Patent Application
20050279337
The present invention relates to a method of controlling the spark current in a spark plug, which spark plug is part of a ignition system, the method providing the possibility to control the intensity as well as the duration of the spark current, and thereby also the energy released by the spark. The invention also relates to a device for accomplishment of such control.
Many of the internal combustion engines of today make use of spark plugs to ignite a mixture of fuel and air. This takes place for example in the Otto engine, in which a premixed mixture of fuel and air is ignited. Spark plugs are also used in direct injection engines, in which fuel and air is mixed directly in the cylinder. Common fuels of today are different types of hydrocarbon compounds, such as petrol, alcohols and various gases such as natural gas and biogas. In the future, hydrogen gas may also be used.
The function of the spark plug is mainly at a given point of time to ignite a small portion of the mixture of fuel and air close to the spark plug, where after the combustion continues in the cylinder by aid of flame spreading.
When a spark arcs between the electrodes of the spark plug, a plasma channel of very high temperature is formed between the electrodes, which plasma channel can have a diameter in the magnitude of 0.1 mm and a typical length of 1 mm. At the surface of the plasma channel, the temperature is beneficial for ignition of the mixture of fuel and air, provided that there is an ignitable mixture close to the spark plug. Normally, the least energy is required to ignite a mixture of fuel and air that is nearly stoichiometric (close to lambda 1). As the fuel-to-air ratio close to the spark plug often varies over time, ignition systems of today normally deliver a spark of relatively long duration, about 1-2 ms. This is to ensure that ignition takes place. The drawback of this procedure is that the point of time for ignition of the fuel and air mixture can vary from cycle to cycle, which results in cycle to cycle variations of the torque delivered by the cylinder, from one combustion cycle to another.
A spark discharge can be characterised by a number of phases. First, the ignition system delivers a high-voltage pulse to the spark plug. When the voltage has risen typically to 10-20 kV, a spark-over takes place between the electrodes. The first phase is called “Breakdown Phase” and has a duration of a few nanoseconds. The voltage over the plug is high, and the current in the plug can be tens of Amperes. The next phase is the “Arc Phase”, typically having a duration of some microseconds. In this phase, the voltage between the electrodes is typically 50-100 V, and the current is in the magnitude of 1-10 A. The last phase is the “Glow Phase”, typically having a duration of a few milliseconds for a standard inductive ignition system. In this phase, the voltage between the electrodes is typically 500-1000 V, and then the ignition coil of the ignition system delivers a current in the magnitude of 10-100 mA dropping relatively linearly to zero. For most ignition systems of today, the major part of the energy release over the spark plug normally takes place during the “Glow Phase”.
To facilitate ignition, especially of lean but also of rich mixtures, as well as inhomogeneous mixtures, the fuel-to-air ratio of which varying over time in the vicinity of the spark plug, it would be beneficial to design an ignition system that delivers a stronger spark, the plasma channel of which being of more intense current and having a larger effective surface during a shorter and more well defined time period. It would in other words be an advantage if an ignition system could be designed that principally operates in “Arc Phase”, and for which the “Arc Phase” is extended in time to be at least some hundreds of microseconds or up to about one millisecond, depending on the desired duration of the spark.
In order for an engine to be able to run on different fuels, it is also an advantage to be able to vary the spark in dependence of how easy the fuel is to ignite. This can also be of interest in other circumstances, such as at cold starting or in damp weather, when ignition of the fuel mixture can be more difficult than what is normal. Today, there are also engines with variable EGR (Exhaust Gas Circulation). At a high EGR, the concentration of inert gases is higher in the cylinder, whereby ignition of the fuel mixture can be more difficult, and in which case it may be of interest to have a more intense spark with a larger energy content.
A more intense spark may however give rise to an increased wear of the spark plug, as well as an increased consumption of electrical energy, which means that it is not desired without cause to have a too intense spark or a spark of too long duration. It would in other words be beneficial if the duration of the spark and the intensity of its current could be varied in dependence of a variety of circumstances. Furthermore, in case the engine runs on very high speed, it may be unnecessary for the spark to have a very long duration, as the purpose of the spark is only to start the combustion in the cylinder.
