Ignition timing control system for internal combustion engine

A system for controlling ignition timing of an internal combustion engine which is capable of securing adequate ignition energy in all engine operation regions including acceleration or high speed engine operation. The retard limit for ignition timing is determined with respect to engine speed. A ratio of dwell angle on an ignition coil to a prescribed angle is calculated and compared with reference value. When the ratio is found to exceed the reference value, the retard limit is changed in advance direction so that ignition timing is caused to be advanced. Misfirings are thus avoided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an ignition timing control system for internal 
combustion engines, and more particularly to an ignition timing control 
system for internal combustion engines which is capable of conducting 
ignition timing retard control so as to enable securement of adequate 
ignition energy in all engine operation regions and specifically to enable 
securement of adequate ignition energy for eliminating any danger of 
misfirings during times of engine speed change such as during acceleration 
and during high speed engine operation. 
2. Description of the Prior Art 
Recent years have seen the proposal of a number of internal combustion 
engine ignition timing control systems which include ar ignition timing 
retard adjustment that is activated during acceleration or the like for 
retarding the ignition timing in crank angle degrees so as to reduce 
engine output and thus suppress vehicle vibration. The assignee, for 
example, applied for patent on such a system under Japanese Patent 
Application No. 1(1990)-45373 filed on Apr. 18, 1989. 
The limit value of the ignition timing on the retarded side is determined 
by the relationship between the required voltage (the voltage necessary 
for breakdown of the insulation (air gap) between the electrodes of the 
spark plug) and the generated voltage (the voltage generated by the 
ignition coil). Specifically, as shown in FIG. 7, the limit 
.theta..sub.igLGG on the ignition timing retard side centers on a battery 
voltage of 14.3 V and is set to increase in proportion as the engine speed 
increases. Therefore, if the ignition timing should be retarded when the 
engine speed is high or when the battery voltage is low, it may be 
impossible to provide the required voltage, which may, in the worst case, 
result in misfirings. 
More specifically, where the TDC (top dead center) interval is represented 
by T as indicated in FIG. 8 and the dwell angle of the ignition coil, that 
is the time during which the current control transistor of the primary 
coil is on, is represented by t, the ON duty ratio (hereinafter sometime 
called simply the "duty ratio") can be represented by t/T. So as to secure 
the energy required for ignition when the battery voltage V.sub.B 
decreases, the ordinary practice is to establish a compensation 
coefficient K as shown in FIG. 9 and to increase the conductive period of 
the transistor to that obtained by multiplying the duty ratio (t/T) by the 
coefficient. As shown in FIG. 10, when the decrease in battery voltage 
becomes large, the duty ratio also becomes large, with the result that the 
amount by which the ignition timing is retarded increases, increasing the 
likelihood of misfirings. More specifically, as will be understood from 
FIG. 10, the shortening of the ignition period (TDC interval) with 
increasing engine speed makes it difficult to secure the required 
conductive period, while, as can be seen in FIG. 11, the required voltage 
grows greater with respect to the generated voltage with increasing 
proximity of the crank angle to TDC. This is because it is a general rule 
that the insulation breakdown voltage increases with increasing 
compression ratio in the combustion chamber, and since retarding the 
ignition timing is tantamount to shifting the ignition timing toward the 
compression point, this means that the required voltage becomes higher. 
Therefore, if retard adjustment, which tends to increase the required 
voltage, should be carried out when the decrease in battery voltage 
becomes large or the engine speed becomes high, there may in some cases 
occur misfirings. Since at this time the aforesaid retard control is 
implemented in view of factors on the power transmission side, if 
conducted during high engine speed operation, as during acceleration or 
the like, it may make it impossible for the generated voltage to satisfy 
the required voltage and, as a result, may in some cases lead to 
misfirings. 
SUMMARY OF THE INVENTION 
In view of the foregoing, one object of the present invention is to provide 
an ignition timing control system for internal combustion engines for 
overcoming the aforesaid problems of the conventional systems. 
Another object of the invention is to provide such a system which is 
capable of conducting ignition timing retard control so as to enable 
securement of adequate ignition energy in all engine operation regions and 
specifically to enable securement of adequate ignition energy for 
eliminating any danger of misfirings during high engine speed operation 
and the like. 
This invention achieves these objects by providing a system for controlling 
an ignition timing of an internal combustion engine, comprising first 
means for detecting operating condition of the engine, second means for 
determining dwell angle of an ignition coil on the basis of the detected 
engine operating condition, third means for determining a retard limit on 
the basis of the detected engine operating condition such that ignition 
timing of the engine is determined within the retard limit, fourth means 
for calculating a ratio of the dwell angle to a prescribed angle and then 
for comparing the ratio with a reference value and fifth means for 
changing the retard limit in the advance direction when the ratio is found 
to exceed the reference value.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will now be explained with reference to a specific 
embodiment. 
