Patent Publication Number: US-9835128-B2

Title: Spark plug fouling detection for ignition system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 14/965,795, entitled “SPARK PLUG FOULING DETECTION FOR IGNITION SYSTEM,” filed on Dec. 10, 2015, now U.S. Pat. No. 9,670,894. U.S. patent application Ser. No. 14/965,795 is a divisional of U.S. patent application Ser. No. 14/077,064, entitled “SPARK PLUG FOULING DETECTION FOR IGNITION SYSTEM,” filed on Nov. 11, 2013, now U.S. Pat. No. 9,249,774. U.S. patent application Ser. No. 14/077,064 claims priority to U.S. Provisional Patent Application No. 61/892,068, entitled “SPARK PLUG FOULING DETECTION FOR IGNITION SYSTEM,” filed on Oct. 17, 2013. The entire contents of each of the above-referenced applications are hereby incorporated by reference in their entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates to an ignition system for detecting spark plug fouling and pre-ignition. 
     BACKGROUND AND SUMMARY 
     Spark plug fouling and pre-ignition caused by hot spark plugs is a significant issue in areas with poor fuel quality control. Fuel additives such as MMT or ferrocene may build up electrically conductive and thermally insulating deposits on the spark plug ceramic. Such build up may cause misfires or pre-ignition (PI). Due to the potential severity of misfires or PI at high speed and load in boosted engines, vehicle manufacturers may recommend very short spark plug change intervals. However, as the issue of misfires and PI due to fuel additive build up is often a geographically and seasonally limited issue, such frequent spark plug changes may be unnecessary for some vehicles. 
     The inventors have recognized the above issues, and offer a system to at least partly address said issues. In particular, the present disclosure provides low cost and easy-to-implement methods and systems for continuously detecting the fouling level present at the spark plug, detecting the occurrence of PI and warning the customer to change plugs only when conditions warrant. In one embodiment, a method includes providing a dwell command on a control wire of an ignition system and generating an indication of a recommendation to change a spark plug of the ignition system based upon a current on the control wire. 
     The present disclosure may offer several advantages. For example, by providing spark plug change recommendations based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage, such recommendations may ensure that spark plug change recommendations are provided in a timely manner. The recommendations supported by measured indications of spark plug fouling may ensure that spark plug change recommendations are not provided too soon, resulting in increased cost for the driver, or too late, resulting in damage to the vehicle. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an engine. 
         FIG. 2  shows a diagram of an ignition system in accordance with an embodiment of the present disclosure. 
         FIG. 3  is a flow diagram of a method of determining spark plug fouling and pre-ignition in accordance with an embodiment of the present disclosure. 
         FIG. 4  shows waveforms of the operation of the ignition system responsive to a dwell command under various conditions in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An ignition system for detecting spark plug fouling and pre-ignition is disclosed herein. The spark plug fouling and pre-ignition detection enables spark plug change recommendations to be provided based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage (e.g., recorded operational mileage, number of combustion cycles, etc.). By measuring voltage at a terminal of the secondary windings of the ignition coil opposite of the spark plug, the level of impedance of the spark plug (indicating a level of fouling) may be determined and utilized to provide spark plug change recommendations. 
       FIG. 1  depicts an engine system  100  for a vehicle. The vehicle may be an on-road vehicle having drive wheels which contact a road surface. Engine system  100  includes engine  10  which comprises a plurality of cylinders.  FIG. 1  describes one such cylinder or combustion chamber in detail. The various components of engine  10  may be controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Combustion chamber  30  is shown communicating with intake manifold  144  and exhaust manifold  148  via respective intake valve  152  and exhaust valve  154 . Each intake and exhaust valve may be operated by an intake cam  51  and an exhaust cam  53 . Alternatively, one or more of the intake and exhaust valves may be operated by an electromechanically controlled valve coil and armature assembly. The position of intake cam  51  may be determined by intake cam sensor  55 . The position of exhaust cam  53  may be determined by exhaust cam sensor  57 . 
