Ignition system improvements for internal combustion engines

Claims

I claim:

1. An ignition system for a spark ignition internal combustion engne comprising:

a pulse transformer having a primary winding and a secondary winding connected to a spark plug;

driver means for supplying a voltage signal to said pulse transformer primary winding thereby including a high voltage signal and said secondary winding applied to said spark plug;

controller means for providing control signals to said driver means, thereby causing said driver means to generate a voltage applied to said secondary winding, and

discharge detection means for sensing the occurrence of electrical discharge across said spark plug and applying a discharge signal to said controller means for controlling the operation of said controller means;

wherein said ignition system detects the existence of auto-ignition characterized by pressure and temperature fluctuations within said engine combustion chamber occurring after piston top dead center which departs from normal combustion, and further comprising timing means for sensing the position of the piston and for providing a timing signal, and wherein said controller means applies a hover voltage of a predetermined maximum level across said spark plug during a period of the cylinder cycle operating cycle after piston top dead center, said hover voltage applied at a level at which discharge across said spark plug occurs in the event that said combustion chamber is experiencing auto-ignition but does not occur if the conditions within the cylinder are of normal combustion, wherein said discharge detection means thereby provides an indication of the existence of auto-ignition.

2. An ignition system according to claim 1 wherein said period of the cylinder of the operating cycle is within the range between 5.degree. and 35.degree. after piston top dead center.

3. An ignition system according to claim 1 wherein said hover voltage comprises a series of pulses each having a maximum voltage at said predetermined maximum level.

4. An ignition system according to claim 1 wherein said means for detecting comprises a shield electrically connected to the spark plug body, a conductor electrically connecting said shield to an electrical ground and a current detector for sensing current in said conductor.

5. An ignition system according to claim 1 wherein said controller means further applies an interrogation pulse across said spark plug gap in a portion of the piston operating cycle in which auto-ignition does not occur as a means of establishing said predetermined maximum level of said hover voltage, said interrogation pulse voltage increasing with respect to time untildischarge occurs and wherein said hover voltage is adjusted in accordance with the pulse width of said interrogation pulse.

6. An ignition system according to claim 5 wherein said interrogation pulse is applied at about piston top dead center.

7. An ignition system according to claim 1 wherein said controller means further retards spark timing upon sensing of auto-ignition and thereafter advances timing whereby said ignition system operates at about the threshold of auto-ignition.

8. An ignition system for a spark ignition internal combustion engine comprising:

a pulse transformer having a primary winding and a secondary winding connected to a spark plug;

driver means for supplying a voltage signal to said pulse transformer primary winding thereby inducing a high voltage signal and said secondary winding applied to said spark plug;

controller means for providing control signals to said driver means, thereby causing said driver means to generate a voltage applied to said secondary winding, and

discharge detection means for sensing the occurrence of electrical discharge across said spark plug and applying a discharge signal to said controller means for controlling the operation of said controller means;

wherein said controller means minimizes the duty cycle of said transformer by terminating the control signal to said driver means upon the detection of discharge by said discharge detector means.

9. An ignition system for a spark ignition internal combustion engine which detects the existence of auto-ignition characterized by pressure and temperature fluctuations within the engine combustion chamber occurring after piston top dead center which departs from normal combustion, comprising:

timing means for sensing the position of the piston and for providing a timing signal,

an ignition spark plug having electrodes presenting an air gap within the combustion chamber,

electrical energy source means for applying voltage to said spark gap air gap,

controller means for receiving said timing signal and controlling said energy source means for applying a hover voltage of a predetermined maximum level across said spark plug gap during a period of the cylinder operating cycle after piston top dead center, said hover voltage applied at a level at which discharge across the spark plug occurs in the even that said combustion chamber is experiencing auto-ignition but does not occur if the conditions within the cylinder are of normal combustion, and

discharge detection means for sensing the occurrence of electrical discharge across said spark plug gap caused by said hover voltage, thus providing an indication of the existence of auto-ignition.

10. An ignition system according to claim 9 wherein said period of the cylinder operating cycle is within the range between 5o and 35o after piston top dead center.

11. An ignition system according to claim 9 wherein said hover voltage comprises a series of pulses each having a maximum voltage at said predetermined maximum level.

