Explaining How Ignition Systems and Components Work
This will explain the types of ignition system components and their function....
All gasoline engines require an ignition of some sort. In it’s most basic form, an ignition system consists of a set of breaker points, a condenser, a coil, a spark plug, and, a voltage source. Let’s set up a sample single cylinder engine with a breaker point system.
First, we need to understand what the different components do.
The coil consists of a set of primary and secondary windings. The primary windings (12 volt or low voltage) are wound around a core made of thin iron plates or solid iron. The secondary windings (High voltage) are wound are around the primary windings separated by a layer of insulation. The current needed to jump across the spark plug and ignite the mixture is generated here. Basically, voltage is applied to the positive side of the primary windings and grounded through the breaker points on the other side. As long as the points are closed, voltage flows through the windings, a magnetic field is created, saturating the secondary windings. The the primary windings are isolated from the case and only ground at the points. When the breaker points are opened, the magnetic field collapses and current is generated (induced) into the secondary windings. This current travels from the coil to the spark plug and jumps the gap, creating the spark. Once the field is collapsed, The coil must re-saturate in order to create another spark.
This is where the points come in. On one side of the points, there is a terminal where the lead from the coil connects. The other side of the points are grounded. There is a camshaft with a lobe that lifts the points and breaks the connection to ground. The width of this gap and the length of time they stay apart is the point gap and dwell. There is a condenser connected to the points to prevent arcing and increase the life of the contact points. Whenever the points are closed, the circuit through the coil is completed. The magnetic field is created, saturating the coil. Open the points, the field collapses and the spark is created, jumping the spark plug and igniting the mix. The opening of the breaker points is timed so the spark is created just before the piston reaches the top of the compression stroke. There is a mark on the crankshaft (flywheel on Harley Davidson Motorcycles) that tells us when the engine is at Top Dead Center (TDC). TDC is when the piston is a the very top of it’s movement on the compression stroke. Since there are 360 degrees in a circle, we can set the timing at any point and know exactly where it’s set by using a timing light. Usually, the spark needs to occur slightly before TDC to make the most power. We also need to have some sort of method to advance and retard the timing to prevent detonation (pinging). As RPM increases, the amount of advance must be increased. Since the pistons are moving faster, there is less time to burn the mix. So, the spark must be started sooner to make the most power and prevent detonation. In our sample engine, we use a simple weight system that moves the breaker point cam in relation to RPM. The amount of advance graphed against RPM is known as the advance curve.
Note: Detonation occurs when the spark explodes the mixture rather than burning it. This explosion is extremely detrimental to your engines health. It sounds like a "pinging" noise when you accelerate. Detonation will destroy an engine. If you hear it, check for proper timing, too low of an octane rating gas, or air leaks.
A magneto ignition works in much the same way, except there is no need for a battery as current is generated by a permanent magnetic armature rotating past the field windings, creating the spark. Magnetos are self contained and create an extremely powerful spark.
On our sample engine, we have a 12 volt battery which provides power to the positive side of the coil through the ignition switch. On the other terminal of the coil, we have a wire from it to the breaker points. At the breaker points, the condenser is wired to the same terminal. An advance unit is used to provide proper spark timing. The coil’s secondary windings are connected to the spark plug. With this basic system, we have a reliable ignition that only occasionally needs the points adjusted and changed. What if we could make it maintenance free, only needing to change the spark plug once in a while?
Lets replace the points with an electronic ignition control. This Pointless Electronic Ignition (PEI), does the job of the points and, the advance unit (some ignitions don’t incorporate the advance). The dwell and timing are controlled in the PEI, this produces a precise, consistent spark. The PEI senses the RPM of the engine and adjusts the timing of the spark accordingly. You have a maintenance free ignition that is extremely accurate.
A limitation on our sample engine is the coil. Our stock coil is fine for everyday use. It provides a good solid spark to the plug. However, if we modify the engine or, the plug becomes worn or slightly fouled, the stock coil is unable to create enough voltage to keep our engine running smoothly. If we replace the stock coil with a performance coil, we get a much hotter spark. Why? How? Well, a performance coil has several important differences from a stock coil. A performance coil may have a larger iron core, more primary windings, better insulation between the windings and, many more secondary windings. The coil may incorporate some or all of these differences. The idea being to create a stronger magnetic field that is able to collapse (decay) quickly and induce a higher voltage through the larger secondary windings. The performance coils also have a much faster saturation (rise) cycle due to the improved insulation between the windings.
