DESC:TECHNICAL FIELD
[0001] The present subject matter relates to spark ignition engines. More particularly, detection of a misfire and a partial misfire in a spark ignition engine using an ion current measurement circuit is disclosed.

BACKGROUND
[0002] Internal Combustion (IC) engines are significantly contributing to pollution and global warming by emitting various exhaust gases into the environment. An IC engine produces power by burning of fossil fuel that emits harmful gases, such as, CO, HC, NOx and hydrocarbons. The emission of gases has been and is deteriorating the environmental condition and hence, vehicle manufacturers are continuously striving to bring in advances in technology to improve the performance of the IC engine and reduce the emissions.
[0003] In addition to the control of emission of gases, the Automobile industry is implementing “On board diagnostic (OBD)” on vehicles for intimating user about the status of vehicle conditions. OBD system across major jurisdictions is being implemented in a phased manner and is subdivided in two categories, viz. OBD I & OBD II. OBD II focuses on three main aspects – Engine misfire detection, Catalytic convertor monitoring, and Lambda sensor monitoring.
[0004] To improve engine combustion efficiency & reduce emissions, it is essential to detect and monitor misfire event in an internal combustion (IC) engine. Misfire occurs when the injected air fuel mixture does not burn at all or partially burns. Misfiring of the IC engine affects the quality of combustion and degrades the performance of a catalyst convertor, thereby leading to increase in emissions, which is undesirable.

BRIEF DESCRIPTION OF DRAWINGS
[0005] The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components.

[0006] Fig. 1 exemplarily illustrates a circuit diagram of an existing ignition system of an IC engine;
[0007] Fig. 2 exemplarily illustrates a circuit diagram of the existing ignition system of an IC engine connected to an ion current measurement circuit;
[0008] Fig. 3 exemplarily illustrates a schematic diagram of a misfire detection system, according an embodiment of the present invention;
[0009] Fig. 4 exemplarily illustrates the misfire detection system comprising an ion current measurement circuit and a signal processor, in accordance to an embodiment of the present invention;
[00010] Figs. 5-6 exemplarily illustrate timing diagrams for the primary voltage in the primary winding of the ignition coil and the ion current during a misfire condition; and
[00011] Figs. 7-8 exemplarily illustrate timing diagrams for the primary voltage in the primary winding of the ignition coil and the ion current during a no misfire condition.

