Ignition control for an engine to prevent overheating and backfiring

An arrangement for protecting an engine from an abnormal condition such as overheating by slowing of the engine through periodic misfiring of the spark plugs. In order to prevent backfiring, the engine is stopped if the slowed condition is such that backfiring might occur.

BACKGROUND OF THE INVENTION 
This invention relates to an engine control device for an internal 
combustion engine and more particularly to an arrangement for slowing the 
speed of an engine in response to an overheating condition and preventing 
of backfiring of the engine through its exhaust system when the slowing 
condition is initiated. 
A wide variety of protective devices are provided for internal combustion 
engines such as spark ignited engines. For example, arrangements have been 
provided for sensing the operating temperature of the engine and for 
effecting a slowing of the engine in response to engine temperatures 
greater than a predetermined value. One popular way in which the engine 
speed is slowed in response to the sensing of an over temperature 
condition is by misfiring the spark plugs of the ignition system. Such 
misfiring will slow the engine speed. An example of an arrangement for 
slowing the engine under such conditions through misfiring of the spark 
plugs is disclosed in U.S. Pat. No. 4,459,951, entitled Overheat 
Preventing System For Internal Combustion Engines, issued July 17, 1984 in 
the name of Motoi Tobinaga et al. Although such arrangements have 
particular utility, most engine ignition systems include a spark advance 
mechanism whereby the time of firing of the spark plug is dependent upon a 
variety of engine running conditions including engine speed. However, when 
the engine is slowed by misfiring, the timing may be such that backfires 
might occur in the engine exhaust system. That is, the delayed timing of 
the firing of the spark plugs can cause hot gases to issue from the 
exhaust port of the engine and create a combustion characteristic in the 
exhaust system commonly known as backfiring or firing back. Such a 
condition is, obviously, undesirable. 
It is, therefore, a principal object of this invention to provide an 
improved arrangement for protecting an engine in response to a dangerous 
condition and, at the same time, insuring that backfiring will not occur 
in the exhaust system of the engine in response to the slowing of the 
engine. 
It is a further object of this invention to provide an improved arrangement 
for protecting an engine in response to an abnormal running condition and 
also insuring that backfiring will not occur in that event in the engine 
exhaust system. 
It is a further object of this invention to provide an arrangement for 
slowing the speed of an engine when an overheating condition occurs and 
also for insuring that backfiring will not occur in the exhaust system due 
to the slowing of the speed of the engine. 
SUMMARY OF THE INVENTION 
This invention is adapted to be embodied in a control for a spark ignited 
internal combustion engine having an exhaust system and including an 
ignition circuit for firing a spark plug at a predetermined time in 
response to the output shaft angle of the engine and the speed of the 
engine. Means are provided for sensing an abnormal engine condition and 
for effecting a change in the firing of the spark plug by the ignition 
circuit in response to the sensing of such an abnormal condition. Means 
further sense a condition that is likely to cause backfiring in the 
exhaust system in response to the operation of the means for effecting the 
change in firing of the spark plug to initiate an operation that prevents 
such backfiring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Referring first to the embodiment of FIG. 1, the ignition circuit for a two 
cylinder internal combustion engine is depicted generally by the reference 
numeral 11. The ignition circuit 11 is particularly adapted for use with 
outboard motors wherein the engine operates on a two stroke crankcase 
compression principle. It is to be understood, however, that the invention 
is capable of use in a wide variety of applications for a wide variety of 
engines. The invention, however, has particular utility in insuring that 
the engine, and particularly its exhaust system, will be protected from 
backfiring conditions when the speed of the engine is slowed due to an 
abnormal condition, such as an over temperature of the engine, by 
misfiring of the spark plug so as to protect the engine. 
The ignition system 11 includes a magneto generator comprised of a charging 
coil 12 and a pair of pulser coils 13 for each of a pair of spark plugs 14 
of the associated engine. As has been previously described, the number of 
cylinders employed can be varied without departing from the spirit and 
scope of the invention and two spark plugs and two cylinders are described 
only as a typical embodiment of the invention. Spark coils or transformers 
15 are associated with each of the spark plugs 14 and have their secondary 
windings in circuit with the terminals of the spark plug 14 in a known 
manner. The ignition circuit 11 includes an ignition control system, 
indicated generally by the reference numeral 16 for operating a 
conventional type of SCR ignition circuit. The charging coil 12 charges a 
charging capacitor 17 associated with each of the spark coils 15 and 
specifically the primary windings thereof through a respective rectifying 
diode 18. There is provided a thyristor 19 in circuit with the charging 
capacitor 17 and a ground circuit. As is well known, the charging 
capacitor 17 is charged during the rotation of the output shaft of the 
engine and when the thyristor 19 is grounded by rendering its gate 
conductive, the capacitor 17 will discharge and generate a voltage in the 
secondary windings of the spark coils 15 for firing the spark plugs. 
