Ignition circuit problem ???

some advice from the petrol heads please (not to exclude wise non-petrol heads)

after a few weeks I have finally got my Jan Ridders SABINE 2 stroke to run. I have issues with it which I think are related to the ignition circuit, after getting better results from changing the battery upwards a couple of times. I am now using a 12v 15AH dry fit battery with a 12v Lucas coil and a condenser from a mini, which I don't know the value of and a v3 micro switch as a contact breaker.

with a fully charged battery the engine will run for about a minute and the fade out. The terminal voltage falls to @ 11.9v under load.

i would welcome some advice on the combination of the components. Is the coils too large for the battery?

thanks in advance

Bob..

My motorbike with an electric starter runs an 11ah battery so yours should be OK. Maybe you have a duff coil, measure it's current consumption it should be no more than 3 - 4A with the points closed and engine stationary. It will be less than this with the engine running.

Derry.

Bob,

In the Stationary engine world we use the choke coil from a fluorescent light fitting which will run with a dry cell battery, very rarely do you see a car coil but if you do it usually connected to a car battery, how hot does the coil get as it could be braking down when hot.

Martin P

Edited By martin perman on 28/06/2013 20:26:16

Hi Bob,
I'm surprised that the battery voltage drops from its fully charged voltage (Which is probably about 13 volts.) to 11.9 volts after just one minute. The ignition circuit should still work well at 11.9 volts. When STARTING the engine in a car the battery voltage will probably be less than this as the starter motor draws a few hundred amps. I suggest checking the current that is being drawn both with the engine stopped (in a position with the contact breaker closed. ) and with it running. I would expect it to be less than 5 amps. The current with it running should be less than when it is stopped. I have an ignition coil which is probably similar to yours so I will do some current measurements tomorrow for comparison. Are you sure that the engine stops due to ignition problems and not a fuel problem ? Could it be it runs for a minute with the contents of the float chamber ? One other thing is I suspect your engine runs at a much lower speed than a car engine so it is possible the coil may be heating up. Check if the coil has heated up at the end of the one minute run.

Les.

Thanks for the comments. The coil does get vey hot. I'll take some current readings tomorrow. Would it break down in such a short time.? Can I test it? I can measure the resistance of the primary and secondary windings. Can I test between with a megger?

Bob..

Bob,

Another thought, are the points closed until needed or open until needed, if shut you maybe shorting the battery.

Martin P

The points are normally closed and open at TDC.

hi Martin I'm very intrested in your sugestion about using flouresant light fitting chokes on the ignition of stationary engines, I have never heard of this before some more information would be much appreciated. Regards Farmer Dave

Make sure the coil isn't rated for use with a ballast resistor, which would drop the nominal 12V from the battery to approximately 7V at the coil during normal running. The resistor is switched out of the circuit when the starter is operated to ensure the coil is not run under-voltage when battery voltage drops under the load of the starter motor.

The coil getting so hot so quickly does suggest it is running at too high a voltage.

Anthony

I've used the ballast from light fittings but only on a low tension setup with an ignitor, don't know how they would fair for a HT sparkplug.

J

Bob,

You say that 'The points are normally closed and open at TDC.'

This is way too long @300 degrees plus, dwell time (period of points being closed) should be about 40 degrees I'm going on the old mini distributor points setting information.

THis would indeed make the coil run hot and drop the battery voltage.

Lofty

Edited By Lofty76 on 29/06/2013 08:50:45

Hi Bob,
Here are some measurements I took from the ignition coil that I have . (Lucas but model number not known.) Primary resistance 3.3 ohms (This would draw 3.6 amps when the points are closed.) Secondary resistance 4.5 K ohms. I am using it with a 0.22 uF capacitor but I think about 0.5 uF is normally used. I am using this with a pulse generator for igniting the burner on my furnace. At the point when the points (High voltage transistor in this case.) the peak voltage on the primary reaches about 180 volts My driver circuit generates a pulse every 80 mS (12.5 Hz) The current is switched on for about 2 mS each cycle. It is driving a spark plug with the gap opened up to about 70 thou and even with this gap it works OK even with the voltage reduced to about 10 volts. Anthony's (LAD machining) point about the coil being designed for use with a ballast resistor is worth checking. (If it has a low primary resistance it would suggest that it is this type.) Lofty's comparison of dwell angle is not a fair comparison as the mini is a 4 cylinder 4 stroke engine and yours is a single cylinder 2 stroke so 40 degrees on a 4 cylinder engine would be the same as 80 degrees on a single cylinder 2 stroke. If your engine is slow revving then it should work OK with a smaller dwell angle which would reduce the average current consumption. The points only need to be closed long enough for the current to have reached a high enough value. The rise of current in the primary is initially controlled by the primary inductance but then is limited by the primary resistance.

Les.

Bob,

Go look at these links **LINK** for low tension
**LINK** for buzz coil.

Martin P

Martin, Les

thanks for your feedback. The coil values are similar to the values I measured yesterday. I plan to replace the capacitor for a known value. I had a bit more success yesterday, getting it to run for @ 3 minutes.

**LINK**

From what I'v read of this motor, it does not have a great reserve of power, so it might be worth doing as I have done with stirling engines, run the motor over with an electric motor for a while with the spark plug out. I build my engines now with the power piston a little less tight in the bore, but even then the power increases with running. Ian S C

Thanks Ian. I agree friction may be an issue. I'm using a battery drill to start it, and the more I've tinkered and run it, it's now running for longer periods. It ran well for @ 5 minutes by the end of yesterday. I would like to re-visit the ignition circuit and reduce the size the dustbin dimensioned coil and the battery. This is a new topic for me so plan some holiday reading and research on the subject, and hopefully details of a circuit to build.

Regards

Bob..

is a v3 micro switch widely used for this?...seems a bit light.. and maybe too slow mechanically....

contact points would typically be 4-6 mm dia. the contacts in a v3 are around 1-2 mm diam and points usally have quite stiff springs to get the flight time down ..v3's are aimed at light operating forces... but as I say if it works elsewhere.. then that might not be the problem..

btw.. many renault ( and others) use a on plug fitted coil..about 30 mm dia ..this might prove more fitting in scale to your engin..**LINK** for example and http://www.qxcomponents.co.uk/CoilCatalogue.pdf

Edited By jason udall on 01/07/2013 17:08:12

Thanks Jason

There is a thread on the 'on the plug' coils in another site to do with model engines, I'm just an occasional visitor to that site, I think there are two types, one twelve volt, the other I think is higher.

Ian S C