FIG. 1 shows the overall arrangement of an ignition timing control system 
for internal combustion engines in accordance with this invention. A 
six-cylinder vehicle internal combustion engine 10 (shown partially in 
schematic representation) has an air intake passage 12 provided at its 
distal end with an air cleaner 14 and at an intermediate portion thereof 
with a throttle valve 16. Air drawn in through the air cleaner 14 has its 
flow rate controlled by the throttle valve 16 and passes through an intake 
manifold 18 to the combustion chambers 20 (only one shown) of the engine 
cylinders. A pipe 24 branches off from the air intake passage 12 at an 
appropriate position downstream of the throttle valve 16. The pipe 24 is 
provided near its far end with a manifold absolute pressure sensor 26 
which detects the engine load by measuring the absolute value of the 
intake air pressure. A coolant temperature sensor 30 is provided i: the 
vicinity of a cooling water passage 28 of the internal combustion engine 
10 for detecting the temperature of the engine cooling water and a 
manifold air temperature sensor (not shown) is provided at an appropriate 
position downstream of the throttle valve 16 for detecting the temperature 
of the air drawn into the engine. A throttle position sensor 32 is further 
provided at an appropriate position on the engine for detecting the degree 
of opening of the throttle valve 16. 
The internal combustion engine 10 has a distributor 36 which includes a 
crankshaft sensor 40 comprised of a magnet which rotates in synchronism 
with a crankshaft (not shown) rotated by the reciprocal motion of pistons 
38 and a stationary member disposed opposite the magnet. The crankshaft 
sensor 40 outputs a signal once every predetermined angle of crankshaft 
rotation. At an appropriate location on a cylinder block 42 of the 
internal combustion engine 10 is provided a piezoelectric detonation 
sensor 44 for detecting vibration produced by combustion knock arising 
within the combustion chambers 20, while at an appropriate portion of the 
vehicle there is provided a vehicle speed sensor 46 for detecting the 
speed at which the vehicle is traveling. The voltage of the battery (not 
shown) is further detected by an appropriately disposed battery voltage 
sensor 48. The exhaust passage 50 of the internal combustion engine 10 is 
provided with a three-way catalytic converter 52 for reducing the amount 
of pollutants in the exhaust gas before it is emitted into the atmosphere. 
Immediately upstream of the three-way catalytic converter 52 is provided 
an oxygen sensor 54 for detecting the oxygen concentration of the exhaust 
gas. The output of the manifold absolute pressure sensor 26 and the other 
sensors 30, 32, 40, 44, 46, 48 and 54 are forwarded to a control unit 60. 
The arrangement of the control unit 60 is illustrated in FIG. 2. The 
outputs from the manifold absolute pressure sensor 26 and the other 
sensors are input to a level converting circuit 62 in the control unit 60 
for adjustment to a prescribed level and are then forwarded to a 
microcomputer 64. The microcomputer 64 comprises an A/D converter 64a, an 
I/O circuits 64b,64f, a CPU (central processing unit) 64c, a ROM 
(read-only memory) 64d, a RAM (random access memory) 64e, a counter for 
computation and a timer (the two last mentioned members not being shown). 
The signals output by the level converting circuit 62 are converted to 
digital values by the A/D converter 64a in accordance with commands from 
the CPU 64c and are then temporarily stored in the RAM 64e. The digital 
outputs of the crankshaft sensor 40 etc. are shaped in a wave shaping 
circuit 66 and then input to the microcomputer 64 through the I/O circuit 
64b. After being sent to the control unit 60, the output from the 
detonation sensor 44 is input to a detonation detection circuit 68 where 
it is discriminated whether or not knock has occurred and the result of 
the discrimination is input to the microcomputer 64 via the I/O circuit 
64b. 
As will ba explained in more detail later, the microcomputer 64 calculates 
the engine speed from the output of the crankshaft sensor 40, judges the 
engine load state from the output of the manifold absolute pressure sensor 
26, retrieves a basic ignition timing map from the ROM 64d and uses the 
same for calculating a basic ignition timing, adjusts the bas-c ignition 
timing on the basis of the intake air temperature and other parameters, 
further adjusts the so-obtained ignition timing if engine knock has 
occurred thereby obtaining the final ignition timing and issues an 
ignition command via the I/O circuit 64f and an output circuit 70 to an 
igniter or other such ignition device 72, thereby causing a spark plug 74 
of a prescribed cylinder selected by the distributor 36 to fire and ignite 
the air-fuel mixture in the associated combustion chamber 20. 
