Waste Spark Ignition
Some engines contain EI systems that fire more than one cylinder at a time. This is known as the waste spark method. These ignition coils have two towers providing spark for two different cylinders simultaneously. One cylinder is on its compression stroke and the other is on its exhaust stroke. The sparkplug that fires during the compression stroke creates power. The plug that fires during the companion cylinder's exhaust stroke is “wasted”; this is because it fires into inert exhaust gases. The next time around the cylinder’s reverse roles. These distributorless EI ignition systems were a major improvement over ignition systems that contained only one coil providing spark for all the engine's cylinders. Because of the coils design, a faulty coil will result in a misfire on more than one cylinder. Scanned data will indicate two misfires, a six cylinder engine may have misfires on cylinders #2 and #5 or the #1 and the #4 at the same time.
Most of today’s engines have (COP) coil over plug ignition systems. These ignition systems are more efficient than single coil or waste spark systems. The ignition module, located in the PCM or in the coil itself controls the amount of on time (dwell). This is important because limiting the amount of on time or dwell reduces unwanted heat when demand is low, yet provides full voltage saturation for high demand accelerating, heavy load, and passing situations. This is true for both types of systems, but particularly effective with COP ignition systems. They are more precise and offer better performance because the PCM is able to control each cylinder individually.