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An ignition system (10) comprises a spark plug (12) having a first end (14) defining a spark gap (16) between a first electrode (18) and a second electrode (20). A transformer (46) comprises a primary winding 44 and a secondary winding (50) also forms part of the system. The secondary winding is con

An ignition system (10) comprises a spark plug (12) having a first end (14) defining a spark gap (16) between a first electrode (18) and a second electrode (20). A transformer (46) comprises a primary winding 44 and a secondary winding (50) also forms part of the system. The secondary winding is connected in a secondary circuit to the first electrode 18 and the secondary winding has a resistance of less than 1KΩ and an inductance of less than 0.25 H. A drive circuit (26) is connected to the primary winding.

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1. A method of monitoring at least one parameter associated with a gaseous substance in a chamber, the method comprising the steps of: utilizing a first electrode and a second electrode, at least one of which is exposed to the substance and which collectively define a gap and form an electrode capac

1. A method of monitoring at least one parameter associated with a gaseous substance in a chamber, the method comprising the steps of: utilizing a first electrode and a second electrode, at least one of which is exposed to the substance and which collectively define a gap and form an electrode capacitor, to generate a corona at the at least one electrode without creating a spark over the gap;causing the corona to change an electrical parameter in a region of the at least one electrode which is indicative of the at least one gas parameter;causing a signal relating to the electrical parameter to be sensed by electronic circuitry connected to the electrodes; andmeasuring the signal sensed by the circuitry, to monitor the at least one gas parameter. 2. A method as claimed in claim 1, wherein the electrodes form part of a spark-plug configured such that energy stored in the electrode capacitor at a corona discharge threshold at any of the electrodes is substantially less than the energy required to create a spark over the gap; and comprising the step of driving the electrodes with a signal to generate said corona, or, to generate said corona before forming a spark over the gap. 3. A method as claimed in claim 2, wherein the signal is a fast rise-time voltage signal, which is one of an edge of a single voltage pulse and an edge of a continuous wave. 4. A method as claimed in claim 3, wherein the rise time of the fast rise-time voltage is high enough to generate a positive or negative corona at one or both of the electrodes. 5. A method as claimed in claim 4, wherein the rise-time is faster than 100 kV/μs. 6. A method as claimed in claim 2, wherein an amplitude of the signal is one of smaller than, equal to and larger than a positive or negative corona threshold voltage of the substance in a region of the spark-gap. 7. A method as claimed in claim 6, wherein the amplitude of the voltage signal is one of smaller than, equal to and larger than a breakdown voltage for the spark-gap. 8. A method as claimed in claim 1, wherein the signal is fed back to a primary side of a transformer, a secondary winding of which is connected to at least one of the electrodes and wherein the measurement is done on the primary side. 9. A method as claimed in claim 1, wherein the gas parameter is monitored before and/or during and/or after ignition of the substance. 10. A method as claimed in claim 1, wherein the gas parameter is used to determine at least one of the timing of and energy in a spark over the gap. 11. A method as claimed in claim 1, wherein the gas parameter is any one or more of pressure in the chamber, composition of the substance and position of a piston moving in the chamber. 12. A method as claimed in claim 2, comprising the step of varying an output power level of a drive circuit for the electrodes between a first lower level suitable to generate said corona for the measurements, and a second higher level to form the spark and to transfer energy for ignition. 13. A method as claimed in claim 12, wherein the second power level is dependent on results of the measurements.