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Thread: Inductive coil tech, coil-on-plug (COP) tech and more.

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2009-12-18 15:37:52
Inductive coil tech, coil-on-plug (COP) tech and more.
How coupled inductive coils work.
When you need to change a low voltage electrical source into a high voltage electrical source like in the case of a gasoline engine's ignition system, an inductive coil is your go-to electrical component.

In the case of most modern automobiles there is a 12 volt electrical system. The spark plugs however require (I'm guessing here) thousands of volts of electricity to jump the gap between the electrode and the ground. We are going to leave amps out of the discussion as they aren't very important.

To get from 12 volts to a couple thousand volts we first need to understand induction. Basically (and I'm going to be simplifying things a lot here) when an electrical current travels through a wire it creates a magnetic field around and all the way down the wire. If you place a second length of wire parallel next to the first wire, the magnetic field will create an electrical voltage in the second length of wire. The voltage in the second wire will be equal to the voltage in the first wire if the wires are extremely close to each other.

(This diagram shows the wires perpendicular and not parallel. That is something I need to fix.)

If you take the second wire and loop it back around and have it pass by the first wire again, both sections of the wire get "pushed" on by the magnetic field and that creates twice the voltage in the second wire.

Now obviously you can repeat this concept and loop the secondary wire by the primary wire 3 times to get 36 volts, 4 times to get 48 volts, 5 times to get 60 volts. That is 12 volts times 5 loops which gives you 5 times the voltage in the other wire at 60 volts.

Now, because of the way magnetic fields are created around lengths of wires, it's better to loop the primary wire to create a stronger magnetic field. So usually the primary wire has many loops in it, and the secondary wire is looped next the primary wire with many loops in it. Because of how close the wires need to be to each other, and the physical limitations of how many loops you can have and how close things can be, most inductive coils use a ferrous core (like iron) to "carry" the magnetic field from the primary wire loops to the secondary wire loops.

In the example above, this inductive coil transforms a high voltage source to the lower voltage secondary wire because the primary coil has more loops or windings than the secondary wires.

Because of this winding, you can see why the electrical schematic symbol for a pair of coupled inductive coils (acting as a transformer) looks the way it does.

Now for an example. If we have the primary wires looped 100 times with 12 volts passing through them and we want to get 2,400 volts out of the other side we will need the secondary wire to be looped 200 times more than the primary loops so we'll want it to have 20,000 loops.
This ratio of primary loops to secondary loops in our example is 100:20,000 or simplified to 1:200. This ratio of loops is called the "turns ratio" and it determines how much more (or less) voltage you're going to get out of the coil compared to the input voltage.

Motorcycle COP and why they are different.
The difference between regular automobile COP and motorcycle COP is important. Now that you know how inductive coil pairs work we can see why they are different. Most automotive coils have a very high turns ratio like our example above. They will take 12 volts and bump it up into thousands of volts. However, coils designed to be used with higher input voltage sources likely have a much lower turns ratio. Specifically coils designed to be used with CDI (Capacitor Discharge Ignition) have a lower turns ratio. CDI is used commonly on motorcycles, lawn mowers, chain saws, small engines, turbine powered aircraft, and in some rare cases on cars. This means that coils sourced from a motorcycle are likely to have a low turns ratio because they are meant to work with 500 volts or more for the input voltage. This is because CDI devices themselves already incorporate inductive coils to raise the input voltage from 12 volts to 500 volts or more. This CDI device feeds power to the coils at 500 volts instead of 12 volts so the coils have a lot more voltage to work with. The turns ratio can be a lot less to create thousands of volts. These systems are likely designed to output 6,000 volts or more compared to the typical car's 2,400 volts. To get 6,000 volts from 500 volts you need 12 times the windings on the secondary coil giving a turns ratio of 1:12 compared to the typical automobile ratio of 1:200.

This means that if you attempt to use a motorcycle coil in an automotive application without the use of a CDI device you're likely getting much less voltage to the spark plug than if you'd used a coil with a higher turns ratio. You're probably dealing with something like 144 volts instead of the thousands you'd normally have. Dan Martin warns of this problem with motorcycle coils in another thread: http://www.sr20-forum.com/tuning/26028-cop-coil-plug-pencil-coils-will-fit-fwd-engine.html#post334814

Advantages of using multiple coils.
COP (coil-on-plug), CNP (Coil-near-plug) and other configurations where a single coil is used for every spark plug provide a couple distinct advantages.