     Fuel injector  66  is shown positioned to inject fuel directly into cylinder  30 , which is known to those skilled in the art as direct injection. Alternatively, fuel may be injected to an intake port, which is known to those skilled in the art as port injection. Fuel injector  66  delivers liquid fuel in proportion to the pulse width of signal FPW from controller  12 . Fuel is delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail. Fuel injector  66  is supplied operating current from driver  68  which responds to controller  12 . In addition, intake manifold  144  is shown communicating with optional electronic throttle  62  which adjusts a position of throttle plate  64  to control airflow to engine cylinder  30 . This may include controlling airflow of boosted air from intake boost chamber  146 . In some embodiments, throttle  62  may be omitted and airflow to the engine may be controlled via a single air intake system throttle (AIS throttle)  82  coupled to air intake passage  42  and located upstream of the boost chamber  146 . 
     In some embodiments, engine  10  is configured to provide exhaust gas recirculation, or EGR. When included, EGR is provided via EGR passage  135  and EGR valve  138  to the engine air intake system at a position downstream of air intake system (AIS) throttle  82  from a location in the exhaust system downstream of turbine  164 . EGR may be drawn from the exhaust system to the intake air system when there is a pressure differential to drive the flow. A pressure differential can be created by partially closing AIS throttle  82 . Throttle plate  84  controls pressure at the inlet to compressor  162 . The AIS may be electrically controlled and its position may be adjusted based on optional position sensor  88 . 
     Compressor  162  draws air from air intake passage  42  to supply boost chamber  146 . In some examples, air intake passage  42  may include an air box (not shown) with a filter. Exhaust gases spin turbine  164  which is coupled to compressor  162  via shaft  161 . A vacuum operated wastegate actuator  72  allows exhaust gases to bypass turbine  164  so that boost pressure can be controlled under varying operating conditions. In alternate embodiments, the wastegate actuator may be pressure or electrically actuated. Wastegate  72  may be closed (or an opening of the wastegate may be decreased) in response to increased boost demand, such as during an operator pedal tip-in. By closing the wastegate, exhaust pressures upstream of the turbine can be increased, raising turbine speed and peak power output. This allows boost pressure to be raised. Additionally, the wastegate can be moved toward the closed position to maintain desired boost pressure when the compressor recirculation valve is partially open. In another example, wastegate  72  may be opened (or an opening of the wastegate may be increased) in response to decreased boost demand, such as during an operator pedal tip-out. By opening the wastegate, exhaust pressures can be reduced, reducing turbine speed and turbine power. This allows boost pressure to be lowered. 
     Compressor recirculation valve  158  (CRV) may be provided in a compressor recirculation path  159  around compressor  162  so that air may move from the compressor outlet to the compressor inlet so as to reduce a pressure that may develop across compressor  162 . A charge air cooler  157  may be positioned in passage  146 , downstream of compressor  162 , for cooling the boosted aircharge delivered to the engine intake. In the depicted example, compressor recirculation path  159  is configured to recirculate cooled compressed air from downstream of charge air cooler  157  to the compressor inlet. In alternate examples, compressor recirculation path  159  may be configured to recirculate compressed air from downstream of the compressor and upstream of charge air cooler  157  to the compressor inlet. CRV  158  may be opened and closed via an electric signal from controller  12 . CRV  158  may be configured as a three-state valve having a default semi-open position from which it can be moved to a fully-open position or a fully-closed position. 
     Distributorless ignition system  90  provides an ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . The ignition system  90  may include an induction coil ignition system, in which an ignition coil transformer is connected to each spark plug of the engine. An example ignition system that may be utilized in the engine of  FIG. 1  is described in more detail below with respect to  FIG. 2 . Universal Exhaust Gas Oxygen (UEGO) sensor  126  is shown coupled to exhaust manifold  148  upstream of catalytic converter  70 . Alternatively, a two-state exhaust gas oxygen sensor may be substituted for UEGO sensor  126 . Converter  70  can include multiple catalyst bricks, in one example. In another example, multiple emission control devices, each with multiple bricks, can be used. Converter  70  can be a three-way type catalyst in one example. While the depicted example shows UEGO sensor  126  upstream of turbine  164 , it will be appreciated that in alternate embodiments, UEGO sensor may be positioned in the exhaust manifold downstream of turbine  164  and upstream of convertor  70 . 
