C D I

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 piston engines). The capacitive-discharge ignition uses capacitor discharge current output to fire the spark plugs.

History

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. In the late 1940s an attempt to make one using mechanical means to trigger the capacitor's discharge was tried. It used extra contact switches in addition to the usual points (contact breaker), but suffered from timing problems and was unreliable. The quest for an 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 unit.

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 spark plugs, high energy throughout the RPM range resulting in better starting, more power and economy, and lower emissions. A few engineers and scientists had built CD ignitions throughout the 1950s, using thyratrons which required a warm-up period, and thyratrons were vulnerable to the effects of vibration as well. Silicon-controlled rectifiers (SCR) or thyristors came later thanks to Bill Gutzwiller and his team at General Electric. These early attempts all suffered from the same problem that made them unable to perform much beyond idling speed.

This was due to 'points bounce' which is a feature of a points-triggered system. In the standard system with points, distributor, 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 thyratron 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. F.L. Winterburn

The design used an inexpensive method that would only recognize 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. It provided a 75 milijoule spark at all engine speeds up to 5,000 rpm on an eight cylinder (10,000 rpm on a four-cylinder) and consumed only four amperes at that speed. 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 induction discharge ignition by the simple swapping of two wires. The Hyland CD ignition was the first commercially produced 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.

Advantages and Disadvantages of CDI

A CDI system has a short charging time, a fast voltage rise (between 3 ~ 10 kV/μs) compared to typical inductive systems (300 ~ 500 V/μs) and a short spark duration limited to about 50-80 µs. The fast voltage rise makes CDI systems insensitive to shunt resistance, but the limited spark duration can for some applications be too short to provide reliable ignition. The insensitivity to shunt resistance and the ability to fire multiple sparks can provide improved cold starting ability.

Since the CDI system only provides a short spark, it's also possible to combine this ignition system with ionization measurement. This is done by connecting a low voltage (about 80 V) to the spark plug, except when fired. The current flow over the spark plug can then be used to calculate the temperature and pressure inside the cylinder. (Source)