The gates of the thyristors 19 are controlled by means of gate circuits 21 
that receive outputs from the pulser coils 13 so as to render the 
thyristors 19 conductive at the appropriate crank angle so as to fire the 
spark plugs. The gate circuits 21 are further controlled by an ignition 
timing operation circuit 22 which, under normal running conditions, sets 
the spark timing in relation to various engine parameters such as actual 
engine speed as sensed by a speed sensing circuit 23, which may sense 
speed as a function of the output of the charging coil 12. There is also 
provided a setable speed signal 24 which supplies a signal to the control 
circuit. Except for the signal 24, the construction of the ignition 
circuit 11 as thus far described may be considered to be conventional and, 
for that reason, further description of it is believed to be unnecessary 
in order to understand the construction and operation of the invention. 
In accordance with the invention, there is further provided a device for 
protecting the associated engine in the event of an overheating condition. 
To this end, there is provided a temperature detector 25 which outputs a 
signal to the ignition timing operation circuit 22 so as to slow the speed 
of the engine in response to the sensing of a temperature greater than a 
predetermined temperature. This is done by misfiring the spark plugs in a 
manner as described in aforenoted U.S. Pat. No. 4,459,951 or as will be 
described in conjunction with the embodiment of FIGS. 2 and 3. Basically, 
this is done by effecting a misfiring of the spark plugs 14 so that they 
will not fire during every revolution of the engine crankshaft 
(remembering that a two cycle engine operation is being referred to). 
However, other forms of protective arrangements for slowing the engine 
speed may be utilized. 
As the engine is slowed by the misfiring of the spark plugs, the timing 
circuit will also tend to retard the spark timing and this combined effect 
may cause a condition known as backfiring and, as aforedescribed, to occur 
in the exhaust system of the engine. In order to prevent this condition, 
there is provided a protective arrangement that operates in accordance 
with the sequence as shown in FIG. 4. 
Referring now to FIG. 4, it will be seen that at the block 26 the running 
of the engine is determined. Then at the block 27, it is determined 
whether or not the temperature sensor 25 has indicated an over temperature 
sensing condition. If it has not, then the program turns back and repeats. 
If, however, at the step 27 it has been determined that there is an over 
temperature condition, the program moves to the step 28 wherein the 
ignition timing operation circuit 22 or a portion of it makes a 
determination to determine if the engine speed V.sub.n is greater than an 
engine speed V.sub.o at which backfiring may occur as set in the signal 
device 24. If the engine speed is above the speed V.sub.o, then the 
program will again repeat. 
If, on the other hand, it is determined that the engine speed is equal to 
or below the speed at which misfiring may occur (V.sub.o) then the engine 
will be stopped at the step 29 by energizing an engine kill circuit 31 
(FIG. 1). The engine kill circuit 31 is effected to ground the output of 
the charging coil 12 so that the charging capcitor 17 will not be charged 
and that firing cannot occur. 
Once the engine has been returned to its normal condition, it will, of 
course, be possible for the operator to restart the engine. 
Referring now to FIG. 2, a second embodiment of the invention is identified 
generally by the reference numeral 51. This embodiment is depicted in 
conjunction with a single cylinder engine, however, as aforenoted, the 
invention can be practiced in conjunction with engines having any number 
of cylinders or spark plugs. 
The engine 51 includes a magneto generator 52 that includes a charging coil 
53 and a pulser coil 54. The charging coil 53 and pulser coil 54 output 
their signals to a capacitor discharge ignition circuit, indicated 
generally by the reference numeral 55. This circuit 55 includes a charging 
capacitor 56 that is charged from the charging coil 53 through a 
rectifying diode 57. As aforenoted, the charging capacitor 56 is charged 
during the rotation of the crankshaft. At the appropriate crankshaft 
angle, as determined by the output from the pulser coil 54, an SCR 58 and 
specifically its gate is rendered conductive by means of a circuit 
including a diode 59 and capacitor resistor circuit 61 so as to ground the 
charging capacitor 57. When this occurs, a spark coil 62 will have a 
voltage induced in its primary winding 63 which is magnified in the 
secondary winding 64 for firing a spark plug 65 in a known manner. A diode 
66 is placed between the charging coil 53 and diode 57 for providing a 
circuit during the negative half way of the charging coil 53. A similar 
diode 68 is provided in the circuit between the charging capacitor 56 and 
the primary winding 63 of the ignition coil and the ground. 
