultaneously, a given fuel injection control signal .tau.3 
is fed to the signal-changeover circuit 58 together with a signal fail 
indicating that the microcomputer 50 is in abnormal state. 
The signal-changeover circuit 58 normally outputs, via the buffer 63, a 
fuel injection signal .tau.p which causes the electromagnetic fuel 
injection valves 17 to open and close in receipt of the fuel injection 
control signals .tau.1 and .tau.2 fed from the microcomputer 50. When the 
backup circuit 56 outputs the signal fail by detecting the abnormal state 
of the microcomputer 50, the signal-changeover circuit 58 outputs the fuel 
injection control signal .tau.3 from the backup circuit 56 instead of the 
fuel injection control signals .tau.1 and .tau.2 so as to control the 
electromagnetic fuel injection valves 17 by the fuel injection control 
signal .tau.3. FIG. 5 shows an example of the signal-changeover circuit 58 
formed of known logic gates. 
FIG. 6 is a timing chart showing an example of engine control according to 
the above-mentioned embodiment. 
Reference is now made to FIG. 7 showing the structure of the power circuit 
60. The structure and operation of the power circuit 60 as well as the 
operation of the wi signal detecting circuit 86 within the microcomputer 
50 will be described. As illustrated in FIG. 7, the power circuit 60 
comprises a constant voltage producing portion 93, which produces the 
first constant voltage Vsub fed to the microcomputer 50 and the second 
constant voltage Vc fed to circuits other than the microcomputer 50, a wi 
signal outputting portion 95 for watching the first constant voltage Vsub, 
an initial signal generating circuit 97 for producing an initial signal 
init using the watch-dog clear signal wdc from the microcomputer 50. 
The constant voltage outputting portion 93 comprises a regulator 101 for 
generating the second constant voltage Vc using the battery voltage +B as 
a power source, and another regulator 102 for generating the first 
constant voltage Vsub using the battery voltage Batt which is not fed 
through the ignition switch 29. 
The wi signal outputting portion 95 comprises an operational amplifier OP1 
which monitors the first constant voltage Vsub using a reference voltage 
Vd1 developed internally. When the first constant voltage Vsub drops below 
a determining voltage V2, then the output signal wi from the operational 
amplifier OP1 is made low, and when the constant voltage Vsub rises above 
another determining voltage V1, which is higher than V2, then the output 
signal wi is turned high. The determining voltage V2 is set as a voltage 
above which it can be determined that the operation of the CPU 70 within 
the microcomputer 50 is normal. Similarly, the other determining voltage 
V1 is set as a voltage above which the CPU 70 can determine that the 
control of fuel injection and so on can be restarted. In this way, these 
two determining voltages V1 and V2 have a difference .DELTA.V therebetween 
to exhibit a hystresis characteristic with which undesirable chattering is 
effectively prevented when the microcomputer 50 is turned on and off. The 
change in the constant voltage Vsub is caused from excessive drop in the 
battery voltage Batt beyond the capacity of the regulator 102. The 
determining voltages V1 and V2 are set to be slightly higher than a 
voltage at which an initial signal init is produced. 
The initial signal generating circuit 97 is used to disable the 
microcomputer 50 by producing the initial signal when the CPU is put in 
runaway state due to a drop in power supply voltage or noise or when the 
constant voltage Vsub has dropped below a voltage where normal opertion of 
the CPU 70 cannot be ensured. This initial signal init is also used as an 
initial signal produced when the electronic control unit 2 is turned on. 
The above-mentioned wi siganl from the wi signal outputting portion 95 is 
fed to an S terminal of the RS flip-flop 82 of the wi signal detecting 
circuit 86 within the microcomputer 50 as shown in FIG. 4. Since the 
output from the inverter 83 is normally of high level, when the signal wi 
once turns low, the RS flip-flop 82 is set so that its output Q assumes a 
low level corresponding to signal 0. The CPU 70 outputs a code set in the 
wi signal detecting circuit 86 to open the bus driver 84 having a gate via 
the decoder 81 so as to read the state of the output Q of the RS flip-flop 
82. Apart from this, the CPU 70 is also capable of writing data into the R 
terminal of the RS flip-flop 82 via the decoder 81. A truth table of the 
RS flip-flop 82 is shown below. 
______________________________________ 
R W Q 
______________________________________ 
1 0 0 
1 1 Qn-1 
0 1 1 
0 0 Qn-1 
______________________________________ 
In the above table, Qn-1 indicates that the output Q holds a state thereof 
at the time just before the state of the terminals R and S have been 
changed. Accordingly, when the signal wi turns low once, the state of the 
output Q of the flip-flop 82 is maintained as it stands even though the 
CPU 70 writes level 1 in the wi signal detecting circuit 86. However, when 
the signal wi turns high with the constant voltage Vsub being above the 
determining voltage V1, the state of the output Q is inverted by the 
writing operation from the CPU 70 to assume high level. The code of the wi 
signal detecting circuit 86, which is read and written by the CPU 70, is 
referred to as the WI port. 
