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11/30/13 Capacitive Discharge Ignition
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Capacitive Discharge Ignition
(CDI)
Introduction
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.
1. AC-CDI - The AC-CDI module obtains its electricity source solely from the alternating current produced bythe alternator. The AC-CDI system is the most basic CDI system which is widely used in small engines.
2. DC-CDI - The DC-CDI module is powered by the battery, and therefore an additional DC/AC inverter circuitis included in the CDI module to raise the 12 V DC to 400-600 V DC, making the CDI module slightly larger.
This AC is rectified to DC to charge the main capacitor. However, vehicles that use DC-CDI systems have
more precise ignition timing and the engine can be started more easily when cold.
Advantages and Disadvantages of CDI
A CDI system has a short charging time, a fast voltage rise (between 3 ~ 10 kV/s) compared to typical
inductive systems (300 ~ 500 V/s) and a short spark duration limited to about 50-80 s. 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.
Block Diagram of CDI Unit
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Important Parts of Digital CDI
1. DC to DC Converter (Chopper)
The DC to DC Converter or Chopper is used to convert supply voltage (6V to 12V) to a High Voltage of around
120V - 300V. It first makes the DC voltage into Pulsating DC (using either PWM from Microcontroller or a 50%
duty cycle Multi-vibrator). This Pulsating DC is fed into the primary of a small transformer which steps up the
Voltage level. The secondary output is then rectified using a simple diode thus making it back to DC, and is used
to charge the main capacitor.
Earlier CDIs had fixed ratio of conversion and used a simple multi-vibrator of 50% duty-cycle. But modern Digital
CDIs use a PWM from the microcontroller which is fed to the primary of the transformer inside the CDI. In PWM,
the duty cycle can be controlled; hence the output of the DC to DC converter can be maintained even at variable
supply voltage. Higher the duty-cycle, higher is the voltage of the DC to DC converter.
2. Main Capacitor
The main capacitor is used to hold the energy used for creating suitable spark at the Spark Plug. It is charged
using the output of the DC to DC converter. The value of the capacitance plays an important role in deciding
charging and discharging times along with the spark energy.
Calculating Value of Capacitance
1. Calculate Minimum Energy required for successful spark.2. Estimate losses over the transmission line (from CDI to Spark Plug).3. Estimate charging and discharging resistance of the Capacitor to be put.4. Keeping both charging time constant (t = R*C) and Total Energy required (E = 0.5 * C * V^2), estimate
Capacitance required.
3. SCR Silicon Controlled Rectifier
A SCR is a four-layer solid state device that controls current. SCRs are mainly used in devices where the control
of high power, possibly coupled with high voltage, like in CDIs. The SCR is used to short the Capacitor to the
primary of the Ignition Coil (outside the CDI). The SCR must be capable of handling the High Voltage of around
300V along with the current associated with it.
The SCR is fired when the Microcontroller gives a pulse at the Gate of the SCR. Once the SCR is fired, it remains
active till the current across it falls to a minimum value. Thus the control of the spark is in the hands of the
Microcontroller.
4. Microcontroller
A microcontroller (sometimes abbreviated C or MCU) is a small computer on a single integrated circuit containing
a processor core, memory, and programmable input/output peripherals. The Program memory in the form of
NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM.
In modern Digital CDI, the Microcontroller has two major functions.
Deciding Advance angle by reading Input from Sensors.
Setting duty cycle of PWM of the DC to DC Converter.
Deciding Advance Angle
The advance angle required for getting optimum performance from the engine is (majorly) RPM dependent. Hence
the system must be aware of the current RPM (and Throttle Position).
Look up tables (called MAPs) are stored in the Program memory of the microcontroller, which give the
appropriate advance angles against the RPM. Multiple MAPs are stored based on different Throttle positions
(Open or Closed). Once the controller calculates the RPM, it can then look up for the appropriate advance
angle.
Calculating RPM
The RPM is calculated with the help of output from the Pulsar Coil. The Pulsar coil is a magnetic sensor which
picks up signals from the two Rots (Reluctance) placed on the rotor of ACG. Following figure shows a typical wave
shape of the pulsar coil (output).
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The controller converts the pulsar coil wave form into two square waves. The conversion is on the basis of rising
edge of the pulsar coil wave.
Since the angles of the two Rots are known (10 degrees for the smaller first Rot and 45 degrees of the larger
second rot), by seeing time difference in G1 and G4, the average RPM can be calculated (Angular Speed =
Angle/Time).
Wave Signal Shaping Circuit
T1 = Time from G2 to G1
T2 = Time from G3 to G2
T3 = Time from G4 to G3
T4 = Time from G4 to G1
T3 > 2*(T1 + T2), must hold for firing of spark plug.
G4 to G1 angle = 310
N(avg) = 60* 310/(2*PI*Time difference)
Which MAP to refer for the advance angle look up, is decided on the basis of input from theThrottle switch.
Example of a MAP
PWM for DC to DC converter
PWM is used for efficient voltage regulation of the DC to DC converter. By switching voltage to the input of the DC
to DC converter, with the appropriate duty cycle, the output will approximate a voltage which is desired.
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Basic Circuit Diagram
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