Ionic current sensing apparatus

A capacitor is connected to an ignition coil for an internal combustion engine so as to be charged by ignition current generated by the ignition coil. After firing of a spark plug in a cylinder by the ignition current, the voltage across the capacitor is applied between the center electrode and the ground electrode of the spark plug to induce an ionic current between the electrodes. The resulting ionic current is measured by a current sensor connected to the capacitor.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for sensing ionic current flowing 
between the electrodes of a spark plug in an internal combustion engine. 
In a spark ignited internal combustion engine, at the time of combustion in 
a cylinder of the engine, ionization takes place in the cylinder. If a 
voltage is applied between the electrodes of the spark plug for the 
cylinder, an ionic current is generated between the electrodes. By 
measuring the ionic current, it is possible to determine whether the 
cylinder in which the spark plug is disposed misfired based on the 
magnitude of the ionic current. Furthermore, the magnitude of the ionic 
current during the combustion stroke of a cylinder is highest when the 
pressure in the cylinder reaches a maximum, so the ionic current can be 
used to monitor pressure variations within a cylinder. 
Conventional ionic current sensing devices have a power supply connected to 
a spark plug so as to apply a voltage between the center electrode and the 
ground electrode of the spark plug to produce an ionic current in the form 
of positive ions. An electric current flowing due to the ionic current 
through a current sensing resistor connected in series with the power 
supply is then measured as an indication of the ionic current. The power 
supply typically generates a voltage of approximately 50-300 volts. Such a 
voltage is usually produced by increasing a battery voltage using a DC-DC 
converter or the like. However, a DC-DC converter requires a transformer, 
so the power supply and the ionic current sensing apparatus as a whole 
ends up being large, heavy, and expensive. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an ionic 
current sensing apparatus for an internal combustion engine which is 
smaller, lighter, and less expensive than conventional ionic current 
sensing apparatuses. 
It is another object of the present invention to provide an ionic current 
sensing apparatus that can be easily applied to a conventional ignition 
system for an internal combustion engine. 
In an ionic current sensing apparatus according to the present invention, a 
high voltage generated in an ignition coil at the time of firing a spark 
plug of an engine is applied to an energy storage element in the form of a 
capacitor. After the spark plug is fired, the capacitor applies a voltage 
between the center electrode and the ground electrode of the spark plug. 
The voltage applied by the capacitor generates an ionic current between 
the electrodes of the spark plug, and the ionic current causes the 
capacitor to discharge. The current produced by the discharge of the 
capacitor is measured by a current sensor connected to the capacitor as an 
indication of the ionic current. 
The voltage applied between the electrodes of the spark plug by the 
capacitor can be either a positive or a negative voltage. However, 
preferably the capacitor applies a positive voltage between the center 
electrode and the ground electrode of a spark plug. 
The present invention can be applied to various types of ignition systems. 
For example, the ignition system can be with or without a distributor and 
with or without breaker points.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A number of preferred embodiments of an ionic current sensing apparatus 
according to the present invention will now be described while referring 
to the accompanying drawings. FIGS. 1-3 illustrate a first embodiment as 
applied to a conventional ignition system for an unillustrated 
multi-cylinder internal combustion engine. Although the engine has a 
plurality of cylinders, the ignition system for only one cylinder is 
illustrated. The ignition system includes a conventional ignition coil 1 
having a primary winding 1a and a secondary winding 1b. One end of the 
primary winding 1a is connected to an unillustrated power supply, such as 
a battery, while the other end is connected to a switching device for 
controlling the flow of current through the primary winding 1a. The 
switching device in this embodiment is a power transistor 2 having its 
collector connected to the primary winding 1a and its emitter grounded. It 
is possible to employ a different type of switching device, such as the 
mechanical breaker points of a distributor. The switching of the power 
transistor 2 is controlled by a conventional, unillustrated control unit 
which applies a control signal to the base of the transistor 2. In this 
embodiment, a separate ignition coil 1 is provided for each cylinder of 
the engine, but it is possible for a single ignition coil 1 to be used for 
a plurality of the cylinders. 
The secondary winding 1b has a positive end and a negative end. The engine 
has a plurality of spark plugs 3, each of which has a center electrode 3a 
connected to the negative end of the secondary winding 1b and a ground 
electrode 3b which is grounded. 