     Controller  12  is shown in  FIG. 1  as a microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random access memory  108 , keep alive memory  110 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  134  coupled to an accelerator pedal  130  for sensing accelerator pedal position (PP) adjusted by a foot  132  of a vehicle operator; a knock sensor for determining ignition of end gases (not shown); a measurement of engine manifold pressure (MAP) from pressure sensor  121  coupled to intake manifold  144 ; a measurement of boost pressure from pressure sensor  122  coupled to boost chamber  146 ; an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position; a measurement of air mass entering the engine from sensor  120  (e.g., a hot wire air flow meter); and a measurement of throttle position from sensor  58 . Barometric pressure may also be sensed (sensor not shown) for processing by controller  12 . In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
     In some embodiments, the engine may be coupled to an electric motor/battery system in a hybrid vehicle. The hybrid vehicle may have a parallel configuration, series configuration, or variation or combinations thereof. 
     During operation, each cylinder within engine  10  typically undergoes a four stroke cycle: the cycle includes the intake stroke, compression stroke, expansion stroke, and exhaust stroke. During the intake stroke, generally, the exhaust valve  154  closes and intake valve  152  opens. Air is introduced into combustion chamber  30  via intake manifold  144 , and piston  36  moves to the bottom of the cylinder so as to increase the volume within combustion chamber  30 . The position at which piston  36  is near the bottom of the cylinder and at the end of its stroke (e.g. when combustion chamber  30  is at its largest volume) is typically referred to by those of skill in the art as bottom dead center (BDC). During the compression stroke, intake valve  152  and exhaust valve  154  are closed. Piston  36  moves toward the cylinder head so as to compress the air within combustion chamber  30 . The point at which piston  36  is at the end of its stroke and closest to the cylinder head (e.g. when combustion chamber  30  is at its smallest volume) is typically referred to by those of skill in the art as top dead center (TDC). In a process hereinafter referred to as injection, fuel is introduced into the combustion chamber. In a process hereinafter referred to as ignition, the injected fuel is ignited by known ignition means such as spark plug  92 , resulting in combustion. During the expansion stroke, the expanding gases push piston  36  back to BDC. Crankshaft  40  converts piston movement into a rotational torque of the rotary shaft. Finally, during the exhaust stroke, the exhaust valve  154  opens to release the combusted air-fuel mixture to exhaust manifold  148  and the piston returns to TDC. Note that the above is described merely as an example, and that intake and exhaust valve opening and/or closing timings may vary, such as to provide positive or negative valve overlap, late intake valve closing, or various other examples. 
       FIG. 2  shows an example ignition system  200  that may be included in the engine  100  of  FIG. 1 . The ignition system  200  includes an ignition circuit for charging an induction ignition coil  202  of a transformer to fire a spark plug  204 , and the spark plug fouling and pre-ignition detecting components, resistors  205  (R 1 ) and  207  (R 2 ), diode  212  (D 1 ), and dwell qualification/detection module  206  for evaluating voltage and/or current output from the ignition system in order to determine a level of spark plug fouling. The ignition circuit includes a spark plug  204  connected to a high voltage terminal of a secondary winding  208  of the ignition coil  202 . The low voltage terminal of the secondary winding  208  is connected to a voltage source  210  (e.g., a voltage of a vehicle battery) via a feed-forward diode  212  (D 1 ) connected in parallel to two resistors  205  (R 1 ) and  207  (R 2 ). At the beginning of ignition coil dwell, the secondary winding  208  of the ignition coil may generate approximately 1000 V peak, termed feed-forward voltage or V ff . V ff  slowly decays over the duration of dwell. The magnitude of the peak of V ff  and the rate of decay depend on the characteristics of the coil and the magnitude of the battery voltage applied to the primary winding  209  of the coil. The total V ff  is distributed between the spark plug  204  and the low voltage end of the secondary winding  208  as determined by the impedance to ground at the spark plug (e.g., the fouling impedance based on the level of spark plug fouling) and the impedance to the voltage source  210  across the feed-forward diode  212 . The feed-forward diode  212  is commonly used in ignition coils to prevent bulk current flow (e.g., arcing) at the spark plug  204  at the start of dwell. The impedance across the diode is determined by the two resistors,  205  (R 1 ) and  207  (R 2 ), placed in series with one another and in parallel across the diode  212 . By selecting values for the resistors, the signal output may be “tuned” to be effective at a selected level of plug fouling for safeguarding the engine from misfires caused by plug fouling and to reliably detect the occurrence of pre-ignition. For example, lower values of resistors will make detection less sensitive (e.g., enable relatively higher levels of fouling to be tolerated) while higher values will make detection more sensitive (e.g., enable relatively lower levels of fouling to be tolerated). 