12. An ignition system according to claim 9 wherein said discharge detection means comprises; a pulse transformer having a primary winding and a secondary winding connected to said plug, a driver circuit for supplying a voltage signal to said pulse transformer primary winding, and means for detecting the short duration high level current flow occurring at the initiation of discharge across said spark plug gap.

13. An ignition system according to claim 12 wherein said means for detecting comprises a shield electrically connected to the spark plug body, a conductor electrically connecting said shield to an electrical ground, and a current detector for sensing current in s id conductor.

14. An ignition system according to claim 9 wherein said controller means further applies an interrogation pulse across said spark plug in a portion of the piston operating cycle in which auto-ignition does not occur as a means of establishing said predetermined maximum level of said hover voltage, said interrogation pulse voltage increasing with respect to time until discharge occurs and said hover voltage is adjusted in accordance with the pulse width of said interrogation pulse.

15. An ignition system according to claim 14 wherein said interrogation pulse is applied at piston top dead center.

16. An ignition system according to claim 9 wherein said controller means further retards spark timing upon sensing of auto-ignition and thereafter advances timing whereby said ignition system operates at about the threshold of auto-ignition.

17. An ignition system for a spark ignition internal combustion engine which detects the existence of auto-ignition characterized by pressure and temperature fluctuations within the engine combustion chamber occurring after piston top dead center which departs from normal combustion comprising:

timing means for sensing the position of the piston and for providing a timing signal,

an ignition spark plug having electrodes presenting and air gap within the combustion chamber,

a pulse transformer mounted directly to said spark plug having a primary winding and a secondary winding, said secondary winding electrically connected to said spark plug electrodes,

a driver circuit for transmitting a pulse of voltage to said pulse transformer primary winding in response to control signals thereby producing a high voltage signal at said spark plug electrodes,

a spark discharge detector including a ground return conductor electrically connected to one of said spark plug electrodes connected to ground, said conductor also connected to ground and further including a current detector for sensing current flow in said conductor, wherein said spark discharge detector senses the short duration high level current flow which occurs at the onset of arcing across said spark plug gap, and

controller means for receiving said timing signals and providing said control signals to said driver circuit, said controller means applying a hover voltage of a predetermined maximum level across said spark plug gap during a period of the cylinder operating cycle after piston top dead center, said hover voltage maximum level being set such that which discharge across the spark plug occurs in the event that said combustion chamber is experiencing auto-ignition, but does not occur if the conditions within the cylinder are of normal combustion, and said controller means modifying spark timing in response to said spark discharge detector.

18. An ignition system according to claim 17 wherein said period of the cylinder operating cycle is within the rang between 5.degree. and 35.degree. after piston top dead center.

19. An ignition system according to claim 17 wherein said hover voltage comprises a series of pulses each having a maximum voltage at said predetermined maximum level.

20. An ignition system according to claim 17 wherein said controller means further applies an interrogation pulse across said spark plug in a portion of the piston operating cycle in which auto-ignition does not occur as a means of establishing said predetermined maximum level of said hover voltage, said interrogation pulse voltage increasing with respect to time until discharge occur and said hover voltage is adjusted in accordance with the pulse width of sai interrogation pulse.

21. An ignition system according to claim 20 wherein said interrogation pulse is applied at about top dead center of the piston during the power stroke.

22. An ignition system according to claim 17 wherein said controller means further retards spark timing when it senses auto-ignition and advances spark timing when it does not sense auto-ignition thereby operating at the threshold of auto-ignition.

23. An ignition system for a spark ignition internal combustion engine which detects the existence of auto-ignition characterized by pressure and temperature fluctuations within the engine combustion chamber occurring after piston top dead center which departs from normal combustion comprising:

timing means for sensing the position of the piston and for providing a timing signal,

an ignition spark plug having electrodes presenting an air gap within the combustion chamber,

a pulse transformer mounted directly to said spark plug having a primary winding and a secondary winding, said secondary winding electrically connected to said spark plug electrodes,

a driver circuit for transmitting a pulse of voltage to said pulse transformer primary winding in response to control signals thereby producing a high voltage signal at said spark plug electrodes,

a spark discharge detector including a ground return conductor electrically connected to one of said spark plug electrodes connected to ground, said conductor also connected to ground and further including a current detector for sensing current flow in said conductor, wherein said spark discharge detector senses the short duration high level current flow which occurs at the onset of arcing across said spark plug gap, and

