Ignition control system for an internal combustion engine

An ignition control system for an internal combustion engine capable of determining appropriate ignition timing in an exact manner without employing a crank angle sensor so as to optimize the combustion of the mixture in the engine cylinders. To this end, a reference ignition timing is first determined based on the operating conditions (e.g., load condition) of the engine, and the internal pressure in the engine cylinder is sensed so that the state of combustion of a mixture in the engine cylinder is detected based on the engine cylinder internal pressure thus sensed. The reference ignition timing is then modified to provide an optimum ignition timing by an amount of modification which is calculated by using a peak point in time of the cylinder internal pressure at which a peak of combustion pressure in the engine cylinder takes place. A specific pressure in the engine cylinder which corresponds to the optimum ignition timing is calculated so that ignition is effected when the internal pressure in the engine cylinder as sensed becomes equal to the specific pressure thus calculated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an ignition control system for an internal 
conbustion engine in which ignition timing is controlled by a 
microcomputer. 
2. Description of the Related Art 
In a known ignition control system for an internal combution engine, when 
the engine is operating, the rotational position of the crankshaft is 
first sensed by a crank angle sensor which is mounted on a camshaft of a 
distributor, the crank angle sensor outputting a signal representative of 
the sensed crankshaft rotational position to a microcomputer including a 
CPU, a RAM and a ROM so that the signal is temporarily stored in the RAM. 
An intake pressure sensor senses the intake pressure in the intake pipe or 
manifold of the engine representative of the engine load condition and 
outputs a signal representative of the sensed intake pressure to the CPU 
through an analog to digital (A/D) converter, which converts the output in 
the form of an analog signal of the intake pressure sensor into a digital 
signal, which is temporarily stored in the RAM. The CPU calculates the 
number of revolutions per minute of the engine based on an ignition period 
which is obtained through the crank angle sensor by using a calculation 
program previously stored in the ROM. Based on the number of revolutions 
per minute of the engine thus calculated and the output signal from the 
intake pressure sensor representative of the engine load condition, the 
CPU determines a target ignition timing from an ignition timing map 
previously stored in the ROM. The target ignition timing is then fed to an 
ignition device to turn on and off a power transistor incorporated therein 
for temporarily interrupting the current supply to the primary side of an 
ignition coil so that a high voltage is thereby generated on the secondary 
side of the ignition coil, thus causing a spark plug to electrically spark 
and ignite the air/fuel mixture in the corresponding engine cylinder. 
With the above-mentioned ignition control system in which ignition timing 
is determined based on the output signal of the crank angle sensor, 
determination of ignition timing can not be made at all in the case of 
failure in the crank angle sensor, with the result that the engine is 
stopped. Further, the mounting position of the crank angle sensor, which 
is mounted on the camshaft in the distributor by a fastener such as a 
fastening belt or the like, may be sometimes displaced from the original 
correct position due, for example, to loosening or slackening of the 
fastening belt or the like so that the exact rotational position of the 
crankshaft can not be sensed. 
SUMMARY OF THE INVENTION 
In view of the above, the present invention is intended to obviate the 
above described problems of the known ignition control system. 
An object of the present invention is to provide an ignition control system 
for an internal combustion engine which is capable of determining 
appropriate ignition timing in an exact manner without employing a crank 
angle sensor so as to optimize the combustion of the mixture in the engine 
cylinders. 
In accordance with the above and other objects, the present invention 
resides in an ignition control system for an internal combustion engine 
having an engine cylinder and a crankshaft comprising: 
cylinder internal pressure sensing means for sensing the pressure in the 
engine cylinder and generating an output representative of the sensed 
engine cylinder pressure; 
first ignition timing determining means for determining a reference 
ignition timing based on the operating conditions of the engine; 
ignition timing modifying means connected to receive the output of the 
cylinder internal pressure sensing means for sensing the state of 
combustion of a mixture in the engine cylinder based thereon and 
determining an amount of modification of ignition timing with respect to 
the reference ignition timing so as to optimize the ignition timing; 
second ignition timing determining means for determining an optimum 
iginition timing based on the reference igintion timing and the amount of 
modification of ignition timing: 
reference signal generating means for calculating a specific pressure in 
the engine cylinder which corresponds to the optimum ignition timing 
determined by the second ignition timing determining means and generating 
a reference signal representative of the specific pressure; 
ignition signal generating means for generating an ignition signal at the 
time when the output of the cylinder internal pressure sensing means 
becomes equal to the output of the reference signal generating means; and 
igniting means connected to receive the output of the ignition signal 
generating means for igniting the engine when it receives an ignition 
signal from the ignition signal generating means. 
Preferably, the ignition timing determining means comprises: engine load 
sensing means for sensing an engine load; ignition period sensing means 
for sensing the ignition period of the engine; and a microcomputer 
connected to receive the outputs of the engine load sensing means and the 
ignition period sensing means, the mircocomputer calculating the number of 
revolutions per minute of the engine based on the output of the ignition 
period sensing means, the microcomputer further determining the reference 
ignition timing based on the sensed engine load and the calculated number 
of revolutions per minute of the engine. 
Preferably, the ignition timing modifying means comprises: peak sensing 
means for sensing a peak point in time at which a peak of combustion 
pressure in the engine cylinder takes place; and a microcomputer connected 
to receive the output of the peak sensing means for calculating an average 
value of the difference between the sensed peak point and the reference 
ignition timing every engine cycle and calculating the amount of 
modification based on the average value of the difference. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following detailed description of 
a presently preferred embodiment of the invention taken in conjunction 
with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will now be described in detail with reference to a 
presently preferred embodiment thereof as illustrated in the accompanying 
drawings. 
In FIG. 1, there is schematically illustrated in block form an ignition 
control system for an internal combustion engine which includes a load 
sensor 1 in the form of an intake pressure sensor for sensing an engine 
load (e.g., the intake pressure in an intake pipe or manifold of the 
engine), and an analog to digital (A/D) converter circuit 2 for converting 
the analog output of the intake pressure sensor 1 into a digital signal 
which is appropriate for precessing by a microcomputer 3. The 
microcomputer 3 comprises a port 4 connected at its input side with the 
output terminal of the A/D converter circuit 2 through a bus, a CPU 5 
connected through a bus with the port 4, a RAM 6 and a ROM 7 connected 
with the CPU 5 through respective buses, and an output port 8 connected 
with the CPU 5. The ROM 7 beforehand stores an ignition timing 
determination program, a number-of-revolutions-per-minute versus 
ignition-timing map, an engine-load versus ignition-timing map and the 
like. The output port 8 of the microcomputer 3 is connected with an 
ignition device 11 in the form of a well-known type igniter through an 
interface 9. The ignition device 11 has a power transistor (not shown) 
coupled with the primary side of an ignition coil 12 so that the power 
transistor is turned on and off to generate a high voltage on the 
secondary side of the ignition coil 12, thereby causing a spark plug 13 
connected with the secondary side of the ignition coil 12 to spark, 
whereby the air/fuel mixture in the engine cylinder is ignited. 
A cylinder internal pressure sensor 14 is provided for sensing the pressure 
in a combustion chamber of an engine cylinder, an interface 15 coupled at 
its input side with the cylinder internal pressure sensor 14, a knocking 
sensor 16 coupled at its input side with the cylinder internal pressure 
sensor 14 for sensing knocking in the engine cylinder based on the output 
of the cylinder internal pressure sensor 14 and at its output side with 
the port 4 for outputting a knocking signal to the port 4 when knocking is 
sensed, a peak position sensor 17 coupled at its input side with the 
interface 15 for sensing the position (i.e., point in time) of the peak in 
the output of the cylinder internal pressure sensor 14 and at its output 
side with the port 4, and a comparator 18 having two input terminals one 
of which is coupled with the interface 15 and the other of which is 
coupled with the output side of a digital to analog (D/A) converter 19, 
and one output terminal coupled with the port 4. The D/A converter 19 is 
coupled at its input side with the CPU 5 through a bus and the port 4. An 
ignition period sensor 20 for sensing the period of ignition of the engine 
is also connected with the CPU 5 through the port 4. 