DETAILED DESCRIPTION OF THE INVENTION
[00012] Misfire events in an IC engine can be categorized as partial or complete, based on the amount of combustion occurring during a particular engine cycle. In most vehicles, identification of misfire is performed by monitoring angular acceleration of engine crank-shaft. However, single cylinder engines with lower capacity (less than 200 cubic centimeter) provide challenges to identify misfire due to low mechanical inertia of the IC engine using the same approach. The problem of misfire identification for the single cylinder IC engines turns out to be more challenging due to presence of various load disturbances on the power train, when the engine is employed in a vehicle.
[00013] Several other techniques are designed for the detection of engine misfire for the single cylinder engines. Such techniques include analysis of instantaneous crankshaft speed, analysis of in-cylinder pressure, analysis of instantaneous crankshaft torque, etc. Evaluation of the crankshaft speed for detection of misfire faces a lot of challenges due to low mechanical inertial and load disturbances in engines. Alternate solutions to address the problem of detection of misfire event in the engine utilize the ion current generated in a sparking event in the engine.
[00014] When air-fuel mixture ignites inside the IC engine cylinder, air particles get ionized. By applying a suitable high-voltage on spark plug, it is possible to measure the ion current as the amount of ion current reflects the level of ionization of the air-fuel mixture. Thus, the flow of ion current depends on the combustion event. The ion current signal can be captured with the help of an ion current measurement circuit. In order to generate the flow of ion current, a “voltage biasing” circuit is necessary to generate the potential difference across the spark plug electrodes after the spark is triggered.
[00015] Fig. 1 exemplarily illustrates a circuit diagram of an existing ignition system 100 of an IC engine. The ignition system 100 consists of an ignition coil 107 with a primary winding 101 and a secondary winding 103, a spark plug 106, and a control circuit 105, for example, an electronic control unit (ECU) with an electrical switching device 104 to produce a high voltage spike required to generate a spark in the engine cylinder. The primary winding 101 is connected between a battery 102 and the electrical switching device 104. When the electrical switching device 104 is in a closed state, the primary winding 101 of the ignition coil 107 stores energy. As soon as the control circuit 105 changes the state of the electrical switching device 104 to open, a voltage e.g. 400V is generated in the primary winding 101 of the ignition coil 107, due to the sudden interruption of current flow in the inductive circuit. A secondary high voltage of around 20-25 kV (depending upon the turn ratio of primary & secondary coils) is generated in the secondary winding 103. The secondary voltage is applied at the spark plug 106, which results into voltage breakdown across gap of the spark plug 106 and a spark current starts flowing into the spark plug 106 through ground connection of the spark plug 106. Subsequently, the spark current generates the flow of an ion current across the spark plug 106 as described in Fig. 2.
[00016] Fig. 2 exemplarily illustrates a circuit diagram 200 of the existing ignition system 100 of an IC engine connected to an ion current measurement circuit 201. As exemplarily illustrated, the ion current measurement circuit 201 is connected to the secondary winding 103 of the ignition coil 107 to provide a biasing voltage to the spark plug 106, which in turn generates the flow of ion current after the sparking event. Also, the existing ion current measurement circuit 201 consists of a capacitor which is charged during trigger of the spark. Once the spark is triggered, the charge held by the capacitor generates a potential difference across the spark electrodes of the spark plug 106, which results in flow of the ion current.
[00017] The designs of ignition coils 107 in the existing ignition system 100 exemplarily illustrated in Figs. 1-2 have a separate ground connection for each of the primary winding 101 and the secondary winding 103 of the ignition coil 107. As shown in Fig. 1, terminal 101B of the primary side 101 and terminal 103B of the secondary side 103 of the ignition coil 107 are electrically isolated. The ion current measurement circuit 201 is connected to the terminal 103B to measure the ion current and subsequently, detect misfire in an IC engine, as shown in Fig. 2.
[00018] However, ignition coils of spark ignition engines are designed to simplify the process of ignition coil production and reduce manufacturing cost of the ignition system. In such ignition coils, the terminals 101B and 103B are connected internally via a common winding connection and there is no provision to connect the ion current measurement circuit 201 to the secondary winding 103 separately. Due to the common electrical connection of the terminals 101B and 103B of the primary winding 101 and the secondary winding 103, the existing ion current measurement circuit 201 gets connected to both the windings 101 and 103. As the electrical resistance of the primary winding 101 is much lower compared to the electrical resistance of the secondary winding 103, the capacitor of the existing ion current measurement circuit 201 would discharge from the primary winding 101 instead of the secondary winding103 and hence no ion current flows in the spark plug 106.
[00019] In applications of the combustion engine, such as vehicles, following are the disadvantages of a misfire condition: Due to the occurrence of a misfire in combustion event, fuel will be wasted as there is no spark for fuel to burn. This will degrade the performance of the vehicle due to reduction in mileage. Misfire has direct impact on power/pick up of vehicle as there is loss of combustion event. The user may feel a sudden jerk in driving due to misfire. These may cause discomfort to driver. Also, due to misfire, the unburnt fuel in the exhaust affects the life of the catalyst in the converter and has direct impact on emissions. Detection and reduction of misfire will have an impact on the durability of vehicle Thus, the known art suffers from failure to detect misfire effectively leading to poor reliability of ignition system and eventually high emissions, rider discomfort, and reduced mileage. Thus, there exists a need for a design of a misfire detection system comprising an ion current measurement circuit for detecting no-misfires, complete misfires, and partial misfires effectively in engines for smooth experience, durability, and adherence to emission norms by the engine, that overcomes above and other known problems in the art.
[00020] With the above objectives in view, the prevent invention discloses a misfire detection system comprising an ion current measurement circuit to precisely detect a complete misfire and a partial misfire in a spark ignition engine, where a primary winding and a secondary winding of an ignition coil are internally connected.
