This article describes a home-made CDI unit for Spartamet and Saxonette motor-assisted bicycles (mopeds). Having been virtually forced to use a Spartamet to travel between home and work for three weeks, it was noticeable that although the moped ran fine, at full throttle and at top speed (15 mph) the ignition began to misfire. The fuel consumption at full throttle also increased dramatically: from 118 mpg at 3/4 throttle to 71 mpg at full throttle. There was a strong suspicion that the higher fuel consumption was related to the misfiring of the ignition; this was confirmed after some further thought and having checked the spark plug and exhaust after several rides.
The ignition starts to skip sparks when the 30 cc two-stroke engine is at full throttle and at top speed. The manufacturer has used this method on purpose to build in an electronic speed limiter to ensure the moped is road legal. However, the carburetor is not limited and it happily continues to deliver the fuel mixture, which ends up unburnt in the exhaust. Apart from the fact that this has a negative impact on the fuel consumption, it doesn’t do the exhaust any good either. There will be more of a carbon build up in the exhaust, which means it has to be replaced sooner. You could, of course, open up the existing CDI (Capacitive Discharge Ignition) unit and modify it, but since this is completely encased in potting compound this is not something we would recommend. Instead, we investigated what was required to produce the sparks without limiting the rpm. The result of this can be seen in the schematic shown here. Since the ignition coil and pickup coil are mounted next to the flywheel of the engine we only have to concern ourselves with the electronics that make a capacitor discharge into a coil at just the right moment. The input is connected to a pickup coil that delivers a single pulse for every revolution of the flywheel. The output is connected to the ignition coil that supplies the high voltage pulse to the spark plug. Capacitor C1 stores the electrical energy and is charged up via D3. When there is a pulse at the input it triggers the thyristor into conduction, which connects C1 to ground so it can discharge into the ignition coil. That is all there is to it!
A single sided PCB has been designed for the circuit (the layout can be downloaded from [1]). However, note that the components are mounted on both sides of the board. This was necessary in order to keep the circuit the same size as the original CDI unit. Its dimensions are 59x38x24 mm. The photos of the prototype make this clearer. First mount D1, D2, DI1 and C2 onto the component side. You should then solder diode D3 and thyristor TH1 onto the board. These should be bent over so they’re level with the board, with D3 ending up on top of D2 and T1 on top of D1 and DI4. The MKP capacitor (C1) ends up alongside the board. The varistor (VR1) and resistor (R1) are then mounted onto the solder side of the board. And finally you should solder the three spade terminals onto the board. For the enclosure you can use a small box from Hammond (001100), Conrad Electronics part number 540830-89, although an acrylic homemade box (cassette or cd case) is an alternative. Once the board has been populated and connected you can check if the ignition produces any sparks. If it all works and the spark plug is sparking happily you can put the circuit in its enclosure and fill it with potting compound. If you fail to do this it is very likely that the circuit will soon stop working properly, since the ignition is subject to quite a lot of vibration.
There are two types of CDI unit in use, one made by Motoplat (red) and one made by Prü- frex (blue). In both cases the earth is connected to the middle connector of the CDI unit. If you accidentally connect the input and output the wrong way round the CDI unit won’t produce a spark. All you need to do when this happens is to swap the red and blue wires over. When the circuit was installed and put into use the effect was immediately noticeable. The engine runs much smoother at full throttle and it no longer misfires. The average fuel consumption was also found to have improved considerably to 166 mpg. Since the little engine has its own mechanical limitations (carburetor, exhaust, compression ratio), the top speed won’t increase by a huge amount: we found it to be about 2 to 2.5 mph higher. The biggest advantages are of course the better running of the engine and the improved fuel consumption.
Internet Link
[1] www.elektor.com/120601
