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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
People in the Mech Eng dept of where I work have mentioned things like twin-spark ignitions, where there are two plugs in each cyclinder. Some sports cars like Alfa's supposedly have this?
Apparently it's not just better for reliability but it sets up two advancing flame-fronts instead of just one that moves through the fuel-air mixture at ignition!
There you have it - I have absolutely no idea what that means or why it would be advantageous - just passing on what i've overheard!
Registered Member #2361
Joined: Sun Sept 13 2009, 09:04PM
Location: Saarbrücken, Germany
Posts: 6
Steve McConner wrote ...
Mates is right, the spark either ignites the mixture or it doesn't. If it does ignite it, you get as much power as you're going to get. If it doesn't, you get obvious misfiring, probably most obvious at high boost, since the higher the compression pressure, the more voltage is needed to make a spark jump.
Therefore, if the spark is already hot enough to ignite the mixture, making it hotter won't somehow "ignite it better".
You can switch to a coil-on-plug system by just connecting the pairs of coils that should fire simultaneously in series. I've done this in the past, when I had four coils, but an ECU with only two outputs.
Oh noes! I thought I was going to get get +50 hp...just kidding.
Joking aside, I'm aware of this. I don't expect gains from this. This is simply to analyze the stock system and see if and where it can be improved upon. Indeed, higher boost will require a higher voltage to initiate the spark. I've run up to 1.4 Bar (gauge) without problems, but I'm planning on running more like 2.0 Bar in the near future. Will the stock system be fine, even then? Maybe, who knows...the key is that I know how it functions and how to improve it if need be.
The only real advantage I see of coil-on-plug is that there would be twice as much time available for dwell as opposed to a wasted spark system. I would assume there would be not as many losses because you are only firing one spark plug instead of two. I wonder, how much energy is lost in firing the wasted spark?
And, why did you wire them in series? You would be effectively doubling the inductance of the circuit, so that dwell time would have to be twice as long in order to obtain the same current.
Harry, unfortunately I still don't know the exact inductance of the primary. However, from browsing a lot online and finding some measured information from a different car of the same make, I would take a guess that it is around 8mH or so. It looks like most aftermarket coils are around 5mH, so this isn't unrealistic.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Speedaddict62 wrote ...
And, why did you wire them in series? You would be effectively doubling the inductance of the circuit, so that dwell time would have to be twice as long in order to obtain the same current.
Well, here's the story. I once was sent to help a bunch of Mech Eng. students who were building a single-seat sports car powered by a Honda CBR600 bike engine. The engine and ECU were purchased from a wrecking yard. Somehow they ended up with an ECU that had only two ignition coil drivers, operating on the wasted-spark system, but an engine with four ignition coils, one per plug.
They had connected the two sets of coil primaries in parallel, which seemed to me quite a sensible thing to do. But the engine wouldn't run. It would fire now and again when cranking, but it never caught. We messed around with it for several days, checking our wiring over and over, but it still wouldn't start.
Eventually I hit on the idea of trying the two sets of coils in series instead of parallel. On the next turn of the key, the engine burst into life with a deafening roar! (Got to love those Scorpion stainless steel exhausts.)
A cookie for anyone who can explain why this worked, and help to spark (hur hur) further debate on ignition systems.
Registered Member #2361
Joined: Sun Sept 13 2009, 09:04PM
Location: Saarbrücken, Germany
Posts: 6
Let me guess...Formula SAE? There is a team at my school, I just haven't really gotten involved yet. Seems like I either have schoolwork or I would rather work on my own car...
I'll have to sleep on your question, I haven't quite figured it out yet, since it seems like wiring in parallel would be the more logical solution.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Hmmm, i'd say that if the coils are connected in parallel then the stored energy from all coils will go into whichever spark plug breaks down at the lowest voltage? That will clamp the voltage and there will be no sparks at the other gaps? Is that anywhere close?
I guess I'm proposing that you can't connect spark gaps in parallel and expect them to share current, only in series and that's my proposed reason the ignition system didn't work as configured?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Richie gets my last cookie! We actually pulled the plugs out of the engine and cranked it to examine the sparks, and we could see them randomly jumping one or other plug of the parallel groups.
Once they were in the cylinders, then it wouldn't be random: the plug that needed to fire would have a higher breakdown voltage because of the compression pressure, so the spark would always go to the wrong plug and the engine wouldn't run at all. Putting the coils in series forces the spark to jump both gaps.