The operation of the system will now be explained with reference to the 
flowchart of FIG. 3. Execution of the program indicated in FIG. 3 is 
initiated in the microcomputer 64 once every prescribed crank angle. 
First, in step S10 a basic ignition timing .theta..sub.igM is determined on 
the basis of a map value retrieved from the ROM 64d of the microcomputer 
64 using the engine speed and the intake air pressure (engine load) as 
address data. Next, in step S12 a water temperature correction value 
determined from the output of the coolant temperature sensor 30 etc., and 
a knock correction value output of the detonation detection circuit 68 are 
combined to obtain a correction value .theta..sub.igCR. The correction 
value .theta..sub.igCR can be either positive (indicating a timing 
adjustment in the advance direction) or negative (indicating a timing 
adjustment in the retard direction). The procedure then moves to step S14 
in which the basic ignition timing and the correction value are added 
together to obtain the final ignition timing .theta..sub.ig. Then 
procedure advances to step S16 in which a reference value 
.theta..sub.igLGG (the aforesaid retard limit) is determined. The 
characteristics of this reference value are shown in FIG. 5 and are 
similar to those shown in FIG. 7 referred to earlier. They are basically 
arranged to be retrievable on the basis of the engine speed and are stored 
in the ROM 64d by an appropriate method in advance and is amended in a 
manner which will be explained with reference to FIG. 4 flowchart. 
Namely, the duty ratio is firstly calculated in step S100. As was explained 
earlier with reference to FIG. 10, the characteristics of the duty ratio 
are stored in the ROM 64d so as to be retrievable using the engine speed 
N.sub.e and battery voltage V.sub.B as address data. This can be 
calculated either as an angular ratio or as a time ratio. Then in the 
following step S102, a reference(critical) duty ratio DUTY.sub.GH is 
retrievee. This is a fixed value, e.g. 82% as is shown in FIG. 10. More 
specifically, the value is a limit value and is made variable depending on 
the operating condition of the engine. The procedure then moves to step 
S104 where the calculated duty ratio and the reference duty ratio are 
compared. When it is found that the calculated value does not exceed the 
reference value, the procedure advances to step S106 wherein the aforesaid 
retard limit is maintained (is not changed), while when it is found that 
the calculated value exceeds the reference value, the procedure moves to 
step S108 in which a unit value Delta IG.sub.X is added to the retard 
limit, shifting the retard limit value in the advance direction by such 
amount. FIG. 6 illustrates the characteristics of the unit value Delta 
IG.sub.X, As shown, this value is appropriately set as a function of the 
engine speed or the duty ratio. Since as the values increases, the 
possibility of misfiring increases. 
Again returning to FIG. 3 flowchart, the procedure then advances to step 
S18 in which the calculated final ignition timing is compared with the 
retard limit and if it is found that final ignition timing exceeds the 
retard limit in the retard direction (i.e. it is less than the retard 
limit, the procedure advances to step S20 in which the final ignition 
timing is restricted to the retard limit and then to step S22 in which the 
limit value is determined as the final ignition timing and output. On the 
other hand if it is found that the final ignition timing has not reached 
the retard limit the calculated ignition timing is output in step S22 as 
it is. As a result, the ignition timing is held on the side of the limit 
where there is no possibility of misfirings occurring and the ignition 
timing can be retarded within the restriction of this limit without danger 
of misfirings. 
Being arranged in the manner described above, this embodiment of the 
invention enables the ignition timing to be held within the region within 
which it is possible to secure adequate ignition energy and, as such, 
makes it possible, as necessary, to conduct appropriate retard control 
within this range while precluding the occurrence of combustion misses. It 
therefore makes it possible to eliminate any danger of misfirings when, 
for example, retard control is conducted during vehicle acceleration for 
the purpose of suppressing vehicle body vibration. Moreover, since the 
possibility of combustion misses can be reliably ascertained solely by 
detection of the duty ratio, this embodiment is also simple structurally. 
While in the embodiment explained in the foregoing, the unit value Delta 
IG.sub.X by which the retard limit is changed is set as a function of the 
engine speed or the duty ratio, it can be made a fixed value. 
The present invention has thus been shown and described with reference to 
the specific embodiments. However, it should be noted that the present 
invention is in no way limited to the details of the described 
arrangements but changes and modifications may be made without departing 
from the scope of the appended claims.