# How does the ignition system control the Energy of the spark?

An automotive ignition system needs to provide a specific amount of energy to properly ignite the air/fuel mixture in the combustion chamber. Energy = voltage * amperage * duration.

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: