Device to provide a regulated power supply for in-cylinder ionization detection by using the ignition coil fly back energy and two-stage regulation

The present invention is directed to a dual charge rate power supply circuit and method for ionization detection. The circuit includes a first diode, first and second capacitors, and first and second current paths. The first diode includes an anode operably connected to a first end of a primary winding. The first capacitor has a second end operably connected to ground and the second capacitor has a first end operably connected to the cathode of the first diode as well as a second end operably connected to ground. The first and second current paths are operably connected between the first and second capacitors and include a second diode, a parallel combination of a first resistor and a third diode, and a second resistor. The first diode is operably connected in parallel with the first capacitor. The second resistor has a first end operably connected to the cathode of the first diode and the parallel combination is operably connected between a second end of the second resistor and the first end of the first capacitor.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention is related to the field of automobile ignition diagnostic systems. More particularly, it is related to the field of supplying power to an ionization detection circuit.

In a spark ignition (SI) engine, the spark plug is inside of the combustion chamber and can be used as a detection device without requiring the intrusion of a separate sensor. Many ions are produced in the plasma during combustion of an engine. For example, H3O+, C3H3+, and CHO+ are produced by the chemical reactions at the flame front and have sufficiently long excitation time to be detected. In addition, a voltage applied across the spark gap attracts free ions and creates an ionization current.

The prior art includes a variety of conventional methods for detecting and using ionization current in a combustion chamber of an internal combustion engine. However, each of the various conventional systems suffers from a great variety of deficiencies.

A typical ionization detector consists of a coil-on-plug arrangement, with a device in each coil to keep a voltage applied across the spark plug electrodes when the spark is not arcing. The current across the spark plug electrodes is isolated prior to being measured. There are two ways to supply regulated power to an in-cylinder ionization detector. A first approach is to use a charge pump powered by a DC power supply such as a battery. A second approach is to use a charge pump powered by ignition flyback energy. The DC power supply and the ignition flyback energy generate a DC bias used by the charge pump to detect ionization current.

Both approaches present disadvantages. A DC power supply is many times too large due to large high-voltage electronics. The flyback energy approach requires a few ignition events to obtain a regulated power supply. This is undesirable for cylinder identification, since cylinder identification uses a regulated power supply at the first ignition event. In addition, the high voltage capacitors used with the flyback energy approach tend to be unreliable due to the high voltage and the high operational temperature.

SUMMARY OF THE INVENTION

In view of the above, the described features of the present invention generally relate to one or more improved systems, methods and/or apparatuses for supplying power to an ionization detection circuit used to detect an ionization current in the combustion chamber of an internal combustion engine.

In one embodiment, the invention comprises a method of charging an ionization detection circuit using a plurality of charge rates.

In another embodiment, the method of charging an ionization detection circuit using a plurality of charge rates comprises charging a capacitor using a first time constant during a time period and charging the capacitor using a second time constant after the time period has elapsed.

In a further embodiment, the invention comprises a dual stage ionization detection circuit including a first diode, first and second capacitors, and first and second current paths. The first diode includes an anode and a cathode with the anode operably connected to a first end of a primary winding. The first capacitor has a first end and a second end with the second end operably connected to ground. The second capacitor has a first end operably connected to the cathode of the first diode and a second end operably connected to ground. The first current path is operably connected between the first and the second capacitor and the second current path is operably connected between the first and the second capacitor. Each of the first and second current paths include a second diode having an anode and a cathode operably connected in parallel with the first capacitor, a parallel combination of a first resistor having a first and a second end and a third diode having an anode and a cathode, and a second resistor having a first and a second end. The first end of the second resistor being operably connected to the cathode of the first diode and the parallel combination operably connected between the second end of the second resistor and the first end of the first capacitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An ionization measuring circuit detects an ionization current in a combustion chamber of an internal combustion engine by applying a bias voltage across a spark plug gap. The present invention provides a regulated power supply that applies a bias voltage across the plug electrodes by harvesting the excess ignition coil leakage and magnetizing energy immediately following turn off of the ignition coil Insulated Gate Bipolar-Junction Transistor (IGBT). The present invention uses a two-stage power supply circuit to harvest the energy.

In addition, the present invention includes a dual-rate charge pump which uses the harvested ignition coil flyback energy to provide a regulated ionization detection power supply at the first ignition event. In other words, the power supply can be ready for ionization detection within tens of microseconds after the start of ignition.

