OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Wipac Magister Condenser Discharge Ignition System

Here is a standard Kettering ignition circuit, as used in pretty much all spark ignition motor vehicles before electronic ignition took over:



Let's compare that with the Magister circuit:


In the Magister, the Feed Coil, Rectifier and Condenser do no more than create a DC voltage source for the coil. After that, the rest of the circuit is the same as the Kettering circuit, except there is no capacitor (condenser) across the points.

So, what advantage does the Magister have? It generates a DC supply that may be at a higher voltage than 12 Volts. However the Magister also has no capacitor across the points, and it only applies its DC voltage to the coil for an instant of time. The Kettering ignition system has that capacitor because it effectively converts the 12 Volt supply to 150 Volts at the moment the spark is developed. The Kettering system also magnetically "saturates" the iron core of the coil by maintaining current through the primary for a substantial period before the points open. When they open, the collapsing flux in the coil's iron core generates a "back EMF" of 150 Volts, and the coil turns the 150 Volts into perhaps 15,000 Volts. There is no saturation of the iron core with the Magister, and as a result the spark generated is of much shorter duration than with a Kettering system. Because of that it is a much lower energy spark - but it may have a very high intensity during its brief duration, depending on the DC voltage applied to the coil primary, and the coil's turns ratio.

The bottom line is that the Magister can fairly be described as a basic form of Capacitor Discharge Ignition system. CDI systems are notable for being able to fire fouled spark plugs. They are also notable for not being able to ignite lean mixtures, or mixtures diluted by exhaust gas due to poor scavenging, or deliberate Exhaust Gas Recirculation, due to the brief low-energy spark. Because port controlled 2 strokes usually have very poor scavenging at light load, I think it is unsurprising that the Magister system was not adopted by many engine manufacturers. However that does not mean that it was a crackpot idea: there was a lot of interest in CDI at about the time it was produced, and fouled spark plugs are a perennial problem with crankcase induction 2 strokes.