mr brightside
New member
After the recent and interesting thread about the benefits of servicing and renewing your ignition components I did some research into the fundamentals of ignition and spark formation, which was confusing at times and left me slightly puzzled as to how to apply the information. However, if the book turns out to be a bit too long winded I could always use its substantial weight to corner balance my car.
Spark formation occurs in three stages, the first of which is the breakdown phase where ionizing streamers bridge the electrode gap and create a plasma channel, this stage is characterised by very high voltage and current values and a very short duration time of 10nS. A shock or blast wave propagates outwards at this point into which some 30% of the breakdown phase energy is applied, however most of this energy is recuperated due to the blast waves transferring most of their energy into a 2mm diameter cross sectional sphere into which the plasma soon expands. This expansion initiates the arc phase when the thin cylindrical plasma strand expands rapidly due to heat conduction and diffusion, and the exothermic effect of the fuel beginning to burn. This is followed by the glow discharge phase when the coil will dump all of its remaining energy into the discharge circuit. There are significant voltage drops in both the arc and glow discharge stages due to heat losses to the spark plug electrodes, 50 and 70 percent respectively. The breakdown phase has the highest power level (1MW), but the energy supplied is small (0.3-1 mJ). The glow discharge has the lowest power level (10W), but the highest energy (30-100mJ) due to its long duration time, and the arc phase lies in between. The arc and glow discharge phases last approximately 4-5 times longer that the breakdown phase.
Kernel temperatures inside the plasma cylinder are far too high to allow combustion material to exist; 60,000K for the breakdown phase, 6000K and 3000K respectively for arc and glow discharge phases. So combustion will only take place around the outer surface of the plasma cylinder, and it's worth noting that increasing the energy input will not produce higher temperatures but larger plasma cylinders during the breakdown and arc stages.
At 20us after discharge is initiated there must be sufficient progress of ignition so as to be self-sustaining, thus the characteristics of the breakdown phase have the biggest impact on good combustion. Energy inputs at this stage are far less likely to be lost as heat and contribute far more towards flame propagation, whilst inputs in the latter two stages are most likely to be lost to heat. The minimum amount of energy required to properly ignite the mixture increases rapidly as the mixture leans out due to improper flame propagation.
In conclusion; the worse the condition of conducting components (rotor arm, dizzy cap, leads and plugs) is, then the more the breakdown phase will be starved of energy resulting in the slower growth of a smaller plasma cylinder leading to poorer flame propagation and less power. Also, the leaner the mixture then the larger the plasma cylinder must grow before successful inflammation will occur and this point is likely to be outside the 20us limit. Obviously ignition timing is absolutely critical. Ignition energy supplied after the breakdown phase has only a modest effect on flame propagation, ergo the timing and quality of the breakdown phase is proportional to good combustion and affected greatly by component condition.
I would welcome any comments/elaborations on my conclusions or findings presented, this is about the best I can do with such complicated material.
Spark formation occurs in three stages, the first of which is the breakdown phase where ionizing streamers bridge the electrode gap and create a plasma channel, this stage is characterised by very high voltage and current values and a very short duration time of 10nS. A shock or blast wave propagates outwards at this point into which some 30% of the breakdown phase energy is applied, however most of this energy is recuperated due to the blast waves transferring most of their energy into a 2mm diameter cross sectional sphere into which the plasma soon expands. This expansion initiates the arc phase when the thin cylindrical plasma strand expands rapidly due to heat conduction and diffusion, and the exothermic effect of the fuel beginning to burn. This is followed by the glow discharge phase when the coil will dump all of its remaining energy into the discharge circuit. There are significant voltage drops in both the arc and glow discharge stages due to heat losses to the spark plug electrodes, 50 and 70 percent respectively. The breakdown phase has the highest power level (1MW), but the energy supplied is small (0.3-1 mJ). The glow discharge has the lowest power level (10W), but the highest energy (30-100mJ) due to its long duration time, and the arc phase lies in between. The arc and glow discharge phases last approximately 4-5 times longer that the breakdown phase.
Kernel temperatures inside the plasma cylinder are far too high to allow combustion material to exist; 60,000K for the breakdown phase, 6000K and 3000K respectively for arc and glow discharge phases. So combustion will only take place around the outer surface of the plasma cylinder, and it's worth noting that increasing the energy input will not produce higher temperatures but larger plasma cylinders during the breakdown and arc stages.
At 20us after discharge is initiated there must be sufficient progress of ignition so as to be self-sustaining, thus the characteristics of the breakdown phase have the biggest impact on good combustion. Energy inputs at this stage are far less likely to be lost as heat and contribute far more towards flame propagation, whilst inputs in the latter two stages are most likely to be lost to heat. The minimum amount of energy required to properly ignite the mixture increases rapidly as the mixture leans out due to improper flame propagation.
In conclusion; the worse the condition of conducting components (rotor arm, dizzy cap, leads and plugs) is, then the more the breakdown phase will be starved of energy resulting in the slower growth of a smaller plasma cylinder leading to poorer flame propagation and less power. Also, the leaner the mixture then the larger the plasma cylinder must grow before successful inflammation will occur and this point is likely to be outside the 20us limit. Obviously ignition timing is absolutely critical. Ignition energy supplied after the breakdown phase has only a modest effect on flame propagation, ergo the timing and quality of the breakdown phase is proportional to good combustion and affected greatly by component condition.
I would welcome any comments/elaborations on my conclusions or findings presented, this is about the best I can do with such complicated material.
]
