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paris

Wed May 22 2013, 08:52PM

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

thats the guy ,thought Id be conservative and say 100kv . :)
so air cores are the way to go hmmm....

Steve Conner

Wed May 22 2013, 09:26PM

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

I've worked on motorbike engines that had the coils mounted directly on the plugs. I don't know about the Skyline GTR, but my humble Swift Sport has one coil per plug, mounted separately with short flying leads to the plugs.

Many modern engines have a "coil pack" that combines one coil per plug and IGBTs to drive them from logic-level signals provided by the ECU, into one big, expensive, unreliable plastic lump.

Another popular device is the double-ended coil. Each end of the secondary winding drives one spark plug. Since the gaps are effectively in series, they always fire together, but only one has anything to ignite. This is known as a wasted-spark system.

I once had a fine head-scratching time trying to get a Honda ECU designed for two double-ended coils to fire four single ones.

Aragorn

Wed May 22 2013, 10:01PM

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

I think the biggest gains to be had here (in power and economy terms) will come from having proper mappable spark timing which you can properly adjust against load and rpm.

Having some massive spark at the wrong time is not going to help you extract maximum power from the engine, and fixed spark timing is a massive compromise, much like Carburettors over EFI.

You should be looking at something like Megajolt, driving an individual coil-per-plug (or coil-on-plug) or a wasted spark type arrangement as Steve mentioned in his post.

While you are right in suggesting that modern engines are built to a price point, THE biggest factor in any modern engine design is fuel economy, and a big part of that means optimising the spark timing to eeke out every last drop of energy from the injected fuel quantity. Those aims are exactly the same as trying to get maximum power.

Ash Small

Wed May 22 2013, 10:56PM

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

paris wrote ...

thats the guy ,thought Id be conservative and say 100kv . :)
so air cores are the way to go hmmm....

I've been considering some form of 'Tesla coil' ignition system, but I think I'm starting to 'get my head round' the 'black art' of flyback design.

Aragorn wrote ...

While you are right in suggesting that modern engines are built to a price point, THE biggest factor in any modern engine design is fuel economy, and a big part of that means optimising the spark timing to eeke out every last drop of energy from the injected fuel quantity. Those aims are exactly the same as trying to get maximum power.

I disagree on two points. Firstly, while correct 'mapping' will give the best performance, a bigger spark helps considerably, and doesn't take so long for full combustion to occur, so can be more retarded, but will also ignite the fuel at more advance. Also, a longer burn spark (or 'burst') will ignite leaner mixtures. The 'mass producers' of vehicles have two objectives. one, keep costs down. two, smaller sparks = longer spark plug life = greater reliability over time.

EDIT: My current thinking is that you only require sufficient voltage to achieve breakdown, from the point of breakdown you want as much current as possible.

paris

Wed May 22 2013, 11:41PM

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

the international harvester link I put up also talks about the thermionic emission theory especially with wasted spark ignition.
1 electrode has to emit electrons thats why they come up with diff metals for spark tips. theres a pencil trick for polarity too.
I like the possibilities your conjuring up even tho I cant see anything at the mo

Ash Small

Thu May 23 2013, 12:35AM

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

This is an interesting article:

I've read quite a few, and it's looking quite promising. I still have a few questions, eg 'right half plane zero'?

etc..If I can't answer them by googling, I'l post some questions tomorrow.

Steve Conner wrote ...

I once had a fine head-scratching time trying to get a Honda ECU designed for two double-ended coils to fire four single ones.

The way to get a bike that was set up for four coils to run the 'wasted spark system' is to use two six volt coils in series (on a 12V system).

The same for double ended coils.

Steve Conner

Thu May 23 2013, 07:45AM

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

Ash, yes, we ended up connecting the primaries of the coil-on-plug units in series pairs to emulate the double-ended coils. It worked fine even though the coils were only getting 6V each.

We started out with them in parallel pairs, but that gave some really weird symptoms. The plugs seemed to spark fine when we tested them out of the engine, but the engine just wouldn't run. Turned out that all the magnetic energy stored in both coils was dumping into whichever gap broke down easiest: the one in the cylinder that wasn't under compression and ready to fire. It was the ultimate wasted-spark system: every single spark was wasted.

I tend to side with Ricardo. I don't think a bigger spark ignites the mixture better, and I don't believe the claims of super ignition systems that add horsepower. Combustion is an all-or-nothing affair. The hot, compressed fuel-air mixture is basically ready to explode. If the spark is strong enough to ignite even one tiny bit of it, the whole lot will go off, and you'll get all the horsepower. If the spark isn't strong enough, nothing will happen.