[00021] It is an object of the present invention to restrict the flow of ion current through the low resistance primary winding. It is another object of the invention to provide a biasing voltage circuit in the ion current measurement circuit that charges the capacitor in the ion current measurement circuit to a suitable value during trigger of a spark in the spark plug and apply this stored charge to the spark plug to generate a potential difference across the spark plug electrodes, after the spark is triggered. This results in the flow of ion current through the ion current measurement circuit and the measurement of the ion current will help in detection of occurrence of a misfire or a partial misfire. The current subject matter enables overcoming the drawbacks of the known art and ensures the misfire is accurately detected thereby enabling reliable control and reduction of emissions for an engine especially of a small capacity.
[00022] In an embodiment, a misfire detection system of an IC engine is disclosed. The misfire detection system comprises an ignition system, an ion current measurement circuit, and a signal processor. The ignition system comprises an ignition coil comprising a primary winding and a secondary winding, where a first end of the primary winding is electrically connected to a first end of the secondary winding, and a spark plug electrically connected in series to a second end of the secondary winding of the ignition coil. A first terminal of the spark plug is connected to the second end of the secondary winding and a second terminal of the spark plug is grounded. The ion current measurement circuit is electrically connected to the first end of the primary winding and the first end of the secondary winding for measuring an ion current generated from the spark plug after a sparking event. The signal processor processes and conditions the ion current measured by the ion current measurement to determine occurrence of a misfire in the IC engine.
[00023] A second end of the primary winding of the ignition coil is operatively coupled to a power source through a first diode. The first end of the primary winding of the ignition coil is operatively coupled to an electrical switching device for a generating a primary voltage in the primary winding and a secondary voltage in the secondary winding of the ignition coil, and the electrical switching device is controlled by a control unit. The first diode allows flow of a primary current in the primary winding to generate the primary voltage and prevents flow of the ion current in the primary winding through the first end of the secondary winding.
[00024] The ion current measurement circuit supplies a bias voltage to the spark plug for generating the ion current. the ion current measurement circuit comprises a biasing resistor and a biasing capacitor connected to the first end of the secondary winding for generating the bias voltage for the spark plug. A first terminal of the biasing resistor is connected in series to the first end of the secondary winding and the biasing capacitor is connected in series to a second terminal of the biasing resistor. The biasing capacitor charges using a spark current during the spark event in the spark plug and the biasing capacitor discharges after the sparking event for providing the bias voltage for the spark plug. The biasing capacitor and the biasing resistor define time constant of the ion current measurement circuit and a rate of charging and discharging of the biasing capacitor.
[00025] The ion current measurement circuit further comprises a voltage control element connected parallel to the biasing capacitor for limiting the bias voltage across the biasing capacitor during charging. In an embodiment, the voltage control element is a zener diode. The ion current measurement circuit further comprises a voltage divider resistor network, comprising a sensing resistor, connected in series between the biasing capacitor and ground for measuring the ion current. The ion current measurement circuit further comprises a second diode connected in parallel to the sensing resistor for providing a low impedance path for a spark current and impeding flow of the ion current.
[00026] In another embodiment, an ion current measurement circuit for measuring an ion current generated from a spark plug of an IC engine is disclosed. In this embodiment, the ion current measurement circuit comprises a biasing resistor connected in series with a first end of a primary winding and a first end of a secondary winding of an ignition coil, a biasing capacitor connected in series to a second terminal of the biasing resistor for generating a bias voltage for the spark plug. A first terminal of the biasing resistor is connected in series to the first end of the secondary winding. A voltage control element is operatively connected in parallel to the biasing capacitor, for limiting the bias voltage across the biasing capacitor during a sparking event using a spark current. A voltage divider resistor network, comprising a sensing resistor, is connected in series between the biasing capacitor and ground for measuring the ion current. A second diode is connected in parallel to the sensing resistor for providing a low impedance path for the spark current and impeding flow of the ion current.
[00027] The biasing capacitor charges using the spark current during the spark event in the spark plug, and discharges after the sparking event for providing the bias voltage. In this embodiment, the spark plug is electrically connected in series to a second end of the secondary winding of the ignition coil, wherein a first terminal of the spark plug is connected to the second end of the secondary winding and a second terminal of the spark plug is grounded. A second end of the primary winding of the ignition coil is operatively coupled to a power source through a first diode and the first end of the primary winding of the ignition coil is operatively coupled to an electrical switching device for generating a secondary voltage in the secondary winding of the ignition coil. The first diode allows flow of the primary current in the primary winding and prevents flow of the ion current in the primary winding through the first end of the secondary winding. The ion current measured is processed and conditioned by a signal processor for determining occurrence of a misfire in the IC engine.