Using a two-stage, dual-rate charge pump produces an improvement in ionization system performance. For example, the ionization detection power supply fully recovers during the flyback period as a result of using a dual-rate charge pump. Since a combustion event happens right after the ignition event, engine speed or rpm is low at that time. At low engine speed, the ignition frequency is commensurately low which may cause the power supply voltage to drop significantly before the next ignition event occurs. The slow charge rate, e.g., at 20 milliseconds, may not be able to build up the ionization detection voltage fast enough to recover to a desired voltage level by the time combustion occurs. This results in poor ionization detection quality. The proposed dual-charge rate power supply of the present invention eliminates this problem by harvesting the excess ignition coil leakage and magnetizing energy immediately following the turn off of the ignition coil or power switch, normally an IGBT22.

The following is a description of how a standard ignition coil charges and then releases energy. Spark ignition systems for internal combustion engines deliver sufficient energy to a spark plug14electrode air gap to ignite the compressed air-fuel mixture in the cylinder. To accomplish this, energy is stored in a magnetic device commonly referred to as an ignition coil12. The stored energy is then released to the spark plug14air gap at the appropriate time to ignite the air-fuel mixture which is the ignition event. A schematic diagram of a typical ignition coil is shown inFIG. 1. The coil12, which is shown as a flyback transformer, consists of primary16and secondary windings18that are magnetically coupled via a highly permeable magnetic core. The secondary winding18normally has many more turns than the primary winding16, which allows the secondary voltage to fly up to very high levels during the “flyback” time.

Energy is stored in the coil by turning on the IGBT22, and applying battery voltage across the primary winding16of the ignition coil12. With a constant voltage applied to the primary inductance (Lpri), primary current (Ipri) increases linearly until primary current Iprireaches a predetermined level as illustrated inFIG. 2. It follows that the energy stored in the coil is a square function of the coil primary current per the following equation:
Energy=½×Lpri×(Ipri)2

Once the primary current (Ipri) has reached a predetermined peak level, the primary power switch IGBT22is turned off. When this occurs, the energy stored in the coil inductance (Lpri) causes the transformer primary voltage to reverse and fly up to the IGBT22clamp voltage, nominally 350 to 450 volts. Since the secondary winding18is magnetically coupled to the primary winding16, the secondary voltage also reverses, rising to a value equal to the primary clamp voltage multiplied by the secondary to primary turns ratio, typically 20,000 to 40,000 volts. This high voltage appears across the electrodes of the spark plug14, causing a small current to flow between the spark plug14electrodes through the electrode air gap. Though this current is small, the power dissipated in the air gap is significant due to the high voltage across the air gap.

The power dissipated in the electrode air gap rapidly heats the air between the electrodes causing the molecules to ionize. Once ionized, the air-fuel mixture between the electrodes conducts heavily, dumping the energy stored in the flyback transformer12in the spark plug14air gap. The sudden release of energy stored in the flyback transformer12ignites the air-fuel mixture in the cylinder.

Turning now to a brief description of in-cylinder ionization detection, different methods of providing a regulated power supply, and the advantages and disadvantages of each method. In-cylinder ionization detection requires a regulated power supply to establish a bias voltage across the spark plug14electrodes. This voltage, which is generally in the 80 to 150 volt DC range, produces an ionization current (Iion) that is nominally limited to a few hundred micro-amps. The resulting ionization current (Iion) is then sensed and amplified to produce a usable signal for combustion diagnostic and control purposes.

Since the magnitude of the ionization current (Iion) is relatively small, the sensing and amplifying electronics are typically located close to the coil12and spark plug14. In addition, the high voltage power supply is located very close to the ionization electronics to avoid bussing high voltages under a car hood. Therefore, means are provided to create the high voltage locally.

There are a number of different ways for providing a regulated power supply for detecting ionization current inside the cylinder. One method of creating the ionization potential is to use a DC—DC converter to create an 80 to 150 volt power supply from the available 12 Vdc at the ignition coil12. This method, though straightforward and reliable, requires several components to implement and, therefore, may be cost and space prohibitive.

Another method for providing a regulated power supply for detecting ionization current inside the cylinder is to charge a capacitor from the collector of the primary IGBT22immediately following IGBT22turn off. A first benefit of this method is that it does not require a separate boost converter to create the ionization bias voltage. A second benefit is that the regulated power supply captures at least part of the energy stored in the transformer leakage inductance and transfers the energy to the energy storage capacitor. Normally, this energy would be dissipated on the IGBT22as heat, raising the operating temperature of switch IGBT22.