The operation of the CPU 70 of the microcomputer 50 in the electronic 
control unit 2 will be described with reference to a flowchart of FIG. 8. 
The CPU 70 is arranged to execute an interrupt service routine shown in 
the flowchart of FIG. 8 at a given interval, such as 4 msec, as a fuel 
injection control on engine start. The contents of processing in each step 
will be described hereinbelow. 
Step 200: It is determined whether the starter motor 32 is driven or not by 
detecting the state of the signal STA. 
Step 210, 220: Logic "1" is written into the WI port, i.e. wi signal 
detecting circuit 86. 
Step 230, 240: It is checked whether the value of WI portion is of logic 
"1" or not. 
Step 260: It is determined whether a variable CTIME corresponding to a 
single asynchronous fuel injection is less then t2 or not. 
Step 270: The variable CTIME is set to t1 which is larger than t2 by 2 or 
more. 
Step 280: The variable CTIME to be used as a count of a counter is reset to 
zero. 
Step 283: A variable INJDLY, which defines a delay time in processing, is 
reset to zero. 
Step 285: It is determined whether the value of the variable INJDLY is 
greater than or equal to 5 or not. Since this control routine is executed 
at an interval of 4 msec, when the variable INJDLY is 5, it means that a 
delay time of 5.times.4 msec is provided. 
Step 288: The variable INJDLY is incremented by 1, i.e. 
INJDLY.rarw.INJDLY+1. 
Step 290: The variable CTIME is incremented by 1, i.e. CTIME.rarw.CTIME+1. 
Step 300: Turn on the fuel injection control signal .tau. to be outputted. 
Step 310: Turn off or maintain the fuel injection control sinal .tau. to be 
outputted. 
The above-mentioned steps are executed in the following order. 
(1) The operation starts at the step 200. When the ignition switch 29 is 
turned on to start the engine 1, the voltage +B of the battery 3 is fed to 
the electronic control unit 2. Since the starter switch 31 of FIG. 2 is 
not closed immediately after the ignition switch 29 is turned on, the 
starter motor 32 is not energized at the very beginning. Therefore, the 
determination in the step 200 results in NO to execute step 210. In step 
210, logic "1" is written in WI port, and then a value t1 is written as 
the variable CTIME in step 270. Then in step 283, the variable INJDLY is 
set to zero, and then the fuel injection control signal .tau.2 is turned 
off in step 310 to complete a first execution of the interrupt routine 
through RTN. 
(2) Meanwhile, the starter switch 31 is closed to cause the starter motor 
32 to receive electrical power from the battery 3 to start driving the 
engine 1. Therefore, when the interrupt routine is started under this 
condition, the determination in step 200 results in YES to execute step 
230 where it is checked whether WI port =1 or not. Since logic "1" has 
been written in the WI port in the former cycle, the value of WI port 
continuously assumes logic "1" unless the constant voltage. Vsub, which is 
fed to the microcomputer 50 as its power supply, drops due to the 
application of load of the starter motor 32. On the other hand, when the 
constant voltage Vsub is below the determination voltage V2, the value of 
WI port is then logic "0". In the case that the battery 3 has sufficient 
capacity so that the constant voltage Vsub does not drop, the 
determination in step 230 results in YES to proceed to step 260 where 
determination of CTIME &lt;t2 is made. Since the value of the variable CTIME 
has been set to t1 in step 270 of the first cycle of the execution of the 
interrupt routine, the determination in step 260 results in NO to execute 
step 310. Accordingly, the fuel injection control signal .tau.2 is 
maintained at its off state to complete the execution of this cycle 
through RTN. 
(3) On the other hand, when the voltage +B of the battery 3 drastically 
drops as the load of the starter motor 32 is applied when the battery 3 is 
in poor condition, and when the constant voltage Vsub fed to the 
microcomputer 50 drops below the determination voltage V2, the 
determination in step 230 results in NO, i.e. WI =1 is not satisfied, to 
proceed to step 220. In step 220, logic "1" is written in WI port, and in 
a subsequent step 240 it is checked again whether WI port is of logic "1" 
or not. Since the value of WI port is not renewed to logic "1" even though 
the CPU 70 writes logic "1" as long as the signal wi is of low level, the 
determination in step 240 results in NO so that the operational flow goes 
to step 310 and then to RTN as described in the above after the constant 
voltage Vsub drops below the determining voltage V2 and before the 
constant voltage Vsub exceeds another determining voltage V1. After the 
constant voltage Vsub exceeds the determining voltage V1 through the 
fluctuation of the load of the starter motor 32, the value of WI port is 
set to logic 1 through the execution of steps 220 and 230, and then the 
determination in step 240 results in YES. This operation is shown in FIG. 