Spark plugs can also be used as sensors to get information about the combustion process. In order to achieve this, a relatively low voltage of e.g. 50-200 V can be applied over the spark plug after a completed spark, while measuring the current that passes between the electrodes of the spark plug. This current as a function of time will give information about the conductivity of the gas close to the spark plug during the combustion process, which among other things can give information about the time progress of the spark plug after the completed spark. Ignition systems utilising the spark plug as a sensor are said to be equipped with an ion-flux system, where the ion-flux is the current flowing between the electrodes of the plug at a given applied voltage over the plug after a completed spark. Normally, this current is low in relation to the spark current. Most often, the ion-flux has a magnitude of 1-1000 μA during a combustion process. In order to be able to make full use of an ion-flux system, it is important that the duration of the spark is short in relation to the time of combustion in the cylinder. This is because the combustion information can be achieved only after the disappearance of the spark current. When the spark current has reached a value close to zero, another phenomenon usually arises which is called coil ringing. When the spark over the plug disappears, there is a strong increase in the impedance between the electrodes of the spark plug, resulting in self-oscillation in the secondary circuit of the ignition coil, of which circuit the spark plug forms part. After the attenuation of this self-oscillation, measuring of the ion-flux can be commenced.
For ignition systems using the spark plug as an ion-flux sensor, it is accordingly important that the duration of the spark is short in comparison with the combustion time in the cylinder, and that the time of coil ringing is short.
From U.S. Pat. No. 5,197,448, U.S. Pat. No. 4,033,316, U.S. Pat. No. 4,136,301, U.S. Pat. No. 4,301,782 and U.S. Pat. No. 4,345,575, is known various methods of supplying energy to the secondary side of an ignition coil, in order to increase the spark current or to extend the spark current after ignition of the spark plug, which methods however have some drawbacks.
U.S. Pat. No. 5,197,448 makes use of a CDI (Capacitive Discharge Ignition) system on the primary side, and a charged capacitor placed on the secondary side of the ignition coil in order to supply energy to the spark plug either via a high-voltage diode or via an ignition coil having a saturatable core in order to increase the current in the spark plug by aid of the energy stored in the capacitor on the secondary side. A typical current of the voltage source is said to be 600 V on the primary side and −600 V on the secondary side. Accordingly, it is evident that costly and complex equipment is required, for example in the form of a 40 kV high-voltage diode (alternatively an ignition coil having a saturatable core). Also, this device will not really make it easy to control neither the spark current nor its duration. Instead, the charged capacitor will discharge quickly, via the diode and the spark plug, and give rise to an intense but short current impulse through the spark plug after it has ignited, with a duration of perhaps some tens of microseconds.
U.S. Pat. No. 4,033,316 utilises a conventional inductive ignition system combined with a voltage source on the secondary side, typically having an amplitude of 1 kV-4 kV, in order to enhance and extend the spark. Neither that invention shows any good method for controlling the spark current nor its duration, but simply a way of extending it. U.S. Pat. No. 4,136,301 discloses a development of U.S. Pat. No. 4,033,316, in which it has been added the use of a DC/DC converter containing among other things a transformer and a rectifier in order to obtain a variable output voltage, such as between voltages 1 kV and 4 kV, in order to adapt the voltage source to the engine conditions in question. Here, a certain possibility is achieved to adapt the spark in relation to the engine conditions, but the invention does not disclose any method of controlling the current in the spark during its duration, in order for example to achieve a specific shape of the current curve or in order to achieve a spark with a predetermined duration.