An embodiment of this method is shown schematically inFIG. 3. As previously described, the energy stored in the coil inductance (Lpri) causes the transformer primary voltage to reverse and fly up to the IGBT22clamp voltage, 350 to 450 volts, when the IGBT22turns off. When this occurs, diode D1is forward biased allowing a current to flow through D1and the current limiting resistor R1into capacitor C1. Zener diode D2limits the voltage on capacitor C1to approximately 100 volts.

A first disadvantage of this method is that the energy storage capacitor C1stores energy at a relatively low voltage, 100 volts, compared to the magnitude of the flyback voltage, approximately 400 volts. Since the energy stored in the capacitor C1is a function of the square of the capacitor voltage, storing energy at a low voltage requires a much higher value of capacitance for a given amount of stored energy than if the capacitor was allowed to charge to a higher voltage. For example, to store 500 μ-joules at 100 volts requires a 0.1 μfd capacitor. To store the same energy at 200 volts requires only a 0.025 μfd capacitor. The capacitance is reduced by a factor of four by doubling the capacitor voltage.

A second disadvantage of this method is that the R1*C1time constant must be short enough to allow a complete recharge of capacitor C1in the short time between IGBT22turn off and spark plug firing, normally less than ten microseconds. At the same time, capacitor C1must be large enough to supply ionization current (Iion) without a substantial drop in the voltage on capacitor C1under worst-case conditions such as low rpm and fouled spark plug. This forces resistor R1to be a relatively small value, tens of ohms, and results in a relatively large capacitor charging current when the IGBT22turns off. Under nominal operating conditions, 2000 to 3000 rpm and a clean spark plug, the discharge on capacitor C1due to ionization is moderate resulting in excess charging current being diverted into the zener diode D2. The product of excess zener diode current and zener voltage constitutes energy wasted in the zener diode D2.

Another method for providing a regulated power supply for detecting ionization current inside the cylinder is to charge an energy storage capacitor with the secondary ignition current by placing the capacitor in series with the secondary winding18of the flyback transformer12. An embodiment of this method is shown schematically inFIG. 4. Spark current flowing in the secondary winding18of the ignition coil12charges the energy storage capacitor C1via diode D1. Once the voltage on capacitor C1reaches the zener voltage, secondary current is diverted through the zener diode D1, limiting the voltage on capacitor C1to approximately 100 volts.

Since capacitor C1is in series with the secondary winding, it is difficult to harvest leakage energy to charge capacitor C1. A portion of the energy which would normally be delivered to the spark gap is now stored in capacitor C1. Therefore, the stored magnetizing energy in the transformer12is increased to compensate for this energy diversion.

Another method provides a regulated power supply for detecting ionization current inside the cylinder by harvesting the excess ignition coil leakage and magnetizing energy in a manner which is more effective than the previously described techniques.FIG. 5is a schematic diagram of the circuit that employs this method. At first glance, the circuit appears to be similar to the second circuit disclosed inFIG. 3described supra in which an energy storage capacitor is charged from the primary winding.

Energy storage capacitor, C2, is added and replaces capacitor C1as the primary energy storage device. As shown inFIG. 5, one terminal of capacitor C2is connected to the cathode of diode D1and the other terminal of capacitor C2is connected to ground. Energy is stored in the coil by turning on power switch IGBT22, and applying battery voltage across the primary winding16of the ignition coil12(Step100inFIG. 6). When the switch IGBT22turns off, the energy stored in the coil leakage and magnetizing inductances causes the transformer primary voltage to reverse. The collector voltage of the IGBT22increases rapidly until the collector voltage exceeds the voltage on capacitor C2by one diode drop, 0.7 volts. At this point, diode D1forward biases, allowing a forward current to flow through diode D1into capacitor C2. When this occurs, energy that is stored in the transformer leakage inductance is transferred to capacitor C2instead of being dissipated on the IGBT (Step110inFIG. 6). Some transformer magnetizing energy may be transferred to capacitor C2as well.

R1, which is now a much larger value, hundreds of kohms, is sized to supply enough current from the high voltage capacitor reservoir C2to satisfy the average ionization current requirements, and to provide adequate bias current to voltage regulator diode D2. Because resistor R1is such a large value, there is a reduced excess current flow in diode D2. This significantly reduces the energy wasted on the voltage regulator diode D2compared to the other techniques previously described.

When the spark plug14fires, the secondary voltage collapses and the magnetizing energy stored in the transformer12is delivered to the spark gap to ignite the air-fuel mixture in the cylinder. Simultaneously, the primary voltage collapses, reverse biasing D1and ending the charging of capacitor C2. At this time, C2is at its maximum voltage, typically 350 to 400 volts. Capacitor C2now acts as the primary energy reservoir to maintain the charge on capacitor C1while supplying current to the ionization circuits and the voltage regulator diode D1(Step120inFIG. 6).