9. In detail, the WI port assumes low level when the signal wi turns low, 
and the state of the WI port returns to high level in response to the 
first data writing by the CPU 70 after the signal wi turns high. 
(4) When the determination in step 240 results in YES, i.e. WI port =1, 
operational flow proceeds to step 280. In step 280, the value of the 
variable CTIME is set to zero under an assumption that conditions for 
starting asynchronous fuel injection have been satisfied. In subsequent 
step 283, the variale INJDLY is set to zero, and then the operational flow 
goes via step 310 to RTN. 
After the above-mentioned processing is performed, the determination in all 
the steps 200, 230 and 260 results in YES to proceed to step 285 whenever 
this interrupt routine is executed until the constant voltage Vsub goes 
below the determination voltage V2. At the beginning the value of the 
variable INJDLY is zero as set so in step 283, and therefore, the 
determination in step 285 results in NO to proceed to step 288 in which 
the variable INJDLY is incremented by 1. Then the operational flow goes to 
step 310 and then to RTN. Therefore, even if the constant voltage Vsub 
exceeds the determination voltage V1, asynchronous fuel injection is not 
immediately performed, and thus fuel injection is not started until this 
interrupt routine is repeated five times with the variable INJDLY being 
counted. 
In a sixth cycle of the interrupt service routine, the determination of the 
step 285, i.e. INJDLY .gtoreq.5 ? , results in YES to proceed to step 290 
in which the variable CTIME is incremented by 1 which variable determines 
the amount of fuel to be injected by the asynschronous injection on engine 
start. In the subsequent step 300, the fuel injection control signal 
.tau.2 is turned on so as to start asynchronous fuel injection on engine 
start. After the completion of step 300, the operational flow goes to RTN 
to terminate the execution of this interrupt routine. When the fuel 
injection control signal .tau.2 turns low, the output signal .tau.p from 
the electronic control circuit 2 becomes active to open the 
electromagnetic fuel injection valves 17. 
(5) When the interrupt routine is executed under the above conditions, 
since the value of WI port is of logic "1" until the constant voltage Vsub 
drops below the determination voltage V2, the determination in step 230 
results in YES, and then in step 260 it is checked whether the variable 
CTIME is less than t2 or not. The value of variable CTIME is set to 0 in 
step 280, and is incremented by 1 each time step 290 is executed. 
Therefore, the determination in step 260 results in YES until the variable 
CTIME reaches t2, i.e. 50 msec in this embodiment, and the value of the 
variable INJDLY is 5 at this time. Thus, the determination in step 285 
results in YES, and steps 290 and 300 follow to perform asynchronous fuel 
injection on engine start through fuel injection control signal .tau.2. 
(6) When 50 msec has lapsed under this condition, then the determination in 
step 260 of CTIME &lt;t2 ? results in NO so that the operational flow goes 
via step 310 to RTN to terminate asynchronous fuel injection. After this, 
the battery voltage +B fluctuates as the starter motor 32 rotates, and 
when the constant voltage Vsub drops below the determination voltage V2 
again and then rises above the higher determination voltage V1, the 
processing is repeated again from the above-mentioned (3). 
(7) The above-mentioned control is continued until the battery voltage +B 
becomes sufficiently high as the result of self rotation of the engine 1 
after ignition takes place so that the constant voltage Vsub never drops 
below the determination voltage V2, or until the starter motor 32 is 
turned off. 
FIG. 10 is a timing chart showing an example of fuel injection control 
performed on engine start which is performed by repeatedly executing the 
interrupt routine of FIG. 8. In detail, the asynchronous fuel injection on 
engine start is performed (see period I in FIG. 10) using the value of the 
variable CTIME as a count of a counter when the constant voltage Vsub goes 
beyond the determination voltage V1 after it drops below the determination 
voltage V2, and is terminated when the variable CTIME equals t2 (see 
period II). Normal fuel injection is performed apart from this 
asynchronous fuel injection such that normal fuel injection is performed 
after the constant voltage Vsub is established (see period III). 
In the above-described first embodiment, the state of the constant voltage 
Vsub, which is power supply voltage fed to the CPU 70, is watched by the 
wi signal outputting portion 95, and when Vsub rises above a voltage, i.e. 
the determination voltae V1, where no problem occurs in connection with 
the resumption of the operation of the CPU 70, asynchronous fuel injection 
of pulse width of 50 msec, which is inherent in engine start, is 
performed. Therefore, even if the constant voltage Vsub varies so that it 
assumes a low voltage with which the normal operation of the CPU 70 cannot 
be ensured, when the constant voltage Vsub exceeds the determination 
voltage V1, the asynchronous fuel injection on engine start is immediately 
started. As a result, fuel injection is accurately performed on engine 
start so that air-fuel mixture is securely sucked into engine cylinders to 
enhance starting characteristic of the engine 1. 