The positive end of the secondary winding 1b is connected to one terminal 
of an energy storage element in the form of a capacitor 4. The other 
terminal of the capacitor 4 is connected to ground through a current 
sensor comprising a current sensing resistor 5. A terminal 6 is connected 
to the junction of the capacitor 4 and the resistor 5 to enable 
measurement of the change in the voltage across the resistor 5 due to 
ionic current flowing between the electrodes of the spark plugs 3. A 
rectifying element such as a diode 7 is connected in parallel with the 
resistor 5 with its anode connected to the capacitor 4 and its cathode 
connected to ground. A Zener diode 8 for clipping the voltage across the 
capacitor 4 and the diode 7 is connected in parallel with the capacitor 4 
and the diode 7 such that its cathode is connected to the positive end of 
the secondary winding 1b and its anode is connected to ground. The Zener 
diode 8 is selected so as to charge the capacitor 4 to a prescribed 
voltage when the secondary winding 1b is energized. A typical value of the 
voltage across the capacitor 4 at this time is 300 volts. 
When the cylinder in which the spark plug 3 is installed is to be ignited, 
the power transistor 2 is turned off by the unillustrated control unit to 
cut off the current flowing through the primary winding 1a, and a high 
voltage (generally 10-25 kV) having the polarity illustrated in FIG. 2 is 
generated in the secondary winding 1b . This voltage causes the spark plug 
3 to discharge, and a discharge current flows from the spark plug 3 along 
the path shown by the arrow in FIG. 2. The discharge of the spark plug 3 
ignites the fuel-air mixture in the corresponding cylinder, and combustion 
takes place. At the same time, the capacitor 4 is charged with the 
illustrated polarity to a voltage determined by the reverse breakdown 
voltage of the Zener diode 8. 
Ionization is produced in the cylinder at the time of combustion of the 
fuel-air mixture, and ions are generated between the electrodes of the 
spark plug 3. At this time, the voltage across the capacitor 4 is applied 
to the electrodes of the spark plug 3, and this voltage causes an ionic 
current to flow between the electrodes. Due to the ionic current, the 
capacitor 4 discharges, and a current flows through the resistor 5, the 
capacitor 4, and the secondary winding 1b in the direction shown by the 
dashed arrow in FIG. 3. This current produces a change in the voltage at 
the terminal 6 corresponding to the magnitude of the ionic current. 
Therefore, by monitoring the voltage at the terminal 6, it can be 
ascertained whether the cylinder is firing properly. 
If the voltage at the terminal 6 indicates that the cylinder is misfiring, 
then the engine can be controlled to let the misfiring cylinder rest. For 
example, the fuel supply to the misfiring cylinder can be cut off, thereby 
preventing uncombusted fuel from being discharged from the engine. 
Preferably, the capacitor 4 is electrically connected to the spark plug 3 
so that the capacitor 4 applies a positive voltage between the center 
electrode 3a and the ground electrode 3b of the spark plug 3, i.e., a 
voltage such that the center electrode 3a is at a higher potential than 
the ground electrode 3b. If a negative voltage is applied to the 
electrodes, the ionic current between the electrodes 3a and 3b is due to 
the flow of positive ions. Since positive ions have a large mass, the 
ionic current is extremely small, and the current flowing through the 
resistor 5 is difficult to measure. In contrast, when a positive voltage 
is applied between the center electrode 3a and the ground electrode 3b, 
the ionic current is caused by the flow of electrons. As electrons have a 
much smaller mass than positive ions, the ionic current is much larger 
(generally 10-50 times as high) than an ionic current due to positive 
ions. Therefore, when a positive voltage is applied across the electrodes 
3a and 3b, a large current flows through the resistor 5, producing an 
easily detectable change in the voltage at the terminal 6. 
The capacitor 4 takes the place of a DC power supply having a DC-DC 
converter such as is required in a conventional ionic current sensing 
apparatus. The capacitor 4 can be much smaller, lighter, and cheaper than 
a DC power supply, so an ionic current sensing apparatus according to the 
present invention can be smaller, lighter, and cheaper than a conventional 
one. 
