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Image courtesy Wikipedia
Your car’s ignition system controls the spark that ignites the air-fuel mixture in the combustion chamber. Ignition systems and their components have changed immensely over the years from magnetos and cardboard coils barely generating enough energy to create a spark to individual coil-on-plug models that deliver massive amounts of energy. Throughout the history of the gasoline engine, we have been able to control the timing and, to an extent, the energy of this spark manually. Now we have to plug in a laptop to do it. Let’s take a look at how automotive ignition system makeup and components have changed over the years.
Image courtesy MSD
Internal combustion engines can have their fuel-air mixtures ignited either by the heat caused by compression, as in diesel engines, or with a spark plug in the combustion chamber, like gasoline engines. Gasoline engines require four basic components to get this done:
• A way to ignite the air-fuel mixture
• Something to generate the spark required to ignite the mixture
• A way to send the spark to the right cylinder at the right time
• Something to transport the spark energy to where it’s needed
Image courtesy Wikimedia Commons
If you put the first spark plug, invented by Étienne Lenoir more than 150 years ago, next to a modern spark plug, you’ll see some minor changes in design and construction - but a spark plug looks like a spark plug no matter how old it is. You’ll see the electrode that attaches to the plug wire. You’ll see the threads that secure it in the cylinder head. You’ll also see the center electrode on the larger end with the gap between it and the outer electrode (plug gap) that creates the spark that makes the engine go vroom.
The above picture illustrates spark plugs through the years, including an early Étienne Lenoire spark plug on the far left.
Image courtesy Wikimedia Commons
Spark plugs have evolved from their crude origins of not being able to create a very big spark and generating huge amounts of electrical noise to achieve the abilities today’s specialty plug, which make huge sparks from multiple electrodes of exotic materials, generates no electrical interference and can last 100,000 miles. Also, instead of “yep, this one fits,” we now have to specify the engine temperature operating range or the engine may not run right because it’s too hot or cold for the plugs.
Above you can see a disassembled Champion spark plug, circa 1920s.
Image courtesy Model T Ford Pix
Early spark plugs generated electrical noise that would interfere with radio reception and be heard as a hiss. It was almost pointless to try using a radio with many of these plugs. Early alternators also induced a whine that rose and fell in pitch with engine speed. To eliminate these, we would install filters between the positive and negative leads on our stereos.
Image courtesy Holley
The interference early ignition systems generated was AC energy that could be filtered out with a filter network we bought or built using capacitors and inductors. Later, manufacturers developed resistor plugs with resistors inside that came close to eliminating most ignition noise.
1927 Roadster magneto image courtesy Flickr
The 12 volts DC on which the rest of your car runs isn’t enough to generate a spark without some help. The first cars used magnetos to generate the energy required to jump the plug gap. These could be driven by the crank or the cam directly or mounted as a belt-driven accessory. Lawnmowers and some aircraft use magnetos to this day, but they are electrically noisy, causing interference with radios and televisions and don’t generate enough energy for the engine to run efficiently. That said, most high level dragsters use magnetos.
Image courtesy Wikimedia Commons
Early ignition coils were made of cardboard or wood and could generate a “massive” 10,000 Volts. Now they’re made with high temperature plastics and can generate up to 100,000 Volts if an engine needs it. We’ve also evolved from using one weak coil, sending energy to a distributor and thence to the plugs, to one strong coil per cylinder, allowing the coils more “dwell time” to recharge between sparks so they can generate bigger, hotter sparks.
In between, we had GM’s HEI system that integrates the coil with the distributor cap, eliminating the need for the power-robbing wire between the coil and the cap. The HEI coil fires directly into the rotor’s electrode from atop the cap. It retains the wires to each cylinder from the cap, however.
Above coils:
Top Left: Accel Super Stock performance coil - courtesy Holley.
Top Right: Coil pack.
Bottom Left: GM HEI coil.
Bottom Right: Accel Supercoil Coil-on-plug – courtesy Holley.
Early spark plug wires resembled early home electrical wiring in that they consisted of an aluminum conductor covered by a fabric/ceramic/asbestos mixture to insulate against short circuits. These had ring terminals at either end that were bolted to the plugs and spark distribution system. If your car has plug wires at all today, they use rubber mixtures to increase isolation while reducing overall diameter, while the center conductor carrying the spark energy is a flexible fabric that can carry large amounts of energy in a small diameter.
Image courtesy Wikipedia
Once we have the ignition coil(s) and the plugs, we need a way to get the spark to move between the two. Since the beginning, we’ve used a distributor to do this. The coil feeds the spark through an electrode in the center of the cap and the electrodes around the cap’s circumference distribute the spark to each plug.
The image above shows a Toyota four cylinder distributor. The drive gear on the bottom meshes with a matching gear on the camshaft to rotate the distributor shaft.
Image courtesy Wikimedia Commons
The distributor houses a shaft, at the top of which is the rotor. It has an electrode that spans the gap between the coil electrode and the plug/cylinder electrodes on the rim of the cap as it rotates. The distributor shaft has a gear or slot drive at the bottom that is driven either by the camshaft, the crankshaft or a belt-driven gear depending on the year, make and version of the engine.
The above pictures shows the underside of a six cylinder distributor cap. In the background you can see the rotor and the electronic ignition module.
Distributors are points of failure and using a single coil didn’t give it time to fully charge in high RPM applications. GM came out with the Distributor-less Ignition System (DIS) in the mid-80s. This replaced the single coil and the distributor with packs of coils (coil packs) feeding two cylinders per coil pack. This gave the coils more time to recharge, thus allowing them to create more energy when called upon to spark.
Image courtesy Wikimedia Commons
Another reason to get rid of distributors is that they’re a maintenance item that requires constant attention. The cap and rotor need to be looked at every now and then to make sure they aren’t developing corrosion. The points (more on these later) need adjustment and/or replacement. Electronic ignition modules fail. The drive mechanism at the bottom of the distributor shaft can wear and cause erratic rotation of the rotor, which can cause the timing to bounce around too much (timing will always fluctuate 5-10 RPM), affecting performance and possibly causing damage.
Image courtesy Summit Racing
Originally, a coil sparked twice per revolution. Engineers halved that to once per revolution and the coil-on-plug/cap was born. Now each plug has its own coil directly connected to it, allowing for maximum transfer of spark energy from the coil to the plug and maximum coil dwell time. This helps generate maximum power per revolution.
Image courtesy Wikimedia Commons
An ignition coil has charge and discharge cycles, with the discharge cycles generating the spark. As the coil charges, an electrical field builds in it, like a battery. In order to cause that field to collapse, we need to interrupt the charging current. This interruption was handled with breaker points until the mid-’70s, when electronic ignition took over.
The points were mounted under the rotor to the base of the distributor cup, with a pivoting arm and cam nub resting against a cam on the distributor shaft. At the end of this arm are the “points.” These are two metallic circles that make and break contact with each other as the engine rotates, opening and closing the coil charging circuit, creating and collapsing the coil field.
In the image above, you can see how the ignition breaker points sit on the breaker plate. The image shows a dual point setup. As the shaft rotates, the nubs (1) hit high points on the cam, causing the points (2) to open.
The points needed to be adjusted every so often to make sure the gas was sufficient to minimize wear and extend life. However, they and their attendant condenser (a medium-size capacitor) still had to be replaced every so often because wear did occur.
Points can’t carry enough current to generate a large enough field in the coil for extra performance. Some manufacturers, such as Accel, made dual-point distributors with heavy-duty points that allowed for coils that can generate 40-50,000 volts. Now you had two sets of points to adjust and the screws liked to loosen a little every thousand miles or so, however, and let me tell you - it wasn’t fun.
The mid-’70s saw the move to electronic ignition begin with points fully phased out within six or seven years. The GM Delco-Remy HEI distributor was also introduced in 1976. Electronic ignition used transistors, which allowed for higher charging currents so the coils they controlled could generate more spark energy: 60-70,000 volts getting to the plugs.
The opening and closing of the charging circuit was handled magnetically. Instead of a cam on the distributor shaft, a reluctor has several “teeth” that allow the coil to charge as it passes a Hall Effect transistor that allows current to flow while it senses the reluctor’s magnetic field. When the tooth passes, the magnetic field collapses and the transistor “shuts off,” stopping current flow and allowing the field to collapse and generate the spark.
The modules containing the power transistors that charge the coils could be housed in the distributor itself or in a module mounted on the firewall or fender. GM’s aforementioned HEI had the module in the distributor and the coil on the cap. Mopar developed a small module that mounted near the distributor on the firewall/bulkhead (I had the Direct Connection performance version in my Charger’s 360), and Ford had bulky “red” and “blue” modules that were usually found on the fender.
Image courtesy GM Performance
Computers started controlling coil firing in the late 90s although the distributor was still used. It took a little longer for distributors to go completely. Now most engines are DIS like the LS in the image above.
Image courtesy ResearchGate
Gasoline can be ignited using just the heat of compression if enough compression can be generated. In fact, more power can be generated with less fuel if combustion is caused by compression. That’s why diesel engines are so much more fuel efficient than gasoline engines. Their main problem is the soot and NOx they generate.
The picture above illustrates the differences between Spark Ignition (SI), Compression Ignition (CI), and Homogenous Charge Compression Ignition (HCCI) systems.
The idea of a homogenous charge compression ignition (HCCI) system has been floated in engineering circles. This would merge compression with spark, giving the best of both worlds. Driving conditions will dictate which ignition system is used to maximum efficiency and power output. Compression is adjusted to the ignition type required. Development of the concept will take at least to the end of the decade, but the concept promises to boost gasoline engine fuel efficiencies to near those of diesel engines without the drawbacks of diesel.
