Ignition system and method

Abstract: The present invention encompasses an ignition system with an ignition dwell signal (207) having charge and discharge states for driving an energy storage device (105) and a spark plug (103). This system applies an essentially periodic switching device (107, 209, 211, 215) for discharging excess energy in the energy storage device (105).

Description: This invention is generally directed to ignition systems of internal combustion engines that include electronic control of spark timing. Solid state ignition systems are in wide spread use today. Many have advanced functions. However, they are deficient in an area that many of the systems claim to excel at, power dissipation, or more succinctly energy management such that power dissipation is minimized. Often ignition system's components are pushed beyond the well defined area of their formal specification in order to optimize their performance. This becomes even more complex and tedious as several analog components, such as the sensing devices as well as power devices are tuned for optimal performance. For economy of scale the circuitry is often fully customized. This usually results in long development cycles as extending the components' performance requires some empirical design practice. Previous designs also rely on active trimming of key components in the production environment adding unnecessary complexity to the manufacturing process. Relying on tuned analog components necessarily compromises optimal energy management. Also, integral to these systems are sophisticated means for determining diagnostic information about the performance of the system for various reasons including managing energy during abnormal operation conditions, such as when a spark plug is fouled or an ignition coil's secondary is shorted to name a few. Hereto previous designs often fall short of optimal performance as some important diagnostic information is not retrieved and applied. GB-A-2,024,941 describes an ignition system including a power transistor connected in series with the primary winding of an ignition tranformer, which transistor acts as an electronic circuit breaker, a measuring resistor for measuring current flowing through the transistor, and a current regulator network for regulating current flow through the resistor in dependence upon the current measured by the measuring resistor. This document also discloses means for measuring a voltage present at the primary winding. A current regulator network stops current flow through the power transistor depending on whether a predetermined maximum current flows through the measuring resistor. The current regulator network fully blocks the power transistor, rather than merely reduce its conduction, upon attaining the maximum current flow through the power transistor. EP-A-447975 describes an ignition system for an internal combustion engine, having a control circuit which is arranged to switch a power transistor progressively from a cut-off condition to a saturated condition. US-A-4944281 describes a circuit for regulating current in an inductive load with a hysteresis comparator connected to a transistor controlling the current flowing in an inductive load, such as an ignition coil. Voltage measured across terminals of a measurement resistor and is compared with a reference voltage established by a source, and a feedback loop establishes a proportional action of the transistor on the current in the load when the transistor's collector voltage exceeds a predetermined threshold. In this way an oscillation is obtained between two values on either side of a nominal value, the two values are being associated with the thresholds of the hysteresis comparator. In accordance with the invention there is provided an ignition system as claimed in claim 1, and a method of ignition control as claimed in claim 5. FIG. 1 illustrates a fault processing apparatus in an ignition control system. FIG. 2 illustrates an ignition control system with discharge of ignition coil energy during the ignition dwell signal's charge state during certain operating conditions. FIG. 3 illustrates details of an ionization detector employed in the present invention. In a preferred embodiment, the present invention overcomes the deficiencies of previous designs by optimally managing energy such that the power dissipation in the ignition system is minimized. In addition indigenous and extraneous system components are protected from abuse. Other treatises such as Deutsch et al. U.S. Patent Application serial number 636,351, IONIZATION CONTROL FOR AUTOMOTIVE IGNITION SYSTEM, filed on 90/12/31 now U. S. Patent number 5,054,461, teach the management of energy while system components are operating normally. The present invention focuses on the management of energy over a broader operating envelope. This includes energy management when system components are not operating properly, such as when an ignition coil's secondary is shorted. FIG. 1 illustrates an ignition system that includes an ignition controller 101, which generates an ignition dwell signal on a line 102, that drives the energy switching element, or driver 107. In order to minimize drive circuitry the energy switching element 107 is a device such as the MPPD2020 type available from Motorola. The energy switching element 107 drives an ignition coil 105, which has a primary winding and a secondary winding. The ignition coil's 105 secondary winding is connected to the spark plug 103. A signal is sensed in the ignition coil's primary by the ionization detector 117 which provides ionization information, in this case an ionization signal 119 to the ignition controller 101 and to a combining device in this case a logical OR gate 115. An additional input to the logical OR gate 115 and the ignition controller 101, is provided by overcurrent information, in this case the overcurrent signal 113 which is provided by the overcurrent detector 111, which is coupled to a current sense resistor 109 and the energy switching element 107. The combining device, in this case the logical OR gate 115, has an output 121 which is connected to the ignition controller 101. This combination of the ionization information and overcurrent information is particularly useful as these functions are designed into a custom integrated circuit which benefits from the reduction in pin count. This is possible as the ionization information and overcurrent information are