Even if the constant voltage Vsub drops below the determination voltage V2 
so that the init signal is produced by the power circuit 60 to reset the 
microcomputer 50, when the constant voltage is restored to be higher than 
the determination voltage V1, asynchronous fuel injection on engine start 
is executed using the fuel injection control singal .tau.2 without waiting 
for the normal or main fuel injection which is carried out with fuel 
injection duration being computed using the rotational speed Ne of the 
engine 1 and other parameters. Therefore, fuel is securely sucked into 
engine cylinders. 
The present invention can be achieved by adding some electrical circuits to 
conventional control apparatus, and since fuel injection control is 
peformed using a single CPU, the fuel injection system does not require a 
start injecter or other additional fuel supply system so that fuel 
injection on engine start can be obtained with simple construction. 
In the first embodiment, when the constant voltage Vsub is continuously 
below the determination voltage V2 so that the CPU 70 cannot produce 
ignition control signal ig, ignition timing and fuel injection are 
controlled by the backup circuit 56. Therefore, the starting 
characteristic of the engine 1 is substantially perfect with voltage range 
where the starter motor 32 is driven. 
A second embodiment of the present invention will be described with 
reference to FIGS. 11 through 15. The second embodiment is a modification 
of the above-described first embodiment, and therefore only different 
portions are described. FIG. 11 shows the relationship between the wi 
signal detecting circuit 86 and the RAM 73 included in the microcomputer 
50 used in the second embodiment. While the wi signal detecting circuit 86 
and the RAM 73 are arranged as shown in FIG. 4 in the first embodiment, 
the output Q of the RS flip-flop 82 of the wi signal detecting circuit 86 
is used to disable writing operation into the RAM 73 as follows in the 
second embodiment. When the siganl Q assumes low level, a control signal 
line WE connected to read/write (R/W) control terminal is rendered high so 
as to disable writing operation into the RAM 73. More specifically, the 
voltage at the read/write control terminal R/W of the RAM 73 is made 
substantially equal to positive power supply voltage Vc through a 
transistor responsive to the signal Q. 
The operation of the second embodiment will be described with reference to 
a flowchart of FIG. 12. The flowchart of FIG. 12 is similar to FIG. 8, and 
therefore, only different steps will be described. The operation shown by 
the flowchart of FIG. 12 is repeatedly executed as an interrupt service 
routine at an interval of 4 msec in the same manner as in the first 
embodiment, and steps which are newly provided in the second embodiment 
are as follows: 
Step 225: A variable CINJ indicative of the number of times of fuel 
injection on engine start is set to 0, and a variable indicative of the 
amount of fuel to be injected by a single injection is set to t1. 
Step 245: It is checked whether the value of the variable CINJ is smaller 
than n or not. 
Step 250: The variable CINJ is incremented by 1, i.e. CINJ.rarw.CINJ+1. 
Remaining steps in FIG. 12 are the same as those shown in FIG. 8 and 
therefore, description thereof is omitted. 
The above-mentioned newly provided steps 245 and 250 are inserted between 
steps 240 and 280 of the flowchart of FIG. 8, while another new step 225 
is added to follow the step 210. The step 270 which follows the step 210 
in FIG. 8 is now arranged to follow the new step 245. Steps 283, 285 and 
288 of FIG. 8 are not used in the second embodiment. 
The operation of the second embodiment will be described in connection with 
those which are different from the first embodiment. In detail, the 
operation described in connection with the first embodiment with reference 
to FIG. 8 under (1) to (3) are also executed in the second embodiment in 
the same manner, with an exception that step 225 is provided for setting 
the variable CINJ to 0 and another variable CTIME to t1. Now the operation 
following (3) will be described. 
(4) When the determination in step 240 is YES, i.e. WI port =1, the 
operational flow goes to step 245 in which it is determined whether the 
variable CINJ is smaller than n or not. This variable CINJ is used as a 
count of a counter indicative of the number of asynchronous fuel 
injections performed on engine start after the starter motor 32 is turned. 
Therefore, the value of the variable CINJ is zero, i.e. the value is the 
same as it has been set in step 225, in the first determination in step 
245. As a result, the determination of CINJ &lt;n ? in step 245 results in 
YES to proceed to step 250. In step 250, the variable CINJ is incremented 
by 1 to count the number of asynchronous fuel injections. With the 
provision of the steps 245 and 250, therefore, asynchronous fuel injection 
is performed n times. The variable CINJ is stored in a given area of the 
RAM 73. In a following step 280, another variable CTIME is set to zero so 
as to set the duration for the asynchronous fuel injection to a 
predetermined value, such as 50 msec in the same manner as in the first 
embodiment. In a following step 290, the value of the variable CTIME is 
incremented by 1, and the renewed value is stored in a given area of the 
RAM 73. Then in step 300, the fuel injection control signal .tau.2 is 
turned on to proceed to RTN to terminate the execution of the interrup 
routine. When the fuel injection control signal .tau.2 is turned on, the 
output signal .tau.p of the electronic control circuit 2 is made active so 
that the electromagnetic fuel injection valves 17 are opened. 
