Open secondary detection via reverse circuit sensing

An open secondary coil of an ignition coil is detected by the use of a reverse current in the primary of the ignition coil to indicate the occurrence of an open secondary coil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to internal combustion engine ignition 
systems and, in particular, to ignition systems with an ignition coil. 
2. Prior Art 
U.S. Pat. No. 3,938,490 discloses an ignition control circuit having an 
open secondary protective circuit for preventing open circuits in the 
secondary winding from destroying portions of the ignition system. The 
patent discloses an open secondary Zener clamp protection circuit, two 
induction emission systems, and teaches using the coil flyback voltage in 
order to regulate the primary circuit coil voltage. An open secondary 
sensor includes a voltage divider network but does not detect a reverse 
current in the primary for generating a signal indicating an open 
secondary circuit. 
U.S. Pat. No. 4,153,032 teaches sensing current, but not the reverse 
current in the primary of an ignition circuit, and then comparing the 
sensed current or voltage with a reference value to control emission. 
However, there is no open secondary detection performed by the sensing 
resistor or the comparator. That is, the primary circuit sense resistor is 
used to control coil energy. The patent teaches only using the sense 
resistor link conducting current as in the forward direction. There is no 
teaching of open secondary detection. 
Various other patent references are also generally related to the operation 
of ignition coils within ignition systems. 
U.S. Pat. No. 4,114,582 teaches a secondary protection system circuit 
utilizing a high voltage connection to the secondary circuit. 
U.S. Pat. No. 4,117,819 teaches using engine rotation speed information in 
order to regulate the percent dwell applied to the ignition system. There 
is no teaching of open secondary detection. 
U.S. Pat. No. 4,359,038 teaches a primary coil drive in the clamp circuit 
which may be incorporated into an introverted circuit to eliminate the 
need for an external Zener clamp for protection. Open secondary detection 
is not mentioned. 
GB No. 2 060 053 A, providing primary current overload protection by using 
a forward current through a sense resistor. Again, open secondary 
detection is not mentioned. 
SUMMARY OF THE INVENTION 
This invention teaches detecting an open circuit condition in an ignition 
coil secondary circuit by sensing reverse current flowing in the ignition 
coil primary circuit. A sense resistor in series with the primary winding 
of the ignition coil provides a reverse current path for the negative 
primary voltage reflected back from the open secondary circuit. The 
reverse current flow produces a negative voltage across the sensing 
resistor which is compared in a comparator in order to produce an open 
secondary signal. 
Advantageously, a signal indicating an open secondary is used in 
conjunction with an engine control system so that fuel can be restricted 
to the cylinders not receiving a spark. If fuel continues to be injected 
into cylinders which do not receive spark, the fuel may pass through the 
cylinder and damage the catalyst. The recent introduction of 
distributorless ignition systems has further increased the significance of 
this type of fault protection, since an open secondary will affect two 
cylinders.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a reverse current sensing open secondary detection 
circuit 10 includes an ignition coil 11 with a primary winding 12 and a 
secondary winding 13. A secondary winding 13 is connected in series to 
spark gaps 14 and 15. Primary winding 12 is connected in series with a 
battery 16, the parallel combination of a Darlington driver 17 and an 
internal protection diode 18, and a sense resistor 19. A base input to 
Darlington driver 17 is connected to battery 16 through the series 
combination of a collector-emitter circuit of a pre-driver transistor 20 
and a series resistor 21. A dwell signal at input 22 is applied to the 
base of transistor 20. The base of Darlington driver 17 is coupled to 
ground through a resistor 23. The base of Darlington driver 17 is coupled 
to the collector of Darlington driver 17 through the series combination of 
a Zener diode 24 and a resistor 25. The intermediate node between diode 24 
and resistor is coupled to ground through a resistor 26. An adjustable 
trim resistor 27 is coupled in parallel across sense resistor 19. An 
adjustable pick up terminal from resistor 27 is connected through a 
resistor 28 to the negative input of a comparator 29. The positive input 
of comparator 29 is connected to ground through a resistor 30. The output 
of comparator 29 is coupled back to the positive input of comparator 29 
through feedback resistor 31. Comparator 29 is coupled to a 5 volt source 
32 and to an offset adjustment resistor 33. The output of the comparator 
29 indicates an open secondary signal. 
