Of the many different aspects of car crafting, ignition-system function is often the most misunderstood. Since it deals with an intangible-electricity-it is very easy to assume one thing is happening while another actually is. But don't worry. Sit back and let Car Craft show you how to dazzle your buddies with ignition-system knowledge.
Scope It To MeIf ignition systems are hard to understand, it's probably because electricity flow cannot be seen. The common voltmeter doesn't do much good as it only assigns a value to the electrical "pressure" in a circuit. To truly understand an ignition system and how the spark plug fires, an oscilloscope (or scope) is required. This device displays an electrical picture of the ignition event as a function of voltage and time.
The scope connects to the coil's primary circuit (the negative post of the coil), the coil wire for secondary output, and the No. 1 spark-plug wire as a synchronization signal. In addition, most automotive scopes require a 12-volt positive and negative connection.
A scope can be thought of as a high-speed visual voltmeter that shows not only the intensity of the voltage, but when the signal occurs. Instead of a bouncing voltmeter needle, the height of the scope pattern indicates the intensity of the voltage. The horizontal axis of a scope indicates the time period in milliseconds (ms), or one thousandths of a second.
With a scope, we can not only see how many volts were required to light the spark plug, but how long it stayed lit, which is very important in diagnosing performance problems with the ignition, engine, and fuel system.
How A Spark Plug FiresOn any ignition system that uses a single coil there are five distinct components of a scope pattern. Refer to the accompanying illustration (below) while reading this explanation to keep it all clear.
The secondary spike (1) is read in kilovolts (kV) and is the energy required to overcome all secondary resistance and bridge the gap of the spark plug under compression. It is impacted by many factors, including the condition of the secondary wires, cap, and rotor; the gap of the spark plug; the cylinder pressure in the bore; and the air/fuel ratio. The proper name for the secondary spike is the ionization voltage. It normally takes 5 to 10 kV (5,000 to 10,000 volts) to get the arc going at idle in a typical engine.
More energy is always required to light the spark plug than to maintain the arc, in much the same manner that it is harder to get a car rolling from a stop than to keep it moving. Section (2) is identified as the firing line. It represents the amount of energy removed from the coil to keep the spark plug arcing and also shows how long the spark plug remained lit in milliseconds. A good electronic ignition system should have the capability to keep the plug burning at idle for at least 1.5 ms. When designing an ignition system, engineers do not reference this reading but instead look to maintain an arc for 20 degrees of crankshaft rotation.
The firing line is important since it would reveal a break in an ignition wire that is smaller than the gap of the spark plug. If a plug wire has a 0.035-inch break and the spark plug gap is 0.045 inch, the ionization voltage may not be impacted but the firing line would be. A normal firing-line height would be between 0.9 and 1.2 kV. The shape of the firing line indicates the air/fuel ratio of the cylinder. Ideally the line should remain flat. One that tilts upward shows a lean mixture and the additional energy required to keep the plug arcing. A firing line that tilts down is the result of an overly rich mixture.