(5) When the interrupt routine is executed under the above conditions, 
since the value of WI port is of logic "1" until the constant voltage Vsub 
drops below the determination voltage V2, the determination in step 230 
results in YES, and then in step 260 it is checked whether the variable 
CTIME is less than t2 or not. The value of variable CTIME is set to 0 in 
step 280; and is incremented by 1 each time step 290 is executed. 
Therefore, the determination in step 260 results in YES until the variable 
CTIME reaches t2, i.e. 50 msec in this embodiment, and the value of the 
variable INJDLY is 5 at this time. Thus, the determination in step 285 
results in YES, and steps 290 and 300 follow to perform asynchronous fuel 
injection on engine start through fuel injection control signal .tau.2. 
(6) When 50 msec has lapsed under this condition, then the determination in 
step 260 of CTIME &lt;t2 ? results in a NO determination so that the 
operational flow goes via step 310 to RTN to terminate asynchronous fuel 
injection. After this, the battery voltage +B fluctuates as the starter 
motor 32 rotates, and when the constant voltage Vsub drops below the 
determination voltage V2 again and then rises above the higher 
determination voltage V1, the processing is repeated again from the 
above-mentioned (3). Each time the above-mentioned control of (4) is 
executed, the value of the variable CINJ is incremented by 1. 
(7) When the variable CINJ reaches n as the result of successive increment, 
the determination in step 245 becomes YES. Then the variable CTIME is set 
to t1 in step 270, and then the fuel injection control signal .tau.2 is 
turned off in step 310 to terminate the execution of the interrupt 
routine. Therefore, when the total amount of fuel injected through 
asynchronous fuel injection on engine start reaches a value given by 
n.times.50 msec, which is determined by a product of fuel injection 
duration (50 msec in this embodiment) corresponding to t2 and n indicative 
of the number of asynchronous fuel injections, further asynchronous fuel 
injection is not performed irrespective of the state of the constant 
voltage Vsub. 
In the above-described control, writing into the RAM 73 of the 
microcomputer 50 is prevented or prohibited when the constant voltage Vsub 
drops below the determination voltage V2, and therefore, the values of the 
variables CINJ and CTIME are unchanged even if the battery votlage +B 
fluctuates due to the variation in the load of the starter motor 32. 
FIG. 13 is a timing chart showing an example of fuel injection control 
performed on engine start which is performed by repeatedly executing the 
interrupt routine of FIG. 12. In detail, the asynchronous fuel injection 
on engine start is performed (see period I in FIG. 13) using the value of 
the variable CTIME as a count of a counter when the constant voltage Vsub 
goes beyond the determination voltage V1 after it drops below the 
determination voltage V2, and is terminated when the variable CTIME equals 
t2 (see period II). The asynchronous fuel injection carried out each time 
the constant voltage Vsub fluctuates up and down beyond the determination 
voltages V1 and V2 is performed n times in total on engine start. Normal 
fuel injection is performed apart from this asynchronous fuel injection 
such that normal fuel injection is performed after the constant voltage 
Vsub is established (see period III). 
In the above-described second embodiment, the state of the constant voltage 
Vsub, which is power supply voltage fed to the CPU 70, is watched by the 
wi signal outputting portion 95 and when Vsub rises above a voltage, i.e. 
the determination voltage V2, where the operation of the CPU 70 can be 
ensured, the contents of the RAM 73 are kept and preserved, and when Vsub 
rises above a voltage, i.e. the determiantion voltage V1, where no problem 
occurs in connection with the resumption of the operation of the CPU 70, 
asynchronous fuel injection of pulse width of 50 msec, which is inherent 
in engine start, is performed. Therefore, even if the constant voltage 
Vsub varies so that it assumes a low voltage with which the normal 
operation of the CPU 70 cannot be ensured, when the constant voltage Vsub 
exceeds the determination voltage V1, the asynchrnous fuel injection on 
engine start is immediately started. As a result, fuel injection is 
accurately performed on engine start so that air-fuel mixture is securely 
sucked into engine cylinders to enhance starting characteristic of the 
engine 1. Furthermore, since the variable CINJ stored in the RAM 73 is 
kept unchanged, the total amount of fuel to be injected on engine start 
can be maintained constant. Therefore, execessive fuel supply is 
effectively prevented so that spark plugs are prevented from getting wet. 
As a result, undesirable misfiring caused from wet spart plugs is avoided. 
Therefore, starting characteristic of the engine 1 is disirably ensured. 