     The dwell qualification and plug fouling/pre-ignition module  206  is connected to the ignition circuit by an input tap connected between the resistors  205  (R 1 ) and  207  (R 2 ) in order to determine the level of plug fouling based upon a rate of decay of the voltage at the location of the input tap, as described in more detail below. A control signal may be provided over a control wire  214  and utilized to start dwell of the ignition coil  202  of the ignition circuit. For example, the control signal may be provided by a Powertrain Control Module (PCM)  215 . At the beginning of dwell, both current sinks  216  and  218  on the control signal are ON (e.g., switch  220  is closed). The dwell signal qualification module  222  receives the control signal and detects the beginning edge of the dwell. At the beginning edge of the dwell, the control signal is forwarded to a solid-state switching device, such as an insulated-gate bipolar transistor (IGBT)  223 , which establishes and disrupts the current flow to the primary windings  209  of the ignition coil  202 . The dwell signal qualification module and solid-state device may form an intelligent driver for dwell control of the ignition coils, including interpretive logic to decode or otherwise interpret the dwell commands provided for control of the ignition coils. 
     The dwell signal qualification module  222  may also instruct a blanking period generator  224  to generate a blanking period (e.g. with a duration of 500 μsec) which holds switch  220  closed to avoid any ringing present on the feed-forward voltage at the beginning of dwell. Accordingly, the blanking period generator may output a logic 1 for a specified time interval during the beginning of dwell. The output of the blanking period generator  224  is provided as an input to a logical OR gate  226  that controls switch  220 . In particular, the logical OR gate  226  may control the switch  220  to remain closed when the output of the OR gate  226  is logic 1 (e.g., when any of the inputs to the OR gate  226  is logic 1). 
     The input tap described above is connected at the node between the two sensing resistors  205  (R 1 ) and  207  (R 2 ), and at the cathode of clamping diode  212  (D 1 ) which will keep the input voltage not less than a diode forward voltage below ground, and that provides a sense voltage (V sense ) to a comparator  228  for comparing the sense voltage to a reference voltage at  230  (e.g., a voltage set ratio-metrically between a battery voltage and ground). The sense voltage is the inverse of the voltage appearing at the high voltage terminal of the secondary windings  208  and its magnitude is related to the ratio between the resistors  205  (R 1 ) and  207  (R 2 ) and the shunting impedance (e.g., the fouling level) of the spark plug  204 . The comparator  228  may be configured to output logic 1 while the sense voltage is less than the reference voltage at  230  and logic 0 while the sense voltage is greater than the reference voltage. 
