This invention relates to an ignition system and more particularly to an ignition system for an internal combustion engine. The invention also relates to an alternative spark-plug, a drive circuit for a spark-plug and associated methods.
It is known that an ignition system for a vehicle comprises a plurality of distributed spark-plugs connected by respective high voltage power cables to a remote and central high voltage generation means. In a known capacitor discharge ignition system, the high voltage generation means comprises a capacitor connected with a power switching device, such as an SCR switch, in series with a primary winding of a transformer. A secondary winding is connected to the high voltage cables. In use, when a piston of the engine reaches a predetermined position, the power switching device is switched to the closed state. Energy in the capacitor is then transferred to the primary winding resulting in a much higher voltage on the secondary, because of the secondary to primary winding ratio. Once the voltage on the secondary reaches the breakdown voltage of a spark-gap between spark electrodes of the plug, a plasma discharge is created between the spark electrodes.
In the known systems, the switching circuit restricts the minimum inductance of the transformer that can be used. The restricting factors are the maximum current rating of the switch, Im, the switching speed of the switch ts, the switching voltage of the switch, Vs, and the cost of the switch. These limitations result in a very high secondary winding inductance, which has several drawbacks including cost. The large inductance normally requires kilometres (ten thousands of windings) of thin copper wire, which is expensive. The systems are inefficient in that the kilometres of thin copper wire have a resistance of a few kilo-ohms. To transfer enough energy for a reliable spark, a large amount of extra energy is required for each spark. Due to the large amount of energy that must be handled as well as the large amount of copper needed, the systems are bulky. The energy loss due to the copper resistance, heats the transformer. This places a severe limit on the maximum amount of energy that can be transferred to the spark and also affects the placement of the transformer for cooling. The fuel efficiency, completeness of combustion, combustion time, exhaust cleanliness and variability in cycle-to-cycle combustion are limited. Because the transformer is large and heats up, it is normally positioned a distance away from the engine. This requires high voltage cables between spark-plugs and the transformer. These high voltage cables generate a large amount of electromagnetic radiation, which may influence other electronic equipment. In order to eliminate the high voltage cables, coil-on-plug systems which comprise an ignition coil at each spark-plug are used. Because these coils are very close to the engine, normally with very little air flow around them, they overheat easily, which makes them unreliable.
Some ignition coils having a very low secondary resistance have been suggested. This is accomplished by using a magnetic path having a high permeability, to reduce the number of windings while keeping the inductance high enough for the switching circuit. The disadvantage of this approach is that the high permeability magnetic material saturates easily and that a large core is therefore required.
Some other ignition systems have a second energy transfer path on the secondary side. They all have the disadvantage that the energy must either go through the secondary winding or through a semiconductor device. If the energy goes through the secondary winding, the transfer is very inefficient due to the high winding resistance. On the other hand, the semiconductor device must be a high voltage (normally above 30 kV), high current (normally above 1 A) device. These devices are expensive and also result in energy loss.
Another disadvantage of all these systems is that the self-resonance frequency of the secondary winding is low (typically less than 20 kHz). The low self-resonance frequency is due to the long length of secondary wire and the large secondary winding inductance. When the secondary winding is connected in a secondary side circuit, the resonance frequency of the secondary side circuit is even lower than the self-resonance frequency of the secondary winding, due to the spark-plug and cable capacitance. Because of the low secondary resonance frequency, it takes some tens of microseconds to charge the spark-plug or electrode capacitance to a breakdown voltage and also some tens of microseconds to dissipate the remaining secondary energy. This limits the number of successive pulses that can be generated in multiple spark ignition systems, which limits the amount of energy that can be delivered during ignition. The efficiency and amount of energy transferred in some ignition systems are increased by placing a capacitor in parallel with the spark-plug. In these systems the secondary resonance frequency will be even lower. Even in systems where an optimal spark time is calculated (as discussed below), the spark cannot be controlled to within a few tens of microseconds. At 6000 rpm, this inaccuracy is larger than one degree in engine rotation.
It is a known technique to use the spark-plug to measure the current in or resistance of the ionized gas after ignition to gain information about the gas temperature, pressure or composition after combustion. This information is then used as one of the inputs to an engine management system to calculate an average optimal spark time. Because of the high loss of the ignition transformer, the measurement must be done on the secondary side of the transformer, which makes the secondary side circuit complex.
Due to cycle-to-cycle variations, the average optimal spark time can be quite different from the optimal spark time for a single cycle. Although there are a number of techniques available to measure the conditions inside the combustion chamber before ignition, none of them are widely used because they all require extra access points to the combustion chamber, are expensive, most have low reliability and are complex.
When using the spark-plug for measurements, the low secondary resonance frequency therefore limits the measuring frequency after ignition and also makes it very difficult, if not impossible, to measure gas properties before ignition.