[00028] In an embodiment, an ignition system of an internal combustion engine is disclosed. The ignition system comprises an ignition coil comprising a primary winding and a secondary winding, a spark plug electrically connected in series to a second end of the secondary winding of the ignition coil, and a first diode electrically connected between a power source and a second end of the primary winding. A first end of the primary winding is electrically connected to a first end of the secondary winding. A first terminal of the spark plug is connected to the second end of the secondary winding and a second terminal of the spark plug is grounded. The first diode allows flow of a primary current in the primary winding and prevents flow of an ion current in the primary winding through the first end of the secondary winding. The ignition system further comprises an electrical switching device electrically connected in series to the first end of the primary winding for generating a secondary voltage in the secondary winding of the ignition coil, wherein the electrical switching device is controlled by a control unit.
[00029] In this embodiment, the first end of the primary winding and the first end of the secondary winding is electrically connected to an ion current measurement circuit for measuring the ion current generated from the spark plug after a sparking event. The ion current measurement circuit supplies a bias voltage to the spark plug for generating measuring the ion current. The measured ion current is processed and conditioned by a signal processor for determining occurrence of a misfire in the IC engine.
[00030] The present subject matter thus discloses an interplay of the ignition coil with a specific ion current measurement circuit intended to work, when the primary side and the secondary side of the ignition coil have common electrical connection. The summary provided above explains the basic features of the invention and does not limit the scope of the invention. The nature and further characteristic features of the present invention will be made clearer from the following descriptions made with reference to the accompanying drawings.
[00031] Fig. 3 exemplarily illustrates a schematic diagram of a misfire detection system 300, according an embodiment of the present invention. As exemplarily illustrated, the misfire detection system 300 comprises an ignition system 303 and an ion current measurement circuit 302. The proposed design of the ignition system 303 with the ion current measurement circuit 302 accommodates the constraint of common electrical connection of the primary winding 101 and the secondary winding 103 of an ignition coil 107. The ignition system 303 comprises the ignition coil 107, a spark plug 106, a first diode 301, and an electrical switching device. The ignition coil 107 comprises a primary winding 101 and secondary winding 103. The ignition coil 107 is a transformer configured to transfer electrical energy in the primary winding 101 to the secondary winding 103 or vice versa. The voltage in the primary winding 101 is a primary voltage and the voltage in the secondary winding 103 is a secondary voltage. The secondary voltage is substantially larger than the primary voltage, and this is achieved by the turns-ratio of the primary winding 101 and the secondary winding 103. Each of the primary winding 101 and the secondary winding 103 comprises a first end 101B and 103B and a second end 101A and 103A respectively. The first end 101B of the primary winding 101 is connected to the first end 103B of the secondary winding 103. The spark plug 106 is electrically connected in series to a second end 103A of the secondary winding 103. A first terminal 106A of the spark plug 106, that is, a terminal of one spark electrode is connected to the second end 103A of the secondary winding 103 and the second terminal 106B of the spark plug 106, a terminal of another spark electrode is grounded. Thus, the spark plug 106 is a grounded spark plug.
[00032] The first diode 301 is electrically connected between a power source 102 and a second end 101A of the primary winding 101. the first diode 301 allows flow of a primary current in the primary winding 101 and prevents flow of an ion current in the primary winding 101 through the first end 103B of the secondary winding 103. The electrical switching device is electrically connected in series to the first end 101B of the primary winding 101 for generating the secondary voltage in the secondary winding 103 of the ignition coil 107. The electrical switching device 104 is controlled by a control unit 105. The electrical switching device 104 may be a transistor, a switch, a relay, or a combination thereof. The control unit 105 may be an engine control unit that generates a spark actuation signal to trigger generation of spark at spark gap of the spark plug 106 for combustion of air fuel mixture in the cylinder of the engine.
[00033] Based on the spark actuation signal, the control unit 105 controls the opening and closing of the electrical switching device 104. When the spark actuation signal is high, the electrical switching device 104 is closed connecting the terminal 101B of the primary winding 101 to ground. This results in forward bias of the first diode 301 and the flow of primary current from the power source 102 through the primary winding 101. The primary current flows from the power source 102, a battery through the first diode 301 and the primary winding 101 towards the ground. The primary winding 101 stores energy during dwell time, that is, the time duration of flow of primary current in the primary winding 101. The control unit 105 may now transmit a low spark actuation and the electrical switching device 104 is opened. As soon as the electrical switching device 104 is opened by the control circuit 105, a transient primary voltage appears at the terminal 101B of the primary winding 101 due to the sudden interruption of the primary current flow in the inductive circuit of the primary winding 101. This leads to a secondary voltage being generated at the secondary winding 103 at termination of dwell time. The secondary voltage is applied to the spark plug 106 and a voltage break down takes place in the gap of the spark plug 106, thus a spark is generated, During the sparking event, the spark generated in the gap of the spark plug 106 ignites the air fuel mixture and the spark current flows through gap in the spark plug 106 through the secondary winding 103. The first diode 301 is reverse biased as the voltage at the terminal 101B is much higher than the battery voltage at the terminal 102.
[00034] The first end 101B of the primary winding 101 and the first end 103B of the secondary winding 103 are electrically connected to an ion current measurement circuit 302. The ion current measurement circuit 302 supplies a bias voltage to the spark plug 106 for generating an ion current in the secondary winding 103. The ion current measurement circuit 302 also measures the ion current generated from the spark plug 106 after the sparking event. Since the first diode 301 connected to the primary winding 101 of the ignition coil 107 is reverse biased, the circuit from the battery to the electrical switching device 104 is not complete. Thus, the ion current does not flow via the primary winding 101.