Capacitor C2is sized to supply average ionization current under worst case conditions, e.g., 600 rpm and fouled spark plug, while maintaining a sufficiently high voltage to regulate the ionization supply bus voltage at 100 volts (Step130) to lower voltage capacitor C1. Since capacitor C1is no longer the primary energy storage element, capacitor C1need only be large enough to limit the voltage drop on the ionization bus to acceptable levels while supplying transient ionization currents. Steady state currents are supplied by capacitor C2.FIG. 6illustrates the steps by which the circuit provides a regulated power supply for in-cylinder ionization detection by harvesting excess ignition coil leakage and magnetizing energy

One of the disadvantages of using a two-stage charging approach is that the ionization detection power supply will not be available after the first ignition event due to the long settling time. The main reason is that the time constant due to resistor R1and C1is relatively large, leading to a long time period before the capacitor voltage settles. For example, assuming resistor R1is 1.8 Megaohms and capacitor C1is 0.1 microfarad, the RC time constant, R1*C1, is equal to 180 milliseconds. If it is assumed that the capacitor voltage settles to an acceptable voltage level within 4 time constants, then the total time before the capacitor C1will be able to supply power to the ionization circuit will be approximately 720 milliseconds. If the engine is running at 300 RPM, 720 milliseconds is equivalent to almost 650 crank degrees. This indicates that the ionization detection power supply will not be available until 650 crank degrees after the first ignition event. Furthermore, using multiple spark events will not reduce the settling time since the same time constant applies.

The present invention combines the signal-stage power supply circuit shown inFIG. 3and the two-stage power supply circuit shown inFIG. 5into a two-stage power supply circuit for ionization detection with dual charge rates. This two-stage, dual rate power supply circuit is shown inFIG. 7. Use of another resistor R2and another zener diode D3make a dual charge rate possible. The circuit disclosed inFIG. 7has two charge time constants (R1+R2)*C1and R2*C1.

The following is a description of the operation of the circuit disclosed inFIG. 7. After dwell control signal70goes from logic “high” to logic “low”, switch IGBT22is turned off. The dwell control voltage70controls the amount of time that the supply voltage is applied to the primary coil. This is known as the dwell time. As a result of IGBT22being switched on and off, the energy stored in the coil leakage and the magnetizing inductances causes the transformer primary voltage to reverse and produce a flyback voltage. The collector voltage of the IGBT22increases rapidly until the collector voltage exceeds the voltage72on capacitor C2by one diode drop, 0.7 volts. At this point, diode D1forward biases, allowing a forward current to flow through diode D1into capacitor C2. When this occurs, part of the energy that is stored in the transformer leakage inductance is transferred to capacitor C2instead of being dissipated in the IGBT22.

Capacitors C1and C2are charged and discharged over four time periods as illustrated inFIG. 8. During the first time period80, the flyback voltage exceeds the voltage72of capacitor C2by a diode drop, 0.7 volts. As a result, the flyback voltage supplies energy to capacitor C2to charge the first-stage power supply capacitor C2. When the voltage72of capacitor C2exceeds the sum of voltage74of capacitor C1and the breakdown voltage of diode D3, the first period80ends and the second period82begins. During this second time period82, the flyback voltage supplies energy to the first-stage power supply capacitor C2directly and to the second-stage power supply capacitor C1through resistor R2. After the flyback voltage drops below the sum of voltage74of capacitor C1and the breakdown voltage of zener diode D3, the second time period82ends and the third time period83begins. During this third time period83, the flyback voltage only charges capacitor C1. After the third period83, the flyback voltage further depletes below the voltage72of capacitor C2. In this fourth time period84, current no longer flows through diode D1. In addition, the output stage, or second-stage, voltage74of the power supply is charged only by the first-stage voltage72of capacitor C2through resistors R1and R2.

As stated earlier, two time constants are used to charge capacitor C1, R2*C1and (R1+R2)*C1. After the voltage72on capacitor C2of the first stage power supply exceeds the sum of the breakdown voltage of zener diode D3and the voltage74across capacitor C1of the second stage power supply, the first time period80ends and the second time period82begins. During the second time period82, the flyback voltage supplies energy to capacitor C1through resistor R2. The time constant for charging capacitor C1is R2*C1. This time constant is valid until the voltage across C1reaches the breakdown voltage of zener diode D2, where zener diode D2starts to conduct and limits the voltage across capacitor C1. In addition, some transformer magnetizing energy is transferred to capacitor C1through resistor R1as well.