     As the logic OR gate  226  is configured to maintain the switch  220  in the closed state when the output of the gate  226  is logical 1, the switch  220  remains closed during the blanking period. After the blanking period, switch  220  is controlled by the output of a voltage comparator  228  and the state of a D flip-flop  232 . The D flip-flop  232  stores and/or outputs the output of the comparator  228  at the end of each dwell (e.g., at the falling edge of a clock signal received from the dwell signal qualification module  222 ) and outputs the stored value at other times (e.g., at a steady state or rising edge of the clock signal). If the D flip-flop  232  stores a logic 0, switch  220  is controlled by voltage comparator  228 . As the feed-forward voltage decays throughout dwell, at some point under moderate levels of fouling at the spark plug, the sense voltage will rise above the threshold level (e.g., above the reference voltage). At this point, current sink  218  is turned off (e.g., switch  220  is opened). This change of the current sink level is detected by a driver integrated circuit (IC) in the PCM  215  and the length of time interval from the beginning of dwell to the switching point (e.g., a decay time) is interpreted as a level of fouling present at the spark plug. This information is communicated to the microprocessor in the PCM  215 . If the microprocessor determines that the level of fouling is too great (e.g., upon comparing the detected level of fouling to a fouling threshold or a decay time to a decay threshold) the microprocessor may warn the driver to replace the spark plugs. For example, the microprocessor may provide a visual, audio, and/or other type of indication to the driver recommending a replacement of the spark plugs. 
     The D flip-flop  232  may be controlled to store the state of the comparator at the trailing edge of dwell. If pre-ignition occurs, such a condition will cause the comparator output to equal logic 1 at the end of dwell (e.g., as V sense &lt;V reference ). This logic 1 is captured at the end of dwell and causes switch  220  to remain closed for the entire following dwell period. During that dwell period, the microprocessor may interpret the closed switch condition as corresponding to an occurrence of pre-ignition (PI) in the previous combustion event and output an indication to replace the spark plugs. 
       FIG. 3  is a flow diagram of a method  300  for controlling an ignition coil and detecting spark plug fouling and/or pre-ignition in cooperation with the configuration of  FIG. 2 , and therefore spark generation, in an engine, such as the engine of  FIG. 1 . For example, the method  300  may be performed by the controller  12  of  FIG. 1  and/or the PCM  215  of  FIG. 2  and utilize measurements and/or outputs provided by the integrated circuits of  FIG. 2 . At  302 , the method  300  includes outputting a dwell command to control an ignition coil, such as the ignition coil  202  of  FIG. 2 . For example, the dwell command may be a pulse having a particular length (e.g., a pulse that is applied for a duration that is longer than a threshold). During the commanded dwell, current is passed through the primary windings of the ignition coil to generate a magnetic field. Responsive to detecting the dwell command at a module, such as the dwell signal qualification module  222  of  FIG. 2 , a blanking period may be generated during which a switch is closed to maintain or set a current sink in an “ON” state, as indicated at  304 . 
     After the blanking period ends, at  306 , a voltage at a sensed location in the ignition circuit (e.g., V sense  of  FIG. 2 ) that has a magnitude related to the fouling level of the spark plug is compared to a reference voltage at  308 . As indicated at  310 , if V sense  is less than the reference voltage (e.g., “NO” at  310 ), the method  300  proceeds to  312  to close or maintain a closed switch, then to  314  to determine whether the trailing edge of the dwell command signal is detected. The trailing edge of the dwell command may include a termination of the pulse to trigger an interruption and/or cessation of current flow through the primary windings of the ignition coil. The interruption of the current flow through the primary windings causes a high voltage pulse across the respective secondary windings of the ignition coil (e.g., to “fire” the spark plug and generate a spark for initiating combustion in a cylinder of the engine). If a trailing edge is not detected, (e.g., “NO” at  314 ), the method  300  returns to  308  continue monitoring V sense . Conversely, if the trailing edge of the dwell command signal is detected (e.g., “YES” at  314 ), a D flip flop (e.g., D flip flop  232  of  FIG. 2 ) is triggered to store the output of the comparison of V sense  to the reference voltage, as indicated at  316 . A condition, in which V sense  is less than the reference voltage at the trailing edge of dwell, is indicative of a pre-ignition event. Since the pre-ignition event prevents the switch from being opened to turn off the current sink during the following dwell or combustion cycle, a switching time from beginning of dwell to the switching point may be determined to be approximately equal to the entire dwell time at  318 . This switching time may be indicative of a pre-ignition event during the previous combustion cycle. 
