Circuit for measuring ionization current in a combustion chamber of an internal combustion engine

This application claims benefit of U.S. Provisional Application Ser. No. 60/423,044, filed Nov. 1, 2002, the entire disclosure of this application being considered part of the disclosure of this application and hereby incorporated by reference.

1. Technical Field

The present invention relates to a circuit for measuring ionization current in a combustion chamber of an internal combustion engine.

2. Discussion

An internal combustion engine produces power by compressing a fuel gas mixed with air in a combustion chamber with a piston and then igniting the mixed gas with an ignition or spark plug. When combustion of the mixed gas occurs in the combustion chamber, the gas is ionized. If, after combustion, a bias voltage is applied between the ignition plug electrodes, then an electric current is produced which passes through the chamber due to the ions generated during the combustion process. This electric current is commonly referred to as ionization current. Since the ionization current varies with respect to the characteristics of the combustion, measurement of the ionization current provides important diagnostic information regarding engine combustion performance.

Several circuits have been proposed for detecting ionization current, however these prior art detection circuits have several shortcomings. In prior art detection circuits, the ignition current (which is produced in response to the combustion of the mixed gas) and the ionization current flow in opposite directions through the secondary winding of the ignition coil, thus requiring the ionization current to overcome the stored energy in the secondary winding of the ignition coil before the ionization current can be detected. As a result, the initiation or, in other words, the flow of ionization current as well as the detection of ionization current is delayed in time. Further, in prior art detection circuits, the ionization current is detected by way of a current mirror circuit which requires a second power source other than the ignition coil. Typically, the second power source supplies a relatively low voltage (e.g. 1.4 volts) to the current mirror circuit. As a result, the magnitude of the mirrored current signal is relatively small and the signal-to-noise ratio is low. Even further, prior art detection circuit designs are complex and, therefore, costly. Accordingly, there is a desire to provide a circuit for measuring ionization current which overcomes the shortcomings of the prior art.

The present invention provides a circuit for measuring ionization current in a combustion chamber of an internal combustion engine including an ignition coil and an ignition plug. The ignition plug ignites an air/fuel mixture in the combustion chamber and produces an ignition current in response to ignition voltage from the ignition coil. A capacitor, charged by the ignition coil, provides a bias voltage which produces an ionization current after ignition of the air/fuel mixture in the combustion chamber. A current mirror circuit produces an isolated current signal proportional to the ionization current.

In one embodiment of the present invention, the ignition coil includes a primary winding and a secondary winding. The ignition current and the ionization current flow in the same direction through the secondary winding of the ignition coil. The ignition current flows from the charged capacitor through the current mirror circuit and the ignition coil to the ignition plug.

Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which:

FIG. 1 is an electrical schematic of a circuit for measuring ionization current in a combustion chamber of an internal combustion engine in accordance with the present invention;

FIG. 2A is a graph of a control signal input to the circuit;

FIG. 2B is a graph of current flow through the primary winding of the ignition coil during circuit operation; and

FIG. 2C is a graph of an output voltage signal from the circuit.

FIG. 1 is an electrical schematic of a circuit 10 for measuring ionization current in a combustion chamber of an internal combustion engine. The components and configuration of the circuit 10 are described first, followed by a description of the circuit operation.

First, with regard to the components and configuration of the present invention, the circuit 10 includes an ignition coil 12 and an ignition or spark plug 14 disposed in a combustion chamber of an internal combustion engine. The ignition coil 12 includes a primary winding 16 and a secondary winding 18 . The ignition plug 14 is connected in electrical series between a first end of the secondary winding 18 and ground potential. The electrical connections to a second end of the secondary winding 18 are described further below. A first end of the primary winding 16 is electrically connected to a positive electrode of a battery 20 . A second end of the primary winding 16 is electrically connected to the collector terminal of an insulated gate bipolar transistor (IGBT) or other type of transistor 22 and a first end of a first resistor 24 . The base terminal of the IGBT 22 receives a control signal, labeled V IN in FIG. 1 , from a powertrain control module (PCM) not shown. Control signal V IN gates IGBT 22 on and off. A second resistor 25 is electrically connected in series between the emitter terminal of the IGBT 22 and ground. A second end of the first resistor 24 is electrically connected to the anode of a first diode 26 .

