ignition

by Stefan in 912 / 914 Technical Questions 7 years ago

how can the cdi box kep 26° btd at all rpm except below ca 1000rpm. what i can see, the cranktrigger for a-system will trig on rising trig and the b-system trig on falling trig. but if you want 26° at all rpm you ned to messure the time betwen the two trigger (180°) so the box can calulate the delay time vs rpm.

Regards

Stefan Amren

Re: ignition

by Roger Lee » 7 years ago

Roger Lee

127 5.2k 4.78k

What you may not know is this system fires on every stroke and not just the compression stroke like many 4 stroke engines. It fires every time that piston comes up so keeping a 26 degree firing is easy and the internal CDI circuitry is setup to fire this way.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: ignition

by Stefan » 7 years ago

Stefan

4 4 2

Yes, i now...waste fire...but how will the cdi box keep the timing 26° all the way upp from idle to maximum rpm. the hardware timing is 0° after fire up it goes over to 26°, i´think
there is i a freqvens/d´well timing in the box, so it can keep correct timing all rpm...

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Re: ignition

by Roger Lee » 7 years ago

Roger Lee

127 5.2k 4.78k

Depending on the ignition modules that you have (standard 4 BTDC at start) or (the soft start 3 ATDC) once the system starts it switches to 26 BTDC and just stays there. It isn't variable and doesn't change with rpm input from the pilot..

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Report
Re: ignition

by Stefan » 7 years ago

Stefan

4 4 2

You got a triggerweel with 1 tooth, ca 23mm long, the box ned to "hit" the 26 °, with softvare. the only way i can find it out how it work is this....when the tooth pass the vr sensor on the flyweel, the vr sensor send a signal to the box, between the capacitator and the coil, there is a d´well circurit, you need to mesure the time vs rpm and d´well the signal from the vr sensor befor the signal trigg the diod and fire the coil...otherwise the box will retard
the ignition linear whit higher rpm

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Re: ignition

by Kevin Stewart » 7 years ago

Kevin Stewart

5 184 0

Hi Stefan,

I'm afraid there is no software in the CDi modules. They are horrifyingly simple in their design. The timing is fixed at 2 different advance angles that are determined by both edges of the trigger tooth on the end of the crank. The amplitude of the pulse from the inductive pick up determines which edge of the tooth triggers the ignition coils.

If you want to see a schematic for the module that I have drawn you can see it here:

https://drive.google.com/open?id=0B6ytVYfsfAiNflZzQnhkMG5SMllWWDlDdm5zX3RiZ3doU0E3T2lWRk5jNjdJaldqUW1MbUk

There are also some pictures of one that I de-potted.

Whilst you might be able to argue that a digital module would perform better it is likely to have a lower MTBF due to it running software. What is interesting is that there is no diversity in the existing modules and some of the failure modes are dormant and difficult to detect. That said I think that the circuit's simplicity is the main reason why it rarely fails when the engine is running. Most of the failure modes reported so far seem to result in no spark at slow cranking rpm so that the engine never starts. You could argue that this is 'fail safe' behaviour. I think it is more by accident than design ;-)

Kevin

Thank you said by: Stefan

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