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.

A further interesting example of the prior art is the German publication <CIT>, which discloses the preamble of the independent claims.

The system described above uses a step down converter stage to limit the maximum input current such as transistor switch and diode. However the step-down converter means that additional components are required. It is an object of the invention to provide a system and method of operation where there are a reduced number of components, thus providing a cheaper alternative system.

According to the invention, an ignition system as defined in claim <NUM> and a method of controlling an ignition system as defined in claim <NUM> are disclosed.

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

<FIG> shows the circuitry of a prior art coupled-multi-charge (CMC)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 11such 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. The voltage and wound on a common core K1 forming a first transformer and secondary windings L3, L4 wound on another common core K2 are forming a second transformer. 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 for measuring the primary current Ip that flows from the primary side is connected between the switches Q1, Q2 and ground, while resistor R2 (<NUM>) 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 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. 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 are 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 <NUM> 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 a similar arrangement to <FIG> with similar reference numerals. The step down converter stage and two coil stages are outlined in broken lines <NUM>, <NUM>, <NUM> respectively. In addition the control unit has input means to determine the voltage at the secondary stages. The figure also shows the connectivity of the spark plug control unit with the engine control unit.

Figure 3a and b shows a plot of the primary and secondary currents against time respectively, during operation of the <FIG> system. The system of <FIG> is using a step-converter-stage to limit the maximum input current and with this the input power e.g. during the Coupled-MultiCharge-Mode where the power is supplied alternately to the coils. When the primary current reaches the maximum primary current threshold, then the switch M1 is pulsed this way that the primary current remains on a constant level. Thus in summary, in the conventional CMC mode, the switches Q1 and Q2 are toggled, to provide continuous spark, and the down-converter function is to limit input current.

According to one embodiment of the invention, the step down converter is eliminated; in other words the switches M1 and D1 are eliminated, and the control of the ignition system implemented in a novel fashion, and in an example, control of the switches Q1 and Q2 are used also to limit the (maximum) input current.

<FIG> shows a figure of the circuitry according to one example of the invention without the step-down converter stage. As can be seen it includes as in the <FIG> circuitry, two coil stages being entirely controlled by switches Q1 and Q2, the control being implemented by the control unit. As can be seen there are connections from the low side of the secondary coils to the control unit which can be used by the control unit to determine the voltages at the stages. However it is to be noted that according to simple embodiments these can be dispensed with. The low side of the secondary coils are connected to diodes D1 and D2 as before. In addition, a line connects the ground side of the diodes to the control unit so that the current in the secondary coils can be measured. According to one aspect, means to implement operation is based solely on selective switching of switches Q1 and Q2.

Figure 5a and 5b illustrate the methodology used in one embodiment to implement control of the system during the charge coupled mode. Figures 5a and b and shows the primary current and secondary current respectively in an ignition cycle.

Accordingly, when the primary current in the CMC phase reaches a maximum primary current threshold, both switches Q1 and Q2 are switched off for a period of time, typically between <NUM> and <NUM> microseconds. During this time the transformers are discharging their stored energy into the spark ignition and so as can be seen from the trace of the secondary current in figure 5b. By the implementation of this algorithm the magnetic circuit cannot go into magnetic saturation and will not end in a fast oscillating system. After expiry of said time period, normal operation of the circuitry in CMC mode is resumed; that is to say the switches Q1 and Q2 are toggled as before.

In an alternative example, which is not part of the invention, the switches Q1 and Q2 are switched off until the secondary current reaches a particular minimum secondary current threshold. Thus in this embodiment the secondary current is measured or estimated (e.g. by the control unit by e.g. the connections shown in <FIG>) and compared with a threshold which may be stored in the control unit. This threshold may be signaled by the ECU and may be variable. After this, again, normal operation of the circuitry in CMC mode is resumed; that is to say the switches Q1 and Q2 are toggled as before. In certain embodiments this secondary current threshold may be a set proportion (e.g. twice as much) of the typically adjusted threshold in standard CMC mode. In other embodiments the primary current may be measured/estimated and compared with a minimum threshold, which once reached, causes resumption in operation (i.e. toggling Q1 and Q2).

Thus in one aspect, there is means to determine e.g. in the charge coupled mode, when the primary current reaches a threshold, and dependent thereon selectively switching on/off one or more of said switches. The threshold value may be stored in the control unit. This may be variable depending on the operation and/or other parameters.

As can be seen in <FIG> there is a line which connects to the control unit which enables primary current to be measured. Alternatively this line may be omitted and, during the recharge-cycle it is possible to determine the primary current based on the previously measured secondary current level for the same coil stage. Together with the battery voltage, the gradient of the primary current is determined and then the primary current can be determined.

In a specific embodiment there is means to determine if the primary current in the charge coupled mode reaches a threshold value, and if so switching both switches Q1 and Q2 off for a pre-set time.

Claim 1:
An ignition system including a spark plug control unit adapted to control at least two stages comprising a first transformer (Tl) including a first primary winding (Ll) 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); said control unit enabled to simultaneously energize and deenergize primary windings (Ll, L3) by simultaneously switching on and off two corresponding first and second
switches (Ql, Q2) to maintain a continuous ignition fire, wherein means to implement operation is based solely on selective switching of the first and second switches (Q1, Q2);
wherein during a Coupled-Multi Charge-Mode, the power is supplied alternately to the coils by toggling the first switch (Q1) with the second switch (Q2);
wherein the ignition system has means to measure a primary current and said spark plug control unit includes means to determine if the measured primary current in the Coupled-Multi Charge-Mode mode reaches a maximum primary current threshold value, and when the measured primary current in the Coupled-Multi Charge-Mode phase reaches said maximum primary current threshold value, both first and second switches (Q1) and (Q2) are simultaneously switched off for a period of time, and thereafter toggling first switch (Q1) with the second switch (Q2) is resumed;
characterised in that the period of time is a pre-set time.