How the Capacitor Discharge System Works
The ignition system plays a crucial role in engine performance and understanding how the system works is essential to effective troubleshooting.
The capacitor discharge - or CD - ignition system provides up to 50,000 volts and a quick spark that reaches its peak voltage in three millionth of a second.This helps fire even wet, worn, or carbon-fouled spark plugs.
Also, another important benefit of CD ignition systems is that each ignition coil is located very close to its spark plug
This eliminates wasteful high-tension distribution systems, and minimizes secondary voltage loss. To understand how CD ignition systems operate, let's take a closer look at a three-cylinder outboard ignition system.
Ignition coils are located close to the cylinder heads and spark plugs. While a capacitor, located in the power pack, is a device that stores electrical energy for use at a later time
- Charge Coil
- First Pole passes the Charge Coil
- Current flows in one direction
- Second pole passes the Charge Coil
- Current flows in opposite direcion
- Continuos output of AC resulting from this process
This energy created is then stored in the capacitor which is located in the power pack and you can think of as a spring-loaded chamber full of electrical energy under your control.
The capacitor needs direct current - or DC - to work properly. Because of the direct current´s property where it only flows in one direction.
To convert AC flowing from the charge coil to DC for the capacitor we use a device called a rectifier.
The rectifier, located in the power pack, uses a bridge of diodes that act like one-way valves for electricity. No matter which way the electricity flows into the the rectifier, it comes out flowing in one direction.
If we take a closer look at the rectifier notice that one wire from the rectifier goes to the capacitor, and the other wire to the engine block. Metal in the engine block serves as a ¨ground¨ - a common return for current flow.
Because electricity can only travel in a complete circuit, the aluminum engine block supplies a convenient path for current to return to the rectifier and capacitor - and eliminates the need to run wires between all of the system components.
In summary, we see the rotating flywheel magnets generate approximately 300 volts of alternating current in the charge coil.
AC flows from the charge coil to the rectifier, where it is converted into direct current. Approximately 300 volts DC flows from the rectifier to the capacitor, where it is stored for later use.
How it all works:
Each cylinder has its own sensor coil and silicon controlled rectifier - or SCR. These two components work together with the sensor magnet to ensure each cylinder´s spark plug fires at the correct time. The sensor magnet - located in the hub of the flywheel - induces approximately three volts
in each sensor coil everytime its poles pass by. Three sensor coils - one for each cylinder - are precisely positioned under the flywheel. Each cylinder´s sensor coil sends a signal to its SCR.
SCR´s located in the power pack, are like tiny electric switches. Together with the sensor magnet and sensor coils, SCRs control the discharge of voltage stored in the capacitor to the ignition coils.
- One connects it to the Capacitor
- One connects it to the Ignition Coil
- One connects its gate to the Sensor Coil.
- The gate controls the SCR
- When sensor coil voltage is applied to the gate
- Current flows from the capacitor, through the SCR, to the ignition coil
- When the capacitor discharges all of its energy
- The switch opens
- And the capacitor begins recharging
For a review we'll look at how the sensor magnet, sensor coils, SCRs, capacitor, and ignition coils work together to fire the cylinders.
- Each sensor is positioned under the flywheel to fire a specific cylinder.
- When the sensor magnet passes sensor coil number one, current flows to SCR one.
- SCR number one discharges the voltage stored in the capacitor to ignition coil number one -which fires spark plug number one.
because the ignition coil and capacitor are both grounded to the engine, the circuit is complete. The remaining two cylinders fire the same way. When the rotating sensor magnet passes sensor coils number two and three, their SCR´s trigger their respective spark plugs to fire.
How can only 300 volts DC - applied to a spark plug- fire a spark plug? Taking into account that 300 volts isn't enough to jump across the gap of a spark plug.
The answer is that the ignition coil is a transformer. It can transform electricity from one voltage to another.
Inside the ignition coil there are loops of wire called windings, wrapped around a ferrite core.
There are two sets of windings: Primary windings, and the secondary windings.
As the capacitor discharges, current flows through the primary windings, inducing a magentic field around both windings and generating voltage in the secondary windings.
Because the ignition coil has only a few primary windings and many secondary windings, It "steps up" or increases the amount of output voltage.
For an input voltage of 300 volts DC appllied to the primary windings generates an output voltage of approximately 50,000 volts in the secondary windings
Because an ignition coil can only transform potential energy - not create it - amperage from the coil decreases as its output voltage increases. This isn't a problem because only a modest amount of current is needed to fire the spark plug.