U.S. Pat. No. 4,301,782 uses a DC/DC converter having a current outlet connected to the high-voltage side of the spark plug, via an inductor, in order to increase the current in the spark plug after ignition. U.S. Pat. No. 4,345,575 uses a high-voltage capacitor charged to a high voltage that is discharged through a resistor connected to the high-voltage side of the spark plug, in order to increase the current in the spark plug after ignition. In common for these to last mentioned methods is the drawback that the energy is supplied to the high-voltage side of the ignition coil, which is more complicated than if the energy is supplied to the low-voltage side of the ignition coil.
All methods mentioned above disclose the use of a high-voltage source in order to affect the spark current, which is a drawback from the point of view that a high voltage is relatively hard to achieve compared to a lower voltage. Furthermore, non of the inventions discloses a method of achieving a spark for which it is easy to control the current intensity and duration in Arc Discharge Mode.
Accordingly, no real good method exists today for controlling the spark current in an ignition device, such as a spark plug in a combustion chamber.
It is an object of the present invention to eliminate or at least minimize any one or some of the problems mentioned above, which is achieved by a method according to claim 1.
Thanks to the invention, there is surprisingly obtained a both simple and cost-efficient method to achieve an ignition system with a controllable spark current and duration of the spark. The cost-efficiency is achieved inter alia by the ability to eliminate high-voltage electronic components, such as a high-voltage diode. Simplicity is achieved inter alia by a lower control voltage being easier to accomplish than a higher voltage.
The invention enables an advantageous ignition system that is able to achieve an intense spark during a predetermined burning time. The spark gap or spark plug operates during the major part of the burning time in Arc Discharge Mode, which results in an intense spark with a large “surface” for igniting a mixture of fuel and air, which is especially advantageous in the cases in which the fuel mixture is harder to ignite, such as at ignition of very lean mixtures.
Furthermore, a preferred embodiment of the invention advantageously enables the spark current as well as its duration to be varied independently of each other but depending on the current operating conditions of the engine or depending on outer circumstances such as fuel quality or weather.
Moreover, thanks to a preferred embodiment of the invention, it is possible to let the control circuit be connected in a phase in which no current is flowing on the primary side of the ignition coil, which means that the requirement is eliminated of trying to minimize the coupling between the voltage source on the secondary side and the primary side, respectively.
The invention is especially advantageous in case the spark plug is used as an ion current sensor, since in that case it is especially important to be able to limit the duration of the spark and be able to limit the time of coil ringing to the initial phase of the combustion, whereby the spark plug can be used as sensor during the major part of the combustion process.
Additional aspects and advantages of the invention will be clarified in connection with the following detailed description.
In the following, the invention will be described in greater detail with reference to the appended drawings, of which:
In the following, the secondary circuit of the ignition system will be described, which secondary circuit differs in circuit design from a conventional inductive ignition system. A first end of the secondary coil 31 is conventionally connected, via a connection 9, with the gap 80 and ground point 82 of the spark plug, such that the secondary coil 31 can affect the voltage over the gap 80 formed by the spark former 91, 81 of the spark plug. A control circuit 6, 7, 14 is arranged between a second end of the secondary coil 31 (included in the ignition coil 3) and ground point 15. This circuit comprises a first part circuit having a second transistor 6 and a second voltage source 14. A suitable DC converter, of a type known per se (not shown), suitably converts the voltage from the voltage source 1 to obtain a second voltage source 14 with a suitable negative voltage, normally about −100 V, which however can be varied within the range of e.g. (−60) —(−140) V. By its collector, also called drain, the transistor 6 is connected with said second end of the secondary coil 31, and by its emitter, also called source, it is connected with the negative pole of the second voltage source. In a parallel circuit with the transistor 6 and the voltage source 14, a diode 7 is arranged, which enables current to flow only in the direction from the primary coil, over the second part circuit, i.e. passing the part circuit including the second transistor.