In the embodiment of FIGS. 1-3, the ignition coil 1 was configured so as to 
apply a negative voltage to a spark plug 3 to produce ignition. FIG. 4 
illustrates a second embodiment of the present invention having an 
ignition coil which is configured so as to apply a positive voltage to the 
center electrode of a spark plug to produce ignition. Like the embodiment 
of FIG. 1, this embodiment has an ignition coil 1 with a primary winding 
1a and a secondary winding 1b. One end of the primary winding 1a is 
connected to an unillustrated power supply such as a battery, and the 
other end of the primary winding 1b is connected to a power transistor 2. 
The secondary winding 1b has a positive end and a negative end. The 
positive end is connected to the center electrode 3a of a spark plug 3 
through a diode 9 having its anode connected to the secondary winding 1b 
and its cathode connected to the spark plug 3. The negative end of the 
secondary winding 1b is connected to one terminal of a capacitor 4, to one 
end of a current sensing resistor 5, and to the anode of a Zener diode B. 
The other terminal of the capacitor 4 is connected to the cathode of a 
diode 7 and to the anode of another diode 10. The cathode of the Zener 
diode 8, the other end of the current sensing resistor 5, and the anode of 
diode 7 are connected to ground. The cathode of diode 10 is connected to 
the cathode of diode 9 and to the center electrode 3a of the spark plug 3. 
A terminal 6 is connected to the junction of the resistor 5 and the 
capacitor 4 to enable measurement of the change in the voltage across the 
resistor 5 caused by ionic current. 
The operation of this embodiment is similar to that of the previous 
embodiment. When the power transistor 2 is switched off to cut off the 
primary current flowing through the primary winding 1a, a high voltage of 
the illustrated polarity is generated in the secondary winding 1b, and a 
discharge current flows from the secondary winding 1b through diode 9 and 
into the spark plug 3 as shown by the solid arrow. At the same time, the 
capacitor 4 is charged with the illustrated polarity to a voltage, such as 
300 volts, determined by the Zener diode 8. 
Discharge of the spark plug 3 produces ionization of the fuel-air mixture, 
and ions are generated between the electrodes of the spark plug 3. The 
positive voltage applied to the spark plug 3 by the capacitor 4 causes an 
ionic current to flow between the electrodes of the spark plug 3. The 
ionic current allows the capacitor 4 to discharge, and an electric current 
flows in the direction shown by the dashed arrow in FIG. 4 through the 
resistor 5, the capacitor 4, and diode 10. This electric current produces 
a change in the voltage at the terminal 6 corresponding to the magnitude 
of the ionic current. As in the previous embodiment, the ionic current 
between the electrodes of the spark plug 3 is due to the flow of 
electrons, so the magnitude of the ionic current is large and can be 
easily measured by the change in the voltage at the terminal 6. This 
embodiment provides the same advantages as the previous embodiment. 
In the preceding embodiments, an ignition coil is connected directly to a 
spark plug. However, the present invention can also be applied to an 
ignition system equipped with a distributor. FIG. 5 illustrates a third 
embodiment of the present invention in which a single ignition coil 1 
provides an ignition voltage for a plurality of spark plugs (only one of 
which is shown) via a distributor 11. As in the preceding embodiments, the 
ignition coil 1 has a primary winding 1a and a secondary winding 1b. One 
end of the primary winding 1a is connected to an unillustrated power 
supply, and the other end of the primary winding 1b is connected to a 
power transistor 2. The distributor 11 has a center electrode 11a that 
rotates in synchrony with the engine (normally at half the rotational 
speed of the engine) and a plurality of stationary peripheral electrodes 
11b disposed around the center electrode 11a. The center electrode 11a is 
connected to the negative end of the secondary winding 1b, and each of the 
peripheral electrodes 11b is connected to the center electrode 3a of one 
of the spark plugs 3 of the engine, only one of which is shown. Each of 
the peripheral electrodes 11b is also connected to the center electrode 
11a by a diode 9 (only one of which is shown) having its anode connected 
to the center electrode 11a and its cathode connected to one of the 
peripheral electrodes 11b. 
The positive end of the secondary winding 1b is connected to one terminal 
of a capacitor 4 and to the cathode of a Zener diode 8. The other terminal 
of the capacitor 4 is connected to one end of a current sensing resistor 5 
and to the anode of a diode 7. The other end of the resistor 5 and the 
cathode of diode 7 are grounded. A terminal 6 is connected to the junction 
of the resistor 5 and the capacitor 4 to enable measurement of the change 
in voltage across the resistor 5 caused by ionic current. This embodiment 
is similar in structure to the embodiment of FIG. 1 except that the 
distributor 11 and the diodes 9 are connected between the spark plug 3 and 
the negative end of the secondary winding 1b. 
