A specific energy must be produced to overcome the quenching effects present in the environment of the combustion chamber and produce a viable flame kernel which will develop into a flame front that advances at a sufficient speed to provide efficient and timely combustion.
In the 2000 edition of "Auto Fundamentals", chapter 8, Ignition Systems, page 122 it notes that in older ignition systems a resistor is used to lower the input voltage
to around 9.5 volts during normal engine operation
Further on it says:
At high speeds, when a hotter spark is needed, the coil receives full battery voltage.
It then notes that:
The majority of modern electronic ignition systems use full battery voltage at all times.
This all brings up a number of questions. When the book talks about needing a
hotter spark at high speeds, it could be talking about either a spark with higher voltage, higher amperage or longer duration. It's my understanding that the duration of the spark is usually fixed at about 1/1000th of a second, so they must be talking about either voltage or amperage.
Now I know the voltage needed to jump the gap can in general vary depending on both the amount of compression and the composition of the A/F mixture with both leaner mixtures and higher compression requiring more voltage, all this based on the application of Paschen's law to determine the breakdown voltage. However, compression is pretty much a fixed value, and under normal operation, even at high speeds, the A/F ratio doesn't vary that much ( excluding wide open throttle or heavy load which are very rich and would require less voltage if I understand correctly ).
So when they talk about requiring a
hotter spark, I can only think that they mean a more energetic spark with higher amperage, which brings me to my question.
How does the ignition system in a modern vehicle built in the last 20 years control the output wattage of the spark ( voltage * amperage ) and what might effect the wattage negatively other than the ignition coil, wires and plugs?
Here is an example ignition system diagram from this book on Fuel Efficiency: