EP2325476A1

EP2325476A1 - Coupled multi-charge ignition system with an intelligent controlling circuit

Info

Publication number: EP2325476A1
Application number: EP09176699A
Authority: EP
Inventors: Frank Lorenz; Marco Loenarz
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2009-11-20
Filing date: 2009-11-20
Publication date: 2011-05-25
Anticipated expiration: 2029-11-20
Also published as: EP2325476B1

Abstract

The invention relates to an ignition system for a combustion engine comprising a spark plug with a pair of gapped electrodes, a first transformer T1 including a first primary winding L1 inductively coupled to a first secondary winding L2 a second transformer T2 including a second primary winding L3 inductively coupled to a second secondary winding L4, secondary windings L2 and L4 being each coupled to the gapped electrodes of the spark plug and a control unit enabled to simultaneously energize and deenergize primary windings L1 and L3 by simultaneously switching on and off two switches Q1 and Q2 to establish an electrical arc across the gapped electrodes and to sequentially energize and deenergize primary windings L1 and L3 by sequentially switching on and off both switches Q1 and Q2 to maintain a continuous ignition fire.

Description

TECHNICAL FIELD

The present invention relates to an ignition system and particularly, to an AC ignition system which is able to create and maintain a continuous spark.

BACKGROUND OF THE INVENTION

The automotive industries have developed gasoline engines that use very lean air-fuel mixtures, that is, having a higher air component, to reduce fuel consumption and emissions. Common combustion principles are either homogeneous lean mixtures or stratified direct injection. To get a safe ignition it is necessary to have a high energy ignition source.
Prior art solutions are generally large, high energy, single spark ignition coils, which have a limited spark duration and energy output. To overcome this limitation and also to reduce the size of the ignition system multi-charge Ignition systems have been developed. Multi-charge systems produce a fast sequence of individual sparks, so that the output is a long quasi-continuous spark. Multi-charge ignition methods have the disadvantage that the spark is interrupted during the recharge periods, which has negative effects, particularly noticeable when high turbulences are present in the combustion chamber. For example this can lead to misfire, resulting in higher fuel consumption and higher emissions. Otherwise, prior solutions such as simple AC ignition systems have also the disadvantage that the primary side is directly coupled to the secondary side of the transformer during the firing, so the transferred energy to the spark plug decreases with higher burn voltages.
Furthermore, it is known from document WO 2007/025367 an ignition system providing power and duration controlled ignition spark. The system comprises a spark controller, first switching energy accumulator, storage capacitor, and second switching energy accumulator with an ignition coil. The ignition system utilizes dual means of switching energy accumulation, internal energy transfer, and three means of energy release to the ignition spark, managed by means of the spark controller depending on engine operating conditions, and provides continuous bipolar ignition spark. Such ignition system is based on the use of an energy accumulator coupled to a storage capacitor in order to feed energy to a single ignition coil. It does not provide any solution to the disadvantage encountered with multi-charge systems.

SUMMARY OF THE INVENTION

One goal of the present invention is to overcome the aforecited drawbacks by providing a multi-charge ignition system without these negative effects and, at least partly, producing a continuous ignition spark over a wide area of burn voltage, delivering an adjustable energy to the spark plug and providing with a burning time of the ignition fire that can be chosen freely.
For that purpose, according to a first aspect, the invention concerns an ignition system for a combustion engine comprising a spark plug with a pair of gapped electrodes, a first transformer including a first primary winding inductively coupled to a first secondary winding, a second transformer including a second primary winding inductively coupled to a second secondary winding, secondary windings being each coupled to the gapped electrodes of the spark plug and a control unit enabled to simultaneously energize and deenergize primary windings by simultaneously switching on and off two switches to establish an electrical arc across the gapped electrodes and to sequentially energize and deenergize primary windings by sequentially switching on and off both switches to maintain a continuous ignition fire. Thanks to the use of a multi-charge ignition system, it allows the control unit to use simultaneously the energy stored in both transformers to create an ignition spark and to use alternatively the energy stored either in one transformer or in the other to maintain a continuous ignition fire while reenergizing the other transformer. The alternation between energizing and deenergizing is done after comparison of the secondary current with a predetermined current threshold representative of the minimum necessary level of energy stored in the transformer that is switched off. When the secondary current falls short to the predetermined current threshold, the switching operation is executed. Thus, the ignition system allows production of a continuous ignition spark with a burning time of the ignition fire that can be chosen freely.
According to an advantageous embodiment, the ignition system further comprises a step-down converter connected to the primary windings and including a third switch and a diode, and said control unit is enabled to switch off said third switch when a primary current exceeds a predetermined current threshold in order to limit the stored energy in the transformer that is switched on by impelling a current over the diode. Due to this step-down converter, the primary current is limited to a predetermined maximum value, so that the transformers cannot go into magnetic saturation.
According to another advantageous embodiment, the control unit is further enabled to compare the secondary current with the predetermined current threshold representative of the minimum necessary level of energy stored in the transformer that is switched off and to adapt this predetermined current threshold to the level of energy stored. Such adaptation of the minimum secondary current level depending on the stored energy in the transformers allows getting a stable controlling circuit.
According to another advantageous embodiment, secondary windings are decoupled one from the other by high voltage diodes, and said control unit is further enabled to detect a burn voltage at the spark plug in the combustion engine, to switch off both corresponding switches when burn voltage is higher than a predetermined burn voltage threshold and switch on high-voltage diodes so as a forward current floats through. Advantageously, said control unit detects burn voltage at the spark plug by measuring the gradient of the secondary current. Detection of the burn voltage allows stopping ignition when it exceeds a predetermined level in order to be able to use ordinary low-cost high-voltage breakdown diodes on the secondary side, i.e. with a breakdown voltage of e.g. 5kV, instead of expensive and too large high-voltage diodes with a breakdown voltage of 30kV or more.
According to another aspect, the present invention concerns a method of producing electrical arcs across a pair of gapped electrodes of a spark plug with an ignition system of claim 1, comprising the steps of:

