First of all a warning: THIS IS A PROTOTYPE!!!
Component values are a first try, but most of them are not critical, and they are dependent of choice of MOSFET transistor and other considerations.
For the same reason, the design was revised a number of times, the circuit diagram is as far as I can see correct, but no guarantee on my part.
If you decide to build it, make a prototype and test it BEFORE you try it on your bike, function testing is easy – spark or no spark??
The system works fine on my motorcycle – some 20 rides (including long ones) and no problem so far – it does not get hot, and there has not been any burnt components.
More generally speaking, this ignition can be used on any two-stroke engine, just adjust the number of switches to match the engine cylinder number. With some more modification, it could also function on a 4-stroke machine.
Prelude
For a number of years, I have used an electronic Boyer Bransden system, fixed ignition timing. It broke down in the end of 2021, and, for a number of reasons, I did not want to replace it.
One: it fires 3 times per cylinder for a complete rotation, meaning that 2 of these are actually wasted, it draws current and present spark plugs and ignition coils with meaningless load.
Two: it is a single system for all spark plugs, meaning electronic failure hits all 3 spark plugs – engine dies.
Three: it has a strange delay of ignition advance point at low revs, presumably to make the engine easier to start, but this makes the engine weak at low revs.
I wanted to try something new, and to fix these small flaws.
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After a lot of research, I ended up with a system as follows:
One: it is cheap
Two: individual circuit for each spark plug – if some electronic component fails, you still have 2 cylinders running.
Three: Flexible and also individual timing for each cylinder.
Four: only one ignition spark per rotation.
Five: low contact resistance – improved efficiency.
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The core of the system is a relatively new component, a small magnet activated Reed contact/switch, that can handle fast switching times. The one I use here for this prototype is rated at 200 Hz, translated to RPM it means 12.000 RPM - and I do not want my old engine to hit that kind of speed.....
There is a French option (costs a little bit more) that can handle 300 Hz, that would allow engine revs to reach 18.000 RPM, if you feel the need.
This reed contact controls an Integrated circuit, which in turn controls a MOSFET transistor, acting as switch for the ignition coil.
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The diagram is quite simple, and most component values are not extremely critical. One of the problems I met while working with different versions was electrical noise in the motorcycle circuit, that could trigger a spark at strange times, you want to avoid this, and the IC turned out to be essential for noise cancellation and precision in spark triggering.
Diagram explained:
Before a spark is triggered, Reed switch is open. R1, R2, R3 and R4 divides the voltage from MC Battery (and charging circuit) – meaning that we have something like 14 – 14.5 Volt from the system. Diode 1 blocks any strange current in a wrong direction, R1 and C1 are a simple noise filter, C1 should be 63V or more – there are transients with high voltage present in the system!!!
The input of the IC (TC 4220) should be high, meaning something like 7 to 9 Volts at input pin 2. R5 is a simple buffer resistor to stabilize the IC. C2 and C3 are a noise filter, C2 should be film type, 100 V is recommended, C3 should be a ceramic type with the same voltage as minimum. When input is high, IC output is also high, it is VERY important that IC is non inverting – TC422x exists in a number of versions, some are inverting. R6 and R7 divides the IC output voltage to reduce to a suitable value for the MOSFET, the MOSFET conducts current, and current flows through ignition coil primary winding. R8 is a Gate buffer, there is a safety Zener Diode between gate and source, here it is 10V, it depends of choice of MOSFET. The other Zener Diode is a 200V safety between drain and source, the ignition coil can generate vicious kickback voltage when it generates the spark, and this can be harmful or lethal for the MOS. Both Zener's should be rated at min. 1.6 W of power.
There is also a safety diode close to the ignition coil – choose a power type – 5 - 10 amperes and 200 V or similar.
When the magnet closes the Reed switch, the IC input goes low, and the output follows. This pulls the MOSFET gate down, and it stops to conduct current in nanoseconds. The magnetic field in the primary coil breaks down, and generates a high voltage burst in the secondary coil – this leads to the spark in the spark plug and the ignition of air/fuel mixture.
