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Ash Small

Sat May 18 2013, 10:00AM

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

I want to build an electronic ignition system. Requirements as follows:

Multiple spark (each time it fires I want it to have a series of sparks to lengthen the effective ignition time for better burning, especially with leaner mixtures). I'm not sure what the optimum duration is.

must be HIGH ENERGY ie BIG SPARKS

I don't need 'advance mapping' or anything at this stage. It will be 'fixed ignition' or 'manual advance/retard'.

Not sure whether to 'wind my own' pickups, use 'off the shelf' pickups, or trigger it with points.

Any ideas how I can achieve these goals SIMPLY and RELIABLY?

johnf

Sat May 18 2013, 10:09AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Ash
Buy a MSD ignition unit
like

Proud Mary

Sat May 18 2013, 10:16AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The transfer efficiency of ignition coils is very low.

This paper claims that adding a peaking capacitor of 90 pF at the plug increases the transfer efficiency from less than 1% to "approaching 50% with peak current discharges of over 1000 amps."

Ash Small

Sat May 18 2013, 12:23PM

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

You should know me better than that by now, John

.

The Interspan unit would probably be my 'unit of choice' if I wanted to buy one

.

I can't get that link to work, PM. I managed to get to the main website, though, but not much technical info there.

This article has quite a bit of info regarding using a capacitor and a couple of MOSFETs to achieve multiple sparks:

I'm still reading it.

I've done some quick calculations, and worked out that I want the 'spark burst' to last 'of the order of' one millisecond (I believe some commercial systems last ~0.5mS), and my first thoughts are (giong right back to 'first principles') that it might be possible to build a resonant 'tank circuit' utilising the spark gap, a capacitor and an inductor, which resembles the primary circuit of an SGTC, which produces a burst of sparks lasting ~1mS each time the capacitor is charged by a coil. The coil will fire at ~50Hz max, I think. (The 1mS burst length figure is a maximum, 0.5mS or less will probably suffice)

Efficiency isn't really an issue here, as an alternator will be fitted, unlike a lot of racing engines which rely on batteries, etc.

Patrick

Sat May 18 2013, 01:53PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well the HEI coils are better at energy transfer than the old oil-can types with the "I" cores, your planning to make your whole system an elaborate SMPS, but I think your better off with some commercial product like the MSD stuff.

Ash Small

Sat May 18 2013, 03:05PM

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

Patrick wrote ...

well the HEI coils are better at energy transfer than the old oil-can types with the "I" cores, your planning to make your whole system an elaborate SMPS, but I think your better off with some commercial product like the MSD stuff.

Yep, HEI coils may well be the thing to use, but I've also thought about using flyback topology with some fast (>=100kHz) magnetics, and just giving it a 1mS blast fifty times a second. 50mS is ~5% duty cycle, so I could really push the Bmax, etc. (I assume a big gap on some fast 'E' cores, or whatever, could work quite well at 5% duty cycle?)

I can't believe that people here on a 'make your own BIG sparks' forum are actually advising me to go out and BUY something

EDIT: Does flyback topology have a Bmax?

Proud Mary

Sat May 18 2013, 04:30PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The link I posted above works for me, Mr Ash, so I don't know what is wrong there.

Here is a paper that describes a set up similar to what you have been describing: High Frequency Ignition System for Gasoline Direct
Injection Engines

Ash Small

Sat May 18 2013, 07:10PM

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

Proud Mary wrote ...

The link I posted above works for me, Mr Ash, so I don't know what is wrong there.

It's working now. It's quite interesting. I use un-suppressed plugs anyway, which, as the article suggests, have ~10pF of capacitance anyway.

The other article is also very interesting, it demonstrates that a 'tesla coil' approach could work quite well. Maybe some form of SSTC, or even DRSSTC circuit could be employed here, operating in 'burst mode', very similar to an interruptor?

Proud Mary

Sat May 18 2013, 11:44PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

There's a paperback book about developments in RF plasma sparking plugs for sale on Amazon here:

Radio Frequency Plasma Spark Plug Development and Engine Testing: Design of a quarter-wave coaxial cavity resonator for use as an ignition source for an internal combustion engine

I think all this stuff about transmission line spark plugs is fab - such a refreshing change to all those dreary laminated cores drowning in oil inside a tin can like so many copper and iron sardines.

Sulaiman

Sun May 19 2013, 07:22AM

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

Although there are possible benefits with multiple ignition sparks
I believe that a reliable spark at the correct time is better.
How will you map the ignition timing to the engine for different speed/load/demand?
I worked on an ignition system that does not need a map,
fit a piezo-sensor to the block to detect pre-ignition ('knocking', 'pinking' etc.)
with a micro and crank position sensor keep advancing the spark until pre-ignition then keep the engine on the edge of pre-ignition.
This gives the maximum power and efficiency for any air/fuel ratio.(best spread of flame front)
This was using a 70's cpu and I don't think that we patented it.
I haven't re-visited this area for decades (can't even remember the best frequency range for pre-ignition detection) but thought it may interest you.
Don't forget, any electronic delays are equivalent to timing delays
(time between 'spark' command and actual ignition spark occurence)
(e.g. @6000rpm 1rev.=10ms, so 100us delay = 3.6 degrees timing error relative to low rpm)

We tried something similar with feedback control of a carburetor air/fuel ratio based on feedback from a CO/NO sensor, but the response was too slow.

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