There is also provided a kill switch 69 that is in circuit between the 
charging coil 56 and the ground for grounding the charging circuit to the 
charging coil 56 and stopping of the engine under the operator control. 
Like the circuit of FIG. 1, the ignition system 51 is provided with an 
arrangement for slowing the speed of the engine by misfiring of the spark 
plug 65 in response to an overheated condition as sensed by a temperature 
sensing switch 71 which closes a circuit and grounds in response to an 
over temperature condition. This protection circuit includes an engine 
speed detecting circuit 72 that is in circuit with the output from the 
pulser coil 54 and which includes a wave shaper circuit 73 that provides 
an output wave form containing a number of pulses indicative of engine 
speed. This is outputted to a frequency to voltage converter circuit 74 
which outputs a voltage V.sub.n indicative of engine speed. 
The frequency to voltage converter 72 outputs its signal to an oscillator 
circuit 75 of an ignition timing operation circuit 76. The ignition timing 
operation circuit 76 is designed so as to cause the spark plug 65 to be 
not fired for increasing time intervals during a given period of time so 
that the spark plug 65 will fire only once every several revolutions of 
the engine until the speed is reduced to a level where the engine will be 
protected. The transmission of speed signal V.sub.n to the oscillator 
circuit 75 occurs only when the temperature switch 71 is closed. This 
causes power to be delivered through a diode 77 to a delay circuit 78 that 
has an output V.sub.r that is also delivered to the oscillator circuit 75. 
The time delay circuit 78 operates like a capacitor in that its output 
signal V.sub.r decays along a curve as shown in FIG. 3. 
The oscillator circuit 75 has an output voltage V.sub.a that is generated 
for a time period which is varied in accordance with the difference 
between the voltages V.sub.n and V.sub.r. The output of the oscillator 
circuit 75 is shown on the curve of FIG. 3 wherein the output extends for 
a period T.sub.1 during a preset time interval T. As may be seen from this 
figure, the time T.sub.1 continues to increase until the voltage V.sub.r 
has decayed to the point t.sub.1 at which it is held constant for a fairly 
substantial time period. During the time T.sub.1 when the oscillator 75 is 
providing its high output, the firing of the spark plug 65 will be 
disabled. 
This disabling is achieved by providing a shunting circuit that prevents 
charging of the capacitor 56. This shunting circuit is indicated generally 
by the reference numeral 79 and includes and SCR 81 that has its output 
connected to a ground by means of a circuit including a resistor 82. The 
SCR 81 has its gate controlled by means of a gate circuit 83 which 
receives the output from the oscillator circuit 75 and which energizes the 
gate of the SCR 81 for a time T.sub.1 as set by the oscillator circuit 75. 
An LED 84 is illuminated at this time through a resistor circuit 85 as 
well as sounding of a buzzer 86 powered by a battery 87 so as to provide 
the operator with an indication that an over temperature condition has 
existed and the engine is being slowed. 
In order to insure that the engine will not encounter a backfiring 
condition, there is further provided a comparator 88 that outputs a signal 
V.sub.s to the gate circuit 83 so as to stop the engine by the permanent 
grounding of the circuit by permanently switching the SCR 81 on to ground 
it. The comparator 85 compares the speed signal V.sub.n with a 
predetermined voltage V.sub.o which is imposed upon it in a suitable 
manner, so as to switch the engine off when the speed falls to the speed 
at which backfiring may occur. 
It should be noted that, like the preceding embodiment, the engine can be 
restarted once it has been stopped, but when the overheating condition is 
still present, the engine will again be stopped when the speed of the 
engine will become equal to the speed at which misfiring may occur. 
It should be readily apparent from the foregoing description that a number 
of embodiments of the invention have been illustrated and described, each 
of which is effective to insure that backfiring will not occur in an 
engine even when the engine speed is being slowed due to an abnormal 
running condition. Although two embodiments of the invention have been 
illustrated and described, various changes and modifications may be made 
without departing from the spirit and scope of the invention, as defined 
by the appended claims.