     The method  300  then determines whether the switching time is greater than a threshold at  320 . If the switching time is less than a threshold (e.g., “NO” at  320 ), the method  300  then returns to wait for the next dwell command. If the switching time is greater than a threshold (e.g., “YES” at  320 ), method  300  then proceeds to  322  to output an indication to the driver to replace the spark plugs responsive to detecting either a fouled plug or a pre-ignition event. For example, if the current on the control wire drops below a predetermined value after a threshold period of time has elapsed after the dwell command is provided, the decay time may be determined to be greater than the threshold. Conversely, if the current on the control wire drops below a predetermined value prior to a threshold period of time has elapsed after the dwell command is provided, the decay time may be determined to be less than the threshold. If the decay time is less than the threshold (e.g., “NO” at  320 ), the method  300  may return to await a next combustion event (e.g., without outputting an indication to replace the spark plugs). Conversely, if the decay time is greater than a threshold (e.g., “YES” at  320 ), the method  300  may proceed to  322  to output an indication to the driver to replace the spark plugs. For example, outputting the indication may include sending an instruction to an icon or display device on an instrument panel to display a visual indicator to the driver regarding the spark plug change recommendation. Outputting the indication may additionally or alternatively include sending an instruction to a speaker system to output an audio indicator (e.g., an audio message, a system beep, etc.) regarding the spark plug change recommendation. After outputting the indication to the driver, the method  300  returns to wait for the next start of dwell command. 
     Returning to  310 , at which the sensed voltage is compared to a reference voltage, if V sense  is greater than the reference voltage (e.g., “YES” at  310 ), the method  300  proceeds to  324  to determine whether the D flip flop is outputting a logic 0. If not, the output of the D flip flop is a logic 1, which indicates that a pre-ignition event occurred in the previous combustion cycle, as discussed above with respect to  316  and  318 . Thus, the method proceeds to  312  to maintain the closed switch and the “ON” state of the current sink. If the D flip flop outputs a logic 0 at  324  (e.g., “YES” at  324 ), the method  300  proceeds to  326  to open the switch and turn off the current sink. By turning off the current sink, the microprocessor may detect a drop in the measured current on the control wire of the circuit (e.g., by receiving a measurement from a current sensor coupled to the control wire) and measure the switching time from the beginning of dwell to the current sink switching point (e.g., the time at which the current sink is switched from the “ON” state to the “OFF” state). The method may then proceed to  314  to determine if the trailing edge of dwell has occurred. 
     Exact selection of circuit components for resistors  205  (R 1 ) and  207  (R 2 ) of  FIG. 2 , the threshold voltage  230  of  FIG. 2 , and the switching time threshold may be based upon attributes of the ignition coil and the range of spark plug fouling deemed unacceptable. For example, 50 M ohms or 10 M ohms of shunting (fouling) impedance at the spark plug may be deemed unacceptable in some embodiments. This range may be judged to give adequate warning of plug fouling prior to misfires occurring. Selection of the blanking period duration (e.g., 500 μsec) may depend on the turn-on characteristics and the total nominal dwell time of the ignition coil. Similarly, selection of the switching time threshold, as evaluated in  320 , may be determined based upon the duration of the blanking period and the total nominal dwell time of the ignition coil. For example, if the blanking period is 500 μsec and the nominal dwell time is 2000 μsec, resistors  205  and  207  (R 1  and R 2 ) and the threshold voltage  230  of  FIG. 2  may be chosen to yield a switching time threshold of 1250 μsec at the desired plug fouling level. 
       FIG. 4  illustrates waveforms  400  reflecting the operation of the ignition system described herein responsive to a dwell command. In the illustrated waveforms, the x-axes correspond to a shared timeline, while each y-axis corresponds to the parameter indicated adjacent to the associated waveform. In  FIG. 4 , waveforms  400  show operation of the ignition system responsive to the dwelling and firing the ignition coil (e.g., ignition coil  202  of  FIG. 2 ) under various spark plug fouling conditions. 