energizing simultaneously both primary windings by switching on corresponding switches;
deenergizing simultaneously both primary windings by switching off corresponding switches to establish an electrical arc across the pair of gapped electrodes;
energizing and deenergizing sequentially primary windings by sequentially switching on and off corresponding switches.

According to an advantageous variant, the method further comprises the steps of comparing the primary current with a first predetermined current threshold; switching off a third switch when the primary current exceeds the first predetermined current threshold and impelling a current over a diode from the primary winding that is switched on.
According to another advantageous variant, the method further comprises the steps of comparing the secondary current with a second predetermined current threshold representative of the minimum necessary level of energy stored in the transformer that is switched off; and switching sequentially on and off both corresponding switches when the secondary current falls short to the second predetermined current threshold.
According to another advantageous variant, the method further comprises the step of setting adaptively said second predetermined current threshold to the level of energy stored in the transformer that is switched off.
According to another advantageous variant, the method further comprises the step of detecting burn voltage at the spark plug in the combustion engine and switching off both corresponding switches when burn voltage is higher than a predetermined burn voltage threshold and switching on high-voltage diodes so as forward current floats through.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear upon reading the following description which refers to the annexed drawings in which:

Figure 1 is an electrical schematic illustration of an ignition system according to a preferred embodiment of the present invention;
Figures 2, 4 and 5 illustrate certain characteristic signals at various points in a use cycle for the exemplary ignition system as illustrated in Figure 1;
Figure 3 is a diagram representing step by step the different control signals sent and received by the control unit of an ignition system as illustrated in Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to Figure 1, a multi-charge ignition system is illustrated for producing a continuous ignition spark over a wide area of burn voltage servicing a single set of gapped electrodes in a spark plug 11 such as might be associated with a single combustion cylinder of an internal combustion engine (not shown).
The multi-charge ignition system uses fast charging ignition coils (L1-L4), including primary windings, L1, L2 to generate the required high AC voltage and wound on a common core K1 forming a first transformer and secondary windings L3, L4 wound on another common core K2 forming a second transformer. The two coil ends of the first and second primary windings L1, L3 may be alternately switched to a common ground such as a chassis ground of an automobile by electrical switches Q1, Q2. These switches Q1, Q2 are preferably Insulated Gate Bipolar Transistors. Resistor R1 for measuring the primary current I_p that flows from the primary side is connected between the switches Q1, Q2 and ground, while resistor R2 for measuring the secondary current I_s that flows from the secondary side is connected between the diodes D1, D2 and ground.
In the present embodiment for extended burn applications, it is assumed that the low-voltage ends of the secondary windings L2, L4 are coupled to a common ground or chassis ground of an automobile through high-voltages diodes D1, D2. The high-voltage ends of the secondary ignition windings L2, L4 are coupled to one electrode of a gapped pair of electrodes in a spark plug 11 through conventional means. The other electrode of the spark plug 11 is also coupled to a common ground, conventionally by way of threaded engagement of the spark plug to the engine block.
The primary windings L1, L3 are connected to a common energizing potential which in the present embodiment is assumed to correspond to conventional automotive system voltage in a nominal 12V automotive electrical system and is in the figure the positive voltage of battery 15.
The charge current can be supervised by an electronic control circuit 13 that controls the state of the switches Q1, Q2. The control circuit 13 is for example responsive to engine spark timing (EST) signals to selectively couple the primary windings L1 and L2 to system ground through switches Q1 and Q2 respectively controlled by signals Igbt1 and Igbt2 respectively. Measured primary current I_p and secondary current I_s are sent to control unit 13.
Advantageously, the common energizing potential of the battery 15 is coupled by way of an ignition switch M1 to the primary windings L1, L3 at the opposite end that the grounded one. Switch M1 is preferably a MOSFET transistor. A diode D3 is coupled to transistor M1 so as to form a step-down converter. Control unit 13 is enabled to switch off switch M1 by means of a signal FET.