When the center electrode 11a of the distributor 11 contacts one of the 
peripheral electrodes 11b and the power transistor 2 is cut off, a 
discharge current flows in the direction shown by the solid arrow in FIG. 
5 and the spark plug 3 is discharged to ignite the air-fuel mixture in one 
of the cylinders. At the same time, the capacitor 4 is charged with the 
illustrated polarity. Then, the positive voltage applied by the capacitor 
4 between the electrodes 3a and 3b of the spark plug 3 produces an ionic 
current between the electrodes, and due to the ionic current, an electric 
current flows in the direction shown by the dashed arrow in FIG. 5 through 
the resistor 5, the capacitor 4, the secondary winding 1b, and the diode 
9. This current produces a change in the voltage at the terminal 6, which 
can be monitored to determine whether the cylinder is firing. As the 
center electrode 11a rotates and successively contacts each of the 
peripheral electrodes 11b, the above process is repeated for each 
cylinder, and the ionic current generated in each of the cylinders can be 
determined based on the voltage at the terminal 6. This embodiment employs 
a capacitor 4 to generate an ionic current, so it provides the same 
advantages as the previous embodiments. 
FIG. 6 illustrates another embodiment of the present invention. This 
embodiment is similar to the embodiment of FIG. 4, but diode 9 of FIG. 4 
has been replaced by a distributor 11 having a center electrode 11a and a 
plurality of peripheral electrodes 11b. The center electrode 11a is 
connected to the positive end of the secondary winding 1b of an ignition 
coil 1, and each of the peripheral electrodes 11b is connected to the 
center electrode 3a of one of a plurality of spark plugs 3, only one of 
which is shown. A capacitor 4, a current sensing resistor 5, a diode 7, 
and a Zener diode 8 are connected to the negative end of the secondary 
winding 1b in the same manner as in the embodiment of FIG. 4. The 
capacitor 4 is connected with each of the peripheral electrodes 11b of the 
distributor 11 through one of more diodes 10, each having its anode 
connected to the capacitor 4 and its cathode connected to the distributor 
11. A single diode 10 can be connected to each of the peripheral 
electrodes 11b, or a separate diode 10 can be provided for each peripheral 
electrode 11b. 
When the power transistor 2 is turned off, an ignition current flows 
through the distributor 11 to one of the spark plugs 3 in the direction 
shown by the solid arrow in FIG. 6, and the capacitor 4 is charged with 
the illustrated polarity. Then, the capacitor 4 applies a positive voltage 
between the electrodes of the spark plug 3 to produce an ionic current, 
and due to the ionic current, an electric current flows through the 
resistor 5, the capacitor 4, and diode 10 as shown by the dashed arrow. 
This current produces a change in the voltage at the terminal 
corresponding to the ionic current, and by monitoring this voltage, it can 
be ascertained whether the cylinder is firing properly. This process is 
repeated in succession for each cylinder of the engine. Like the previous 
embodiments, this embodiment employs a capacitor 4 to apply a voltage for 
generating an ionic current, so it provides the same advantages as those 
embodiments. 
FIG. 7 illustrates another embodiment of the present invention. This 
embodiment differs from the embodiment of FIG. 1 only in that a diode 12 
is inserted between the positive end of the secondary winding 1b and the 
Zener diode 8 with its anode connected to the secondary winding 1b and its 
cathode connected to the cathode of the Zener diode 8. The solid arrow 
indicates the direction of current when the spark plug 3 is firing, while 
the dashed arrow indicates the direction of current through the resistor 
5, the capacitor 4, and the secondary winding 1b when ionic current is 
generated between the electrodes 3a and 3b of the spark plug 3. The 
operation of this embodiment is essentially the same as that of the 
embodiment of FIG. 1 and provides the same advantages. 
FIG. 8 illustrates the voltage in the secondary winding 1b, the secondary 
current when the spark plug 3 fires, and the ionic current that flows 
between the electrodes of the spark plug 3 as a function of time during 
the operation of the embodiment of FIG. 7. 