     Waveform  402  corresponds to a dwell command, which may be issued from a controller, such as controller  12  of  FIG. 1 . As indicated, the dwell signal has a duration extending from time T 0  to time T 4 . Waveform  404  corresponds to a voltage at the high voltage terminal of the secondary windings of an ignition coil (e.g., secondary windings  208  of  FIG. 2 ), which connected to the spark plug. As indicated, the voltage may decay from a peak level (e.g., approximately 1000 volts) responsive to a level of fouling on the spark plug. Upon termination of the dwell command at time T 4 , the current provided to the primary windings of the ignition coil may be interrupted, producing a pulse of approximately −30000 volts to be provided to the spark plug for generating a spark. 
     Waveform  406  corresponds to a sensed voltage (e.g., V sense  as illustrated in  FIG. 2 ) and current on a control wire (e.g., control wire  214  of  FIG. 2 ) measured responsive to the dwell command of waveform  402  during ideal conditions, in which there is no pre-ignition event or spark plug fouling. As illustrated, the sensed voltage remains approximately equivalent to the battery source voltage throughout the measurement period (e.g., without dropping and/or ramping up to the battery voltage responsive to the dwell command). The current on the control wire (I control ) reflects the operation of current sinks coupled to the control wire (e.g., current sinks  216  and  218  of  FIG. 2 ). The time between T 0  and T 1  corresponds to a blanking period, as described at  304  of method  300  illustrated in  FIG. 3 . During the blanking period, which begins at the rising edge of the dwell command and ends after a predetermined amount of time has elapsed since the start of the dwell command, both current sinks are maintained in an “ON” state, as a switch controlling the second current sink is closed. 
     After the blanking period ends at time T 1 , V sense  is measured and compared to a reference voltage (e.g., as described at  310  of  FIG. 3 ). As illustrated in  FIG. 2 , the reference voltage may be smaller than the battery voltage, and one example value of a reference voltage is indicated on the y-axis of the waveforms of  FIG. 4 . Since the sensed voltage is greater than the reference voltage at time T 1  (e.g., when the blanking period ends), the switch is opened, turning the second current sink off (e.g., in response to the execution of  326  as illustrated in  FIG. 3 ). The switching time may therefore be determined to be equal to the blanking period, if measured from the start of the dwell command to the time at which the second current sink is switched off (e.g., time T 1 ). It is to be understood that the waveform  406  provides the control current during a condition in which pre-ignition was not detected during the previous combustion cycle (e.g., the sensed voltage was greater than the reference voltage at the trailing edge of the dwell command for the previous combustion cycle). At time T 4 , the current drops again responsive to the cessation of the dwell command, which results in a decrease in current provided to the control wire and a decrease in current at the first current sink. 
     Waveform  408  corresponds to a sensed voltage (e.g., V sense  as illustrated in  FIG. 2 ) and current on a control wire (e.g., control wire  214  of  FIG. 2 ) measured responsive to the dwell command of waveform  402  during a condition in which there is no previous or current pre-ignition event, however a relatively moderate amount of spark plug fouling is present. As illustrated, the sensed voltage drops at the beginning of dwell due to the impedance at the spark plug caused by the fouling. As the fouling during the condition described in waveform  408  is relatively moderate, the sensed voltage may quickly ramp up to the battery voltage, surpassing the reference voltage at time T 2 . The current on the control wire (I control ) reflects the operation of current sinks coupled to the control wire (e.g., current sinks  216  and  218  of  FIG. 2 ). As the sensed voltage does not exceed the reference voltage until time T 2 , both current sinks remain on and the current is maintained at a peak level until time T 2  (at which point, the second current sink is turned off and the current drops). Thus, the switching time  410  under the moderate fouling may correspond to the amount of time that elapses between time T 0  and time T 2 . As described above, at time T 4 , the current may drop (e.g., no current may flow on the control wire) responsive to the cessation of the dwell command. 