In operation, the control circuit 13 is operative to provide an extended continuous high-energy arc across the gapped electrodes. During a first step, switches M1, Q1 and Q2 are all switched on, so that the delivered energy of the power supply 15 is stored in the magnetic circuit of both transformers (T1, T2). During a second step, both primary windings are switched off at the same time by means of switches Q1 and Q2. On the secondary side of the transformers a high voltage is induced and an ignition ignition spark is created through the gapped electrodes of the spark plug 11. During a third step, switch Q1 is switched on and switch Q2 is switched off (or vice versa). That means that the first transformer (L1, L2) stores energy into its magnetic circuit while the second transformer (L3, L4) delivers energy to spark plug (or vice versa). During a fourth step, when the primary current I_p increases over a limit (I_pmax), the control unit detects it and switches transistor M1 off. The stored energy in the transformer (L1, L2 or L3, L4) that is switched on (Q1, or Q2) impels a current over diode D3 (step-down topology), so that the transformer cannot go into the magnetic saturation, its energy being limited. Preferably, transistor M1 will be permanently switched on and off to hold the energy in the transformer on a constant level. During a fifth step, just after the secondary current I_s falls short of a secondary current threshold level (I_smin) the switch Q1 is switched off and the switch Q2 is switched on (or vice versa). Then steps 3 to 5 will be iterated by sequentially switching on and off switches Q1 and Q2 as long as the control unit switches both switches Q1 and Q2 off.
As illustrated in Figure 2, on the upper graph, the trace represents primary current I_p along time. On the lower graph, the traces represent the secondary current I_s and the secondary voltage U_s at the gapped electrodes of the spark plug. The different steps 1 to 5 of operation of the control circuit have been reported on Figure 2. During step 1, i.e. M1, Q1 and Q2 switched on, the primary current I_p is increasing rapidly with the energy storage in the transformers. During step 2, i.e. Q1 and Q2 switched off, the secondary current Is is increasing and a high voltage is induced so as to create an ignition spark through the gapped electrodes of the spark plug. During step 3, i.e. Q1 and Q2 are switched on and off sequentially, so as to maintain the spark as well as the energy stored in the transformers. During step 4, comparison is made between primary current I_p and a limit I_pmax. When I_p exceeds I_pmax M1 is switched off, so that the "switched on" transformer cannot go into the magnetic saturation, by limiting its stored energy. During step 5, comparison is made between the secondary current I_s and a secondary current threshold level I_smin. If I_s < I_smin, Q1 is switched off and Q2 switched on (or vice versa). Then steps 3 to 5 will be iterated by sequentially switching on and off Q1 and Q2 as long as the control unit switches both Q1 and Q2 off. Because of the alternating charging and discharging of the two transformers the ignition system delivers a continuous ignition fire.
Figure 3 is a diagram showing in more details step by step the different control signals sent and received by the control unit of an ignition system as illustrated in Figure 1.
During a step S0, the control unit checks whether there is a high EST signal. If so, during a step S1, control signals Igbt1, Igbt2 and Fet are switched on, so both transformers are charged at the same time. The delivered energy of the power supply is stored in the magnetic circuit of the transformers. During a step S1.1, the control unit checks whether there is a low EST signal. Until this is the case, the transformers are charging.
When a low EST signal is detected, during a step S1.2, the control unit will preferably detect the maximum primary current I_pmax and set the secondary current threshold I_smin dependent on the I_pmax (I_smin=f(I_pmax)). A greater primary current will result in a greater secondary current threshold and vice versa.
Then during a step S2, both control signals Igbt1 and Igbt2 are switched off. On the secondary side of the transformers a high voltage is induced (up to 30-40kV). After a short time a spark gap at the spark plug breaks down and the secondary voltage decreases to a burn voltage (e.g. ≈ 0.5 kV). The high voltage diodes (D1, D2) are protected from too high voltages, because both diodes are conducted in forward direction during this critical breakthrough period.
Then during a step S3, Igbt1 remains off and Igbt2 is switched on. Thus coil T1 is recharged and coil T2 is firing. If the primary current I_p exceeds the primary threshold I_pmax, the control signal FET is switched off for a short time during a step S4. Thus, the primary current I_p is limited to a maximum value and cannot rise up very quickly to non-controllable values; and therefore the magnetic circuit cannot go into magnetic saturation.