FIG. 2 is a flowchart showing a main routine for the operation of the 
ignition control system of FIG. 1. As illustrated in this flowchart, the 
output of the intake pressure sensor 1 representative of the sensed 
pressure in an intake pipe or manifold of the engine is converted from an 
analog value into a digital value by the A/D converter 2, and the digital 
value thus converted is then input to the CPU 5 of the microcomputer 3 
through the port 4 in Step S1. Subsequently in Step S2, the CPU 5 
calculates the number of revolutions per minute of the engine based on the 
period of ignition which is sensed by the ignition period sensor 20. The 
control program then proceeds to Step S3 where a reference ignition timing 
corresponding to the intake pressure sensed by the intake pressure sensor 
1 and the number of revolutions per minute of the engine as previously 
determined in Steps S1 and S2 is determined from an ignition timing map 
stored in the ROM 7. Thereafter in Step S4, the CPU 5 determines from a 
combustion pressure peak map stored in the ROM 7 a specific crank angle 
corresponding to an optimum combustion pressure peak based on the sensed 
intake pressure and the calculated number of revolutions per minute of the 
engine. In Step S5, the specific crank angle thus determined is converted 
by the microcomputer 3 into a corresponding point in time which is then 
set into a built-in timer counter. After S5, the main routine is 
interrupted so that the control program jumps into a 
combustion-pressure-peak-position determining routine as illustrated in 
FIG. 3(a). 
Thus, in Step S6 as shown in FIG. 2(a), a difference in time between a 
point in time at which a peak in the combustion pressure appears and the 
value previously set into the timer counter is calculated, and in Step S7, 
the difference thus calculated is averaged every engine cycle. After Step 
S7, the control program returns to the main routine, i.e., it goes to Step 
S8 in FIG. 2 where an amount of modification of ignition timing is 
calculated based on the averaged value of the difference. Subsequently in 
Step S9, a specific ignition timing is determined as the sum of the 
reference ignition timing and the amount of modification of ignition 
timing. Then is Step S10, the pressure in the combustin chamber of the 
engine cylinder corresponding to the thus determined specific ignition 
timing is calculated from the intake pressure in the intake pipe or 
manifold as sensed by the intake pressure sensor 1 in the following 
manner. 
Designating the pressure in and the internal volume of the combustion 
chamber in the engine cylinder as P and V, respectively, the following 
equation is established: 
EQU P.multidot.V=constant. 
Here, 
EQU V=r(1-cos .theta.).multidot.S+.alpha. (1) 
where 
r=the radius of rotaion of the crank shaft 
.theta.=the crank angle measured from the point at which iginition occurs; 
S=the cross-sectional area of the piston; and 
.alpha.=the volume of the combustion chamber at TDC. 
In this connection, it is to be noted that r, S and .alpha. are 
respectively of constant values. 
Accordingly, the internal pressure P in the combustion chamber of the 
cylinder (hereinafter referred to as cylinder internal pressure) is 
described as follows. 
EQU P={r(1-cos .theta..sub.0).multidot.S+.alpha.}/{r(1-cos 
.theta.).multidot.S+.alpha.}.multidot.P.sub.0 (2) 
where 
P.sub.0 =a reference pressure (equal to the pressure in the intake pipe or 
manifold); and 
.theta..sub.0 =a reference crank angle (corresponding to the crank angle at 
which an intake valve is closed). 
Substituting known values for the constants in equation (2) above, the 
cylinder internal pressure P is calculated as a function of the crank 
angle .theta.. Thus, a specific cylinder internal pressure for ignition is 
ignition is to take place. 
Thereafter in Step S11, the CPU 5 outputs the result of the above 
calculation (i.e., the specific cylinder internal pressure) thus obtained 
to the D/A converter 19 through the port 4. The D/A converter 19 converts 
the digitalized input from the CPU 5 into an analog value which is then 
input to one of the input terminals of the comparator 18. 
On the other hand, imposed on the other input terminal of the comparator 18 
is the output of the cylinder internal pressure sensor 14 through the 
interface 15. When the output of the cylinder internal pressure sensor 14 
becomes equal to the calculated specific value which is input to the one 
input terminal of the comparator 18 from the CPU 5, the comparator 18 
outputs a matching signal to the CPU 5 through the port 4. When the CPU 5 
receives the matching signal, the main routine is interrupted so that the 
control program jumps into a usual ignition interrupt routine as shown in 
FIG. 3 (b) wherein ignition is effected in a conventional manner. That is, 
the ignition signal, which is output from the CPU 5 to the ignition device 
11 through the output port 9 and the interface 10, is inverted into the 
ignition mode (e.g., from a low to a high level) so as to start the 
current supply to the ignition coil 12 in Step S12, and at the same time a 
timer incorporated in the ignition device 11 is actuated in Step S13. When 
a predetermined time set in the timer has elapsed, the current supply to 
the ignition coil 12 is stopped so that a high voltage is developed on the 
secondary side of the ignition coil 12 thereby to cause the spark plug 13 
to spark. In this manner, it is possible to precisely control ignition 
timing by use of the cylinder internal pressure without employing any 
crank angle sensor.