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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks, PM, I've been reading some papers and associated patents relating to QWCCR, but I think I'll stick to conventional spark plugs for now.
Sulaiman, The piezzo-sensor concept also sounds interesting. I can implement 'mapping' or other forms of advance/retard control later if required. All it involves is a sensor at ~40 degrees BTDC, and an algorythm to delay the spark. (along with other sensors like gas flow meters, Llamda sensors, etc.)
Initially I'll run with fixed ignition, or manual advance/retard. (A lot of racing engines actually run with fixed ignition, especially if maximum torque is at low RPM.)
Multiple spark ignition systems do give a better burn, especially at leaner mixtures where ignition is more difficult, and will allow more advance before pinking, etc.
As I see it, there are basically three ways to achieve multiple sparks:
Some form of spark tank that is charged initially, and then rings until it decays.
A high speed coil that is fired repeatedly for a short burst of sparks.
Gradually charging a tank until breakdown is achieved, then continuing to supply power to maintain the discharge for a brief period.
Most of the systems currently available seem to be similar to the third option mentioned above, but I may experiment with the other two options.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, the simplest solution I can think of, and I've no idea if it will work, is to trigger a 555 monostable with the ignition sensor, points, or whatever, the monostable then turns on a 555 astable for ~one millisecond, the astable is set to ~150kHz, so in one millisecond it switches ~150 times. This drives a 1200V MOSFET, which switches a flyback (wound on N87 core material?) primary which is fed from ~300V(?). Flyback secondary connects to spark plug.
]pdf_n87.pdf[/file] (N87 datasheet)
This will give a burst of 150 sparks in one millisecond every time it is triggered, or will it?
Can anyone point out any expected problems?
I know I'll be 'pushing' the ferrite, but duty cycle is less than 5% so a bit of overheating shouldn't be a problem.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
Each pulse will have an oscillatory tail of damped waves, so you'd have to check that the gap is extinguished when you want to be.
Are we talking a millisecond or two here? That won't be a problem at all. Would putting a snubber on the primary help?
Might it actually work if I use a suitable ferrite and get the gap right?
I don't know what effect (if any) the combustion of hydrocarbon gases inside the arc might have on the extinction of the arc. You'd have to ask someone like Sulaiman that has a good understanding of ignition systems. :)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Five to ten milliseconds shouldn't be a problem here, but I can 'scope it when I get it going.
Ah, Mr Ash, I found out some info that may help you while I was supposed to be getting on with something else this morning:
There may be time constraints in implementing your multi-spark ignition system. Turbulence may suck the burning mixture away from the plug and if fresh un-combusted mixture is present, it may be ignited by the next spark. Apparently, the sparks need to be less than a few degrees apart to work effectively. If you look at the relationship between time, angle, and r.p.m., you can see the time constraints imposed: At 1000 r.p.m. the crank will travel 6° in only 1ms. 1° will pass in just 170 µs.
Perhaps multi-sparking may only be effective at low engine speeds.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The method Sulaiman described, adjusting ignition timing on the fly with feedback from a knock sensor, is pretty much standard in modern car engines.
Ignition of lean mixtures is still a "hot" research topic as far as I know. The classical ignition system produces a quite long duration spark that does a better job of it than CDI. One old trick was to set up the fuel injection system for a "stratified charge": a pocket of relatively rich mixture around the spark plug that would ignite easily and set off the rest.
Lean burn engines ran into some problems when limits were introduced on NOx emissions. A lean mixture makes more NOx and a catalyst does nothing about it.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Ash Small wrote ...
Five to ten milliseconds shouldn't be a problem here, but I can 'scope it when I get it going.
Ah, Mr Ash, I found out some info that may help you while I was supposed to be getting on with something else this morning:
There may be time constraints in implementing your multi-spark ignition system. Turbulence may suck the burning mixture away from the plug and if fresh un-combusted mixture is present, it may be ignited by the next spark. Apparently, the sparks need to be less than a few degrees apart to work effectively. If you look at the relationship between time, angle, and r.p.m., you can see the time constraints imposed: At 1000 r.p.m. the crank will travel 6° in only 1ms. 1° will pass in just 170 µs.
Perhaps multi-sparking may only be effective at low engine speeds.
I've read that some of the systems on the market produce 'up tp ten sparks in 0.5mS', and as a starting point I chose 150 sparks in one mS, as this equates to 150kHz, which is easily attainable by the flybacks used in 150kHz monitors, so should be achievable.
This is only a starting point, though, I could reduce the burst length to 0.5mS and still have 75 sparks. Any residual ringing won't be an issue for another 360 degrees or so (on a four stroke engine).
I'm also not concerned with emissions, only increasing power and achieving smoother running, although any increase in burn efficiency will improve emissions. I also don't want additional sensors (at this point) as reliability = simplicity.
In my opinion the 'knock sensor' method is not ideal because a better ignition system would allow more advance, and therefore more power (more reliable ignition at more advance).
The engine in question here was successfully raced (second and two thirds in national championships) with fixed ignition @32 degrees BTDC, and was fitted with an Interspan ignition unit (see link above). (the length of the spark burst is adjustable on these units- they run from a battery pack for reliability, so you set the burst length so you can get to the end of the race. This won't be an issue for me as I'm fitting an alternator).
The engine will also be fitted with carburettors, not fuel injection. Probably SU's, probably two, as it is a parallel twin with a 180 degree crankshaft, which means (I think) that there will be an overlap where the inlet valves on both cylinders will be open at the same time, which means a single carb won't work (from what I've read elsewhere). I will also need two coils, one for each cylinder, as I can't use the 'wasted spark' system used on most 360 degree twins.
I could just fit points and coils, or a single spark electronic unit, but if a multi-spark unit is as simple to build as I've outlined above, I may as well build it as it should have benefits. I can always add 'advance mapping' later if I decide to.
As far as I'm aware I don't need to worry about emmissions regulations, but a multi-spark system should improve efficiency.
The task is now to design and build a suitable flyback transformer, which only needs to run at a maximum of <=5% duty cycle. (realisticly, if I reduce burst length to 0.5mS, and if it's running at cruising speed, it's only ~1% duty cycle, so I should really be able to 'push' the ferrite, etc).
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Perhaps it would help if you drew up an equivalent circuit for the transformer, the HT cable, and the plug, with all the different inductances, capacitances, and resistances, just to make sure it could really fire 150 pulses in a millisecond.
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Electronic ignition thread