mutually exclusive in time. Finally, fuel control line 125 is derived from the ignition control 101 for modifying the fuel flow to the engine during certain conditions detected by the preferred embodiment of the present invention. This may include shutting off fuel to a particular cylinder that has exhibited an abnormal operating condition so that raw fuel isn't passed through the engine unburned deteriorating the catalytic converter's condition and expelling undesired emissions. These abnormal operating conditions may include an open or shorted ignition coil primary, an open driver, an open or shorted ignition coil secondary, an open spark plug wire, a defective or fouled spark plug, and other system component malfunctions. In summary, FIG. 1 is a broad illustration supporting the teaching of recognition and combining of the detected ionization and overcurrent conditions in the energy switching element 107. This figure is important to better understand the overall energy management function of this invention. Both the ionization detector and the overcurrent detector of FIG. 1 are shown in FIG. 2 in detail. In FIG. 2, we find an ignition controller 101' which generates an ignition dwell signal 207, comprised of charge and discharge states, which is then coupled to a latch 205 and a multiplexer 215. The latch 205 derives its other input from a comparator 203. The purpose of the latch 205 is to ensure the proper signal selection throughout the ignition dwell signal 207 period. The comparator 203 compares a current limit reference 201 to a voltage representative of the current in ignition coil's 105 primary which is developed across the current sense resistor 109. The latch 205 is set if the signal representative of the current in the ignition coil exceeds the current limit reference 201. The ignition signal 207 is used to clear the latch 205 when the discharge cycle starts. An additional input to the comparator 203 is supplied by an intervention signal 219 from the ignition controller 101'. A resistor 221 is employed to isolate the potential impedance swamping effect of the current sense resistor 109 which typically has a very low resistance compared to the relatively high resistance of the intervention signal 219. The ignition coil 105 drives comparator 211, which derives its other input from the voltage limit reference 209. The comparator 211 in turn derives an alternative ignition dwell signal 213 which drives the multiplexer 215. This circuit 211, 215 acts as a clamping mechanism, limiting the value of the voltage at the junction of the ignition coil's 105 primary and the energy switching element 107, which in turn will prevent a spark. The control line 216 for the multiplexer 215 is derived from the latch 205. The multiplexer 215 in turn derives the signal 217 which drives the energy switching element 107. The intervention signal 219 would be invoked for instance when the engine was rotating slowly, such as in the cranking sequence, such that a particular cylinder's ignition coil's primary would not be over charged. The ignition controller 101' would issue the intervention signal 219 to the ignition drive causing the alternative ignition dwell signal 213 to drive the energy switching element 107, resulting in the discharge of the energy in the ignition coil's primary, preventing a spark. This alternative ignition dwell signal 213 is also invoked when the current in the ignition coil 105 exceeds the value preset by the current limit reference 201. The alternative dwell signal 213 discharges, or depletes, energy in the ignition coil 105, thus implementing a soft shutdown, by repetitively switching the element 107 on and off while limiting the voltage at the junction of the ignition coil primary and the switching element 107. In summary, FIG. 2 is supportive of the teaching of the discharge of ignition coil energy during the charge state of the ignition dwell signal 207. This figure is also important to better understand the energy management function of this invention. In FIG. 3 we find a detailed illustration of an ionization detector 117'. This ionization detector 117' uniquely and accurately extracts the ionization information from the ignition coil's 105 primary. This information is later applied to understand the actual performance of the ignition system. Resistor 301 derives its input from the ignition coil 105. The resistor 301 in turn drives the scaling resistor 302. These elements, 301 and 302 in turn drive the transmission gate 303. The transmission gate 303 derives its control input from a latch 305 that is driven by a logical NOR gate 307 and a latch 205. The purpose of the latch 305 and the transmission gate 303 is to enable the sampling of the signal from the ignition coil 105 during a certain period of the ignition signal 123 provided from the ignition controller 101". A filter element, in this case a capacitor 309 is then coupled to the transmission gate 303 and in turn coupled to a comparator 313 and a comparator 317. The voltage limit reference 311, the comparator 313, the comparator 317, the amplifier 315 and the latch 319 form the basic elements necessary for a window comparator. The amplifier 315 is used to scale the voltage provided from the battery in order to provide an accurate representation of the ionization signal over various operating conditions. The output of this circuit is the ionization signal 119 which is applied in the present embodiment. In summary, FIG. 3 supports the teaching of more detail concerning operation of the ionization detector. Element 215 of FIG. 2 is not repeated here and is understood to be located in the ignition controller 101". Additionally the ionization information line 119 in FIG. 3 is identical to line 119 in FIG. 1, and the ionization detector 117' in FIG. 3 is a detailed version of the ionization detector 117 in FIG. 1. The efficiency of this system is primarily due to the digital control of energy management controlled by its enhanced diagnostic capability. It would be obvious to one of ordinary skill in the art that this concept is extendible to multiple cylinder designs. The technique of slowly depleting or discharging energy from an ignition coil through the drive circuit is often referred to as soft shutdown and is intended primarily to prevent firing a particular cylinder. Previous systems inadequately accomplished this through linear control techniques which unnecessarily heat the ignition coil and