The system of ignition is intended for ignition of fuel-air mix in each cylinder at a given time. In petrol engines it is reached at the expense of the electric spark (electric discharge) created between spark plug electrodes.
Reliable work of ignition in all range of operation of the engine is an essential factor for effective work of catalytic converter.
Ignition by a spark of a small cloud of fine fuel-air mix is enough to start burning process.
The spark plug determines length of a spark which also depends on type and a design of system of ignition, and also on conditions under which there is ignition.
For emergence of a spark tension between electrodes of a candle has to increase sharply from zero up to tension necessary for formation of an arch. After emergence of the spark category tension falls to level necessary for distribution of a spark, at the same time there is an ignition of fuel-air mix.
For obtaining energy of sparking the ignition coil working by the principle of the autotransformer and accumulating energy of ignition is used. When giving current on primary winding of the coil of ignition energy of current turns into energy of magnetic field until interruption in supply of current. Magnetic field at the same time sharply decreases, directing the high-tension current arriving on high-voltage wires on one spark plug in a secondary winding of the coil of ignition.
Between the initial moment of ignition of working mix and its full combustion there pass about 2 ms. Therefore formation of a spark has to happen earlier, than the piston will reach VMT that provides optimum combustion in all range of frequency of rotation of the engine. The moment of ignition is defined depending on the frequency of rotation of a bent shaft, loading, design features of the engine, quality of fuel and working conditions at the moment (start, idling, size of opening of a butterfly valve, etc.).
The spark plug serves for transfer of the high-tension current developed in the ignition coil to electrodes in the combustion chamber where this current makes the spark category for ignition of working mix.