how to trigger MS on a small 25cc engine?

[speedracing944]

I am working on a project at engineering school where we are converting a small 25cc 4 stroke engine to run on gas phase propane. I am using MicroSquirt to meter the fuel while maintaining the original ignition system on the engine for simplicity. The MS is all wired and I synced it with the laptop and MegaTune software last friday. I am getting signals from all the sensors.

The question is how do I trigger the MS off of our ignition coil? The coil is mounted right above the flywheel. The flywheel has magnets mounted inside and must magnetically charge the coil as it passes underneath. The only wires coming off of the coil are
1) The ignition cable which goes to the sparkplug
2) a ground wire which is switched (the swith is closed when the engine is running and open when you want the engine to shut off) to allow the coil to generate spark

This isn't a typical vehicle coil which I am used to seeing, it is much more rudementary in execution.

So how do I trigger the MS?

This is the last piece of the puzzle before we attemp to start the engine.

Speedy:)

[BillClem]

You may need to use an oscilloscope to see if that grounding wire is the 'hot' end of the first or primary mag coil winding. Most of these magneto coils are basic transformers with a primary and secondary winding but there are a number of different magneto configurations, some which use a set of points under the flywheel running from a cam on the crankshaft (wasted spark at crankshaft speed) and also mag coils that are internally electronically interrupted as well. But any of these will have a signal on the grounding shut-off wire (reason for a condenser) - so a 10K ohm resistor pull-up to positive on that wire would give a signal at engine speed and still allow the coil to inductively charge via the permanent magnets in the flywheel. The switched wire is normally a coil grounding or shut-off and basically closes the points so there is no primary voltage to induce the secondary high spark voltage.

You may also use a tickler coil on the spark plug lead but again; use an oscilloscope to check out the resulting signal strength - don't fry something....

The configuration of MS explains these tach inputs in detail....

Bill C.

[speedracing944]

So I should use an oscilloscope on the switched ground wire to test for a sine wave and what the voltage is? I should expect the voltage increases with engine rotational velocity correct?

This coil is not linked to a cam. It is just bolted right above the flywheel which has magnets mounted in it.

[BillClem]

So I should use an oscilloscope on the switched ground wire to test for a sine wave and what the voltage is? I should expect the voltage increases with engine rotational velocity correct?

Yes and Yes, the voltage will be increase with speed of the flywheel magnet.

The tickler coil may be your best and most tailorable tach signal. Wind about ten turns of single strand around the high tension spark plug lead, ground one end and begin examining this signal on your o'scope. You can add or subtract turns; use some enhancing circuits to shape and limit the signal such as a Schmidt trigger....or gates, or op amps to make it a nice square wave, spike or whatever...

This coil is not linked to a cam. It is just bolted right above the flywheel which has magnets mounted in it.

Then it is either reed switch triggered, electronically triggered or just triggered by pure excited coil. Probably the latter....especially if there are two magnets in the flywheel. That is a field reversal type that collapses the magnetic field very quickly which induces an intensive high spark voltage.

Does that mag coil have another solid grounded lead plus the 'shut off' lead (plus the high tension lead) for a total of three wires from the coil?

[speedracing944]

there are what looks like 2 sets of magnets close to each other on the flywheel.

If I remember correctly there are only 2 wires from the coil. I will check for sure when I get back to school.
1) the spark plug wire going to the spark plug
2) the ground wire which goes to a 2 position, normally open switch. When I want to start the engine I switch this switch to closed position before I rotate the engine.

Speedy:)