Model Engine Maker

Supporting => Engine Ancillaries => Topic started by: geoff_p on May 16, 2014, 08:04:16 PM

Title: Another ignition question
Post by: geoff_p on May 16, 2014, 08:04:16 PM
I'm attempting to make an experimental horizontal, gear-less, 4-stroke engine of my own 'design' (a euphemism for many excess tapped holes where yet another daft idea didn't work.)  The ignition cam is directly on the crankshaft, there being no half-speed shaft, so clobbers the 'points' every revolution.  The wasted-spark principle, see later.

From an idea by Jan Ridders, I've used a standard microswitch for points, connected to a 'Pegasus'-brand electronic ignition, which I bought from Hobby King in Singapore.  The original Hall-effect sensor was lost some time ago!

My problem is that there is a spark on points opening, and another on points closing (about sixty-degrees later) so I'm actually getting 4-sparks per revolution: one at firing point; another during the power-stroke, which is only trying to re-ignite what's already burning; the third spark is at the end of exhaust but the fourth one is during inlet/suction, and I feel this is giving a problem.

Presumably, even with a Hall-effect sensor, there would be two-sparks because the sense wire has to go high- and then low-voltage at some point?

Question:  is there any way to suppress the second spark of each pair?  (I can live with the wasted-spark at the end of exhaust.)


Title: Re: Another ignition question
Post by: AOG on May 16, 2014, 08:51:48 PM
Try putting a diode inline with the switch. That should help prevent reverse current flow when you open the switch.

Title: Re: Another ignition question
Post by: PStechPaul on May 16, 2014, 09:51:55 PM
It sounds like you are getting contact bounce when the switch closes. This would be unusual with a microswitch, but it's possible that the contacts or mechanism have been damaged. Also, perhaps the ignition circuit does not have proper debouncing. You might be able to fix this with a small capacitor (like 0.005 uF = 5 nF) across the switch contacts.
Title: Re: Another ignition question
Post by: gbritnell on May 16, 2014, 10:07:15 PM
Hi Geoff,
There should only be a spark when the Hall turns on or off depending on how the ignition was made. This will be the same with points or micro switch. I'm guessing that the switch is defective in some way causing a bounce or poor connection thereby giving the second spark. The only way to really tell is to operate the switch at slow speed and see what happens.
Title: Re: Another ignition question
Post by: geoff_p on May 18, 2014, 03:27:16 AM
Thanks, all of you.
Turning the engine over by hand (very slowly) allowed me to see there is a succession of sparks even before the micro-switch snaps over.  Then again as it begins its return journey.  I also tried putting a small capacitor across the points, followed by a diode (instead of the capacitor) but neither had any effect.

My voltmeter across the points showed 4.90V initially.  This dropped to near-zero for a brief moment as the micro-switch was moving but before it 'snapped' across to the other position, whereupon it returned to 4.9V.  Implication?  Jiggered micro-switch, confirmed by the resistance of the switch staying at several Meg-ohms even after the 'snap'.

So I replaced that switch with another brand-new one with much better results, although there is still a spark at each of opening- and closing-points.

The spark doesn't seem as 'strong' as before - I suspect I have now jiggered the CDI itself.  Oh well, time to save up for a new unit.