A control unit 4 is arranged to control both transistors 5, 6. The control unit receives input signals from a parent engine control member that determines the timing of the spark, its duration and current strength, or that alternatively chooses between some different pre-programmed shapes of the spark curve. The control unit 4 comprises a logic unit 41 for time control, in order to control the different units with optimal timing. The control unit 4 can also contain a regulating member 42 for current regulation of the current through the primary coil 30 and the secondary coil 31, respectively, which currents can be measured e.g. over a suitable measuring resistance arranged in series with the primary coil and the secondary coil, respectively (not shown in the drawings). In addition, the control unit 4 comprises drive units for control members in the control loop. By this control unit 4, the transistor 5 is controlled in a way that is conventional for an inductive ignition system, i.e. the transistor is brought to conduct for a predetermined time, often called “dwell time”, whereby current flows through the primary coil 30 of the ignition coil 3, such that energy is accumulated in the ignition coil. When the transistor 5 is shut off, a high voltage pulse is formed over the secondary coil 31 of the ignition coil, which forms a spark over the gap 80 of the spark plug, whereby the energy accumulated in the ignition coil is discharged in the secondary circuit of the ignition system. According to the invention, a control circuit 6, 7, 14 has been added to the secondary side 31 of the ignition coil, for controllability of the duration and current intensity of the spark. After a spark having been established over the spark plug, this control circuit supplies energy to the secondary circuit. By connecting and disconnecting a voltage source 14 in series with the ignition coil 3, during the burning time of the spark and by aid of the transistor 6, the current intensity of the spark as well as its duration can be controlled. The voltage source 14 should be of the same magnitude as the sum of the voltage drop over the spark plug and the resistive voltage drop in the ignition coil. If the voltage drop over the spark plug is assumed to be about 60-80 V and the resistive voltage drop over the ignition coil some tens of Volts, the control voltage can suitably be about 100 V, although it may be varied for example in the range of 50-150 V. If the control voltage is lower than the just mentioned voltage drop, the supplied energy can only be used to extend the duration of the spark current, not to increase the spark current above the maximum value generated by the ignition coil. If the control voltage is higher than the just mentioned voltage drop, the spark current can be increased too. By connecting and disconnecting this voltage source 14, during the burning time of the spark and by aid of a suitable control method, the shape of the spark current curve can be controlled as a function of time in order to achieve a desired shape of the curve (see
To achieve a short and intense spark with a low voltage drop, the ignition coil 3 has a very low inductance on its secondary side 31 (typically 1-100 mH), as compared to a standard ignition coil (typically 1-100 H). A low inductance also means few turns in the coil, whereby a more coarse wire can be used which also gives a low inner resistance in the coil (typically 1-10 Ohm), instead of a standard resistance in the magnitude of kOhm. If about the same amount of energy is accumulated in this ignition coil 3 according to the invention, as in a standard ignition coil, the maximum current delivered by the coil 3 will be 1-2 magnitudes larger than the current from a standard ignition coil, which means a maximum current in the magnitude of 1 A or more, as compared to the 10-200 mA delivered by a standard ignition coil. This means that for the major part of the time, the spark plug 8 will be in Arc Discharge Mode, with a typical voltage drop of 50-100 V over the spark plug, which differs from a standard ignition system in which the spark plug operates in Glow Discharge Mode for the major part of the time, with a typical voltage drop over the spark plug in the magnitude of 500-1000 V. The time that the ignition coil 3 according to the invention can deliver a spark in Arc Discharge Mode depends on the design of the ignition coil and the amount of energy accumulated in the ignition coil 3, but typically the time is some hundreds of microseconds.
The turns ratio between primary coil and secondary coil in the ignition coil 3 is suitably about 1:20, but can be varied for example between 1:8 and 1:30, depending on the current/voltage ratio desired over the first transistor 5 and the maximum voltage required over a spark plug 8 to which the ignition system is connected.
In order to be able to accumulate a suitable amount of energy, e.g. 20-100 mJ, in an ignition coil 3 with an inductance that is so much lower than usual, a switch transistor 5, e.g. a IGBT, is suitably used in the primary circuit. The switch transistor 5 has a relatively high current and voltage threshold failure levels compared to a conventional inductive ignition system. A typical voltage threshold failure level of this transistor 5 can be 1700-2500 V, but voltages in the range of 1000 V-5000 V can also be used. Depending on the spark current requirements, the transistor should be able to handle a primary current in the magnitude of 10-200 A. The higher the current, the lower is the voltage threshold failure level.
