Spark Plug/Ignition Coil: DC Pulse

Question

If you have the following circuit with a step up voltage from L1 to L2, then when do you get a spark across the spark plug? I have three scenarios.

Scenario 1. You have the switch open, then you close it right away you get a spark.

Scenario 2. You have the switch open, then you close the switch. This allows the magnetic lines of flux to expand. A current then is induced for L2. Once a little time passes you reopen the switch, and then you get a spark.

Scenario 3. Neither of these, I am dead wrong.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Scenario #2 is the correct one.

Without some type of voltage step up a 12V battery can not create a high enough change in current to create a spark. If instead current is set up through the coil and then the path is broken the change in current is orders of magnitude higher.

To make this circuit practical the addition of a capacitor across the switch is needed to make the switch break clean. This would work like a condenser in a point and breaker automotive ignition.

Answer 2

Just to add - scenario 2 is correct, but feel free to spam the button. That is, feel free to repeatedly send pulses to generate a spark continuously while the engine is in that part of the cycle.

The only reason breaker points and reluctors didn't do that... is that they couldn't. They were unable to mechanically. This was bad, because if the first spark didn't cause ignition, you had to wait til the second spark, which was often too late for effective combustion.

But go back even further, to the Model T's and hit-and-miss engines, and you find the amazing buzzer coil. The coil itself had an NC relay contact at the top if it, so the coil saturating interrupted the coil (spark) and as the field collapsed the NC contact re-closed, repeatedly, having that same "spam of sparks" effect. It guaranteed ignition even in extreme conditions such as hand-cranking speed.

Answer 3

Neither scenarios is correct! When the switch closes, the current in the primary coil increases at a relatively slow rate due to a ballast resistor (not shown in you circuit). This energizes the coil. When the switch opens, the current in the coil tend to keep flowing (similar to mechanical inertia) and would then produce ionization which allows the current to flow and produce a spark across the switch while opening. To prevent this, a capacitor (condenser) is placed across the switch thus allowing the current to bypass the opening switch and charge the capacitor. If the cap were not there, the energy in the coil would all be dissipated in the gap across the switch and no energy would be transferred to the secondary. The presence of the capacitance (with the appropriate value) allows to current to quickly reduce to zero in the primary. This abrupt change in primary produces a voltage in the secondary (V=L*di/dt) where L is the inductance of the secondary coil. This voltage is large enough to ionize the spark plug gap and a current flows until the stored coil energy is dissipated. Then the process starts over. The spark occurs when the switch (known as points in automotive terms) is immediately opened. This was developed by Kettering and now bears his name in automotive history.

Answer 4

I have observed another scenario: the switch stays closed until the spark is desired and then opens. Thus, the coil normally has current going through it. The switch being closed creates the field in the coil. Opening the switch collapses the field and discharges through the spark plug gap.

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However, the concept of a mechanical opening and closing of a switch is very "last century". The concepts of dwell (pulsewidth) and advance introduce variables which CDI igniters work to solve. These CDI units utilize various schemas of input from the engine via inductive or Hall effect "pick up coils".

Depending on the budget or how clever the engineers are, the input can be from one pulse per cycle (or camshaft rotation on 4 stroke), once per engine rotation, or a multi toothed setup based on either cam or crankshaft rotation. Or even a combination of sensors on both camshaft and crankshaft rotation. The racing homologation special production Ducati 996 is example of the last option. While the road-going model only uses the camshaft pickup, there is also a crankshaft pickup with 4 triggers, which allegedly allow the ECU to discern mid-cycle acceleration.