Auto Ignition and CDI Capacitors

Auto Ignition capacitor

All older vehicles had an ignition coil connected to spark gaps of engine. The coil was in fact an autotransformer, whose primary coil was normally shorted by an ignition switch (or a breaker point, mounted on a cam). At a precise moment during rotor movement, governed by a cam switch, the switch was opened momentarily. This sudden voltage appearing on primary creates a high voltage of 12-20 KV at spark plug, and petrol / air mixture gets ignited.

The switch contacts will get a very high voltage across them, and a resultant arcing and damage to points, in the absence of any protective mechanism. This was provided by the capacitor in parallel with switch. The capacitor opposes any change in voltage and discharge current is thus diverted through the capacitor, thereby protecting the ignition switch.

Capacitance value of this capacitor was between 0.1 to 0.33 μF, and voltage rating varied between 200 V to 400 V. Being mounted very close to engine, its operating temperature was high, and often they were rated nearly 100 ℃. Most common capacitor construction was impregnated paper/ metal foil/ oil (extended foil construction) element encased in steel can. The capacitor also has to withstand high level of vibrations.

This type of auto ignition circuit was discontinued almost 30 years back, and a CDI (Capacitor Discharge Ignition) unit took its place.

CDI Unit

Most ICE based vehicles of today carry a CDI (Capacitor Discharge Ignition) unit to generate spark across spark gap which makes the petroleum-air mixture to ignite and impart motive power to the automobile. CDI unit is common in most 2-wheelers, cars, lawn mowers, turbine powered aircraft and many small engine applications. CDI is a timed energy storage and discharge switch. This stores energy in a capacitor, and a timed pulse sensor triggers it to discharge the capacitor into the primary of HV coil.  The design allows faster charging times and higher energy storage compared to older ignition devices.

CDI capacitor is basically a thyristors switched ignition capacitor used in most ICE petrol driven vehicles. The capacitor is charged via electrical circuit of vehicle, and then is discharged through the primary of HV coil of ignition coil. The coil has secondary with large number of turns, and produces very high voltage of 12 to 20 KV in response to sudden discharge of capacitor through primary. This causes a spark necessary to ignite petrol/air mix in the engine.

CDI units can be either on AC or DC types. AC-CDI units get energy from Ac supply from alternator, while DC-CDI units get powered from battery. Advantages of CDI are fat charging times, higher energy storage, and suitability for high engine speeds. However, ignition spark durations are reduced, and CDI units are a source of EMI interference, which has to be taken care of.

CDI capacitor

The CDI capacitor is rated from 0.47 μF to 3.3 μF and 250 to 600 V DC, and is usually an MPP (Metallized Polypropylene) / MPET (metallized Polyester) capacitor, with upper temperature rating up to 100℃. It dumps power of the order of a few hundred watts for few milliseconds. The capacitor discharge current is quite high, and must carry high frequency charge/discharge cycles for many years.

This makes the capacitor duty very onerous, and design has to made with this in view. CDI capacitor differs from other metallized film capacitors. It must carry high current surges repeatedly, and also bear higher charging currents for fast charging. End connections also must be sufficiently heavy for heavy surge currents. Usually it is made with lower resistivity metallized film (thick metallization) to withstand the high current density. Metallized Polypropylene or metallized Polyester film is used for these capacitors. End connection spray has also to be thicker and capable of bearing the heavy surges with ease. Working temperature of these capacitors is quite high, at about 100°C. Either epoxy coated or box type construction can be used.

Advantages of CDI

• Fast charging times
• Short transient response, ensuring shorter spark duration of 0.1 to 0.3 ms and faster voltage rise
• Capacitor has high current carrying capacity by design, High pulse capacity due to end spray design
• Can accept large voltage fluctuations in power source (engine / alternator)
• Improved end contact in capacitor leads to better pulse behaviour.

Limitations of CDI

• Rapid discharging of capacitor. CDI capacitor has to be specially designed for this duty.
• Short spark duration may result in ignition failure in case of lean mixture conditions.
• EMI interference noise

Alternatives to CDI system                   

Alternatives to CDI units include magneto ignition system and inductive ignition system. Magneto ignition systems have entire ignition system, including ignition coil and points, under a magnetized flywheel. Inductive ignition system use a transistor to switch charging current to the coil off, and on at appropriate synchronized time, eliminating the spark burn out problem of points due to fast voltage rise.