[00035] Also, the spark current generated during the sparking event does not flow from the secondary winding 103 via the primary winding 101, the battery at terminal 102, and the ground connection, even though the resistance of the circuit connected to the primary winding 101 is lower than the secondary winding 103. The secondary winding 103 is provided with a ground connection through the ion current measurement circuit 302 as shown in Fig 3. The spark current flows from ground, through the gap of the spark plug 106, the secondary winding 103, the ion current measurement circuit 302, and the ground. The spark current generates the bias voltage in the ion current measurement circuit 302. The ion current measurement circuit 302 supplies the bias voltage to the spark plug 106 and initiates the flow of ion current in the secondary winding 103 through the ion current measurement circuit 302 as will be disclosed in the detailed description of Fig. 4.
[00036] Fig. 4 exemplarily illustrates the misfire detection system 300 comprising the ion current measurement circuit 302 and the signal processor 401, in accordance to an embodiment of the present invention. As exemplarily illustrated, the ion current measurement circuit 302 comprises a network of a voltage control element 302A, a diode 302D, a voltage divider resistor network of resistors 302E and 302F, a biasing resistor 302B and a biasing capacitor 302C. The biasing resistor 302B is connected in series with the first end 101B of the primary winding 101 and the first end 103B of the secondary winding 103 of the ignition coil 107. A first terminal 302B1 of the biasing resistor 302B is connected in series to the first end 103B of the secondary winding 103. The biasing resistor 302C is connected in series to a second terminal 302B2 of the biasing resistor 302B for generating the bias voltage for the spark plug 106. The biasing resistor 302C charges using the spark current during the sparking event in the spark plug 106, and discharges after the sparking event for providing the bias voltage to the spark plug 106. The biasing resistor 302C and the biasing resistor 302B define time constant of the ion current measurement circuit 302 and a rate of charging and discharging of the biasing resistor 302C.
[00037] The time constant of the ion current measurement circuit 302 is based on the values of the biasing resistor 302B and the biasing capacitor 302C. The values of the biasing resistor 302B and the biasing capacitor 302C are such that the biasing capacitor 302C does not get fully charged during the sparking event using the spark current. If the time constant of the ion current measurement circuit 302 is low (~10ms) based on the values of the resistance and capacitance of the biasing resistor 302B and the biasing resistor 302C, the spark current flows through the voltage control element 302A and there can be loss of spark energy.
[00038] The voltage control element 302A is operatively connected in parallel to the biasing resistor 302C, for limiting voltage across the biasing resistor 302C during the sparking event. The voltage control element 302A may be a variety of components but not limited to a zener diode, a transistor, a shunt regulator, or any combination thereof. The zener diode 302A is connected in parallel to the capacitor 302C to maintain the suitable bias voltage required to produce the ion current.
[00039] The voltage divider resistor network 302E and 302F, comprising a sensing resistor 302F, is connected in series between the biasing resistor 302C and ground for measuring the ion current. A second diode 302D is connected in parallel to the sensing resistor 302F for providing a low impedance path for the spark current and for impeding flow of the ion current. The resistors 302E and 302F in the voltage divider resistor network offer higher resistance to the spark current. The second diode 302D is forward biased to allow the spark current to flow to the ground. The spark current generated flows from the ground, through the gap of the spark plug 106, the secondary winding 103, the biasing resistor 302B, the biasing resistor 302C, the second diode 302D, and the ground.
[00040] The biasing resistor 302C of the ion current measurement circuit 302 is configured to provide the bias voltage across the gap of the spark plug 106 via the secondary winding 103 to ionize the air particles and hence produce the ion current. The ion current flows from ground through the voltage divider resistor network 302E and 302F, the biasing resistor 302C, the biasing resistor 302B, the secondary winding 103, and the gap of the spark plug 106, to the ground. The voltage divider resistor network with the resistors 302E and 302F forms a sensing circuit for the ion current. The voltage measured across the sensing resistor 302F is indicative of the ion current. The flow of the ion current produces a negative voltage across the sensing resistor 302F. This voltage is a measure of the ion current flow and is sensed as an ion current signal. The signal processor 401 processes and conditions the measured ion current signal for detecting the occurrence of a misfire in the engine. This ion current signal is further processed through the filtering circuitry of the signal processor 401 to extract the desired information. The resistance of the sensing resistor 302F is selected to provide a voltage magnitude suitable for the signal processor 401 to sense and process the ion current signal. That is, the sensing resistor 302F may be selected to ensure that a sufficiently large voltage is induced by the ion current flow to avoid noise susceptibility or detection errors. The sensing resistor 302F is employed to reduce voltage levels of electrical noise experienced by the signal processor 401 while sensing and processing the ion current signal. The voltage across the sensing resistor 302F is obtained by applying voltage divider across the resistors 302E and 302F. Thus, the voltage of the ion current signal is influenced by the resistance of both resistors 302E and 302F.
[00041] The biasing resistor 302B is connected between the ignition coil 107 and the biasing capacitor 302C to define the time constant of the ion current measurement circuit 302. The voltage across the biasing capacitor 302C is restricted to the maximum rated voltage of the zener diode 302A. The bias voltage across the biasing resistor 302C also limits the maximum value of primary voltage in the primary winding 101. The value of biasing resistor 302B is critical to maintain the primary voltage value to its desired condition for sparking event to take place.