With twin output coils, the two output leads should come from the two ends of the secondary, so the gaps are effectively in series. The Alfa twin spark engines probably have one coil like this per cylinder. I believe the two flame fronts get the mixture burnt quicker, which helps to avoid detonation. (If you want to run 30psi of turbo boost, you'll know all about detonation!)
Yes, it was Formula SAE. What do you mean, your school has a team and you haven't joined? :P
Registered Member #2361
Joined: Sun Sept 13 2009, 09:04PM
Location: Saarbrücken, Germany
Posts: 6
Ah ha, very interesting! Good to know!
I suppose in my case COP wouldn't be a real advantage, unless I was able to figure out a way to trigger the coils independently, which I doubt the stock ECU is capable of. Otherwise, if you have to run them in series, it would just double the time required to get them charged, so any benefits in dwell time would be null. At least in a waste spark system, having the gaps in series doesn't effect the dwell time of the primary.
A quicker burn time would also allow for less timing, which would be beneficial for power, since the piston would be doing less negative work on the compression stroke.
Haha yes I know about detonation, or I will! My engine has a factory knock sensor, but I'm thinking of building some det cans to see how the output compares to the ECU's. I'm also running E85 which is quite a bit more tolerant than gasoline. I've even thought of running toluene (80's F1 rocket fuel!) if I could it in bulk for a decent price.
Yeah, I haven't really joined. Well, I did, and then I stopped going to the meetings...poor choice of mine, but I had no idea what I was doing and no one cared to show me. Now I'm out of the country, so I really have no chance of joining the team this year, but next year I'll be back.
So Harry, roughly 8mH and .86 ohms on the primary. What's next?
EDIT: After doing a lot more reading, I finally understand the fundamental difference between the stock ignition and CD ignition (what MSD is). Basically, the capacitive discharge (CD) system charges a capacitor, then discharges it through the ignition coil. This way very high voltage (500V) can be used to charge the ignition coil, and because the capacitor can be recharged relatively quickly, at low rpm more than one spark is possible. Something like that wouldn't be so hard to make, would it? It would also make COP possible. Oh, the possibilities...
Registered Member #1121
Joined: Thu Nov 15 2007, 02:34AM
Location:
Posts: 4
OK - Something I can finally contribute here :)
I am one of the co-creators of the MegaSquirt engine controller, which is a DIY-oriented controller for fuel injection and ignition (more at ). Some time back we did some investigation on how to characterize ignition coils to help determine the best coil to use for a given application. As many have posted here, there is more to a coil than just how big the spark is...
A few years back we did some research on the topic. The result was a program (visual C++ application) that can be used to help determine some of the key aspects of ignition coils. There is a archive file located here:
There is a pretty detailed document writeup included that shows the equations used, and how to take simple measurements of an ignition coil's primary and secondary resistance and inductance (and coupling coefficient). With this, along with the driver used and the primary circuit voltage/resistance, it will estimate the secondary voltage, spark time, etc. There is also a spark breakdown voltage model (obtained from a SAE document, referenced) that is a function of gap and cylinder pressure. There is also a chart of several different coil types with measurements.
The above testing is for standard induction coil types. Most modern vehicles use coil-on-plug setups. Probably the one that packs the most bang for the buck is the LS2/Yukon coil type, its model D585. You can find these all over Ebay for $20.00 - they are a HV experimenter's dream! These have all of the ignitor electronics built-in, so all you have to do is supply +12 volts and a TTL trigger input. Apply +5 to the trigger input to dwell the coil, and when the trigger transitions to zero the coil fires. The secondary current peaks at 120ma (with a 5ms primary dwell time).
A few years back I posted a video of tests I performed on LS1 and LS2 (Yukon) coils, it is here:
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Nice work BowlingSuperior, thanks for posting :)
In the video, you talk about keeping the control wiring separate from the power wiring. Don't these ignition coils have a separate terminal for the HV return? (ie, the other end of the winding that drives the plug?)
If so, you should run that green wire from your spark gap straight back to that terminal. And when installed on a car, you should connect the return terminal straight to the engine block near the spark plug.
If the HV return is shared with the power ground, you should run the green wire right to the power ground terminal on the coil. But I'd be surprised if it was, since it would inject interference from the spark into the wiring loom.
Bear in mind that the exposed core could be the HV return. I'd suggest ohming it out with a meter.
Following this line of investigation might explain why you blew up one of your coils.
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Automotive Ignition Coil Mods