controller means for receiving said timing signals and providing said control signals to said driver circuit, said controller means providing an interrogation control signal at a period of the piston operating cycle where auto-ignition does not normally occur, said interrogation control signal causing an interrogation pulse voltage at said spark plug gap to increase over time until said interrogation voltage reaches the breakdown voltage which causes discharge across said gap, said breakdown voltage being affected by pressure and temperature conditions that said spark plug electrodes are exposed to, and for measuring the phase difference between the leading edge of said interrogation control signal and said breakdown current which said phase difference is an indication of said conditions, said controller means further applying a hover voltage of a predetermined maximum level across said spark plug gap during a period of the cylinder operating cycle after piston top dead center, said hover voltage applied a a level at which discharge across the spark plug occurs in the event that said combustion chamber is experiencing auto-ignition but does not occur if the conditions within the cylinder are of normal combustion, and said controller means further for adjusting the level of said hover voltage in response to s id phase difference to thereby adjust said hover voltage in response to said interrogation pulse breakdown voltage.

24. An ignition system according to claim 23 wherein said period of the cylinder operating cycle is within the range between 5.degree. and 35.degree. after piston top dead center.

25. An ignition system according to claim 23 wherein said hover voltage comprises a series of pulses each having a maximum voltage at said predetermined maximum level.

26. An ignition system according to claim 23 wherein said interrogation pulse is applied at about piston top dead center.

27. An ignition system according to claim 23 wherein

controller means further applies an ignition control signal at a point of the piston operating cycle before top dead center to initiate combustion in said chamber, and wherein when said breakdown is sensed during said hover, indicating the occurrence of auto-ignition, said controller means retards spark advance to reduce the likelihood of auto-ignition.

28. An ignition system according to claim 27 wherein said controller means further incrementally advances spark timing when auto-ignition is not sensed.

29. An ignition system for a spark plug ignited internal combustion engine for improved lean air/fuel mixture burn limits comprising:

a pulse transformer having a primary winding and a secondary winding connected to the spark plug;

driver circuit means for supplying a voltage pulse to said pulse transformer primary winding thereby inducing a high voltage in said secondary winding;

controller means for generating control signals causing said driver circuit means to cause said pulse transformer to output a series of pulses from said secondary winding thereby causing a series of spark discharges starting less than 100 microseconds apart, whereby the multiple firing of the spark plug enables leaner air/fuel mixtures to be combusted as compared with a single spark discharge; and

discharge detection means for sensing the occurrence of electrical discharge across the spark plug electrodes and wherein said controller means terminates said control signal upon the detection of said discharge.

30. An ignition system according to claim 29 wherein said discharge detection means comprises; a pulse transformer having a primary winding and a secondary winding connected to the spark plug, a driver circuit for supplying a voltage signal to said pulse transformer primary winding, and means for detecting the short duration high level current flow occurring at the initiation of discharge across the spark plug gap.

31. An ignition system according to claim 30 wherein said means for detecting further comprises a shield electrically connected to the spark plug body, a conductor electrically connecting said shield to an electrical ground, and a current detector for sensing current in said conductor.

32. An ignition system for a spark plug ignited internal combustion engine for improved lean air/fuel mixture burn limits comprising;

a pulse transformer mounted to the spark plug and having a primary winding and a secondary winding connected to the spark plug,

driver circuit means for supplying a voltage pulse to said pulse transformer primary winding thereby inducing a higher voltage in said secondary winding,

controller means for generating control signals causing said driver circuit means to cause said pulse transformer to output a series of pulses from said secondary windings thereby causing a series of spark discharges across electrodes of said spark plug, said discharges starting less than 100 microseconds apart, whereby the multiple firing of the spark plug enables leaner air/fuel mixtures to be combusted as compared with a single spark plug discharge, and

discharge detection means for sensing the occurrence of electrical discharge across the spark plug electrodes and providing a signal to said controller means for terminating said control signal upon the detection of said discharge.

33. An ignition system according to claim 32 wherein said discharges are about 70 microseconds apart.

34. An ignition system according to claim 32 wherein said means for detecting further comprises a shield electrically connected to the spark plug body, a conductor electrically connecting said shield to an electrical ground, and a current detector for sensing currnt in said conductor.

