Where does the spark come from?
Welcome to this week’s tech talk at Venom Motorsports!
I have been getting emails from customers asking me...Tim, where does the spark come from?
Today we are going to spend some time discussing that mystery.
Please click on our “Ignition System Component” video link below to view ignition system components and their location on our Venom 125 cc ATV.
Keep in mind that this information applies to all of our fine gas powered Super Pocket Bikes, Dirt Bikes and ATVs!
Ok onto this week’s topic.
Tim...Seems like a complex discussion where do we begin?
Well once upon a time in a land far far away...there was this little electrical charge that wanted to become a big one. Ok seriously, it’s not that bad.
To begin with you need to understand the KABOOM equation.
Spark + Fuel + Air = KABOOM!
When KABOOM occurs the piston is pushed downward in the engine cylinder, which turns the crankshaft, which turns the gears in the transmission, which drives the chain, which turns the rear wheel gear, which makes you go zoom! So KABOOM is most important!
Today we are looking at the Spark side of the KABOOM equation.
Firstly let’s begin identifying the various components needed to produce a spark and then explain their function.
The ignition system components are;
Alternator, CDI, Coil, Spark Plug.
Once we understand how they work individually we can put the puzzle together to understand how the system as a whole functions to produce the spark in the engine cylinder and ignite the air/fuel mixture.
Tim...is this alternator the same as the one in my car?
Yes, it is an AC alternating current generating device, like your car. Both use an alternator as it can produce more electrical energy for its size than a comparable DC generator.
Once the AC current is produced it is rectified to a DC voltage. That means that the full wave form of AC current is clipped to a half wave form referred to as Direct Current. Typically the electronic component used to achieve this is called a half wave rectifier or simply rectifier.
The electrical charge is produced on your bike by the alternator that has a rotating armature attached to the crankshaft that rotates within a stationary winding called a stator found in your lower engine casing. The electrical charge is then fed up into the CDI “Capacitor Discharge Ignition” chip from the stator by two wire connections. One feeds the CDI a 15-25 volt AC charge and the other a much smaller millivolt AC charge in the range of 5 to 25 millivolts which is the trigger charge for the CDI. The trigger charge indicates the engine rpm to the CDI which then determines the best time to discharge the ignition capacitor.
Tim...What is this CDI thing you keep talking about?
The CDI “Capacitive Discharge Ignition” is the brains of your ignition system. The CDI controls when voltage produced in the alternator is going to be released to flow into the “Coil”. The ignition capacitor acts in a manner similar to a battery, building and releasing an electrical charge very quickly from the CDI chip to the ignition coil.
The CDI does this in response to the trigger charge sent to it from the alternator. This signal charge acts on the electronic control circuitry in the CDI to determine when the “spark” charge is to be released to the coil.
This is what controls the engines timing...which means when the spark plug is going to fire in relation to the position of the piston inside the engine cylinder. Ideally we want the spark to fire just before the piston reaches top dead center of the compression stroke. More on four stroke engine design in future blogs.
Timing is critical for an engine and “high performance” CDI ignition chips are very clever as they alter the engine timing in relation to engine rpm. This allows for a higher than normal rpm range for the engine and smoother engine acceleration.
In the good old days, “Yup! I am a tad vintage” we use to use a magneto and set of points to achieve ignition timing. Today a CDI replaces that technology and does a much better job.
The CDI is a smart high tech gadget!
Tim...Does the “Ignition Coil” make our spark?
Well the “Ignition Coil” commonly just referred to as a “coil” produces the high DC voltage needed. However, it is by forcing this high charge across an air gap in the spark plug that actually produces the spark.
Ignition coils are actually a form of transformer called a Step Up transformer. These are used to step up or increase the voltage provided to them.
You see the CDI releases a DC electrical charge into the primary side of the coil. The primary side of the coil is basically small coil of wire. When the charge flows through the wire in the primary side of the coil it produces a magnet field.
This magnet field then acts on the secondary windings of the coil inducing a very high DC voltage in the secondary windings that is then directed through the high tension wire to the spark plug.
Making this high DC voltage jump from the end of the electrode to the grounding tip of the spark plug is what produces the actual spark.
Tim...How can I remember all that stuff?
I try to keep it simple and focus on what each component does.
The alternator produces the electrical charge, the CDI controls the electric charge, the coil makes the electrical charge larger and the spark plug makes the electrical charge jump across and air gap. Not so bad!
You can watch our video that explains where the spark comes from in details by clicking here
Check out our blog next week as we look at the second aspect of the KABOOM equation Fuel!
Spark + Fuel + Air = KABOOM!
We will take a look at the fuel system on your bike from tank to carburetor and chat about different types of gasoline and how octane levels affect your bikes performance.
If you have any tech questions about our bikes, please feel free to email them to...
Have a great day!
Technical Support Specialist
Venom MotorSports Canada