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 the 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.
These distributorless EI ignition systems were a significant improvement over ignition systems that contained only one coil providing the spark for all the engine's cylinders. Because of the design, a faulty coil results 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). Limiting the amount of dwell reduces unwanted heat when demand is low but still provides full saturation for acceleration, heavy load, and passing situations. COP ignition systems offer better performance because the PCM controls each cylinder individually.