While coils are extremely efficient at upping the voltage (while decreasing the amperage in direct proportion) there is still some power loss in the device. This creates heat in the coil. As rpm rise and ignition events happen faster a single coil can suffer from decreased output as it gets heat soaked or fail completely. Having multiple coils cuts down the number of ignition events the coil has to deal with, helping to keep heat under control.

Having a coil for every spark plug also removes the need for an ignition distributor, eliminating this relatively unreliable, problematic, rotating mechanical device from the system. In the case of COP, you also eliminate the need for spark plug wires.

Coils for every plug allows for tuning of ignition events on a per-cylinder basis if you have the need and equipment for this.

Coils do not work instantly. Coils need a certain amount of time (2-15 milliseconds) to "charge". At high rpm the charge time required can exceed the amount of time available to charge the single coil preventing the spark from firing. This is common on high rpm engines or engines with many cylinders. Multiple coils eliminate this problem.

Wasted spark: what is that?
Technically, any system that fires the spark plug every revolution on a 4-stroke engine is a "wasted spark" system. The name comes from the fact that the spark plug is fired on the compression stroke to provide ignition, and then also fired again on the exhaust stroke (this spark event is "wasted" because it doesn't ignite anything). This happens usually for one main reason. When you don't have cam angle information.

When you don't have cam angle information you will only have crank angle information from a crank angle sensor. The ECU will know where the pistons are at all times, but will never know if the engine is on the compression stroke or the exhaust stroke. Because of this, the ECU has to fire the ignition on both strokes to make sure you provide spark when you need it.
Last edited by BenFenner on 2011-02-09 at 20-42-43.
2009-12-18 16:42:29
2009-12-19 01:05:02
Ben I just passed my electrical engineering course and your shoving mutual inductance laws at me already! I'm on Christmas break damm*t!

Anyways, very good read.

Would we use a wasted spark setup? I'm curios if we can modify the distributor to provide cam angle information to the COP's. If this has been answered already say the word.

2009-12-19 01:23:01
Honestly, I'm probably going to have to change a couple of the diagrams as I've gotten the specifics of some of it wrong (the general idea is the same).

I can't see the SR20 crowd having any real need or desire to deal with wasted spark because we have a cam angle sensor already. For the FWD guys it's inside the distributor, and the RWD guys have a dedicated one elsewhere. I think that answers both of your questions. =]
2009-12-20 12:49:35
So instead of getting M/C cops (unless you want to get a CDI box) the best bet is to get AEM pencils?

Are the CBR cops you're using a low turn ratio?
2009-12-20 14:21:05
Originally Posted by unijabnx2000
So instead of getting M/C cops (unless you want to get a CDI box) the best bet is to get AEM pencils?
No, I don't think so. There is no evidence that the AEM pencil coils are any different from the motorcycle stuff. I'm almost positive they are the exact same thing. AEM insists you use them with a CDI device and I can only assume it is because they are identical to the M/C stuff.

Originally Posted by unijabnx2000
Are the CBR cops you're using a low turn ratio?
Likely yes. Had I known at the time that the K20 coils would fit (or any other coil actually) I probably wouldn't have done what I did. However, the simple truth is that they are working incredibly well so far. The plugs are gapped at 0.030" and I have no ignition issues at 11 psi (303 WHP). Problems might arise if/when I try to do more and I might have to change things but for now everything is working great.
2010-05-03 16:42:56
have you tried to up the gap any to see where the limit is with the coils?
2010-05-03 18:56:00
No I haven't done that.

I've since changed ECUs from VEMS to AEM and because AEM doesn't hanndle the 2-wire COP I've had to change the wiring around a bit. I'm getting to the point where I want to avoid some minor issues with COP mounting and I want a better look in the engine bay so I'm thinking about going with a SR20VET valve cover and VET or S15 COP to just really cap off this part of my build.

We'll see.
2010-05-03 18:59:42
Going to update us?
You still using the transistors?
Or you take out the COP altogether?
Last edited by unijabnx2000 on 2012-07-04 at 22-37-28.
2010-05-03 19:05:56
I'll update everyone on what I do with my ignition setup (in my build thread is really where people should look). As for testing the limitations of these CBR coils as far as spark plug gap goes I'm not really in the position to do that right now. I can't open up the gap and see how wide I can go, nor can I up the boost and see how much boost it will take for them to misfire. The car is not operational right now, and frankly I don't have the desire to find out their limitation. I have many other things to do.

Yes I am still using the four transistors. I needed them to make the COP work with AEM and until I replace the CBR COP with some other coils I'll still be using them. Like I said, I'm thinking about S15 or VET COP because they have a high winding ratio and they mount with a bolt to the VET valve cover (which I want to get as well).

I plan on using COP from now on and for ever as long as I can.
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