The capacitor 4, the current sensing resistor 5, diodes 7 and 12, and the 
Zener diode 8 are all very compact, so as shown in FIG. 9, it is possible 
to combine them into a single current sensing unit 13 small enough to be 
mounted on a spark plug 3 together with the power transistor 2. The spark 
plug 3, the power transistor 2, and the current sensing unit 13 can be 
made integral with one another so as to enable their being installed on an 
engine as a single member. This arrangement not only reduces the size and 
weight of an ionic current sensing apparatus, but it reduces the length of 
wiring which is required for connecting the various components. 
The embodiment of FIG. 7 employs two diodes 7 and 12. FIG. 10 illustrates 
another embodiment of the present invention in which a single diode 7 
performs the function of both of diodes 7 and 12 of the embodiment of FIG. 
7. This embodiment is similar to the embodiment of FIG. 7 except that 
diode 12 is omitted, and the cathode of the Zener diode 8 is connected to 
the positive end of the secondary winding 1b, while its anode is connected 
to the anode of diode 7 instead of to ground. The operation of this 
embodiment is essentially the same as that of the embodiment of FIG. 7. 
The solid arrow indicates the direction of current when the spark plug 3 
is firing, and the dashed arrow indicates the direction of current through 
the resistor 5, the capacitor 4, and the secondary winding 1b when ionic 
current is flowing between the electrodes of the spark plug 3. This 
arrangement permits a further decrease in the size and weight of the 
apparatus. 
A separate circuit comprising a capacitor 4, a resistor 5, a diode 7, a 
Zener diode 8, and optionally a diode 12 can be provided for each cylinder 
of the engine. Alternatively, as shown in FIG. 11, a single such circuit 
can be used to detect ionic current in all the cylinders. In FIG. 11, each 
cylinder of an engine is equipped with a separate ignition coil 1 having a 
secondary winding 1b connected to the center electrode 3a of a 
corresponding spark plug 3. The junction of diode 12 and capacitor 4 is 
connected to the secondary winding 1b of each ignition coil 1. Whenever 
one of the spark plugs 3 fires, the capacitor 4 is charged with the 
illustrated polarity. The capacitor 4 then applies a positive voltage 
between the electrodes 3a and 3b of each of the spark plugs 3. However, 
since only one of the cylinders is in its combustion stroke at a given 
time, an ionic current will be generated between the electrodes of only 
the spark plug 3 for the cylinder in its combustion stroke. FIG. 12 
illustrates the primary current flowing through the primary winding 1a of 
each of four ignition coils 1 for a four-cylinder engine and the ionic 
current which is sensed. Since it can be easily determined which cylinder 
of the engine is in its combustion stroke at any given time, it can be 
determined to which cylinder the measured ionic current corresponds. 
Therefore, it can be ascertained if any of the cylinders is misfiring. 
FIG. 13 illustrates another embodiment of the present invention. This 
embodiment is identical to the embodiment of FIG. 5 except that a diode 12 
is inserted between the Zener diode 8 and the positive end of the 
secondary winding 1b. The anode of diode 12 is connected to the secondary 
winding 1b and to the capacitor 4, while its cathode is connected to the 
cathode of the Zener diode 8. Each of the peripheral electrodes 11b of the 
distributor 11 is connected to the center electrode 3a of one of a 
plurality of spark plugs 3, and the center electrode 11a of the 
distributor 11 is connected to each of the peripheral electrodes 11b by 
corresponding diodes 9 having their anodes connected to the center 
electrode 11a and their cathodes connected to the peripheral electrodes 
11b. The solid arrow shows the direction of current when one of the spark 
plugs 3 is firing, and the dashed arrow shows the direction of current 
when an ionic current is flowing between the electrodes of one of the 
spark plugs. The operation of this embodiment is essentially the same as 
that of the embodiment of FIG. 5 and provides the same advantages. 
Although in the above-described embodiments, an engine to which the present 
invention is applied is illustrated as having four cylinders, the number 
of cylinders is arbitrary, and the present invention can be applied to any 
spark ignited engine having one or more cylinders. 
In the illustrated embodiments, a current sensing resistor 5 is used to 
sense electric current caused by an ionic current in a cylinder, but other 
current sensing devices can instead by used.