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Ash Small

Thu May 23 2013, 11:56AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

dex wrote ...

Ash Small wrote ...

Specifically, what impulse shape is best?

in what sense "the best"?

The best for lower breakdown voltage.

dex

Thu May 23 2013, 12:15PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

Ash Small wrote ...

The best for lower breakdown voltage.

for spark gap,short impulse,systems i don't know.speaking generally,it is probably qcw mode with a long excitation time.

Steve Conner

Thu May 23 2013, 12:26PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think the voltage will be limited by what would flash over the spark plug insulator on the outside. Maybe 50kV or so in clean dry lab conditions. Out on the road, who knows?

If I wanted big sparks, I'd just buy a heavy duty double-ended ignition coil like a MSD Blaster, and use plain copper ignition leads, not the ones with resistive cores. It will kick out a lot of RF interference, but with a good-sounding engine there is no need for a radio.

Drive it with an optical sensor picking up reflective patches on the flywheel, and some variant of the old Lucas transistor ignition amplifier, using an IGBT. (Ignition IGBTs with built-in overvoltage protection are available and used in almost all modern car ignition systems.)

Ash Small

Thu May 23 2013, 02:39PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

I think the voltage will be limited by what would flash over the spark plug insulator on the outside. Maybe 50kV or so in clean dry lab conditions. Out on the road, who knows?

If I wanted big sparks, I'd just buy a heavy duty double-ended ignition coil like a MSD Blaster, and use plain copper ignition leads, not the ones with resistive cores. It will kick out a lot of RF interference, but with a good-sounding engine there is no need for a radio.

Well, the MSD Blasters claim 45kV. A double ended coil with wasted spark is no good here because it's a 180 degree crankshaft, so I'll need two of everything. I use copper cored HT leads anyway (although the suppressed ones do make good HV resistors

).

I'll have to drive it from the exhaust camshaft, to get the correct timing (pickups/points set at 90 degrees on the camshaft = 180 degrees +540 degrees (720 degrees total) on the crank. I believe the reluctor shape on the old Lucas units provided the advance/retard curve, the amplifiers themselves had no advance mapping or anything (Lucas RITA units).

That is the 'Plan B', if I need one, though.

Aragorn

Thu May 23 2013, 02:44PM

Registered Member #18516 Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38

Your point about modern engines running smaller sparks doesnt agree with my findings in the world of VAG 1.8T engines, and i'd expect you'd find the same in general across most manufacturers.

The first models using that engine built ~95-'00 had simple dumb coils (one per cylinder), with an external IGBT module like steve describes, in this case mounted on top of the airbox with a big heatsink.

These worked well enough, but later they switched to a combined Coil/IGBT unit for each cylinder. Many folk have converted to the newer system in search of stronger sparks, because when tuning the engine waay beyond how it left the factory, the dumb-coil setup starts to struggle, requiring the spark gap to be closed right down to prevent missfires.

These later coils still suffer issues, at big power levels, and very recently, people have started experimenting using the coils from the newer direct injection "TFSI" engines, which are an almost direct drop in replacement for the combined coil/IGBT units, and have found further increases in spark strength. The TFSI engines use larger spark gaps from the factory (1mm vs 0.8mm), and as a result the coils are more powerful to match. The end result being you can retain the stock gap, and get smoother running without missfires at big boost pressures.

Theres most certainly a move towards stronger sparks, certainly in turbocharged engines, and concerns over electrode wear are largely mitigated with most vehicles coming from the factory with platinum spark plugs, which in the factory tuned engines can last for 60-80k miles.

Ash Small

Thu May 23 2013, 04:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Aragorn wrote ...

Many folk have converted to the newer system in search of stronger sparks, because when tuning the engine waay beyond how it left the factory, the dumb-coil setup starts to struggle, requiring the spark gap to be closed right down to prevent missfires.

I didn't mean they use smaller sparks, just that they can't justify the cost of more powerful systems with platinum plugs, or whatever, on most mass-produced vehicles, and therefore resort to compromises like anti-knock sensors, etc.

Closing up the gap increases the dwell angle, which allows the coil to 'charge' to a higher voltage before discharge. Turbo engines (or any engine with a higher effective compression ratio will require a higher voltage spark before discharge occurs.

I think we're mostly in agreement regarding the advantages of bigger/longer sparks here.

Sulaiman

Thu May 23 2013, 07:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Steve, " and some variant of the old Lucas transistor ignition amplifier "

I was part of the team that developed that amplifier !
the designer of the green logo on the heatsink earned more than my annual salary at the time!
This project is why I left, it was to have been part of a simple reliable fully electronic system to replace the complex un-reliable electronic/mechanical system in production,
unfortunately the designer of the old system was promoted to a position where he could keep his baby in production - effectively blocking development.

+1 to using ignition igbt.

I think that;
a double-ended coil would not give the spark performance of two separate coils
you will have significant insulation problems, especially in wet conditions
smearing everything in "vaseline" (petroleum jelly) helped, and can be cleaned off.
some spark plug types are much better than others .. I'm out of date
choosing the 'temperature' (hot/cold) of the plug is also important for reliable ignition.

Theoretically,
a small cap near the plug with inductance between the ht/lead/cap and plug
forms an rf resonant tank .. more plasma-like than arc-like .. difficult to physically implement reliably, or at least it used to be, not up to date.

Ash Small

Thu May 23 2013, 07:36PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

I was part of the team that developed that amplifier !

Are we talking about the RITA system, Sulaiman? If so, can you confirm whether the ignition advance was controlled by the shape of the reluctor? This is what I was told when I bought mine. ( I think mine was the '5PU' type with 'AB11' amplifier.)

paris

Thu May 23 2013, 09:37PM

Registered Member #3042 Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121

Ash,
61 - 64 Harley had 2 coil/2 points/2 caps and 1 lobe.
Crane ign today uses the fly weights but replaces points with a plastic disc with 2 small magnets that trigger a sensor.
180 degrees must be BMW .....or zundapp altho theyre a 170 degree v twin

Ash Small

Thu May 23 2013, 10:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

paris wrote ...

180 degrees must be BMW .....or zundapp altho theyre a 170 degree v twin

No. Although the crank on a BMW IS 180 degree, the cylinders are also opposed by 180 degrees, so a BMW actually fires like a 360 degree twin.

This is a parallel twin with a 180 degree crank, so one piston rises as the other falls. (recent innovations have also seen 90 degree, and even 76 degree cranks, in attemps to improve balance factors).

I have read, this afternoon, that I could run a wasted spark, as long as I don't run it on methanol.....but I've only read that once so far.

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