Otherwise, since the secondary voltage U_s depends on the ambient conditions at the spark plug (e.g. airflow), the control circuit will preferably detect during a step 4.1 the secondary voltage U_s using the gradient of the secondary current (dI_s/dt=f(U_s)). In order to protect the high-voltage-diodes on the secondary side from too high voltages, the control circuit will advantageously switch both Igbt1 and Igbt2 off, if the secondary voltage (respectively dI_s/dt) reaches a maximum limit (U_s>U_smax). In case such event occurs the control circuit detects the gradient of the secondary current any more and will fall back to the normal operational mode, if the secondary voltage falls below the maximum limit (Us<Usmax), i.e. one Igbt control signal being on, the other one being off. In case the secondary voltage would remain at a very high level, the system then works in a normal multi-charge mode where both Igbt signals are on respectively off at the same time, (see figures 4 and 5). Another way to detect the secondary voltage is to measure the voltage at the drain connector of the transistors Q1, Q2.
During a step 5, if the secondary current I_s falls short off the secondary current threshold I_smin (I_s < I_smin) the Igbt 2 control signal is switched off and Igbt 1 is switched on. Then during a step 6, the Igbt control signals are alternately switched on and off, steps 1.2, 4, 4.1 and 5 being iterated by the control unit.
The burn voltage at the spark plug in a combustion engine is variable, because of the turbulences at the ignition spark. When the secondary voltage U_s becomes higher, the firing transformer has to deliver more energy to the ignition spark. Then, the transformer, which is recharging, cannot safe enough energy until the secondary current I_s falls short to the secondary current threshold I_smin. Consequently, the average energy level in the transformers decreases. To get a stable controlling circuit, the burn voltage U_s is advantageously detected and the secondary current threshold I_smin set adaptively to a level that depends on the stored energy in the charging coil. This situation has been shown in Figure 4.
Figure 4 illustrates three traces which represent primary current I_p, secondary current I_s and the secondary voltage U_s along time. In this example, the secondary voltage or burn voltage is around 2kV, i.e. twice greater than in the example of Figure 2. The steps for creating and maintaining an ignition spark are mainly the same as for the example of Figure 2. However, in this preferred embodiment, the secondary current threshold I_s is set dependent on the primary current I_p (see step S1.2 explained above). This function is useful to prevent the system to get to an oscillating system. When the secondary current threshold I_smin reaches a minimum set value, the system switches into a normal multi-charge mode to deliver a higher power level to the spark plug until the burn voltage decreases.
When the burn voltage U_s becomes too high (e.g. the ignition spark is blown out), the high-voltage diodes on the secondary side can breakdown. One possible solution is to increase the breakdown voltage of the diodes, e.g. to 30kV or more. But these diodes are expensive and/or not available for automotive applications. Therefore, to minimize the breakdown voltage of the high-voltage diodes on the secondary side, the burn voltage U_s has to be detected by the control unit. A convenient way to do so is to detect the gradient of the secondary current (dI_s/dt). If the gradient is too high, the control unit switches both transistors Q1 and Q2 off. Thus, the diodes are safe, because both are switched on and through the diodes float a forward current. This situation has been shown in Figure 5.
Figure 5 illustrates the same traces as Figure 4, namely the primary current I_p, the secondary current I_s and the secondary voltage U_s along time. In this example, the secondary voltage or burn voltage is around 4.8kV, i.e. five greater than in the example of Figure 2. The controlling unit detects the detects the gradient of the secondary current (dI_s/dt). Over a predetermined value, e.g. 4kV, since the gradient is to high, the ignition system switches both transistors Q1 and Q2 off, so as to deliver a higher power level to the spark plug until the burn voltage decreases called normal multi-charge mode. If the burn voltage does not decrease the system remains in this normal multi-charge mode.
According to this preferred embodiment, such intelligent control unit saves the high-voltage diodes on the secondary side for too large burn voltages, allowing using safely high-voltage diodes with only a breakdown voltage of 5kV which are easily available and at low price on the market.
Having described the invention with regard to certain specific embodiments, it is to be understood that these embodiments are not meant as limitations of the invention. Indeed, various modifications, adaptations and/or combination between embodiments may become apparent to those skilled in the art without departing from the scope of the annexed claims.

