Capacitor discharge ignition

Capacitor discharge ignition (CDI) or thyristor ignition is a type of automotive electronic ignition system which is widely used in outboard motors, motorcycles, lawn mowers, chainsaws, small engines, turbine-powered aircraft, and some cars. It was originally developed to overcome the long charging times associated with high inductance coils used in inductive discharge ignition (IDI) systems, making the ignition system more suitable for high engine speeds (for small engines, racing engines and rotary engines). The capacitive-discharge ignition uses capacitor discharge current to the coil to fire the spark plugs.

The history of the capacitor discharge ignition system can be traced back to the 1890s when it is believed that Nikola Tesla was the first to propose such an ignition system. In U.S. patent #609250 first filed February 17, 1897, Tesla writes 'Any suitable moving portion of the apparatus is caused to mechanically control the charging of a condenser and its discharge through a circuit in inductive relation to a secondary circuit leading to the terminals between which the discharge is to occur, so that at the desired intervals the condenser may be discharged through its circuit and induce in the other circuit a current of high potential which produces the desired discharge.' The patent also describes very generally with a drawing, a mechanical means to accomplish

This was put into practice starting in 1906 on the Ford Model K. The Model K had dual ignition systems, one of which was the Holley-Huff Magneto, or Huff System, manufactured by the Holley Brothers Company. It was designed by Edward S. Huff with US patent #882003 filed July 1, 1905 and assigned to Henry Ford. The system used an engine driven DC generator that charged a capacitor and then discharged the capacitor through the ignition coil primary winding. An excerpt from the 'Motorway' Jan 11 1906, describes its use on Ford six cylinder cars: 'The efficiency of the Ford Magneto is shown by the fact that the instant it is switched in the car will pick up speed and, without changing the position of the ignition control lever, will run at least ten miles an hour faster.'

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