     Waveform  412  corresponds to a sensed voltage (e.g., V sense  as illustrated in  FIG. 2 ) and current on a control wire (e.g., control wire  214  of  FIG. 2 ) measured responsive to the dwell command of waveform  402  during a condition in which there is no previous or current pre-ignition event, however a relatively high amount of spark plug fouling is present (e.g., the spark plug is more fouled than the condition represented by waveform  408 ). As illustrated, the sensed voltage drops at the beginning of dwell due to the impedance at the spark plug caused by the fouling. As the fouling during the condition described in waveform  408  is relatively high, the sensed voltage may stay at ground for longer than conditions in which the spark plug is more moderately fouled, and ramp up to surpass the reference voltage at time T 3 . The current on the control wire (I control ) reflects the operation of current sinks coupled to the control wire (e.g., current sinks  216  and  218  of  FIG. 2 ). As the sensed voltage does not exceed the reference voltage until time T 3 , both current sinks remain on and the current is maintained at a peak level until time T 3  (at which point, the second current sink is turned off and the current drops). Thus, the switching time  414  under the high level of fouling may correspond to the amount of time that elapses between time T 0  and time T 3 . The switching time  414  is longer than the switching time  410  since the level of fouling is higher during the condition represented by waveform  412  in comparison with the condition represented by waveform  408 . For example, the switching time  414  may be determined to be longer than the switching threshold (e.g., resulting in a “YES” at  320  of  FIG. 3 ) while switching time  410  may be determined to be shorter than the switching threshold (e.g., an acceptable level of fouling, resulting in a “NO” at  320  of  FIG. 3 ). Accordingly, the switching time  414  may result in an output of an indication to the driver to replace the spark plugs, while the switching time  410  may result in no such indication. As described above, at time T 4 , the current may drop (e.g., no current may flow on the control wire) responsive to the cessation of the dwell command. 
     Waveform  416  corresponds to a sensed voltage (e.g., V sense  as illustrated in  FIG. 2 ) and current on a control wire (e.g., control wire  214  of  FIG. 2 ) measured responsive to the dwell command of waveform  402  during a condition in which pre-ignition event occurs. In particular, the sensed voltage corresponds to sensed voltage during a pre-ignition event, and the current on the control wire corresponds to the measured current during the next combustion cycle directly following the pre-ignition event (e.g., pre-ignition has occurred before the trailing edge of dwell in previous combustion cycle). As illustrated, the sensed voltage remains at the battery voltage level until just prior to the trailing edge of the dwell command at T 4 , at which point the voltage drops to below the reference voltage level. Shown below the sensed voltage are the current on the control wire for the current dwell cycle and the current on the control wire for the next consecutive dwell cycle. The current on the control wire (I control ) reflects the operation of current sinks coupled to the control wire (e.g., current sinks  216  and  218  of  FIG. 2 ). During the current dwell cycle, the current drops to the lower level at T 1 , as expected with no fouling present. Just prior to the end of dwell however, the current jumps to the higher level due to V sense  being less than the reference voltage (resulting in a “NO” at  310  of  FIG. 3 ). At the end of dwell, T 4 , the D flip-flop captures the pre-ignition event and holds the current on the control wire at the high level through the entire following dwell period as illustrated by I control  (next consecutive dwell cycle). Thus, the switching time  418  responsive to the pre-ignition event may correspond to the amount of time that elapses between time T 0  and time T 4 . The switching time  418  is longer than the switching times  410  and  414  due to the pre-ignition event and is reported at the combustion cycle following the pre-ignition event. Accordingly, during the reporting combustion cycle, the switching time may be determined to be above a switching threshold and an indication to change the spark plugs may be output (e.g., via a display or other visual indicator of the vehicle). As described above, at time T 4 , the current may drop (e.g., no current may flow on the control wire) responsive to the cessation of the dwell command. 
     The above-described ignition systems and routines thereby provide a mechanism for detecting spark plug fouling and pre-ignition events. Accordingly, spark plug change recommendations may be provided based on evidence of malfunction or degradation, rather than a predetermined period of time or amount of vehicle usage (e.g., recorded operational mileage, number of combustion cycles, etc.). Such recommendations may ensure that spark plug change recommendations are provided in a timely manner, rather than too soon (e.g., resulting in increased cost for the driver) or too late (e.g., resulting in damage to the vehicle). Further, by determining the level of spark fouling at a controller based upon a measurement of current on a control wire, the condition may be detected without an additional wire (e.g., other than the control wire for providing dwell commands) from each ignition coil to the controller. 
     Note that the example control and measurement routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.