FIG. 14 shows a modification of the above-described second embodiment. This 
modification differs from the second embodiment in that new steps 550 and 
560 are additionally provided to the flowchart of FIG. 12. The step 550 is 
provided for reading, via the input port 52, engine coolant temperature 
Thw indicated by an output signal from the coolant temperature sensor 33 
(see FIG. 2). Another step 560 is provided for setting the above-mentioned 
value n used in step 445. These new steps 550 and 560 may be provided 
prior to steps 210 and 225 as shown to be executed after the ignition 
switch 29 is turned on and before the starter motor 32 is turned on. This 
value n indicative of the number of times of asynchronous fuel injections 
to be performed on engine start is determined in accordance with the 
detected coolant temperature Thw. For instance, the value of n may be 
determined using a map, one example of which is shown in FIG. 15. 
With the provision of the steps 550 and 560, the total amount of fuel 
(corresponding to n.times.50 msec) to be injected into engine cylinders on 
engine start through the asynchronous fuel injection performed after the 
starter motor 32 is turned on, can be changed depending on the engine 
coolant temperature Thw. 
As a result, the modification of the second embodiment is advantageous when 
the engine 1 is started under low temperature condition. In detail, the 
total amount of fuel is increased when the coolant temperature Thw is low 
because most portion of injected fuel is apt to attach to inner wall of 
the intake pipe or to intake valve when the engine 1 is started under 
completely cooled state. On the other hand, when the coolant temperature 
Thw is not low, the total amount of fuel is suppressed to prevent the 
spark plugs from getting wet. In this way, the starting characteristic of 
the engine 1 is properly ensured. 
The circuit arrangement shown in FIG. 11 for prohibiting writing into the 
RAM 73 is one example, and therefore, it may be replaced with another 
structure. For instance, the initial signal to the microcomputer 50 may be 
produced with the output signal wi from the wi signal detecting circuit 86 
and the initial signal init from the initial signal generating circuit 97 
being ANDed. 
A third embodiment of the present invention will be described with 
reference to FIGS. 16 through 18. The third embodiment differs from the 
above-described embodiments in that the normal fuel injection is 
prohibited during engine start. In the third embodiment, when init signal 
outputted from the power circuit 60 immediately after power is applied, 
disappears, then the CPU 70 performs initialization, and then executes a 
normal fuel injection control routine as one of various controls of the 
engine 1. In the above-mentioned intialization, the contents of internal 
registers of the CPU 70 are cleared, and various flags including normal 
fuel injection prohibiting flag, which will be described hereinlater, are 
reset to initial value, such as 1. 
FIG. 16 shows this normal fuel injection control routine. In a first step 
150, various operating conditions of the engine 1 are read. As the 
operating conditions, engine rotational speed Ne, intake air quantity US, 
the coolant temperature Thw, the battery voltage +B and so on are used. 
Then in a following step 155, it is checked whether the battery voltage +B 
is equal to or higher than a predetermined voltage V3 such as 7 V or not. 
This voltage V3 is determined as a third watching voltage above which the 
operation of the CPU 70 is ensured. 
If +B .gtoreq.V3 is satisfied in the determination of step 155, the 
operational flow proceeds to step 160 to compute the amount of fuel to be 
injected through normal fuel injection using the engine operating 
conditions. This fuel amount is represented by time length .tau.1 
corresponding to duration of fuel injection. The fuel amount to be 
injected through normal fuel injection is determined in accordance with 
engine load, such as Q/Ne wherein Q is intake air quantity. Then 
determined fuel amount is corrected through well known warm up fuel 
increase correction performed using the coolant temperature Thw, and 
acceleration fuel increase correction so as to obtain final fuel amount. 
In a subsequent step 165, a flag F indicative of the prohibition of normal 
fuel injection is reset to 0 since the condition for performing normal 
fuel injection (+B .gtoreq.V3) has been satisfied. Then in a subsequent 
step 170, normal fuel injection is carried out using the fuel amount 
(injection duration) .tau.1 obtained in step 160. 
On the other hand, when the determination in step 155 results in NO, 
namely, when the battery voltage is below approximately 7 V, the 
operational flow proceeds to step 180 in which the flag F is set to 1. 
Then in step 190, normal fuel injection is interrupted, if it has already 
started, and further normal fuel injection is prohibited. After steps 180 
and 190 are completed, the flow goes to NEXR to terminate the present 
control routine. 
Reference is now made to FIG. 17 showing an interrupt service routine used 
in the third embodiment. This interrupt routine is periodically executed 
at an interval of 4 msec in the same manner as in previous embodiments. In 
FIG. 17, steps other than step 205 are the same as those in the flowchart 
of FIG. 8. The step 205 is provided for checking whether the flag F is 1 
or not. If the flag F is 1, the operational flow proceeds to a step 230. 