drive circuit. This improved embodiment does not suffer from this excessive heating. Once the soft shutdown sequence is invoked it is locked in until the completion of the ignition dwell signal's 207 charge cycle. When the ignition dwell signal discharge cycle commences this system may either fire the cylinder or continue to deplete the energy in the respective ignition coil's 105 primary such that no firing occurs. One advantage of the present embodiment over previous systems is that while applying a single sense resistor to sense multiple channel ignition coil currents, individual ignition drivers can be soft stalled while other ignition drive circuits function normally. Also multiple ignition channels can overlap if the current limit reference 201 is set high enough. This technique further benefits the user as the energy in the ignition coil can be charged to a higher than normal level as desirable during certain operating conditions such as low speed. Conventional systems need to account for this overhead in their power dissipation budget yielding inefficient designs. The combined signals at the output of the logical OR gate 121 can be applied to diagnose faults as follows. If the ignition coil's 105 primary is shorted, as the ignition dwell signal 207 transitions to its charge state, the energy in the ignition coil 105 will rise very rapidly. This is sensed by the voltage rise across resistor 109. When compared with the current limit reference 201 the comparator 203 sets the latch 205 driving the logical OR gate 121. If the output of the logical OR gate 121 transitions high within a small period of time as the ignition dwell signal 207 transitions to its charge state this indicates a shorted ignition coil 105 primary. If the ignition coil's 105 primary, or the driver 107 is open, there will be no current flow in the ignition coil 105, resulting in no ionization detected. As a result the output of the logical OR gate 121 will be continuously low during the ignition dwell signal's 207 charge state. If the ignition coil's 105 secondary is shorted, across itself or to ground, or the spark plug's 103 gap is abnormally small, the ignition coil's 105 discharge time will be longer than normal and the overcurrent detector will detect an abnormally high current flow during the ignition dwell signal's 207 charge state. As a result, the output of the logical OR gate 121 will transition high within a small period of time, but longer than the period expected for an ignition coil's 105 shorted primary. If the ignition coil's 105 secondary is open, or the spark plug's 103 gap is abnormally wide during the ignition dwell signal's 207 discharge state then the ionization signal 119, thus the output of the logical OR gate 121 will have a significantly shorter output. If the output of the logical OR gate 121 is continuously high a circuit malfunction is indicated. If the logical OR gate 121 is continuously low there is either a circuit malfunction or an open in the ignition coil's primary.

Claims: An ignition system comprising an ignition coil (105) that has a primary winding and a secondary winding, the secondary winding being coupled to a spark plug (103), a switching element (107) and means (101) for generating an ignition dwell signal that has a charge state and a discharge state, the ignition dwell signal coupled via the switching element (107) to provide energy to, the ignition coil, the ignition system further comprising: means (117; 209, 211; 117') for determining ionization information for said spark plug (103)including means for measuring a voltage present at the primary winding of said ignition coil; means (109, 111; 109, 201, 203, 205) for determining when current flowing in the primary winding of said ignition coil (105) exceeds a reference value; and means (101, 107) for providing energy to the primary winding of the ignition coil (105) during the charge state of the ignition dwell signal, the ignition system characterised by means for depleting energy from the primary winding of the ignition coil, to provide a soft shutdown by continuously reducing an average current in the primary winding of the ignition coil during, and until the end of, the charge state of the ignition dwell signal, responsive to said means (117; 209, 211; 117') for determining ionization information and said means (109, 111; 109, 201, 203, 205) for determining when current flowing in the primary winding of said ignition coil (105) exceeds the reference value. A system in accordance with claim 1 further characterized by means (115) for combining an output (119) from said means (117; 209, 211; 117') for determining ionization information and an output (113) from said means (109,111,109,201,203,205) for determining overcurrent information. A system in accordance with claim 2 wherein said means (115) for combining further comprises means for logically OR'ing . A system in accordance with claim 2 further characterized by means (125) for controlling fuel responsive to said means (115) for combining. A method of ignition control having an ignition controller (101) generating a ignition dwell signal (102; 207; 123) that has a charge state and a discharge state for driving, via a switching element (107), an ignition coil (105) with a primary winding and a secondary winding, the secondary winding being coupled to a spark plug (103), the method comprising the steps of: sensing voltage at the primary winding of said ignition coil (105) and providing an ionization voltage (119; 213) indicative of said sensed voltage; sensing current in the primary winding of said ignition coil (105) and providing a current sense voltage indicative of said sensed current; and controlling current to the primary winding of said ignition coil (105) when the current sense voltage is less than a predetermined limit, the method characterised by said controlling current step being responsive to said step of sensing voltage, for depleting energy from the primary winding of the ignition coil for providing a soft shutdown by continuously reducing an average current from the primary winding of said ignition coil (105) during, and until the end of, the charge state of the ignition dwell signal, after the current sense voltage is greater than the predetermined limit.

Inventors: DEUTSCH ROBERT W, SUN KOUSHUN, DEUTSCH, ROBERT W., SUN, KOUSHUN
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1992
Main classification: F02P3
IPC codes: F02P3