Direct & distributorless Ignition
Modern engine designs have abandoned the high-voltage distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary (low-voltage) pulse to individual coils for each spark plug, or one coil for each pair of companion cylinders in an engine (two coils for a four-cylinder, three coils for a six-cylinder, four coils for an eight-cylinder, and so on).
In traditional remote distributorless systems, the coils are mounted together in a transformer oil filled 'coil pack', or separate coils for each cylinder, which are secured in a specified place in the engine compartment with wires to the spark plugs, similar to a distributor setup. General Motors, Ford, Chrysler, Hyundai, Subaru and Toyota are among the automobile manufacturers known to have used coil packs. Coil packs by Delco for use with General Motors engines allow removal of the individual coils in case one should fail, but in most other remote distributorless coil pack setups, if a coil were to fail, replacement of the whole pack would be required to fix the problem.
More recent layouts utilize a coil located very near to or directly on top of each spark plug (Direct Ignition, 'DI' or coil-on-plug). This design avoids the need to transmit very high voltages, which is often a source of trouble, especially in damp conditions.
Both direct and remote distributorless systems also allow finer levels of ignition control by the engine computer, which helps to increase power output, decrease fuel consumption and emissions, and implement features such as Active Fuel Management. Spark plug wires, which need routine replacement due to wear, are also eliminated when the individual coils are mounted directly on top of each plug, since the power is transported a very short distance from the coil to the plug.
Four-stroke 2-cylinder engines can be built without a distributor, as in the Citroen 2CV of 1948 and BMW boxer twin motorcycles. Both spark plugs of the boxer twin are fired simultaneously, resulting in a wasted spark on the cylinder currently on its exhaust stroke.
Four-stroke 4-cylinder engines can be built without a distributor, as in the Citroen ID19. Two coils are used with one coil firing two of the spark plugs simultaneously, resulting in a wasted spark on the cylinder currently on its exhaust stroke, and the other coil used for the other two cylinders.
Four-stroke one-cylinder engines can be built without a distributor, as in many lawn mowers. The spark plug is fired on every stroke, resulting in a wasted spark in the cylinder when on its exhaust stroke.