[00042] The values of the biasing resistor 302B and the biasing capacitor 302C are configured in such a way that the time constant (T) of circuit is equal to 1.5ms (~RC). The biasing capacitor 302C requires approximately 5T duration (~ 7.5 ms) to charge fully using the spark current. The maximum time for the trigger of the spark is more than the time constant (t) and less than charging time (5T) of the biasing resistor 302C. Thus, the spark current would always flow through the biasing capacitor 302C as the biasing capacitor 302C would never be fully charged and reach an open circuit state. The charged biasing capacitor 302C applies the bias voltage to the gap of the spark plug 106 after the trigger of the spark, which causes flow of the ion current through the sensing resistor 302F. The ion current is measured across the sensing resistor 302F and further processed through the signal processor 401 to get desired information on occurrence of a misfire or a partial misfire in the engine. The signal processor 401 detects the occurrence of misfire based on the trend of the ion current signal during a misfire condition and no misfire condition. The trends in the ion current signal during a misfire condition and no misfire condition are exemplarily illustrated in Figs. 5-8.
[00043] Figs. 5-6 exemplarily illustrate timing diagrams for the primary voltage in the primary winding 101 of the ignition coil 107 and the ion current during a misfire condition. Fig. 6 is an enlarged view of the timing diagram illustrated in Fig. 5. The misfire condition may be a partial misfire condition or a complete misfire condition. Some amount of ion current flows due to incomplete combustion during the partial misfire. During a complete misfire condition due to the absence of spark, there is no flow of the ion current. As can be seen, at an instant when the sparking event ends after a dwell time, the ion current starts flowing in the ion current measurement circuit. However, when there is a misfire in the circuit, the spikes in the ion current signal are absent or suppressed as shown in Figs. 5-6.
[00044] Figs. 7-8 exemplarily illustrate timing diagrams for the primary voltage in the primary winding 101 of the ignition coil 107 and the ion current during a no misfire condition. Fig. 8 is an enlarged view of the timing diagram illustrated in Fig. 7. As can be seen, at an instant when the sparking event ends after a dwell time, the ion current starts flowing in the ion current measurement circuit. During normal combustion cycle, when there is no misfire in the ignition system 303, some oscillations are observed in the ion current signal at the end of trigger of spark. These oscillations occur due to the ionization process. The combustion event thus is identified by the presence of oscillations in the ion current signal. The misfire in the combustion cycle is found out by the absence of oscillations in ion current circuit. Even though oscillations are not absent in Figs. 5-6, some amount of ion current is flowing through the ion current measurement circuit 302. The signal processor 401 performs analysis of the waveforms to understand the features of the ion current signal that are to be extracted and supervised in order to implement a robust misfire detection strategy. The signal processor 401 may detect the presence and absence of oscillations in the waveforms and the frequency of oscillations to determine an occurrence of a misfire, a partial misfire, and a no-misfire in the IC engine.
[00045] Embodiments of the misfire detection system disclosed here provides technical advancement in the field of IC engine performance in the following manner: The interplay of the ignition system with common connection between the primary winding and the secondary winding of the ignition coil, the ion current measurement circuit, and the signal processor effectively detects the occurrence of misfire in the IC engines, with reduced energy losses. The biasing capacitor, if fully charged, behaves as an open circuit and there would not be any path for the flow of the spark current through the biasing capacitor and there will be loss of the spark energy. To avoid this, the values for the biasing resistor and the biasing capacitor are selected in a manner to never fully charge the biasing capacitor. The second diode provides a path for the spark current and limits the voltage across the voltage divider resistor network to protect the biasing resistor, the biasing capacitor, and the signal processor from over-voltage conditions.
[00046] Also, the capacitance of the biasing capacitor is chosen to ensure that the bias voltage applied to the spark gap remains relatively constant during an engine cycle. That is, the rate of discharge of the biasing capacitor is ensured to not drop drastically when the ion current is being measured by the sensing resistor. If the primary voltage is less than a predetermined value, it would not be able to generate spark in the secondary side. The value of the biasing resistor is selected to maintain the primary voltage value to its desired condition for the sparking event to take place. The capacitance of the biasing capacitor ensures an appropriate bias voltage is applied to the spark plug. During normal operating condition (dwell time), the first diode is forward biased and there is no affect in the ignition coil operation. During sparking and ion current flow, the first diode gets reverse biased as the voltage at the first end of the secondary winding is much higher than battery voltage. This prevents the discharge of biasing capacitor from the primary winding and allows the flow of ion current through the secondary winding.
[00047] Further, all the components of the ion current measurement circuit are positioned in close proximity of the ignition system and the signal processor to avoid voltage drop due to long wiring running between them. The voltage divider resistor network limits loss of spark current and ensures adequate ion current flows through the sensing resistor to measure the voltage level of the ion current signal. Magnitude and shape of the ion current signal provides information about the misfire event. Thus, the ion current measurement is indicative of the combustion event as well as the quality of combustion.
[00048] Misfire detection using the misfire detection system will intimate user of the IC engine or a vehicle employing the IC engine about the misfire events in the spark ignition engine and by investigating and rectifying the causes of misfire, the performance of the vehicle is improved, thereby improving reliability, durability, mileage of the vehicle, and comfort offered to the user of the vehicle. Misfire detection will intimate driver about the misfire events in the vehicle and by investigating and rectifying the causes of misfire, the user can improve the performance of vehicle. Also, timely detection of misfire in the engine and rectification of the fault reduces catalyst and lambda performance degradation in the vehicle. In addition to detecting the occurrence of the misfire, the misfire detection system of the present invention can perform knock detection, determine spark plug timing and perform spark duration measurement, perform analysis of quality of combustion in the engine, and aid in spark plug maintenance of the engine, using the ion current measurement circuit.
[00049] Improvements and modifications may be incorporated herein without deviating from the scope of the invention.
LIST OF REFERENCE NUMERALS

