Ignition system

Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-air mixture just before each combustion stroke. Gas turbine engines and rocket engines normally use an ignition system only during start-up.

Diesel engines use compression ignition to ignite the fuel-air mixture using the heat of compression and therefore do not use an ignition system. They usually have glowplugs that preheat the combustion chamber to aid starting in cold weather.

Early cars used ignition magneto and trembler coil systems, which were superseded by Distributor-based systems (first used in 1912). Electronic ignition systems (first used in 1968) became common towards the end of the 20th century, with coil-on-plug versions of these systems becoming widespread since the 1990s.

Magneto and mechanical systems

Ignition magneto systems

thumb|right|Ignition magneto

An ignition magneto (also called a high-tension magneto) is an older type of ignition system used in spark-ignition engines (such as petrol engines). It uses a magneto and a transformer to make pulses of high voltage for the spark plugs. The older term "high-tension" means "high-voltage".

Used on many cars in the early 20th century, ignition magnetos were largely replaced by induction coil ignition systems. The use of ignition magnetos is now confined mainly to engines without a battery, for example in lawnmowers and chainsaws. It is also used in modern piston-engined aircraft (even though a battery is present), to avoid the engine relying on an electrical system.

Induction coil systems

As batteries became more common in cars (due to the increased usage of electric starter motors), magneto systems were replaced by systems using an induction coil. The 1886 Benz Patent-Motorwagen and the 1908 Ford Model T used a trembler coil ignition system, whereby the trembler interrupted the current through the coil and caused a rapid series of sparks during each firing. The trembler coil would be energized at an appropriate point in the engine cycle. In the Model T, the four-cylinder engine had a trembler coil for each cylinder.

Distributor-based systems

thumb|upright|Rotor contacts inside distributor cap

An improved ignition system was invented by Charles Kettering at Delco in the United States and introduced in Cadillac's 1912 cars. The Kettering ignition system consisted of a single ignition coil, breaker points, a capacitor (to prevent the points from arcing at break) and a distributor (to direct the electricity from the ignition coil to the correct cylinder). The Kettering system became the primary ignition system for many years in the automotive industry due to its lower cost and relative simplicity.

Electronic systems

thumb|Honda Civic Si 1990. Spark plug wires of cylinders 3 and 4 each has a Hantek-HT25 connected. On the dash board, cables from the HT25s connect to an Owon HDS2202S. Ignitions, enclosed in red circles, have a sawtooth shape. Between successive ignitions, are three pulses from [[Wasted spark|wasted sparks and crosstalk.]]

The first electronic ignition (a cold cathode type) was tested in 1948 by Delco-Remy, while Lucas introduced a transistorized ignition in 1955, which was used on BRM and Coventry Climax Formula One engines in 1962. Pontiac became the first automaker to offer an optional EI, the breakerless magnetic pulse-triggered Delcotronic, on some 1963 models; it was also available on some Corvettes. in 1971, Chrysler (after a 1971 trial) in 1973 and by Ford and GM in 1975. Other systems do away with the distributor as a timing apparatus and use a magnetic crank angle sensor mounted on the crankshaft to trigger the ignition at the proper time.

Engine Control Units

Modern automotive engines use an engine control unit (ECU), which is a single device that controls various engine functions including the ignition system and the fuel injection. This contrasts earlier engines, where the fuel injection and ignition were operated as separate systems.

Gas turbine and rocket engines

Gas turbine engines (including jet engines) use capacitor discharge ignition, however the ignition system is only used at startup or when the combustor(s) flame goes out.

The ignition system in a rocket engine is critical to avoiding a hard start or explosion. Rockets often employ pyrotechnic devices that place flames across the face of the injector plate, or, alternatively, hypergolic propellants that ignite spontaneously on contact with each other.