Ignition Mods


The purpose of an ignition system is to ignite the air-fuel mixture, which has been introduced into the combustion chamber, so as to produce the required power. Since this is the first step to the entire process, it is the most critical, hence this process must be carried out with utmost precision and with no compromise at all. This will ensure 100% performance and will also bring down wastage of the mixture, thus reducing fuel consumption and pollution considerably.

Let us look at the different ignition systems and how they work, this will give us a better understanding of what needs modifcation.

 

CB Point ignition system

The RD350 made use of an ignition system which comprised of points, ignition coils and a condenser. This was also known as the CB (contact breaker) point ignition system. Although this system was able to put out nice hot sparks at lower revs, the spark considerably weakened at higher revs, thereby bringing down the performance quite considerably. This was the main drawback of this ignition system. Also the points used were spring controlled points that were activated by means of an irregular cam which rotated at the end of the crankshaft, external to the alternator. This essentially meant a large amount of mechanically active parts. As a result the ignition timing of the engine would get altered over a period of time due to constant mechanical activity and also wear and tear.

Let us see how this system actually functions,

The coil is really a transformer with two windings which are called the primary and secondary. The turns ratio is around 100 or 150 to one, meaning that for every single turn in the primary winding there are 100-150 turns in the secondary. This means that for every volt put into the primary winding when the points open there will be 100 to 150 volts in the secondary, which is connected to the spark plug.

The ignition points are simply a switch that's opened and closed by a small cam on the end of the crankshaft (in a two-stroke). While the points are closed, current from the battery flows in the series circuit comprised of the coil primary and the points. Nothing happens when there is steady current flow in this circuit. But when the points are pushed open by the points cam, current flow abruptly stops and a very useful phenomenon comes into play: the sudden ending of current flow in the inductor that is the coil primary causes a collapse of the magnetic field that had been set up by the steady current flow. The sudden collapse of this field generates an "inductive kick" (properly called counter-emf or back-emf) which is of much higher peak voltage than the battery voltage that had been causing the steady current flow. If the battery voltage is 12 volts, the peak voltage of the counter-emf will be around 200 volts across the primary of the coil. With a turns ratio of 100:1 the peak voltage in the secondary coil will be 20,000 volts which should be enough to fire the spark plug.

So what does the condenser do? First of all, it's properly called a capacitor, has been since the late 1940s. A capacitor can be thought of as a device that stores energy and blocks direct current (DC) but allows alternating current (AC) to pass through it. The inductive kick we generated with the opening of the points starts in the primary of the coil, flows through the condenser which is connected across the open points, and passes on to ground (the chassis of the bike) to complete the circuit back to the battery.

 

Capacitor Discharge / Electronic Ignition

Popularly known as the CDI, this is a modern day ignition system and almost all the bikes currently in Indian markets make use of this ignition system. Although this a much more developed system than the CDI, the one drawback of the system is that its spark depends on the velocity of the magneto, it is directly proportional. This means that hotter sparks at higher revs. Nothing wrong with that, the only problem is starting a cold engine with this system. Since at lower revs and especially at start-up, the spark is at its weakest\

Here we still have the ignition coil, as in the other systems, but there is no steady current flow in it. Instead, there is a capacitor in the black box that's charged to several hundred volts by an electronic oscillator that steps up the 12 volts DC from the battery. Then, at the right time, a switch (usually optical or magnetic) that's actuated by a disk on the end of the crankshaft puts out a pulse that causes a switching transistor (also in the black box) to dump the capacitor's several hundred volts into the coil. The coil is now just acting as a transformer that takes the 300 volt pulse and transforms it to about 30,000 volts for the spark plug (remember the 100:1 turns ratio). There are three major advantages over coil-points-condenser systems:

The ignition coil in an OEM (original equipment) CD ignition system has less inductance than a conventional ignition coil, allowing an even faster rise time, but aftermarket systems often use the original standard ignition coil. If the aftermarket CD system can handle the higher switching current of the CD-type coil it will produce a hotter spark.

 

Using the CDI on the RD

Fitting this system on the RD will definitely make a difference. A popular practice is to incorporate the CDI systems from popular 100cc bikes like the RX and the shogun. Although this is not too bad a practice since it will permanently eliminate the pain of setting the points every 500kms, the only problem is the design of the systems. These systems were designed to work on bikes which made use of a single engine, thereby these systems fire only a single coil for every revolution that the magnetic drum makes. The RD on the other hand needs a system which should ideally fire two coils for every revolution with an interval of 180 degrees.

It is this inadequacy in the system that brings about a power loss in the bike. This essentially means that although you can build up speed and power gradually, there wont be a blast of power when you whack open the throttle. However don't misunderstand that these systems are completely useless. For street and city riding conditions these systems are very suitable, and I would certainly recommend one for the RD.

 

Transistorized Ignition systems

These were ignition systems which were first brought about by Martek and Newtronics, however they have now been brought into our markets by AUTOESCORTS.

However both these kits are different. The Martek / Newtronics systems made use of the HALL effect which provided actuation by means of optical coupling. The Autoescorts' kit on the other hand makes use of magnetic coupling.

The kit is a very simple system. It basically consists of two pulsar coils which have been located 180 degrees apart. A magnet is then placed on a cam which rotates between these 2 coils. Every time the magnet passes the coil, it excites the base of a transistor which is connected to the coil. This excitation leads to a voltage output at the emitter terminal of the transistor. Since this voltage output is very small, it is then amplified by means of an amplifier and then fed to the primary of the ignition coil. The voltage is then multiplied 'n' number of times depending on the turns ratio of the coil, and this voltage is then given to the spark plug which produces the spark.

This kit has been installed on many RDs in Mumbai with success. The power train is much smoother, and the bike runs very well. Also the emission levels are brought down drastically with considerable decrease in the fuel consumption. The kit is also known to make cold starts much easier as it puts out a hotter spark that the CDI. 

Although this kit is not a complete electrical upgrade, it is highly recommended as it provides better performance with little or no compromise at all.

 

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