In operation, an open secondary circuit condition is detected by reverse 
current sensing of the primary circuit. Reverse current sensing refers to 
sensing ignition module reverse current exhibited when the ignition coil 
is acting as a current source to the battery. In this mode, current is 
drawn out of the ignition primary winding 12 and a negative voltage with 
reference to ground may be detected across current sense resistor 19. By 
proper adjustments of comparator 29 offset voltage and hysteresis 
feedback, a single ended supply comparator 29 can be used to detect the 
negative voltage drop. 
This detection scheme uses reflection of stored energy in secondary winding 
13 of ignition coil 11 to primary winding 12 and through protection diode 
18 and Darlington driver device 17. Under normal spark firing conditions, 
most of the primary winding energy is magnetically coupled across to 
secondary winding 13 where the coil energy is discharged across spark gaps 
14 and 15. Due to a lack of a complete secondary circuit during an open 
secondary condition, the coil energy cannot be dissipated as spark but is 
instead used to charge the capacitance of secondary winding 13. When the 
breakdown voltage of the secondary capacitance is exceeded, the secondary 
voltage collapses and an inductive-capacitive resonant oscillation occurs. 
This voltage is reflected back into primary winding 12. 
This reflection induces a negative voltage on primary winding 12 thereby 
biasing the protection diode 18 in parallel with Darlington driver device 
17. With a current path established, a significant amount of remaining 
energy is conducted out of the ignition coil 11 through primary winding 
12, diode 18 and sense resistor 19. The current flow amplitude and 
direction are exceptionally distinguished and unmistakable. Using a 
current sense resistor 19 in series with Darlington driver 17 emitter, the 
negative voltage with reference to ground is detectable with great 
reliability. Using a linear voltage comparator 29, a digital pulse can be 
generated to indicate an open secondary. Open circuit secondary detection 
by reverse primary current sensing is advantageously immune to erroneous 
fault detection. In addition, optional processing of the duration and 
relative time of occurrence with respect to the end of dwell may be used 
to further enhance the reliability of detection. That is, a longer 
duration secondary detection signal provides increased noise immunity for 
an indication of an open secondary condition. The output of linear voltage 
comparator 29 is applied to a time measuring device 34. Device 34 measures 
the duration of the secondary detection signal and using threshold 
criteria determines whether or not to indicate an open secondary detection 
signal at the output of device 34. 
Referring to FIG. 2A, the signal A on top indicates primary charge current 
and the signal B on the bottom indicates an open secondary detection 
signal, both with respect to time. When the primary coil charge current of 
signal A has a negative magnitude, there is indicated a reverse current, 
i.e. a current which has a direction the reverse of normal coil charging 
current. If this reverse current is sufficiently large then at the same 
time there is a corresponding rise in signal B to indicate open secondary 
detection. Such detection occurs at time 60 and time 61 on FIG. 2A. 
Referring to FIG. 2B, time 61 is expanded so that the rise of signal B 
corresponding to the reverse current of the primary coil, is more clearly 
seen. Indeed, the rise in signal A now has a duration which can be 
detected by time measuring device 34 of FIG. 1 and there can be 
determination whether there should be an indication of an open secondary 
detection circuit. That is, a brief noise spike would not have sufficient 
duration in signal B to cause indication of an open secondary. 
FIG. 6 is a block diagram indicating how fuel flow is reduced to engine 
cylinders in response to detection of an open secondary. Ignition coil 11 
is coupled to a spark controller and open secondary detection circuit 50. 
Circuit 50 is shown in more detail in FIG. 1. Circuit 50 is coupled to a 
fuel controller circuit 51 which in turn is coupled to fuel injection 
components 1, 2, 3, and 4 associated with cylinders 1, 2, 3, and 4, 
respectively. In operation, when spark controller and open secondary 
detection circuit 50 has detected an open secondary, it provides an output 
signal to fuel controller circuit 51 to initiate fuel flow reduction. Fuel 
controller circuit 51 then cuts out fuel at cylinders 1, 2, 3, and 4 as 
appropriate. Reducing fuel flow in a cylinder which is not sparking is 
advantages because it reduces gasoline flow without combustion. Such 
gasoline flow may damage the engine, or the catalyst for reducing 
emissions, or cause reduced fuel economy. 
Various modifications and variations will no doubt occur to those skilled 
in the arts to which this invention pertains. For example, a particular 
dwell control of the ignition coil may be varied from that disclosed 
herein. These and all other such variations which basically rely on the 
teachings through which this disclosure has advanced the art are properly 
considered within the scope of this invention.