Suitably, the second transistor 6 has a voltage threshold failure level of 150-400 V. The current rating for this transistor 6 depends on the current intensity in the desired spark, but is normally in a range of 1-5 A.
A control unit 4 is also shown, which among other things comprises a logic unit 41 for time control, in order to achieve optimal control timing for the different units, and preferably also a regulating member 42 for current regulation in the primary and secondary circuits. Moreover, it comprises drive units for regulating elements in the regulating circuit. Accordingly, the control circuit controls the ignition current, the duration of the spark and the triggering time for various control variables.
During the time that a spark exists in the gap 80, either one of the second 6 and third 12 transistors will be active, depending on if it is desired to supply more energy to the spark or not. At the end of the spark formation, these two transistors 6, 12 will be shut off and then a spark current will charge the capacitor 16. After the completion of the spark, this capacitor 16 will give a positive voltage over the spark plug 8, and thereby the generated low current in the spark plug can be used for ion current measuring.
Accordingly, it is shown that the circuit A comprises a diode A1 enabling fast charging of the capacitor 16 at the end of the spark current, as well as a resistor A2 that together with the diode A1 prevents fast discharge of the capacitor 16 during the duration of the spark. The just mentioned resistor A2 is also used in order for the ion current to be able to pass without a large voltage drop.
The circuit B is used to measure the ion current and to augment it to a useable measuring signal that can be used for control and monitoring of the combustion engine. The capacitor 16 is the voltage source driving the ion current, and the Zener-diode B1 in parallel with the capacitor 16 is used in connection with the charging of the capacitor 16 in order to determine the voltage value of the capacitor. The circuit also includes a measuring resistance B2 that the ion current passes, augmented by an associated augmenter B3. The inlet terminal of the augmenter is protected by an additional protection diode B4 that limits the voltage over the measuring resistance B2 at charge of the capacitor 16 and at discharge of the capacitor 16 during the duration of the spark.
Specifically,
Accordingly, it is clear that by aid of the invention different types of spark currents can be achieved in varied ways, allowing for adaptations to any type of situation that is desired.
All ignition devices according to the invention can be controlled by aid of a control signal in order to control the point of time of the spark, another control signal determining the duration of the spark and a third control signal that determines its current intensity. As an alternative, a control signal can be used that orders any one of a number of pre-programmed shapes of the spark curve. It is also conceivable that these control signals are combined to a single control signal that by aid e.g. of a suitable pulse code determines both time point and current intensity, as well as duration of the spark.
The invention is not limited to that described above, but can be varied within the scope of the claims. Accordingly, it is realised, among other things, that the control principles of the invention can be used also to control the spark current in a conventional ignition system, mainly operating in Glow Discharge Mode with a higher voltage drop over the spark plug. Part of the benefits is however lost in that case, since the duration of the spark is often adequately long and there normally is no greater need of further extending its duration. Moreover, an energy source of relatively high voltage output, in the magnitude of 500-1000 V, is needed in this case in order to beneficially affect the shape of the spark current curve, which means that implementation becomes more costly.
It is also realised that the invention not necessarily has to be used in connection with each spark plug, but that the invention also may be arranged in a central unit that is connected by ignition cables to the various spark plugs of the engine.
It is realised that the concept of ignition system is not to be interpreted with limitations. This is especially the case since the person skilled in the art knows that different parts of the ignition system can be supplied as modules. By the concept of ignition system according to the claims it is accordingly understood that at least one or some essential components are included to work the invention.
IGBT stands for Insulated Gate Bipolar Transistor.
MOSFET is the English denotation of a field effect transistor having a metal oxide drive.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0401629-1 | Jun 2004 | SE | national |