On the other hand, if the flag F is not 1, the operational flow goes to 
step 270. The initial value of the flag F is 1, and this flag F is set to 
either 1 or 0 depending on the battery voltage +B in the above-described 
normal fuel injection control routine of FIG. 16. When the flag F is set 
to 1, this indicates the prohibition of normal fuel injection. 
The operation of the interrupt service routine of FIG. 17 will be described 
hereinbelow. 
(1) The operation starts at the step 200. When the ignition switch 29 is 
turned on to start the engine 1, the voltage +B of the battery 3 is fed to 
the electronic control unit 2. Since the starter switch 31 of FIG. 2 is 
not closed immediately after the ignition switch 29 is turned on, the 
starter motor 32 is not energized at the very beginning. Therefore, the 
determination in the step 200 results in NO to execute step 210. In step 
210, logic "1" is written in WI port, and then a value t1 is written as 
the variable CTIME in step 270. Then the fuel injection control signal 
.tau.2 is turned off in step 310 to complete a first execution of the 
interrupt routine through RTN. 
(2) Meanwhile, the starter switch 31 is closed to cause the starter motor 
32 to receive electrical power from the battery 3 to start driving the 
engine 1. Therefore, when the interrupt routine is started under this 
condition, the determination in step 200 results in YES to execute step 
205 to check whether the value of the flag F is 1 or not. Since the 
initial value of the flag F is 1, the determination in step 205 results in 
YES to execute step 230 where it is checked whether WI port=1 or not. 
Since logic "1" has been written in the WI port in the former cycle, the 
value of WI port continuously assumes logic "1" unless the constant 
voltage Vsub, which is fed to the microcomputer 50 as its power supply, 
drops due to the application of load of the starter motor 32. On the other 
hand, when the constant voltage Vsub is below the determinatin voltage V2, 
the value of WI port is then logic "0". 
The value of the flag F is 1 until the first determiantion (step 155 in 
FIG. 16) of whether normal fuel injection is to be carried out or not is 
performed, and thus the determination in step 205 results in YES. However, 
in the case that the battery 3 has sufficient capacity so that the 
constant voltage Vsub does not drop, the determination in step 230 results 
in YES to proceed to step 260 where determination of CTIME&lt;t2 is made. 
Since the value of the variable CTIME has been set to t1 in step 270 of 
the first cycle of the execution of the interrupt routine, the 
determination in step 260 results in NO to execute step 310. Accordingly, 
the fuel injection control signal .tau.2 is maintained at its off state to 
complete the execution of this cycle through RTN. 
On the contrary, if it is determined in the normal fuel injection control 
routine that the battery voltage +B is sufficiently high, the value of the 
flag F is reset to 0, and thus the determination in step 205 results in 
NO. As a result, the operational flow goes through steps 270 and 310 to 
RTN so as to turn off the fuel injection control signal .tau.2 in the same 
manner as in the above without performing asynchronous fuel injection on 
engine start at all. Accordingly, when the battery voltage +B is 
sufficiently high, only normal or main fuel injection is performed and the 
asynchronous fuel injection is not carried out. 
(3) On the other hand, when the voltage +B of the battery 3 drastically 
drops as the load of the starter motor 32 is applied when the battery 3 is 
in poor condition, and when the constant voltage Vsub fed to the 
microcomputer 50 drops below the determination voltage V2, the value of 
the flag F is set to 1 so that the determination in step 205 results in 
YES, and the determination in subsequent step 230 results in NO, i.e. WI 
=1 is not satisfied, to proceed to step 220. In step 220, logic "1" is 
written in WI port, and in a subsequent step 240 it is checked again 
whether WI port is of logic "1" or not. Since the value of WI port is not 
renewed to logic "1" even though the CPU 70 writes logic "1" as long as 
the signal wi is of low level, the determination in step 240 results in NO 
so that the operational flow goes to step 310 and then to RTN as described 
in the above after the constant voltage Vsub drops below the determining 
voltage V2 and before the constant voltage Vsub exceeds another 
determining voltage V1. After the constant voltage Vsub exceeds the 
determining voltage V1 through the fluctuation of the load of the starter 
motor 32, the value of WI port is set to logic 1 through the execution of 
steps 220 and 230, and then the determination in step 240 results in YES. 
This operation is shown in FIG. 18. In detail, the WI port assumes low 
level when the signal wi turns low, and the state of the WI port returns 
to high level in response to the first data writing by the CPU 70 after 
the signal wi turns high. 
