Waste Spark Ignitions
Like most automotive systems, the ignition system has undergone many changes. One example is the Distributorless Ignition System, or “DIS” for short. This system introduced in the 1980s. The distributor, coil, cap and rotor the conventional system were replaced by coil packs. The new DIS system proved to be more reliable and required less maintenance.
The earliest DIS featured a bank of coils; one coil for every two cylinders. Each pair of coils would provide power to two spark plugs. Each of the two paired coils would fire the paired spark plugs simultaneously, one on the compression stroke and the other on the exhaust stroke. Since one plug is fired on the exhaust stroke, serving no real purpose, the system was known as a “waste spark system.” This style of ignition has negative and positive polarity sides to the coil; meaning the spark plug also has positive or negative polarity.
It is important to know that in a waste spark ignition system that spark plug electrode wear occurs differently based on the polarity. For plugs in a positive polarity position, more electrode wear will occur at the ground electrode. Plugs in a negative polarity position will experience more wear at the center electrode.
As a result of this, vehicles equipped with waste spark ignitions often use double precious metal spark plugs, such as the OE Iridium® or OE Platinum®. These dual precious metal plugs feature either iridium or platinum on the center-firing electrode and platinum on the ground electrode. Because these electrode materials are harder, gap erosion is reduced, allowing for a longer service interval. Since these materials are denser, this will reduce gap erosion, thus leading to a much longer service interval. The use of nickel-alloy or single precious metal spark plugs in vehicles equipped with waste spark ignitions will result in more aggressive gap erosion and shorter spark plug life.