The circuit 10 further includes a capacitor 28 . A first end of the capacitor 28 is electrically connected to the cathode of the first diode 26 and a current mirror circuit 30 . A second end of the capacitor 28 is grounded. A first zener diode 32 is electrically connected across or, in other words, in parallel with the capacitor 28 with the cathode of the first zener diode 32 electrically connected to the first end of the capacitor 28 and the anode of the first zener diode 32 electrically connected to ground.

The current mirror circuit 30 includes first and second pnp transistors 34 and 36 respectively. The pnp transistors 34 and 36 are matched transistors. The emitter terminals of the pnp transistors 34 and 36 are electrically connected to the first end of the capacitor 28 . The base terminals of the pnp transistors 34 and 36 are electrically connected to each other as well as a first node 38 . The collector terminal of the first pnp transistor 34 is also electrically connected to the first node 38 , whereby the collector terminal and the base terminal of the first pnp transistor 34 are shorted. Thus, the first pnp transistor 34 functions as a diode. A third resistor 40 is electrically connected in series between the collector terminal of the second pnp transistor 36 and ground.

A second diode 42 is also included in the circuit 10 . The cathode of the second diode 42 is electrically connected to the first end of the capacitor 28 , the emitter terminals of the first and second pnp transistors 34 and 36 . The anode of the second diode 42 is electrically connected to the first node 38 .

The circuit 10 also includes a fourth resistor 44 . A first end of the fourth resistor 44 is electrically connected to the first node 38 . A second end of the fourth resistor 44 is electrically connected to the second end of the secondary winding 18 (opposite the ignition plug 14 ) and the cathode of a second zener diode 46 . The anode of the second zener diode 46 is grounded.

Referring now to FIGS. 1 and 2 , the operation of the circuit 10 is described. FIG. 2A is a graph of the control signal V IN from the PCM to the IGBT 22 versus time. FIG. 2B is a graph of the current flow (I PW ) through the primary winding 16 of the ignition coil 12 versus time. FIG. 2C is a graph of an output voltage signal from the circuit 10 versus time. As mentioned above, the IGBT 22 receives the control signal V IN from the PCM to control the timing of 1) the ignition or combustion and 2 ) the charging of the capacitor 28 . In this circuit configuration, the IGBT 22 is operated as a switch having an OFF, or non-conducting, state and an ON, or conducting, state.

Initially, at time=t 0 , the capacitor 28 is not fully charged. The control signal V IN from the PCM is LOW (see FIG. 2A ) thereby operating the IGBT 22 in the OFF, or non-conducting, state. Primary winding 16 sees an open circuit and, thus, no current flows through the winding 16 .

At time=t 1 , the control signal V IN from the PCM switches from LOW to HIGH (see FIG. 2A ) thereby operating the IGBT 22 in the ON, or conducting, state. Current from the battery 20 begins to flow through the primary winding 16 of the ignition coil 12 , the conducting IGBT 22 , and the second resistor 25 to ground. Any of a number of switches or switching mechanisms can be used to conduct current through the primary winding 16 . In a preferred embodiment IGBT 22 is used. Between time=t 1 and time=t 2 , the primary winding current I PW , (illustrated in FIG. 1 with a dotted line) begins to rise. The time period between time=t 1 and time=t 2 is approximately one millisecond which varies per type of ignition coil.

At time=t 2 , the control signal V IN from the PCM switches from HIGH to LOW (see FIG. 2A ) thereby operating the IGBT 22 in the OFF, or non-conducting, state. As the IGBT 22 is switched OFF, flyback voltage from the primary winding 16 of the ignition coil 12 begins to quickly charge the capacitor 28 up to the required bias voltage. Between time=t 2 and time=t 3 , the voltage at the first end of the secondary winding 18 connected to the spark plug 14 rises to the voltage level at which the ignition begins. The time period between time=t 2 and time=t 3 is approximately ten microseconds. The first resistor 24 is used to limit the charge current to the capacitor 28 . The resistance value of the first resistor 24 is selected to ensure that the capacitor 28 is fully charged when the flyback voltage is greater than the zener diode.