During the second charge period82, the voltage74settling time of capacitor C1is primarily dependent on the time constant R2*C1. By selecting a relatively small time constant, capacitor C1can be fully charged during the second charge period82.FIG. 8shows that after turn-off of the dwell control signal70the voltage74of the second-stage power supply capacitor C1can be charged from 0 to 100 volts in approximately 13 microseconds. Therefore, the ionization detection power supply can be ready to supply power for ion detection right after the start of the ignition event.

After the first-stage power supply voltage72across capacitor C2falls below the sum of the breakdown voltage of zener diode D3and voltage74of capacitor C1, the second charge period82is complete and the third charge period83begins. During the third83and fourth charge periods84, capacitor C2continues to provide the energy to maintain the second-stage power supply voltage74across capacitor C1at the desired voltage level which is around 100 volts in the illustrated implementation. During the third charge period83, the voltage across zener diode D3is below the breakdown voltage of zener diode D3so the current path to capacitor C1changes. Current now flows from the first-stage power supply capacitor C2through resistors R2and R1into the second-stage power supply capacitor C1. Thus, the charge time constant of the circuit then becomes (R1+R2)*C1when the voltage of C1is below the breakdown voltage of zener diode D2. The time constant changed because the current path to capacitor C1changed.

In summary, the first current path comprises a first resistive value R2, but does not include the second resistive value R1because the current path through resistor R1is effectively shorted by the low impedance path provided by zener diode D3. The second current path comprises both first resistive value R2and second resistive value R1. In the dual-stage, dual charge rate power supply circuit, the value of resistor R1is much greater than the value of resistor R2. As a result, during the flyback period capacitor C1can be charged very quickly by a larger current with very small time constant. However, between ignition events a much smaller current flows to maintain the charge of capacitor C1due to the addition of a second resistive value R1. If the value of resistor R2is too large, capacitor C1will not charge quickly enough on the first ignition event. On the other hand, if the value of resistor R1is too small, excessive current will flow through zener diode D2and the charge on capacitor C2will deplete prematurely.

The following are some of the advantages provided by the dual-stage, dual charge rate power supply circuit for ionization detection.

First, the dual-stage, dual charge rate power supply circuit for ionization detection uses the energy stored in the transformer leakage inductance for two purposes. First, to capture part of the transformer leakage inductance energy as a supplemental energy source for the ionization electronic circuit after capacitor C1is charged. Secondly, to charge capacitor C1with a fast charge rate, i.e., with a short settling time. This allows for a minimal recovery time of the ionization detection power supply.

Second, the dual-stage, dual charge rate power supply circuit for ionization detection reduces the dissipation and resulting heating of the primary IGBT22by diverting the leakage energy into both capacitors C1and C2instead of allowing the leakage energy to be dissipated in the IGBT.

Third, the fast charge rate during the second charge period82allows the ionization detection power supply to recover fully during the flyback period. In the example circuit used to generateFIG. 8, the output supply voltage74of capacitor C1was charged from 0 to 100 volts in approximately 6 microseconds or 0.0216 crank degrees at 600 RPM. This ensures that the high quality power is made available immediately after the ignition event. In addition, the fast charge rate provides an advantage particularly when the engine is operated at a low speed because the amount of delay caused by the settling time of the ionization power supply when measured in crank angles is greater at lower speeds.

Fourth, storing part of the flyback energy at a high voltage in capacitor C2allows a smaller capacitor C1to be used. In the circuit used to generate the waveforms inFIG. 8, the value of capacitor C2was 100 nF. Since energy stored in a capacitor increases as the square of the capacitor voltage, a higher capacitor voltage allows use of a smaller capacitor in the ionization detection circuit of the present invention than has been previously disclosed in the prior art.

Fifth, the dual-stage, dual charge rate power supply circuit for ionization detection reduces the energy wasted on the voltage regulator diode D2by increasing the value of the current limiting resistor R1such that the voltage regulator diode D2does not see large reverse currents.

Sixth, the fast charge rate during the second charge period82also allows the ionization detection power supply to be ready when an ignition event occurs which allows cylinder identification using the ionization current signal during the ignition event.

The following table provides the typical values and ratings for components and time constants of the demonstrating circuit shown inFIG. 7.

While the invention has been disclosed in this patent application by reference to the details of preferred embodiments of the invention, it is to be understood that the disclosure is intended in an illustrative rather than in a limiting sense, as it is contemplated that modification will readily occur to those skilled in the art, within the spirit of the invention and the scope of the appended claims and their equivalents.