Patent Description:
Ignition engines that use very lean air-fuel mixtures have been developed, that is, having a higher air composition to reduce fuel consumption and emissions. In order to provide a safe ignition it is necessary to have a high energy ignition source. Prior art systems generally use 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.

An improved multi-charge system is described in European Patent application <CIT> which discloses 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.

One drawback of current systems is the high primary current peak at the initial charge. That current peak is unwanted, it generates higher copper-losses, higher EMC-Emissions and acts as a higher load for the onboard power generation (generator / battery) of the vehicle. One option to minimize the high primary current peak is a DC/DC converter in front of the ignition coil (e.g. <NUM> V). However this introduces extra cost.

<CIT> discloses a conventional multi charge ignition system.

It is an object of the invention to minimize the high primary current peak without the use of a DC/DC converter.

In one aspect is provided a multi-charge ignition system as claimed in claim <NUM>. The system includes a spark plug control unit and at least two coil stages. The spark plug control unit is configured to control the two coil stages so as to successively energise and de-energise said coil stage(s) to provide a current to a spark plug. The two stages comprise 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).

The high sides of the first and secondary primary windings are connected to a common energizing potential. The first and second secondary windings are arranged in parallel.

A first switch means M1 is electrically connected between a voltage supply high side and the high side of the first primary winding, a second switch Q1 is electrically connected between the first primary winding and the power supply low side supply/earth, a third switch is connected between the junction of the first switch and high side end of the first inductor and a point between the low side of the second primary winding and low side supply/earth. A fourth switch is located between the low side of the second primary winding and said point, and a fifth switch is located between said point and low side supply/earth.

In a further aspect is provided a method of operating a system as above including in a non-operational state, setting all switches M1 M2 M3 Q1 Q2 to off.

In a further aspect is provided a method of operating a system as above including, during an initial ramp-up phase, switching switches Q1, Q2, M3 to on, and M1,M2 to off.

In a further aspect is provided a method of operating a system as above including, after said initial ramp up stage, switching Q1 and Q2 to off.

In a further aspect is provided a method of operating a system as above including during a coupled multi-charge phase, setting the switches alternately to/from the following settings a) Q1/M1 on, Q2/M2/M3off and b) Q1/M1/M3 off, Q2/M2 on.

In a further aspect is provided a method of operating a system as above including, in a step-down phase, setting the switches a) Q2/M1/M3 on, Q1/M2 off and toggling M2/M3.

In a further aspect is provided a method of operating a system as above including, in a step-down phase Q1/M2/M3 on, Q2/M1 on and toggling M1/M3.

The invention will now be described by way of example and with reference ot the following drawings of which:.

<FIG> shows the circuitry of a prior art coupled-multi-charge ignition system for producing a continuous ignition spark over a wide area of burn voltage servicing a single set of gapped electrodes in a spark plug <NUM> such as might be associated with a single combustion cylinder of an internal combustion engine (not shown). The CMC system uses fast charging ignition coils (L1-L4), including primary windings, L1, L2 to generate the required high DC-voltage. L1 and L2 are wound on a common core K1 forming a first transformer (coil stage) and secondary windings L3, L4 wound on another common core K2 are forming a second transformer (coil stage). The two coil ends of the first and second primary <NUM> 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 may be optionally present for measuring the primary current Ip that flows from the primary side and is connected between the switches Q1, Q2 and ground, while optional resistor R2 for measuring the secondary current Is that flows from the secondary side is connected between the diodes D1, D2 and ground.

The low-voltage ends of the secondary windings L2, L4 may be 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 <NUM> through conventional means. The other electrode of the spark plug <NUM> 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 may correspond to conventional automotive system voltage in a nominal 12V automotive electrical system and is in the figure the positive voltage of battery. The charge current can be supervised by an electronic control circuit <NUM> that controls the state of the switches Q1, Q2. The control circuit <NUM> is for example responsive to engine spark timing (EST) signals, supplied by the ECU, 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 Ip and secondary current Is may be sent to control unit <NUM>. Advantageously, the common energizing potential of the battery <NUM> 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 or any other semiconductor switch (e.g. MOSFET) is coupled to transistor M1 so as to form a step-down converter. Control unit <NUM> is enabled to switch off switch M1 by means of a signal FET. The diode D3 or any other semiconductor switch will be switched on when M1 is off and vice versa.

In prior art operation, the control circuit <NUM> 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 <NUM> 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 spark is created through the gapped electrodes of the spark plug <NUM>. During a third step, after a minimum burn time wherein both transformers (T1, T2) are delivering energy, 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 Ip increases over a limit (Ipmax), 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 Is falls short of a secondary current threshold level (Ismin) the switch Q1 is switched off and the switch Q2 is switched on (or vice versa). Then steps <NUM> to <NUM> 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.