At time=t 3 , an ignition voltage from the secondary winding 18 of the ignition coil 12 is applied to the ignition plug 14 and ignition begins. Between time=t 3 and time=t 4 , combustion of the air/fuel mixture begins and an ignition current I IGN (illustrated in FIG. 1 with a dash-dot line) flows through the second zener diode 46 , the secondary winding 18 of the ignition coil 12 , and the ignition plug 14 to ground. At time=t 4 , the ignition is completed and the combustion of the air/fuel mixture continues.

At time=t 5 , the combustion process continues and the charged capacitor 28 applies a bias voltage across the electrodes of the ignition plug 14 producing an ionization current I ION due to the ions produced by the combustion process which flows from the capacitor 28 . The current mirror circuit 30 produces an isolated mirror current I MIRROR identical to ionization current I ION . A bias current I BIAS (illustrated in FIG. 1 with a phantom or long dash-short dash-short dash line) which flows from the capacitor 28 to the second node 48 is equal to the sum of the ionization current I ION and the isolated mirror current I MIRROR (i.e., I BIAS =I ION +I MIRROR ).

The ionization current I ION (illustrated in FIG. 1 with a dashed line) flows from the second node 48 through the first pnp transistor 34 , the first node 38 , the fourth resistor 44 , the secondary winding 18 of the ignition coil 12 , and the ignition plug 14 to ground. In this manner, the charged capacitor 28 is used as a power source to apply a bias voltage, of approximately 80 volts, across the spark plug 14 to generate the ionization current I ION . The bias voltage is applied to the spark plug 14 through the secondary winding 18 and the fourth resistor 44 . The secondary winding induction, the fourth resistor 44 , and the effective capacitance of the ignition coil limit the ionization current bandwidth. Accordingly, the resistance value of the fourth resistor 44 is selected to maximize ionization signal bandwidth, optimize the frequency response, and also limit the ionization current. In one embodiment of the present invention, the fourth resistor 44 has a resistance value of 330 k ohms resulting in an ionization current bandwidth of up to twenty kilohertz.

The current mirror circuit 30 is used to isolate the detected ionization current I ION and the output circuit. The isolated mirror current I MIRROR (illustrated in FIG. 1 with a dash-dot-dot line) is equal to or, in other words, a mirror of the ionization current I ION . The isolated mirror current I MIRROR flows from the second node 48 through the second pnp transistor 36 and the third resistor 40 to ground. To produce a isolated mirror current signal I MIRROR which is identically proportional to the ionization current I ION , the first and second pnp transistors 34 and 36 must be matched, i.e., have the identical electronic characteristics. One way to achieve such identical characteristics is to use two transistors residing on the same piece of silicon. The isolated mirror current signal I MIRROR is typically less than 300 microamps. The third resistor 40 converts the isolated mirror current signal I MIRROR into a corresponding output voltage signal which is labeled as V OUT in FIG. 1 . The resistance value of the third resistor 40 is selected to adjust the magnitude of the output voltage signal V OUT . The second diode 42 protect the mirror transistor 34 and 36 by biasing on and providing a path to ground if the voltage at node 38 crossed a threshold. A third transistor can also be used to protect the mirror transistor.

FIG. 2C illustrates an output voltage signal V OUT resulting from a normal combustion event. The portion of the output voltage signal V OUT from time=t 5 and beyond can be used as diagnostic information regarding combustion performance. To determine the combustion performance for the entire engine, the ionization current in one or more combustion chambers of the engine can be measured by one or more circuits 10 respectively.

In the present circuit 10 , the ignition current I IGN and the ionization current T ION flow in the same direction through the secondary winding 18 of the ignition coil 12 . As a result, the initiation or, in other words, the flow of the ionization current as well as the detection of the ionization current is quick. In the present circuit 10 , the charged capacitor 28 operates as a power source thus the circuit 10 is passive or, in other words, does not require a dedicated power source. The charged capacitor 28 provides a relatively high bias voltage from both ionization detection and the current mirror circuit 30 . As a result, the magnitude of the mirrored, isolated current signal I MIRROR is large and, thus, the signal-to-noise ratio is high. Finally, the present circuit 10 is less complex and less expensive than prior art detection circuits.

The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.