50,000 volts DC is more than enough voltage to jump across the spark plugs gap and ignite the fuel-air mixture in the cylinder. Now that we have our engine running, how do we stop it?
All we have to do is connect a wire from the rectifier to a switch grounded to the engine block. Now, when we close the switch, current from the rectifier takes the path of least resistance - and flows to the engine block instead of the capacitor.
The switch also shorts out the capacitor by connecting both ends of the capacitor together - effectively releasing any charge that might be stored in the capacitor. It's really double protection - with no rectifier current reaching the capacitor, and the capacitor shorted out, current cannot reach the spark plugs, and the engine stops running.
Variable spark timing : engines require different timing of the spark at different speeds. At high RPM the spark must occur earlier in the combustion cycle.
How does the ignition system accomplish this?
The sensor coils are located in a movable assembly timer base. Linkage connects the timer base to the throttle control. As the throttle is advanced, the timer base moves counter-clockwise.
The flywheel rotates clockwise, so moving the timer base counter-clockwise causes the spark to occur earlier - or advance.
CD ignition systems used on two, four, six and eight-cylinder outboards operate much like the system we just examined.
Of course, with more cylinders, you need more SCR´s. V6 models have two charge coils under the flywheel and two capacitors in the powerpack - -one set for each bank of cylinders.
Overspeed protection - For example, when an engine exceeds its rated speed, the power pack causes the engine to misfire by shorting out some of the spark impulses.
This feature limits engine rpm and protects it from damage.
QuikStart and S.L.O.W. -
QuikStart advances the spark timing of an outboard so it runs at a fast idle until it warms up. QuikStart advances spark timing when the engine is operating below approximately 1,100 RPM. The temperature Switch - Located on the cylinder head, grounds out at a preset temperature - signaling the power pack when the engine is warmed up. Idle timing cannot be verified when the ignition is in the QuikStart mode.
If the timing jumps back and forth between two readings, the engine´s probably in the QuikStart mode.
The S.L.O.W. feature - Speed Limiting Operational Warning gradually limits engine RPM if the outboard overheats, protecting it from damage.
A temperature switch, located on the cylinder head, signals the power pack to limit engine RPM by creating a connection to ground when the engine reaches a preset temperature.
On outboards with optical ignition, an optical sensor replaces the sensor coils and magnets. Power for the optical sensor comes from the coil located on the stator assembly.The analyzer verifies the optical sensor is working, and allows both idle and maximum timing to be adjusted without running the engine.
Procedures for troubleshooting and repairing CD ignition systems are fairly simple and straightforward.
You check the condition of individual components, and replace those that don't meet service manual specifications.
An Ohmmeter is used to check the resistance of the charge coil, sensor windings, and ignition coils, and to verify that none of the components have shorted to ground.
A peak-reading voltmeter is used to check the voltage output of the sensor coils, charge coil, and power pack. When troubleshooting and performing repair procedures, always be sure you are using the correct service manual for the year and model engine you are servicing.
These procedures are not general in nature, and change from year to year. You can destroy a good component simply by following the wrong testing procedure.
Outboard ignition systems have come a long way since Ole Evinrude marketed his first outboard in 1909.
CD ignition systems help provide boaters with smoother running, more durable, and more reliable engines. By understanding how CD ignition systems work - and how to properly troubleshoot and repair them - you will keep your customers satisfied!
The capacitor discharge – or CD – ignition system provides up to 50,000 volts and a quick spark
that reaches its peak voltage in three millionths of a second.
Advantages of the CD ignition system are:
• Lower maintenance
• No distributor
• Higher voltage
• Quicker spark
• Shorter high voltage path
The main components of the CD ignition system are:
• The flywheel and its magnets – one on the hub (sensor coil magnets) and a set on the
rim of the flywheel (charge coil magnets).
• Charge coil – generates approximately 300 volts of AC electricity to be stored in the
• Sensor coil – generates approximately 3 volts signal to trigger the power pack to release
the electricity to each ignition coil.
• Powerpack – contains circuitry to change AC electricity into DC electricity, and the stop
circuit. The powerpack uses the signals from the sensor coils to direct current to each
cylinder’s ignition coil at the proper time.
• Ignition coil – transforms approximately 300 volts into 50,000 volts to fire the spark plug.
• Spark plug – ignites the fuel/air mixture in the cylinder.
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