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:

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1 Answer 1


Most electronic ignition systems are INDUCTIVE on cars modern vehicles. The energy available for the spark is stored in the magnetic field of the coil. This energy is released when the coil current is broken. The old mechanical breaker points have been replaced with a solid state switch. The energy is proportional to current squared before saturation. So up to a point, more current means more spark energy. This primary coil current takes time to build up. The build up is exponential limiting out due to the finite DC resistance of the primary coil. So up to a point more time means more primary current which means more spark energy. Dwell time can control spark energy.

On a capacitive discharge ignition or CDI system the spark energy is stored in a capacitor instead of the coil. Energy stored in a capacitor is proportional to voltage squared. The voltage that the cap is charged to before being dumped can be and is varied to change spark energy.

1970s bad reliability and crossfire and poor lean mixture performance are the main reasons that you don't see CDI on cars. Its great for firing oily plugs so its popular on 2 Strokes.

  • I actually think I understand what you are saying.
    – Ppoggio
    Mar 24, 2016 at 6:34

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