Automotive ignitions systems have seen many transitions over the years. Historically, the designs have matured from a magneto to today’s coil-over-spark plug designs. The progression follows the emergence of solid state electronics as well as the phasing out of mechanical components in favor of electrical components.
The goal of the ignition system is to fire a spark plug. In order to do so, a large voltage must be pulsed to the spark plug to make a spark fire across the spark gap. The basic spark plug remains the same. However, the firing circuit has gone through many changes.
Spark Plugs Ignite the Air-Fuel Mixture in the Cylinder1
The early magnetos are still in use today in many small engines such as those commonly found in lawn mowers. This style of ignition circuit generated energy by having magnets move past an armature. A magnet in motion creates an electrical current in the armature passing the current through a primary coil. The basic operation of an inductor is not to rapidly change current. When the points open [or the field ceases providing current in some designs], the inductor voltage reverses as the inductor keeps the current flowing. The voltage on a secondary coil is amplified by the turns ratio between the primary and secondary. This voltage is then ported through the spark plug wire causing the plug to fire. The location of the magnets are such that the cylinder fires slightly before the piston reaches top dead center on the compression stroke.
As engines gained more cylinders, the magneto became impractical to locate on the flywheel, so an electromechanical device called a distributor was invented. A distributor uses gearing from the engine to fire a signal from a central connector outward to a perimeter connector in the distributor cap. A spinning rotor carries the electrical pulse from the coil input in the center connector to the spark plug connection on the outer connector. On a V8 engine, the wires coming from a distributor resemble the eight tentacles of an octopus.
A distributor routes voltage between the spark plugs and the coil via wires, a cap, and a rotor3 .
Inside the distributor is a set of points. The points are closed and build up current on the coil during a portion of the engine rotation cycle. In a V8 engine, an octagon ring on the center distributor shaft mechanically separates the points opening them and interrupting the current in the coil causing the aforementioned flyback of the inductor and generation of the spark.
Points had difficulty handling 12 volt circuits so a ballast resistor was used to prolong the life of the points. Often times, the ballast resistor was bypassed during starting for maximum firing power.
As solid state electronics became more robust, high voltage transistors replaced the mechanical points. At first bipolar junction transistors [BJTs] were used. Later on, insulated gate bipolar junction transistors [IGBTs] were used as they require less drive current.
An Ignition Circuit That Has an IGBT as a Main Switching Device
Solid state electronics solved many problems for ignition systems. Unlike mechanical systems, there were no moving parts to wear out. Triggering could be accomplished by magnetic sensing of a location on the distributor shaft. The assembly could be located within the distributor or moved to a control box or “brain box” for creating pulses. Solid state ignitions were more immune to shorting from moisture. A famous car commercial during the 1970’s showed an engine starting even though fire hoses were pouring water under the hood directly on the distributor.
Ignition modules often failed. Furthermore, routing the high spark energy through long wires resulted in losses due to resistance. As a result, coil packs were created. The first coil packs were located near the spark plugs and fired two plugs. Now the spark was more direct and could be on longer as the coil powered on two cylinders in the same time period the distributor fired eight cylinders. With the loss of the distributor, the timing system no longer was linked to the engine via gearing. Designers went back to sensing the camshaft or directly on the flywheel like the magneto. However, sensing required much less energy than the ignition power of a magneto. The sensed signal is sent to the ignition control module for processing and creating the spark trigger pulses that are in turn sent to the coils.
Today’s engines have the coil mounted directly on the spark plug. This offers the shortest path for the energy to traverse. Also, the high power signals of spark plugs generate a lot of noise and EMI. By routing low level signals directly to the coils, interference is greatly reduced.