Some comments about components.
I recommend the MOSFET transistor to be minimum 200 V, 6 – 8 A, meaning something like a 100 – 125 W device, in a TO 220 housing or larger – as long as it is a switching type MOSFET, it is fine. You can choose more expensive devices, higher currant, lower on-resistance and so on, it is up to you. Suitable IGBT's are also an option.
IC1 should be non-inverting, TC4220, TC4222 or similar. All resistors are standard, 1 to 5 % tolerance, ¼ or 1/3 W, no substantial current runs through them.
Capacitors and diodes should be so to speak over-voltage, to be on the safe side, ZD1 and ZD2 values are important, and related to the choice of MOSFET transistor. Some MOSFET transistors have an internal protection diode between source and drain, but in this case, I prefer to be more safe than sorry, component breakdown in the middle of a long journey/vacation is not what you want.
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When it comes to PCB mechanics, pictures says more than many words – everything was mounted on a experimental board, and fastened to a ex-CPU cooling fin. No heat worth mentioning - hand-warm after several hours of riding – mostly due to the fact that transistors operate as switches, either fully on or fully off – the transistor does not work as a resistor, accumulating thermal energy.
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Mechanics
When it comes to mechanical solutions, my suggestion is, that you start with the rotor. It can be made quite easy, and you can also use a very small neodymium magnet. In this way, the rotor is small and light, easy to balance.
When the rotor is finished, make a round aluminum mounting plate, fix center hole and oval cut-out, and mount your rotor. Turn the engine, so it is aligned with one timing mark. Test with one reed for contact point – test with a battery and a small bulb, or an ohm-meter to find the exact spot. Make 2 marks for the screws at the middle of the oval holes in the reed switch, to allow for adjustment each way. Then drill 3 x 2 holes for the reeds, spaced exactly 120 degrees. Mount one reed, check timing again, and mark the oval cut-outs for engine mounting screws.
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More mechanics – the layout allows for general adjustment for all 3 reed switches, AND for individual adjustment of each switch. To get some sort of precision, you will need a stroboscope ignition timing device, and align the timing mark on the engine – see pictures!!
When it comes to timing, I would like to mention A. Graham Bell: Two-stroke Performance Tuning – an excellent book about what to do when it comes to serious engine set-up. Standard advance figure for Suzy is 24 degrees, and it is my guess that this figure is related to a desire to get maximum power at typically 6500 RPM, to have an impressive horsepower figure for motorcycle magazines. Personally, I rarely go up to 6500 RPM, I need more power at 3000 – 5000 RPM.
Adjustment notes: When you turn the alu mounting plate and adjust ALL switches, according to my calculations, 0.87 mm movement at the mounting screws corresponds to one degree change.
The diameter for the timing indication plate is smaller, and you can't measure anything while the engine is running, you have to trust your strobo light and your eyes.
Again, according to my calculations, 0.523 mm offset indicates one degree change.
A. Graham Bell has diagrams and measurements suggesting something like 25 – 27 degrees of advance figure at low revs, and I will not argue with this. As a consequence, my ignition is set at around 25 – 25.5 degrees general, and the middle cylinder, which has always had a somewhat darker spark plug, at 25.5 – 26 degrees. For me, this works very well, clearly more power with low revs, and no problem whatsoever in the 4000 – 5000 RPM area. Suzy has not been up on a test bench to verify this, but, generally speaking, if you are convinced/if you sense that a change (in this case an increase) of power is noticeable, and this is clearly the case for me, we are talking about a power increase of around 10% or more.
If you want to have more control, an option is to test the bike on a test bench of some sort.
Bottom line for me is clear: this is an improvement!!
Price: depends of components and what you choose to use, the MOSFETs, the reed switch and the IC are the most expensive components. MOSFETs can go from 5 Euros each, reed's are maybe 7 - 10 Euros, the IC something similar – all dependent on supplier. The rest are cheap components. All in all, it should not cost more than max. 70 to 100 Euros in total.