<FIG> shows timeline of ignition system current; <FIG> shows a trace representing primary current Ip along time. <FIG> shows the secondary current Is. <FIG> shows the signal on the EST line which is sent from the ECU to the ignition system control unit and which indicates ignition time. During step <NUM>, i.e. M1, Q1 and Q2 switched on, the primary current Ip is increasing rapidly with the energy storage in the transformers. During step <NUM>, 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 <NUM>, 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 <NUM>, comparison is made between primary current Ip and a limit Ipth. When Ip exceeds Ipth M1 is switched off, so that the "switched on" transformer cannot go into the magnetic saturation, by limiting its stored energy. The switch M1 is switched on and off in this way, that the primary current Ip is stable in a controlled range. During step <NUM>, comparison is made between the secondary current Is and a secondary current threshold level Isth. If Is < Isth, Q1 is switched off and Q2 switched on (or vice versa). Then steps <NUM> to <NUM> 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. The above describes the circuitry and operation of a prior art ignition system to provide a background to the current invention. In some aspects of the invention the above circuitry can be used. The invention provides various solutions to enhance performance and reduce spark-plug wear. <FIG> show the operating states of the respective coils by virtue of the switch on and off times.

<FIG> shows a schematic circuit according to one example - it is similar to that of <FIG>. In order for enhanced clarity the primary side of the circuit is shown separately to the secondary side of the circuit. e.g. the primary coils are shown separate from the secondary coils. It is to be understood however that the two cores shown in the figure K1 and K2 are each represented twice but in reality there is only one of each; inductor coils L1 and L2 share the same common core K1 and L3 and L4 share the same common core K2.

In the example a power switch M1 is located similarly arranged to M1 in the <FIG>. This switch is located between the power e.g. battery high side and the high side of the coil L1. Low sides of the inductor coils L1 and L3 are connected through ground via switches Q1 and Q2. A further power switch is connected between the high side of inductor L1 and the low side of inductor L3. A further power switch M2 connects the switch Q2 to earth.

On the secondary side the two secondary coil which are arranged in parallel each have a diode in series connecting the low sides of the coils to earth via the shunt resistor R2, R2 is used to measure the secondary current.

Any of the switches M1, M2, M3, Q1 or Q2 may be controlled by the ECU and/or spark control unit (not shown).

The circuit needs only one additional power switch. The two transformers are connected symmetrically to the battery.

<FIG> shows an alternative example with preferred switches.

The circuits may include means to measure the voltage at the high voltage HV-diodes (D1 and D2), though this is optional, the supply voltage (Ubat) can additionally and optionally be measured.

The operation of the circuit according to the examples such as <FIG> and <FIG> may be implemented as follows with reference to the flow charts of the drawings. Also at the end of the description is a list of the abbreviations/definitions.

<FIG> shows a flow chart of the main loop
At the beginning all power switches are off. The coil is waiting in a loop for the control signal (EST signal) from the ECU. When EST is high "Initial Charge" is starting. The process then proceeds to the Initial Charge process.

<FIG> shows a flow chart for this phase. For the initial charge both coil stages are connected in series: Q1, Q2, M3 are on: The current flows through L3, L1 and R1. With this energy is stored in both transformers. The primary current is measured via R1, if the current is too high both IGBTs are switched off as a safety feature. The Tdwell-time is detected, if the time is too high both IGBTs are switched off; this is a safety feature. Typical Tdwell time for a CMC-coil is between <NUM> and <NUM>. Both transformers are charged as long as the EST-signal of the ECU is high. At the falling edge:.

<FIG> shows a flow chart for this phase. This program section is used between each toggle cycle. The main goal of this system is to maintain a continuous secondary current and with this to toggle between two characteristic stated:.

<FIG> shows a flow chart of this phase. The main goal of this phase on is to measure different current and voltages and to react on it, if the corresponding value is out of range.

<FIG> shows the flow chart of this phase. This phase is initated when the voltage at the HV-diodes is too high and is needed to protect the HV-diodes of too high voltages by switching on both transformers. This is similar to the initial charge phase.

<FIG> shows a flow chart of the "MultiIgbtEnd" phase. Here the secondary current is ramped down to zero, this is needed to minimize the spark plug wear. The following steps are taken:.

<FIG> shows the IpmaxStepDown phase. This function/phase is needed to limit the primary current to a maximum value. In this mode the current flows in a freewheeling path and with this feature the current is limited and with this the stored energy. This function is called during CMC-cycle, where one coil is charged and the other coil is discharged / firing.

The table of <FIG> below shows the timing: Inside the step-down-state M1 and M3 are toggled (T), when Q1 is switched on resp. M2 and M3 when Q2 is switched on. The "MultiIgbtNxt" refers to the CMC-Mode (MultiCharge Mode).

Claim 1:
A multi-charge ignition system including a spark plug control unit and at least two coil stages, said spark plug control unit being configured to control said at least two coil stages so as to successively energise and de-energise said coil stage(s) to provide a current to a spark plug, wherein said two stages comprise:
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);
the high sides of the first and secondary primary windings being connected to a common energizing potential;
wherein said multi-charge ignition system further comprises:
a first switch (M1) is electrically connected between a power supply high side and the high side of the first primary winding,
a second switch (Q1) is electrically connected between the low side of the first primary winding (L1) and the power supply low side supply/earth,
a third switch (M3) is connected between the junction of the first switch (M1) and the high side end of
the first primary winding (L1) and a point between the low side of the second primary winding (L3) and low side supply/earth, and
further including a fourth switch (Q2) located between the low side of the second primary winding (L3) and said point, and
a fifth switch (M2) located between said point and low side supply/earth.