Ignition circuit information

[A.Clampitt]

I've been lurking around this group for some time , but decided that 
rather than learn a new MCU/Operating system etc . I would proceed with 
the HC11 , which for my purposes fits the bill ( 4 cyl sequential) I'm 
nearly to the pre production stage with a dual HC11 system ( maybe I 
should have stuck with the '332 !) 
	The ignition interfaces are now nearing the end of a 1000 Hr 
soak test , and seem to be coping well with the 40 days and 40 nights 
(approx) of simulated 7000 RPM .

	If anyone is interested I can supply details of the circuits ( 
one using a single package Darlington transistor and the other using two 
HV transistors in a Darlington arrangement [ slightly cheaper]) and if 
there is enough interest possibly the boards .

GENERAL 

	My experiments with the circuits , using a Bosch HEC 715 coil 
( designed for electronic ignition ) show that, with a nominal 13.6 
Volts supply ,the coil takes about 2mS to fully charge to the 3.5Amp 
level ( the circuit limits the max current , boards can be set up via 
sense resistors to the limit of the output Darlingtons ), discharge in 
air initiates at about 40 KV followed by around 0.5mS of "ringing" ( 
multiple spark discharge !!!) and then another 1.5mS of steady discharge 
at 20KV and then collapse of the spark .
	The above indicates that with a 3.5A coil current in a Bosch HEC 
715  ( probably a little on the low side in practice ) the minimum cycle 
time for full energy spark discharge is around 4mS , or 250 sparks/sec 
this equates to 7500 RPM on a 4cylinder 4 stroke or 3750 RPM on a V8 
	Exceeding this rate causes the 1.5mS of steady spark discharge 
to be shortened as the coil is recharged , the coil can still reach full 
charge as it starts with a partial charge and needs less time to reach 
full charge , however the total energy per spark is reduced .
	If the system is set at 5A coil current , the charge time 
increases to around 3mS  giving a min full charge/discharge cycle of 5mS 
 equivalent to 6000 RPM (4cyl,4St) 3000 RPM V8 
 
	Coils designed for Electronic ignition have smaller resistance 
and hence larger static currents , this is one reason that one should 
not use an "Electronic" coil on a points based ignition , the HEC715 has 
a static current of 15 amps , so would dissipate 200 Watts if the key 
was on , point closed and engine not running .
	The effect of this lower resistance is to allow the coil to 
charge quicker , experiments with a selection of "junk yard"  non 
electronic ignition coils showed  lower staic currents  ( around 4-7 
Amps) and much longer charge times to my reference 3.5Amps ( 4-6mS )  
and slightly increased spark discharge times ( 2.5mS total) , with a 6mS 
charge time and a 2.5mS spark discharge time , the total full charge 
discharge cycle comes in at 8.5mS or around 3500 RPM on a 4cyl 4 stoke .
	This is not the full story as the spark energy is a function of 
both coil current and coil inductance , My experiments are in air 
not in the operating cylinder .

	Anyone out there know if there are benefits from having the 
spark continue for the full 2-2.5mS , or to put it another way waht is 
the minimum spark duration required for satisfactoy ignition/combustion 
?
	Hope this has been of some interest , if not don't waste more 
bandwidth telling me about it !