Ignition/CDI

7050 Views 9 Replies 3 Participants Last post by 88nightrider, Nov 8, 2010

88nightrider

Discussion Starter · Nov 6, 2010

Fist of all, I must admit that I have not had the opportunity to take a look at the flywheel and the windings behind it on this bike

Was wondering if some one could explain the workings of the CDI on the Rebels?
Is the timing curve static (same all the way through the RPM range)? Or does the CDI adjust the timing curve?
^^If static: How many degrees (°) BTDC?
How many volts (V) does the "Igniton Pulse Generator" generate?
And it fires once every revolution of the crank? Or twice?

I remember that on my Yamaha DT had a flywheel with four magnets. Would not that make four ignition pulses as they all looked the same?

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Buickguy

· #2 · Nov 6, 2010

I don't know how many volts the ignition pulse generates but it does fire once per revolution (fires every 360 degrees of rotation). It fires both plugs at the same time (wasted spark system) Each cylinder is only ready for combustion every 720 degrees of rotation.
I believe the timing advance is controlled by the CDI module. There is a test for advance in the service manual but if it fails the test it says to replace the CDI module.

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88nightrider

Discussion Starter · #3 · Nov 6, 2010

Thank you Buickguy

So the Ignition Pulse Generator only generates one (1) pulse every 360°?
Do you know how many degrees BTDC it is placed?
Thinking about the possibility of using it as a crank position sensor. If the voltage is to high that can be solved (I think)

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Buickguy

· #4 · Nov 6, 2010

I don't know how it is placed but the ignition fires:
5 degrees BTDC @ 1300 rpm
30 Degree BTDC @ 3500 rpm

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88nightrider

Discussion Starter · #5 · Nov 6, 2010

Ok. Thank you again.
5° sounds about right
1300rpm is idle i guess on these bikes
30° @ 3500 sounds pretty ideal. hmm. all good

Spark Advance

Advance refers to the precise crankshaft position where the ignition is initiated by a spark from the spark plug. It is always referenced to the crankshaft position in degrees (the symbol for degrees is °, the same as temperature). Since there are 360° in a crankshaft revolution (or any complete circle), one intake stroke, which takes ½ a revolution, is 180°. Normally the advance is specified as 'before top dead center' (BTDC). This means the number of degrees the crankshaft would have to turn to reach the very top of it travel from the spark point.

Sparking before TDC is necessary because the fuel and air take some milliseconds to burn. Typical values range from 5 degrees BTDC at idle to 35 degrees at wide open throttle (WOT) and possibly even higher under cruise conditions. The flame front moves at about 50 mph (~73 feet/second or ~880 inches per second) at high cylinder pressures and appropriate AFRs. The pistons can travel a considerable distance in the time it takes for the fuel to burn all the way from the spark plug to the most distant regions the cylinder. For example, at 880 in./sec and a 3.5" bore, if the spark was centrally located, the burn would take 1.75/880 = 2.0 milliseconds.

If the burning takes 2 milliseconds to reach maximum pressure, at 3000 rpm the piston & crank will travel 36° in that time. There is an optimal point (peak pressure position - ppp) in the piston movement when we want the burning gases to reach their peak pressure (usually about 17° ADTC), so we need to start the burn early to get the peak pressure where we want it (in this case 36°-17° = 19° BTDC).

Needs calculating for this particular engine. But seems pretty optimal

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Buickguy

· #6 · Nov 6, 2010

Here's an interesting read on Kettering, TCI, and CDI ignitions.
Both the Kettering and TCI are inductive ignition wher the CDI is capacitive.
It's explained very well.
Electronic Ignition Overview

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88nightrider

Discussion Starter · #7 · Nov 6, 2010 (Edited)

Thanks again
That explained a lot about the CDI system

Gives food for thought

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Guba

· #8 · Nov 7, 2010

Ignition pulse generator voltage is a MINIMUM of 100 volts.

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88nightrider

Discussion Starter · #9 · Nov 7, 2010

Guba said:
Ignition pulse generator voltage is a MINIMUM of 100 volts.

You say minimum. Is the voltage the same through out the whole RPM range?
Lets say it is 100V at idle. Is it 100V at 10k rpm then? If so there should be no problem.
If its not, then I must look at the possibility of electric field readings. Or set up a new sensor...

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88nightrider

Discussion Starter · #10 · Nov 8, 2010

Got some info on the "Igniton Pulse Generator". Apparently its a VR sensor.

The VR sensor is an induction type sensor, it is passive;, i.e. it does not require a power source, and has a small magnet built in. It basically works like a dynamo. The output of this sensor varies with the speed of the engine. At idle the output is approximately .6 volts, at mid RPM it is closer to 3 volts, and at high revs it goes to almost 50 volts

And it should be usable to be used as a crank trigger sensor for the ECU

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