100- Existing ignition system
101- Primary winding
101A- second end of primary winding
101B- first end of primary winding
102- power source
103- secondary winding
103A- second end of secondary winding
103B- first end of secondary winding
104- Electrical switching device
105- control unit
106- spark plug
106A- first terminal
106B- second terminal
107- ignition coil
201-existing ion current measurement circuit
300- misfire detection system
301-first diode
302-ion current measurement circuit (present invention)
302A- voltage control element
302B-biasing resistor
302B1-first terminal
302B2-second terminal
302C-biasing capacitor
302D-second diode
302F-sensing resistor
303-ignition system (present invention)
401- signal processor
,CLAIMS:I/We claim:

1. A misfire detection system (300) of an internal combustion engine comprising:

an ignition system (303) comprising:

an ignition coil (107) comprising a primary winding (101) and a secondary winding (103),
wherein a first end (101B) of the primary winding (101) is electrically connected to a first end (103B) of the secondary winding (103), and
a spark plug (106) electrically connected in series to a second end (103A) of the secondary winding (103) of the ignition coil (107), wherein a first terminal (106A) of the spark plug (106) is connected to the second end (103A) of the secondary winding (103) and a second terminal (106B) of the spark plug (106) is grounded;

an ion current measurement circuit (302) electrically connected to the first end (101B) of the primary winding (101) and the first end (103B) of the secondary winding (103) for measuring an ion current generated from the spark plug (106) after a sparking event, and

a signal processor (401) configured for processing and conditioning the ion current measured by the ion current measurement circuit (302) to determine occurrence of a misfire in the IC engine.

2. The misfire detection system (300) of claim 1,
wherein a second end (101A) of the primary winding (101) of the ignition coil (107) is operatively coupled to a power source (102) through a first diode (301), and
wherein the first end (101B) of the primary winding (101) of the ignition coil (107) is operatively coupled to an electrical switching device (104) for a generating a primary voltage in the primary winding (101) and a secondary voltage in the secondary winding (103) of the ignition coil (107), and
wherein the electrical switching device (104) is controlled by a control unit (105).

3. The misfire detection system (300) of claim 2, wherein the first diode (301) allows flow of a primary current in the primary winding (101) to generate the primary voltage and prevents flow of the ion current through the first end (103B) of the secondary winding (103) into the primary winding (101).

4. The misfire detection system (300) of claim 1, wherein the ion current measurement circuit (302) comprises a biasing resistor (302B) and a biasing capacitor (302C) connected to the first end (103B) of the secondary winding (103) for generating a bias voltage for the spark plug (106).

5. The misfire detection system (300) of claim 4,
wherein a first terminal (302B1) of the biasing resistor (302B) is connected in series to the first end (103B) of the secondary winding (103), and
wherein the biasing capacitor (302C) is connected in series to a second terminal (302B2) of the biasing resistor (302B).

6. The misfire detection system (300) of claim 4,
wherein the biasing capacitor (302C) charges using a spark current during the spark event in the spark plug (106), and
wherein the biasing capacitor (302C) discharges after the sparking event for providing the bias voltage for the spark plug (106).

7. The misfire detection system (300) of claim 4, wherein the biasing capacitor (302C) and the biasing resistor (302B) define time constant of the ion current measurement circuit (302) and a rate of charging and discharging of the biasing capacitor (302C).

8. The misfire detection system (300) of claim 4, wherein the ion current measurement circuit (302) further comprises a voltage control element (302A) connected parallel to the biasing capacitor (302C) for limiting the bias voltage across the biasing capacitor (302C) during charging.