35. An ignition system for a spark ignition internal combustion engine comprising

a pulse transformer having a primary winding and a secondary winding connected to said plug,

driver circuit means for supplying a voltage signal to said pulse transformer primary winding thereby inducing a high voltage pulse in said secondary winding applied to said spark plug gap,

controller means for providing control signals to said driver circuit means,

discharge detection means for sensing the occurrence of electrical discharge across said spark plug gap caused by said pulse transformer secondary voltage, and

said controller means receiving a discharge signal from said discharge detection means and causing said control signal to be curtailed thereby minimizing the duration of said secondary winding pulse.

36. An ignition system according to claim 35 whereby said controller means causes the duty cycle of said pulse transformer to be minimized.

37. An ignition system according to claim wherein said duty cycle is about 1 percent.

38. An ignition system according to claim 35 wherein said discharge detection means comprises; a pulse transformer having a primary winding and a secondary winding connected to said plug, a driver circuit for supplying a voltage signal to said pulse transformer primary winding, and means for detecting the short duration high level current flow occurring at the initiation of discharge across said spark plug gap.

39. An ignition system according to claim 35 wherein said means for detecting comprises a shield electrically connected to the spark plug body, a conductor electrically connecting said shield to an electrical ground, and a current detector for sensing current in said conductor.

40. An ignition system for a spark ignition internal combustion engine which senses abnormal conditions within the engine combustion chamber comprising;

a pulse transformer having a primary winding and a secondary winding connected to said plug,

driver circuit means for supplying a voltage signal to said pulse transformer primary winding thereby inucing a high voltage pulse through said secondary winding,

controller means for providing a control signal to said driver circuit means whereby at the onset of said control signal, the voltage at a spark plug rises with respect to time until a breakdown voltage is reached causing electrical discharge across the spark plug, and wherein normal operation of said engine will result in said breakdown voltage having a magnitude within a voltage range, and wherein abnormal conditions within said combustion chamber will result in a breakdown voltage which is outside said range,

discharge detection means for sensing the short duration, high level current flow occurring at the initiation of discharge across the spark plug, and

wherein said controller means measures the breakdown voltage thereby providing a means for detecting said abnormal con itions.

41. An ignition system according to claim 40 wherein said controller means measures said breakdown voltage by detecting the elapsed time between the leading edge of said control signal and the time of electrical discharge across said spark plug.

42. An ignition system according to claim 40 wherein a measured breakdown voltage below said range indicates the presence of pre-ignition and a measured breakdown voltage above said range indicates abnormal spark plug condition.

BACKGROUND OF THE INVENTION

This invention relates to improvements in spark ignition systems of internal combustion engines having the capabilities of improving engine operation by permitting combustion of extremely lean fuel mixtures, more accurately setting and controlling spark timing, and further enabling ignition system components to be designed more efficiently.

In order to initiate combustion of an air/fuel mixture within an internal combustion engine chamber, a spark ignition system is used which generates a high energy arc at the appropriate time in the engine operating cycle. The onset of the arc across a spark plug gap is timed to occur at a predetermined number of degrees of engine crankshaft rotation before the piston reaches top dead center (TDC). If spark timing is established properly, the flame front emanating from the spark plug will cause a pressure peak to develop within the combustion chamber which occurs just after top dead center of the piston during its power stroke. If the spark is initiated too late in the operating cycle (retarded timing) the developed pressure within the combustion chamber will not be efficiently converted to work output. On the other hand, if the spark is initiated too early in the cycle (advanced timing), extemely high and potentially damaging pressure and temperature rises can occur in the combustion chamber which are also not efficiently converted into useful work.

Excessively advanced spark timing can lead to several different types of combustion chamber phenomena. Auto-ignition of the end gases is a condition where the end gases (the unburnt fuel-air mixture that is being ignited by the movement of the flame front) explodes spontaneously when the engine combustion temperatures and pressures become too high. When auto-ignition occurs in the cylinder of the spark engine, pressure therein rises and falls alternately due to the sudden release of chemical energy and temperature rapidly increases. If the rate of energy release is sufficiently high, vibrations within the exploding gas will force the cylinder walls to vibrate, resulting in a characteristic sound referred to as "pinging", or other audible sounds. The rapid fluctuations in pressure and temperature of gases within the combustion chamber caused by auto-ignition occur well after top dead center.

