VIDEO: How Much Energy Does It Take To Fire A Coil?
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VIDEO: How Much Energy Does It Take To Fire A Coil?

The condition of a spark plug can tell you a lot. This video is sponsored by Standard Motor Products.

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CC: When analyzing the performance of an ignition coil, some technicians will use a scope to analyze the firing event.  A secondary ignition waveform is not just about the coil, it is also about the resistance of the air and fuel mixture inside the combustion chamber, as well as the cylinder pressure.

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The firing line is the amount of voltage required to ionize the plug gap. Lean mixtures and higher cylinder pressures will increase the required voltage

Once the gap is ionized, the burn line is where spark is occurring across the plug.

It is important to look closely at the burn line. The “hashy” look is the changing resistance across the plug gap as the spark moves through the layers of fuel and air.

Now, imagine the air and fuel inside the combustion chamber as ohm resistors between the spark plug electrodes. If you increase the distance between the electrodes, you are increasing the amount of air between the electrodes and the value of the resistor between them. As cylinder pressure increases and fuel mixture changes, the amount of energy required to fire the plugs also increases. The resistance also changes because the amount of fuel changes the resistance.

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Less fuel means higher resistance between the two electrodes of the spark plug. In the waveform, you can see this as the spikes change in height. Also, the burn line might be higher and shorter. But the sign that there is a lean cylinder is the spike on the end of the burn line. In some cases where the fuel injector is making the cylinder run lean, the two peaks might be the same height when the throttle is snapped or the engine is put under load. This is the leftover energy in the coil because the resistance in the cylinder was too great for the coil to fully fire.

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The shape of the waveform will not only be influenced by the air fuel mixture, it will also be influenced by the condition of the spark plugs. This is why it is critical to look at the condition of the spark plugs as part of any coil diagnostic procedure.  It is also possible on many COP systems to analyze the secondary ignition waveform on all cylinders so the technician can compare the pattern on various cylinders and hopefully pick the faulty one out of the good ones. 

To summarize:  You can have a strong ignition coil such as this one from Blue Streak Import Ignition Coils, but still have a weak looking pattern depending on the air/fuel mixture and in-cylinder pressures.  It is important to understand how all these things come together to produce a strong combustion event.

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This video is sponsored by Standard Motor Products

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