[QUOTE=kwren;586816]Previously posted:
Quote:
Originally Posted by iizbeastie
The added spark simply ignites more of the fuel initially
I could see a possible advantage in that.
Keith 
No degree here but it looks like I guessed why they work better.
Starting the fuel ignition with a larger beginning would make the complete fuel air mixture burn closer to the same time.
As far as a capacitor is concerned, the pulse associated with the capacitor discharging through the coil through the spark plugs gap to ground was the reason fuel was ignited for decades before the electronic ignition. Remember the old "points and condenser" The pulse occurred when the points opened.Taking time to build up voltage on the capacitor was never a consideration. a charge on a capacitor is instantaneous and discharges a sharp sign wave as you can tell on an oscilloscope. It works with either ac voltage or "pulsating" dc voltage for a circuit for current flow.
There are two different approaches in life. Learning the words... or understanding the message.
Hopefully we each have a bit of each!
Keith |
Keith I think we are probably the only ones old enough to remember 'points ignition'
The capacitor was used across the points to provide a lower resistance path, as the points started to break. This prevented the rising primary voltage from arcing across the opening points, wasting energy and slowing the secondary voltage rise time.
It was the time that it took for the coils voltage to rise to around 8kv that produced all the problems that the older ignition systems had. It took about 8kv to overcome the resistance of the fuel-air mixture, once it was ionised, the resistance falls, and the voltage drops to about 3kv.
It gets Technical from here. The ignition coil has two windings, the primary (12 volts), and the secondary (Hi-voltage), they are both wound around the same iron core. The primary circuit produces the magnetic field around both windings. When the primary circuit is turned off the magnetic field collapses back through both windings, generating a voltage in both.
It is the voltage that is generated in the primary winding that slows the magnetic fields collapse. As the magnetic field moves through the primary windings it generates a current in them, that will try to put the magnetic field back up. This is the 'Inductance reactance' produced by the windings themselves.
As the voltage generated depends on the speed of flux movement. The Hi-voltage generated by the coil is slow to rise, and its voltage is reduced.
'Capacitor Discharge' ignition system were developed to overcome this slow rise time, that caused fouled plugs not to fire. This system used a higher, inverter generated, primary voltage (375 volts DC) to charge a capacitor, that was dumped into the secondary windings, producing the Hi-voltage, as the magnetic field was rising, instead of collapsing, to get around the Inductance reactance problem, giving a very fast voltage rise time, that would fire any fouled plug. As was needed by outboard motors.
The ignition system that the SVX uses is the Bosch developed 'Hi-energy system'. This gets around the slow Hi voltage rise time by reducing the amount of windings that are used in the primary circuit, to reduce the 'Inductance reactance' that slowed the Hi-voltage rise time.
To produce the same magnetic field strength with less windings meant that the current flowing in the primary had to be increased. A primary resistance of 0.7 ohm would allow a current of 20 amps to flow, but the Igniter unit that controls the coil waits till the current gets to 4/5 amps, then limits this current flow, till it turns it off to fire the spark.
Edit: I forgot about another feature that our coils have to reduce the voltage rise time. it is a diode that is in the secondary circuit, to prevent the secondary from generating a current, as the magnetic field is rising.
This is the scan from the book.
The benefits of this type is a strong magnetic field with very little inductance reactance to slow the rise time. A fast strong spark that needs no add-ons.
Harvey.