A slight degree of auto-ignition is believed by many engine designers to be desirable because it generates turbulence which hastens the combustion process at a time when the speed of the flame emanating from the spark plug is decreasing. Slight auto-ignition can reduce hydrocarbons left unburnt by the spark-triggered ignition process and simultaneously utilizing the energy released when they are burnt, resulting in lower hydrocarbon emissions as well as improved fuel economy. For these reasons, engine designers often seek to calibrate ignition systems so that spark is advanced to about the threshold of auto-ignition. Care must be taken, however, to avoid excessive auto-ignition which leads to high combustion chamber temperatures which can eventually heat the spark plug electrodes to the point where they initiate the combustion process independently of the spark, thus leading to a phenomena known as pre-ignition. Pre-ignition is marked by extremely high cylinder temperatures and pressures near TDC and can cause significant engine damage, including perforation of the piston. Pre-ignition is frequently referred to as "knock" due to the characteristic audible sound which it generates. Generally, it can be stated that auto-ignition leads to pre-ignition, and subsequently, pre-ignition leads to furthe auto-ignition.

A number of factors influence the timing threshold of generating auto-ignition, including inlet air temperature, engine speed and load, air/fuel ratio, fuel characteristics, and a host of other variables. Spark timing further directly affects engine fuel efficiency and noxious emissions output. Due to the significance of accurately controlling spark timing, numerous engine control systems in present use have microprocessor based closed-loop spark timing control systems which simultaneously measure a number of parameters such as exhaust composition, coolant temperature, and the occurrence of spark knock. These systems proces these data to set timing to near a predicted auto-ignition threshold. The present spark knock detectors used with spark controllers are typically a piezoelectric transducer which senses the intense vibrations caused by spark knock. These knock detectors, however, are not sensitive enough t detect incipient engine auto-ignition which may produce a barely detectable engine vibration and therefore the threshold of auto-ignition is not sensed by such transducers. Accordingly, there is a need to provide a spark ignition control system which enables the detection of incipient auto-igniton, thus enabling more precision in setting spark timing in a closed-loop system.

Designers of spark ignition internal combustion engines for motor vehicles are constantly striving to enable the engines to burn leaner air/fuel mixtures (i.e., lower fuel concentration). An air/fuel mixture of approximately 15 to 1 (respectively) is referred to as a stoichiometric mixture and provides just enough oxyge to completely burn the fuel charge. Adding excess air to the combustion chamber, however, has been found to reduce noxious engine emissions such as oxides of nitrogen and hydrocarbons, etc. There are limits, however, to the extent to which the mixture can be leaned before the spark will not produce an exothermic reaction within the combustion chamber. The presen lean limit for most present motor vehicle engines is approximately 20 to 1 air/fuel ratio. Engine designers are striving to increase the lean burn limit of engines, which is theoretically believed to be extendable to about 27 to 1 air/fuel ratio. There is a need therefore to extend the lean limit of spark ignition internal combustion engines.

In newer generation ignition systems, a transformer generally known as an ignition coil is mounted directly on each of the ignition spark plugs and is often referred to as a coil-on-plug (COP) ignition arrangement. The size and mass of such devices placed on the spark plugs is greatly affected by thermal requirements. Windings which operate with high duty cycles (i.e., periods of winding energization cmpared with dwell periods) must be large and massive enough to prevent excessive heating of the winding. Conversely, low duty cycle operation enables the winding to be made more compact and lighter in weight. Reductions in size of COP windings is further desirable to reduce engine packaging constraints. There is accordingly a need to provide an ignition system having a device mounted on the spark plug which is of minimum size and weight.

SUMMARY OF THE INVENTION

In accordance with the present invention, an improved ignition system is provided having the capabilities of providing the above-mentioned desirable features. A solution to the previously discussed problems results from the use of novel ignition system elements which take advantage of various phenomena which occur during the combustion and spark ignition processes which are detected and utilized by the subject ignition system. As a means of more clearly describing the features of the present invention, these principals are outlined below.