9. The misfire detection system (300) of claim 8, wherein the voltage control element (302A) is a zener diode.

10. The misfire detection system (300) of claim 8, wherein the ion current measurement circuit (302) further comprises a voltage divider resistor network (302E and 302F), comprising a sensing resistor (302F), connected in series between the biasing capacitor (302C) and ground for measuring the ion current.

11. The misfire detection system (300) of claim 10, wherein the ion current measurement circuit (302) further comprises a second diode (302D) connected in parallel to the sensing resistor (302F) for providing a low impedance path for a spark current and impeding flow of the ion current.

12. A misfire detection system (300) of claim 1, wherein the signal processor (401) determines the occurrence of the misfire based on oscillations in the measured ion current.

13. An ion current measurement circuit (302) for measuring an ion current generated from a spark plug (106) of an IC engine, the ion current measurement circuit (302) comprises:

a biasing resistor (302B) connected in series with a first end (101B) of a primary winding (101) and a first end (103B) of a secondary winding (103) of an ignition coil (107),

a biasing capacitor (302C) connected in series to a second terminal (302B2) of the biasing resistor (302B) for generating a bias voltage for the spark plug (106), wherein a first terminal (302B1) of the biasing resistor (302B) is connected in series to the first end (103B) of the secondary winding (103);

a voltage control element (302A) operatively connected in parallel to the biasing capacitor (302C), for limiting voltage across the biasing capacitor (302C) during a sparking event;

a voltage divider resistor network (302E and 302F), comprising a sensing resistor (302F), connected in series between the biasing capacitor (302C) and ground for measuring the ion current;

a second diode (302D) connected in parallel to the sensing resistor (302F) for providing a low impedance path for a spark current and impeding flow of the ion current.

14. The ion current measurement circuit (302) of claim 13, wherein the biasing capacitor (302C) charges using the spark current during the spark event in the spark plug (106), and discharges after the sparking event for providing the bias voltage to the spark plug (106).

15. The ion current measurement circuit (302) of claim 13, wherein the spark plug (106) is electrically connected in series to a second end (103A) of the secondary winding (103) of the ignition coil (107), wherein a first terminal (106A) of the spark plug (106) is connected to the second end (103A) of the secondary winding (103) and a second terminal (106B) of the spark plug (106) is grounded.

16. The ion current measurement circuit (302) of claim 13,

wherein a second end (103A) of the primary winding (101) of the ignition coil (107) is operatively coupled to a power source (102) through a first diode (301), and
wherein the first end (101B) of the primary winding (101) of the ignition coil (107) is operatively coupled to an electrical switching device (104) for generating a secondary voltage in the secondary winding (103) of the ignition coil (107).

17. The ion current measurement circuit (302) of claim 16, wherein the first diode (301) allows flow of the primary current in the primary winding (101) and prevents flow of the ion current in the primary winding (101) through the first end (103B) of the secondary winding (103).

18. The ion current measurement circuit (302) of claim 13, wherein the ion current measured is processed and conditioned by a signal processor (401) for determining occurrence of a misfire in the IC engine, based on oscillations in the measured ion current.
19. An ignition system (303) of an internal combustion engine comprising:
an ignition coil (107) comprising a primary winding (101) and a secondary winding (103),
wherein a first end (101B) of the primary winding (101) is electrically connected to a first end (103B) of the secondary winding (103);

a spark plug (106) electrically connected in series to a second end (103A) of the secondary winding (103) of the ignition coil (107), wherein a first terminal (106A) of the spark plug (106) is connected to the second end (103A) of the secondary winding (103) and a second terminal (106B) of the spark plug (106) is grounded; and

a first diode (301) electrically connected between a power source (102) and a second end (101A) of the primary winding (101), wherein the first diode (301) allows flow of a primary current in the primary winding (101) and prevents flow of an ion current in the primary winding (101) through the first end (103B) of the secondary winding (103).

20. The ignition system (303) of claim 19, further comprises an electrical switching device (104) electrically connected in series to the first end (101B) of the primary winding (101) for generating a secondary voltage in the secondary winding (103) of the ignition coil (107), wherein the electrical switching device (104) is controlled by a control unit (105).

21. The ignition system (303) of claim 20, wherein each of the first end (101B) of the primary winding (101) and the first end (103B) of the secondary winding (103) is electrically connected to an ion current measurement circuit (302) for measuring the ion current generated from the spark plug (106) after a sparking event.

22. The ignition system (303) of claim 21, wherein the ion current measurement circuit (302) supplies a bias voltage to the spark plug (106) for generating the ion current.

23. The ignition system (303) of claim 22, wherein the measured ion current is processed and conditioned by a signal processor (401) for determining occurrence of a misfire in the IC engine, based on oscillations in the measured ion current.