To be sure, some engines need super ignition systems. A F1 engine at full speed needs almost 1,000 sparks per second, which requires a lot of power. One coil per plug helps a lot, by splitting the problem into 8 ignition systems each firing at 125Hz.

And a nitro-burning dragster probably needs some serious voltage, because the charge is so dense and highly compressed, giving it a high breakdown strength. Although I've heard it said that nitromethane is so volatile, and the compression so high, that once the engine really got going, turning off the ignition would hardly make a difference.

But a twin bike engine running off pump gas is nothing special from an ignition point of view.

dex

Thu May 23 2013, 09:31AM

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

Ash Small wrote ...

paris wrote ...

what about the guy who holds the record using cadillac coils , hes up around 100Kv . he might have a few ideas

I've not read about that, but Terry Fritz got 250kV out of an HEI coil after removing the core completely:

13" rod gap has 50% breakdown voltage somewhere between 175 kV and 230 kV (single shot).
therefore,200 kV figure would be better estimate.
except gap geometry and polarity ,breakdown voltage depends also on impulse shape.

Ash Small

Thu May 23 2013, 09:42AM

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

Steve Conner wrote ...

If the spark is strong enough to ignite even one tiny bit of it, the whole lot will go off, and you'll get all the horsepower. If the spark isn't strong enough, nothing will happen.

But a twin bike engine running off pump gas is nothing special from an ignition point of view.

I've run twin bike engines with every type of ignition system there is, fixed ignition, manual advance/retard, mechanical auto-advance and electronic ignition with auto-advance, magnetos, and points and coil.

The best were magneto and electronic ign., and a correctly mapped advance does have advantages, but a weak spark ALWAYS causes problems with erratic running. a bigger/longer spark results in a hotter flame front which travels through the mixture quicker, resulting in quicker and more complete combustion under any conditions.

I basically have three choices here:

fit points and coils.

fit/build some form of electrinic ignition pickups and amplifier.

look into building something along the lines of the MSD/Interspan multi-spark racing units mentioned at the beginning of this thread.

We are talking here about big twins and singles here, fairly low revving engines that produce bucket loads of torque at low RPM, and a lot of people who race them find that fixed ignition, but with a big spark gives the best results. I was initially surprised myself when I found out that Hunt magnetos, which many people fit to Harleys for better performance are fixed ignition. As you said yourself, Steve, a longer duration spark will ignite leaner/weaker mixtures, and a higher voltage spark will overcome dirty/fouled plugs, etc., so there are advantages to be had.

I certainly wouldn't advise fitting fixed ignition to an F1 engine, or any other engine where maximum power is at high RPM, but I'm sure that one of the variables in the complex engine management systems they use is variable length sparks, etc. especially when they are running in 'fuel conservation mode'.

I've pretty much ruled out using a complex advance mapping system here, because fitting all the required sensors, and everything else involved just isn't worth the effort. This is a big, low revving twin that will run quite happily on fixed ignition, or manual advance/retard.

When I used to run fixed ignition on twins, if I was going on the motorway, I'd pull over and adjust the timing to 38 degrees BTDC, then, when I came off the motorway, I'd pull over and retard it again, to make the engine more flexible around town or the country lanes or whatever. You don't need 100 plus BHP in those conditions, just a flexible, low revving engine with loads of torque.

This engine, which is by far the biggest capacity twin I've had, was raced successfully with fixed ignition of 32 degrees. It doesn't have anywhere I can easily fit a magneto to, but did come with Interspan pickups for a 360 degree crank with 'wasted spark'. It also came with a 180 degree crankshaft and camshafts. the two choices I have are to either fit two sets of points, or two pickups. (or buy a 360 degree crankshaft and cams, easier said than done).

Due to these constrictions, and baceause the timing won't always be at the optimum advance, and because of the risks of fouled plugs, and to allow me to run leaner mixtures, a hotter, longer lasting spark sounds like the best plan, although I wouldn't necessarily reccommend the same approach for a high revving multi-cylinder engine.

If I am to build some form of electronic ignition amplifier, a multi-spark system does seem to make the most sense. After all, the racers wouldn't use them if they didn't have advantages. The downside, as I've mentioned before, is accelerated erosion of the electrodes.

EDIT: When it was racing, this engine ran on methanol, but multi-spark systems are also used with petrol/gasoline.

dex wrote ...

except gap geometry and polarity ,breakdown voltage depends also on impulse shape.

Can you enlighten us further regarding this point? Specifically, what impulse shape is best?

dex

Thu May 23 2013, 11:35AM

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

Ash Small wrote ...

Specifically, what impulse shape is best?

in what sense "the best"?

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