For spark ignition systems, there are believed to be three distinct phases of electrical discharge at the spark plug gap. These phases are referred to as the breakdown, arc, and glow phases. In the breakdown phase, which is the initial phase of discharge, a high voltage applied to the spark electrodes at around 10 KV leads to extremely high current flow, on the order of 1,000 amps or more, which occurs within an extremely short period of time, typically in the nanosecond range. Due to the exceptionally short duration of the current flow of this phase, it could not be detected accurately in the past due to the unavailability of measuring instruments having a sufficient dynamic response. Due to the high frequency of the current flowing during the breakdown phase, it is also referred to as the ignition r.f. (radio frequency) current. Once a conductive path at the electrode gap is established during the breakdown phase, the discharge transitions to the arc phase, characterized by low voltage across the gap and moderate current flow over a relatively long duration. During the final or glow phase, low currents and low gap voltage occur. Many investigators including Maly and Vogel (for example, see their article entitled, "Initiation and Propagation of Flame Fronts in Lean CH.sub.4 -Air Mixtures by the Three Modes of the Ignition Spark") have asserted that it is the breakdown phase of discharge which produces the effect of in initiating an expanding flame front at the spark plug gap, and that the arc and glow phases not only do not contribute to the combustion process, but are actually undesirable from the standpoint of spark plug electrode erosion.

The ignition system according to the present invention employs a pulse transformer system which operates in a distinctly different manner than a conventional ignition coil. Each spark plug has its own ignition pulse transformer attached at its outer end, rather than the conventional arrangement in which a single coil provides a high voltage discharge for each of the multiple spark plugs. In conventional ignition systems using a fly-back type transformer (referred to as a coil), energy is stored as a magnetic field in the coil which is developed through current flow in the primary winding, which when stopped, induces a voltage in the secondary winding due to the collapse of the magnetic field. Although fly-back transformer coil designs have been used successfully for decades, they are electrically inefficient. A principal advantage of the pulse transformer design according to this invention is that inductance of the secondary circuit is greatly decreased which provides the means of firing the spark plug for a much shorter time (typically 50 microseconds), thereby permitting multiple firing. Additionally, the lower impedance allows for the firing of partially fouled spark plugs. In the pulse transformer, spark discharge energ is not stored as a magnetic field. Rather, the pulse transformer merely acts as a quick response step-up transformer which provides a secondary output in response to a voltage spike transmitted to the primary winding.

The described embodiments of the present invention make use of a ferrite toroidal detector which senses the presence of breakdown current within the ignition system as a means of detecting the occurrence and timing of spark discharge, and also as a means of controlling pulse transformer operation to minimize the duration of the arc and glow phase of discharge.

Paschens' law characterizes the relationship between breakdown voltage, and the pressure and temperature existing within the combustion chamber at the point of discharge. This relationship is expressed as: ##EQU1## where V.sub.b is breakdown voltage

K is a constant

P is cylinder pressure

T is cylinder temperature

As previously described, the process of auto-ignition is marked by abnormal pressure and temperature fluctuations occurring after TDC, and typically occurs from 5.degree. to 20.degree. after TDC. As a means of detecting the existence of auto-ignition, the ignition system in accordance with this invention applies a predetermined voltage (a so called "hover" voltage) across the spark plug during the time in the engine operating cycle where the pressure and temperature variations of auto-ignition are likely to occur. Since the temperature and pressure conditions occurring cylindrically during auto-ignition relate to a breakdown voltage in accordance with Paschens' law which is momentarily less than the breakdown voltage for normal combustion, spark discharge during that period of the cycle can be made to occur during auto-ignition conditions and not occur during normal ignition conditions, provided that electrode hover voltage is properly selected.

A sensor circuit in accordance with this invention which includes the above mentioned toroidal detector senses the existence of breakdown current, thus signally the occurrence of dischage. This signal is then used to retard, in steps, the engine ignition timing, either for all or individual cylinders, until the auto-ignition stops. Another routine would incrementally advance timing to again develop auto-ignition which would again be corrected, resulting in a timing "dither" at about the threshold of auto-ignition. Therefore, by energizing the spark plug in a predetermined operating window of cycle time after TDC, the spark plug can act as an auto-ignition detector which is far more sensitive than conventional piezoelectric engine knock sensors. Such energization of the spark plug after TDC is not intended to contribute to the combustion process, but is provided only as a means of detecting the existence of auto-ignition. The use of a pulse transformer in accordance with this invention enables the hover voltage to be adjusted to a desired level which is not easily achievable using a conventional fly-back transformer ignition system configuration.

