Do you know how the spark plugs in an electronic ignition system works?
Are you sure you do?
Recently, while texting with my friend Roger, he and I both realized that, despite both of us having owned cars with electronic ignition, we didn’t really know how spark plugs works, at least not beyond the broadest strokes of “Ignition make spark. Spark make fire. Fire make engine go.” We decided to dig deeper and find out.
Electromechanical Ignition Systems
In the dark ages before electronics became cheap, ubiquitous, and ruined our attention spans while tearing our social fabric asunder, ignition systems were electromechanical, and at their heart was the distributor. These electromechanical devices are responsible for helping the ignition coil generate sparks and for directing the sparks to the appropriate cylinders.
Since not everyone reading this is going to be familiar with how distributor-based ignition systems work, let’s briefly go through it. As you probably know, the fuel-air mixture that enters the cylinders of a gasoline engine is ignited by high-voltage sparks jumping across the electrodes on the ends of the spark plugs that protrude into the cylinders. The high voltage necessary for those sparks is generated by the ignition coil, a small transformer that turns the 12 volts from the car’s battery into many thousands of volts.
Since ignition timing is vitally important to the efficient operation of an engine, the system needs something that will trigger the ignition coil to generate sparks at just the right moment in the engine’s cycle. That’s where the distributor comes in.
The distributor is directly coupled to the rotation of the engine, and as it itself rotates, it opens and closes a switch commonly referred to as “points.” Each time the points close, 12 volts from the battery flows into the primary winding of the ignition coil, creating a magnetic field in the core of the coil. As the distributor continues rotating, the points open, breaking the coil’s connection to the battery, and causing that magnetic field to collapse. As the field collapses, it creates a voltage spike across the secondary winding of the coil.
At the same time, the distributor’s rotor — a rotating electrical contact — is connecting the high voltage output of the coil to the spark plug of the cylinder that is ready for combustion.
As the field collapses, it creates a voltage spike across the secondary winding of the coil. At the same time, the distributor’s rotor connects the high-voltage output of the coil to the spark plug of the cylinder that is ready for combustion. Here’s a handy gif that shows all of it in operation:
Electronic Ignition Systems
With that knowledge of distributor-based systems fresh in our minds, let’s consider electronic ignition systems again and note the differences between them and their predecessors. One obvious difference is that there is no distributor — its functions have been taken over by the ECU, which keeps ignition timing matched to engine rotation by way of a crank position sensor. Well, early electronic ignition systems did have distributors, but by the 1990s, when coil-on-plug-based systems came out, then distributors went away.
Another difference is that many of these modern systems have a coil on each spark plug rather than a single, central coil providing the high voltage for all the spark plugs. And that takes us back to the discussion I was having with Roger. How does one of these work?
On an older-style ignition coil, you are likely to find three terminals: positive, ground, and high voltage. It’s simple enough: 12 volts from the battery goes to the positive germinal, the negative terminal is connected to ground, and the high voltage terminal is connected to the distributor cap.
On the other hand, modern ignition coils have four or even five terminals, and looking at one of these coils doesn’t reveal much other than which terminal is high voltage (it’s the one that attaches to the spark plug).
By looking around online, I found that the other terminals comprise a 12-volt connection from the battery, a connection to ground, and one or two other mysterious terminals labeled IGT and sometimes IGF.
It turns out they stand for IGnition Trigger and IGnition Feedback, and that they are connected directly to the ECU. The IGT terminal receives a signal from the ECU telling the coil when to spark. The IGF terminal, which not all coils have, passes a signal back to the ECU to tell it whether it successfully fired off a spark. That’s how an ECU can sense a misfire on a cylinder. So, what’s actually going on inside of one of these kinds of coils? Unfortunately, their internal components are encased in epoxy, protecting them from vibration and moisture, but also making it difficult to examine them. Helpfully though, someone created this schematic of an ignition coil from an LS1 that we can look at for clues.
The specific components used in electronic ignition coils vary from manufacturer to manufacturer, but the important thing to note here is the piece of the circuit labeled “igniter.” That bit of circuitry is an electronic switch and takes the place of the contacts in a distributor. Instead of connecting the coil’s primary winding to 12 volts as a distributor rotates, the circuitry connects the primary winding to 12 volts when it receives a 5-volt signal from the ECU. That was an interesting bit of information because it meant it would be pretty simple to control one of these ignition coils without an ECU.
Look Ma, No ECU
A couple of weeks after the text conversation I had with Roger, my dad asked for my assistance changing the spark plugs on my mom’s Toyota minivan, so I stopped by their house to help out and found out he was also replacing the van’s ignition coils. I ended up taking one of the old coils home with me. I texted Roger that I now had a coil we could experiment with, so he set about building a board that would let us wire an Arduino (an easy-to-program microcontroller) to the coil, and I designed and 3D printed a connector that plugs into the coil so we wouldn’t have to go to the junkyard to pull one out of another Toyota.
The next day, we met up and married my coil to his Arduino board, and when we connected everything to power, we got the result we wanted. The ignition coil was making sparks, about 8 per second.
It was a good first experiment, but we wanted to test the limits of the coil. We modified the board and the code running on the Arduino and we pretty soon had a setup that let us take the coil up to the red line of a Toyota Sienna — 7,000 rpm. In a four-stroke engine, like the one in the Sienna, a spark is only needed for ignition every other rotation, so a coil-on-plug coil on an engine spinning at 7,000 rpm will need to make 3,500 sparks per minute or about 58 sparks per second. A knob that we wired to the Arduino board let us change the frequency of those parks, right up to what would be the red line, and the coil handled it without protest.
A Spark Plug Symphony
We didn’t want to stop there. I had mentioned to Roger that if we could trigger the coil at a high enough frequency, we would be able to generate musical notes. Neither of us knew if that was going to be possible though. For reference, middle C on a piano is about 261 hertz, meaning the coil would have to be triggered 261 times a second to generate that note, and that would be almost five times the Sienna’s redline. Would the coil be able to handle that kind of abuse? There was only one way to find out, so Roger rewrote his code, and here is the result:
Final Thoughts
Ok, so that was fun, but is any of this useful for anything? A musical ignition coil might not be, but the idea of using an Arduino to control ignition coils has merit, and we were not the first to think of this. In fact, since 2013, Speeduino, an open-source ECU system that uses an Arduino as its core, has been under ongoing development. I’ve never used one, so I can’t speak to how well they work, but it looks like a very interesting project. In any case, knowledge is power, and hopefully, you know a little bit more about electronic ignition systems than you used to. If you’re interested in reading more, I recommend stopping by Roger’s blog, where he has dedicated several posts to this ignition coil, the driver board he built for it, and the code he’s running on it.
Awesome article!