(4) When the determination in step 240 results in YES, i.e. WI port=1, 
operational flow proceeds to step 280. In step 280, the value of the 
variable CTIME is set to zero under an assumption that conditions for 
starting asynchronous feul injection have been satisfied. After this, the 
operational flow proceeds to step 290 in which the variable CTIME is 
incremented by 1 which variable determines the amount of fuel to be 
injected by the asynschronous injection on engine start. In the sebsequent 
step 300, the fuel injection control signal .tau.2 is turned on so as to 
start asynchronous fuel injection on engine start. After the completion of 
step 300, the operational flow goes to RTN to terminate the execution of 
this interrupt routine. When the fuel injection control signal .tau.2 
turns low, the output signal .tau.p from the electronic control circuit 2 
becomes active to open the electromagnetic fuel injection valves 17. 
(5) When the interrupt routine is executed under the above conditions, 
since the value of WI port is of logic "1" until the constant voltage Vsub 
drops below the determination voltage V2, the determination in step 230 
results in YES, and then in step 260 it is checked whether the variable 
CTIME is less than t2 or not. The value of variable CTIME is set to 0 in 
step 280, and is incremented by 1 each time step 290 is executed. 
Therefore, the determination in step 260 results in YES until the variable 
CTIME reaches t2, i.e. 50 msec in this embodiment. Steps 290 and 300 
follow to perform asynchronous fuel injection on engine start through fuel 
injection control signal .tau.2. 
(6) When 50 msec has lapsed under this condition, then the determination in 
step 260 of CTIME t2 ? results in NO so that the operational flow goes via 
step 310 to RTN to terminate asynchronous fuel injection. After this, the 
battery voltage +B fluctuates as the starter motor 32 rotates, and when 
the constant voltage Vsub drops below the determination voltage V2 again 
and then rises above the higher determination voltage V1, the processing 
is repeated again from the above-mentioned (3). 
(7) The above-mentioned control is continued until the battery voltage +B 
becomes sufficiently high as the result of self rotation of the engine 1 
after ignition takes place so that the value of the flag F assumes 0 and 
the constant voltage Vsub never drops below the determination voltage V2, 
or until the starter motor 32 is turned off. 
FIG. 18 is a timing chart showing an example of fuel injection control 
performed on engine start which is performed by repeatedly executing the 
interrupt routine of FIG. 17. In detail, the asynchronous fuel injection 
on engine start is performed (see period I in FIG. 18) using the value of 
the variable CTIME as a count of a counter when the constant voltage Vsub 
goes beyond the determination voltage V1 after it drops below the 
determination voltage V2 under a condition where the battery voltage +B is 
below the predetermined voltage V3, and is terminated when the variable 
CTIME equals t2 (see period II). Normal fuel injection is prohibited at 
this time. On the other hand, normal fuel injection is carried out when 
the power supply voltage of the electronic control unit 2 is established 
with the battery voltage +B becoming above the predetermined voltage V3, 
through normal fuel injection control (see period III). 
In the above-described third embodiment, the state of the constant voltage 
Vsub, which is power supply voltage fed to the CPU 70, is watched by the 
wi signal outputting portion 95 when the battery voltage +B drops below 
the predetermined voltage V3, and when Vsub rises above a voltage, i.e. 
the determination voltae V1, where no problem occurs in connection with 
the resumption of the operation of the CPU 70, asynchronsou fuel injection 
of pulse width of 50 msec, which is inherent in engine start, is 
performed. Therefore, even if the constant voltage Vsub varies so that it 
assumes a low voltage with which the normal operation of the CPU 70 cannot 
be ensured, when the constant voltage Vsub exceeds the determination 
voltage V1, the asynchrnous fuel injection on engine start is immediately 
started. As a result, fuel injection is accurately performed on engine 
start so that air-fuel mixture is securely sucked into engine cylinders to 
enhance starting characteristic of the engine 1. 
Even if the constant voltage Vsub drops below the determination voltage V2 
so that the init signal is produced by the power circuit 60 to reset the 
microcomputer 50, when the constant voltage is restored to be higher than 
the determination voltage V1, asynchronous fuel injection on engine start 
is executed using the fuel injection control singal .tau.2 without waiting 
for the normal or main fuel injection which is carried out with fuel 
injection duration being computed using the rotational speed Ne of the 
engine 1 and other parameters. Therefore, fuel is securely sucked into 
engine cylinders. 
Moreover, when the asynchronous fuel injection on engine start is carried 
out, the normal or main fuel injection is prohibited, and therefore, 
excessive fuel supply due to simultaneous fuel injection by both types of 
fuel injections can be effectively prevented. This is achieved since each 
of normal fuel injection and asynchronous fuel injection is exclusively 
performed using the flag F. As a result, only necessary amount of fuel is 
supplied to the engine on engine start. 
From the foregoing description it will be understood that fuel injection is 
accurately performed on engine start without using conventional start 
injectors or an additional fuel supply system so that desirable starting 
characteristic of an internal combustion engine is ensured according to 
the present invention. The above-described embodiments are just examples 
of the present invention, and therefore, it will be apparent for those 
skilled in the art that many modifications and variations may be made 
without departing from the scope of the present invention.