Due to cylinder-to-cylinder variations for a given internal combustion engine, it may be desirable in some circumstances to calibrate each cylinder since the pressure and temperature characteristics of one cylinder may vary from the next, and thus erroneous indications of auto-ignition could result if a fixed hover voltage is used as a sensor of auto-ignition. A calibration reading can be generated in which the spark plug is energized through a range of voltages during a segment of the cylinder operating cycle after TDC, but before the pressure and temperature variations caused by auto-ignition manifest themselves (for example, on the order of 5.degree. after TDC). The voltage at which breakdown occurs during this calibration period can be used to adjust the hover voltage for that cylinder. Therefore, in accordance with this invention, a total of three periods of spark plug energization may occur in a single piston power stroke cycle; a first discharge to initiate spark, a second calibration discharge, and a third period for the etection of auto-ignition.

Upon spark plug discharge, a small ball or "kernel" of ionized gases is formed. Since the fluid within the combustion chamber is turbulent, this small kernel moves away from its point of origination at the spark plug gap. If the air/fuel ratio is sufficiently rich, this kernel will induce an exothermic reaction in which the kernel grows rapidly and becomes a spherical flame front which moves away from its point of origination. If, on the other hand, the air/fuel ratio within the combustion chamber is excessively lean, the high temperature kernel of ionized gas will be quenched by the surrounding fluid so that it decreases in size and disappears as it moves away from the spark plug and no significant energy is derived from the mixture within the combustion chamber.

The air/fuel ratio at which the threshold of endothermic-exothermic reaction occurs is referred to as the lean burn limit of the engine. The present inventor has found that if the spark plug can be caused to discharge a number of times over a very short duration of time (in the microsecond range), the lean burn limit of the engine can be improved (i.e., moved to leaner mixtures). By rapidly firing pulses at the plug, a successive train of kernels of ionized gas is generated which are blown away from their point of origination. Due to the close proximity of the kernels to each other, quenching to the surrounding fluid is minimized. A reduction in quenching permits the ionized gas kernels to exist longer in the combustion chamber which has been found to permit leaner mixtures to combust. The use of a pulse transformer configuration enables such rapid multiple firing which is not possible using conventional fly-back transformer systems due to their significant secondary winding inductance.

In accordance with this invention, rapid multiple firing of the plug is achieved by sensing the existence of breakdown current which signifies the discharge event. This signal is used to immediately curtail that discharge cycle and begin another firing cycle, enabling multiple discharges to occur in a very short time duration.

In accordance with another feature of this invention, the size and mass of the pulse transformer is minimized for structural and packaging efficiency. Inherently, the use of pulse transformers minimizes size and mass which is a significant concern since they are placed on the end of a spark plug where their mass exerts a cantilever loading on the plug due to engine and vehicle vibration. A reduction in size is further advantageous in that it reduces engine packaging constraints. The size and mass of a pulse transformer of this type are greatly affected by thermal requirements. In accordance with this invention, the detection of breakdown current is employed as a means of immediately curtailing the flow of primary current to the spark plug coil; thus, reducing its duty cycle. Since the breakdown phase of spark discharge produces the useful work in initiating combustion, the arc and glow phases can be curtailed without adversely affecting combustion operation. These inventors believe that duty cycles on the order of one percent are possible using an ignition system operated in this manner. Such a low duty cycle enables pulse transformers to be of minimum size and mass; thus, improving engine packaging and structural efficiency.

Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph relating voltage versus time for the electrical discharge at a spark plug causing ignition of the combustible mixture.

FIG. 2 is a graphical representation of current versus time illustrating the existence of the breakdown or "r.f." current occurring at initial discharge at the spark plug electrodes.

FIG. 3 is a graphical representation of pressure within a combustion chamber with respect to crankshaft position showing normal combustion and pressure fluctuations caused by auto-ignition, and further showing a relationship between crankshaft position and spark plug energization voltage at ignition, calibration, and hover energization periods.

FIG. 4 is a schematic diagram of an ignition system in accordance with the present invention particularly adapted for a four cylinder spark ignition engine.

FIG. 5 is an electrical schematic diagram of the pulse transformers and pulse transformer drivers shown in FIG. 1.

FIG. 6 is a cross-sectional view of the ignition pulse transformer according to this invention mounted to a spark plug.

FIG. 7 is a graph showing the relationship of control signals, secondary voltage and r.f. current versus time during the multiple discharge cycle for combustion initiation.