Now that we’ve covered the basics, let’s talk about Harley Davidson motorcycle ignitions. Prior to 1978, Harley Davidson used a breaker point system. From 1978 to 1980, an early electronic ignition, called V-fire I was used. It was prone to failure and was often replaced with points. Harley Davidson’s current dual-fire, PEI (V-Fire II, III), system has been used since the 1980 models. This system has a preprogrammed advance curve and rev limiter. Both spark plugs fire at the same time. While the basic design remains the same, many improvements have been made to the system. Better electronics, advance curves and reliability improvements. A Vacuum Operated Electric Switch (VOES) was added in the 1984 Evolution models. The VOES switch senses the vacuum in the engine and advances the spark at idle. This produces a smoother idle and allows for a more aggressive advance curve to be set in the ignition module. All Harley models use the dual fire system except for the injected Harley models.
There are several types of performance ignitions. Dual fire, single fire and Multiple fire. Most Harley Davidson motorcycle models include an adjustable advance curve that allows you to select the correct curve that gets you the most power without detonation. If you’re not familiar with detonation, you may want to read about it in our hop-up article. They also include RPM or rev-limiters, to prevent over-revving your engine by selectively cutting the spark to the plugs or, retarding the timing. Some are module replacement type, where just the ignition module is replaced, there are systems that replace the module and the pickup, Systems where the pickup and module are combined, and systems with multiple boxes that must be mounted.
Dual fire ignitions and modules. The least expensive performance ignitions are then non adjustable module replacement type such as the Screaming Eagle. There are both adjustable and non adjustable modules. Some modules provide a choice between single and dual fire. Bear in mind that you still need a performance coil to get the full benefit from the module.
Single fire ignitions fire each plug separately. This allows for precise timing and for a very intense spark. Most Harley models have adjustable advance curves and rev-limiters. Some models even have the ability to set the timing for the rear cylinder.
Multiple Spark Discharge (MSD) or Capacitor Discharge (CD) ignitions create multiple sparks instead of one spark. This system is less likely to have the spark "blown out" or detonation since there are many sparks instead of one. The downside of this type of system is the amount and size of the components which makes it a challenge to install. These systems also cost a lot more than the single and dual fire performance systems.
Dual fire vs. Single fire vs. Multiple fire. Which is better? Why? Do I need to change from a stock system? Well, the answers depend on your overall hop-up plan. The stock system puts out about 20-25k volts across two plugs each time it fires. Most performance systems put out 35-45+k volts to the plug(s). The common theory is that firing the plug on the empty cylinder increases vibration, robs available spark to the firing plug and can ignite unburned mix causing the empty cylinder to "fight" the firing cylinder. The fact that the injected models have single fire ignitions defeats Harley’s claims of "purging" the empty cylinder. A performance ignition system will increase power, and is essential in your overall hop-up plan. Even a stock Harley will benefit from a performance ignition. If you’re adding pipes, cam and, carburetor, make sure you include an ignition in the plan. If all you want is a hotter spark and never plan to add a cam, carburetor or high flow exhaust, then a non adjustable module and a performance coil may suit you fine. If your plans on adding a cam or doing other hop-ups, we suggest in investing in an adjustable module as a minimum. In our experience, single fire ignitions will provide a smoother running engine and allow for any high performance changes you may decide on later. If you plan on doing some racing or, are building a very hot street Harley Davidson motorcycle, you may want to use an MSD or CD type of ignition.
Again, if you plan on never hopping up your Motorcycle, a Screamin’ Eagle or Dyna module along with a hotter coil may be exactly what you need. These modules provide better advance curves than stock and the hotter coil will give a much better spark. Single fire vs. Dual? Make mine Single fire thank-you.
Can an older points equipped v-twin be fitted with an electronic ignition? You bet! There are a lot of choices here. Basically, all 1971 and up alternator big twins and Sportsters can be converted to electronic ignition.
Last edited by chopper; 02-26-2008 at 04:47 PM.
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