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, gas 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 to discharge current to the ignition 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 609,250 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 its purpose.

Originally invented for Henry Fords six cylinder racer in 1905, the first production use of a CDI system was put into use in 1906 as standard equipment with the Ford Model K. The Model K utilized 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.'

It was the Robert Bosch company which was the pioneer of the first electronic CD ignitions. (Bosch is also responsible for the invention of the high-tension magneto.) During World War Two, Bosch had fitted thyratron (tube type) CD ignitions to some piston engined fighter aircraft. With a CD ignition, an aeroplane engine did not need a warm up period for reliable ignition and so a fighter aircraft could take flight more quickly as a result. This early German system used a rotary DC converter along with fragile tube circuitry, and was not suited to life in a fighter aircraft. Failures occurred within only a few hours. The quest for a reliable electronic means of producing a CD ignition began in earnest during the 1950s. In the mid-1950s, the Engineering Research Institute of the University of Michigan in cooperation with Chrysler Corporation in the United States worked to find a method to produce a viable solution.

They were unsuccessful, but did provide much data on the advantages of such a system, should one be built. Namely; a fast voltage rise time to fire fouled or wet spark plugs, high energy throughout the RPM range resulting in better starting, more power and economy, and lower emissions. A few engineers, scientists, and hobbyists had built CD ignitions throughout the 1950s using thyratrons. However, thyratrons were unsuitable for use in automobiles for two reasons. They required a warm-up period which was a nuisance, and were vulnerable to vibration which drastically reduced their service life. In an automotive application, the thyratron CD ignition would fail in weeks or months. The unreliability of those early thyratron CD ignitions made them unsuitable for mass production despite providing short term benefits. One company at least, Tung-Sol (a manufacturer of vacuum tubes) marketed a thyratron CD ignition, model Tung-Sol EI-4 in 1962, but it was expensive. Despite the failings of thyratron CD ignitions, the improved ignition that they gave made them a worthwhile addition for some drivers. For the Wankel powered NSU Spider of 1964, Bosch resurrected its thyratron method for a CD ignition and used this up until at least 1966. It suffered the same reliability problems as the Tung-Sol EI-4.

It was the SCR, Silicon-controlled rectifier or thyristor invented in the late 1950s that replaced the troublesome thyratron, and paved the way for a reliable solid-state CD ignition. This was thanks to Bill Gutzwiller and his team at General Electric. The SCR was rugged with an indefinite lifetime, but very prone to unwanted trigger impulses which would turn the SCR 'on'. Unwanted trigger impulses in early attempts at using SCRs for CD ignitions were caused by electrical interference, but the main culprit proved to be 'points bounce'. Points bounce is a feature of a points-triggered system. In the standard system with points, distributor, ignition coil, ignition (Kettering system) points bounce prevents the coil from saturating fully as RPM increases resulting in a weak spark, thus limiting high speed potential. In a CD ignition, at least those early attempts, the points bounce created unwanted trigger pulses to the SCR (thyristor) that resulted in a series of weak, untimed sparks that caused extreme misfiring. There were two possible solutions to the problem. The first would be to develop another means of triggering the discharge of the capacitor to one discharge per power stroke by replacing the points with something else. This could be done magnetically or optically, but that would necessitate more electronics and an expensive distributor. The other option was to keep the points, as they were already in use and reliable, and find a way to overcome the 'points bounce' problem. This was accomplished in April 1962 by a Canadian, RCAF officer F.L. Winterburn working in his basement in Ottawa, Ontario. The design used an inexpensive method that would recognize only the first opening of the points and ignore subsequent openings when the points bounced.

A company was formed in Ottawa in early 1963 called Hyland Electronics building CD ignitions using the Winterburn design. The discharge capacitor within the CD ignition had the ability to provide a powerful spark in excess of 4 times the spark power of the Kettering system using the same coil, with the exception that spark energy could be maintained at high rpm unlike the Kettering system. The Hyland unit consumed only four amperes at 5000rpm (8cyl) or 10,000rpm(4cyl). Dynamometer testing during 1963 and 1964 showed a minimum of 5% increase in horsepower with the system, with 10% the norm. One example, a Ford Falcon, had an increase in horsepower of 17%. Spark plug lifespan was increased to at least 50,000 miles and points lifespan was greatly extended from 8,000 miles to at least 60,000 miles. Points lifespan became a factor of rubbing block (cam follower) wear and the life cycle of the spring with some lasting almost 100,000 miles.

The Hyland unit was tolerant of varied points gaps. The system could be switched back to standard inductive discharge ignition by the swapping of two wires. The Hyland CD ignition was the first commercially produced solid-state CD ignition and retailed for $39.95 Canadian. The patents were applied for by Winterburn on September 23, 1963 (United States patent# 3,564,581). The design was leaked to the United States in the summer of 1963 when Hyland exposed the design to a US company in an effort to expand sales. Afterward, numerous companies started building their own throughout the 1960s and 1970s without licence. Some were direct copies of the Winterburn circuit. In 1971 Bosch bought the European patent rights (German, French, British) from Winterburn.