Claims

An ignition system for a combustion engine comprising:
- a spark plug with a pair of gapped electrodes;

- a first transformer (T1) including a first primary winding (L1) inductively coupled to a first secondary winding (L2);

- a second transformer (T2) including a second primary winding (L3) inductively coupled to a second secondary winding (L4);

- secondary windings (L2, L4) being each coupled to the gapped electrodes of the spark plug;

- a control unit enabled to simultaneously energize and deenergize primary windings (L1, L3) by simultaneously switching on and off two corresponding switches (Q1, Q2) to establish an electrical arc across the gapped electrodes and to sequentially energize and deenergize primary windings (L1, L3) by sequentially switching on and off both corresponding switches (Q1, Q2) to maintain a continuous ignition fire.
An ignition system according to claim 1, wherein it further comprises a step-down converter including a switch (M1) and a diode (D3), said control unit being enabled to switch off said switch (M1) when a primary current (I_p) exceeds a first predetermined current threshold (I_pmax) in order to limit the stored energy in the transformer (T1 or T2) that is switched on by impelling a current over said diode (D3).
An ignition system according to claim 1 or 2, wherein the control unit is enabled to compare a secondary current (I_s) with a second predetermined current threshold (I_smin) representative of the minimum necessary level of energy stored in the transformer that is switched off and for switching sequentially on and off both corresponding switches (Q1 and Q2) when the secondary current (I_s) falls short to the second predetermined current threshold (I_smin).
An ignition system according to any of claims 1 to 3, wherein the control unit is further enabled to compare the secondary current (I_s) with a second predetermined current threshold (I_smin) representative of the minimum necessary level of energy stored in the transformer (T1 or T2) that is switched off and to adapt said second predetermined current threshold (I_smin) to the level of energy stored in said switched off transformer (T1 or T2).
An ignition system according to any of claims 1 to 4, wherein secondary windings (L2, L4) are decoupled one from the other by high voltage diodes (D1, D2), and wherein said control unit being further enabled to detect a burn voltage at the spark plug in the combustion engine, to switch off both corresponding switches (Q1, Q2) when burn voltage is higher than a predetermined burn voltage threshold and switch on both high voltage diodes (D1, D2) so as a forward current floats through.
An ignition system according to claim 5, wherein said control unit detects burn voltage at the spark plug by measuring the gradient of a secondary current I_s or by detecting the drain voltage at the switches (Q1, Q2).
A method of producing electrical arcs across a pair of gapped electrodes of a spark plug with an ignition system of claim 1, comprising the steps of:
- energizing simultaneously both primary windings (L1, L3) by switching on corresponding switches (Q1, Q2);

- deenergizing simultaneously both primary windings (L1, L3) by switching off corresponding switches (Q1, Q2) to establish an electrical arc across the pair of gapped electrodes;

- energizing and deenergizing sequentially primary windings (L1, L3) by sequentially switching on and off both corresponding switches (Q1, Q2).
The method according to claim 7, wherein it further comprises the steps of:
- comparing the primary current (I_p) with a first predetermined current threshold (I_pmax);

- switching off a switch (M1) when the primary current (I_p) exceeds the first predetermined current threshold (I_pmax) and impelling a current over a diode (D3) from the primary winding that is switched on.
The method according to claim 7 or 8, wherein it further comprises the steps of:
- comparing the secondary current (I_s) with a second predetermined current threshold (I_smin) representative of the minimum necessary level of energy stored in the transformer that is switched off;

- switching sequentially on and off both corresponding switches (Q1, Q2) when the secondary current (I_s) falls short to the second predetermined current threshold (I_smin).
The method according to claim 9, wherein it further comprises the step of:
- setting adaptively said second predetermined current threshold (I_smin) to the level of energy stored in the transformer that is switched off.
The method according to any of claim 7 to 10, wherein it further comprises the step of:
- detecting burn voltage at the spark plug in the combustion engine and switching off both corresponding switches (Q1, Q2) when the burn voltage is higher than a predetermined burn voltage threshold and switching on high-voltage diodes (D1, D2) so as a forward current floats through.
The method according to claim 11, wherein said control unit detects burn voltage at the spark plug by measuring the gradient of the secondary current (I_s) or by detecting the drain voltage at the switches (Q1, Q2).