There has always been a bit of mystery about the HT circuit for a petrol engine. Indeed, Electricity is one of the mysteries of the universe – we are still not certain what it is.
However, the High Tension circuit takes advantage of an electro-magnetic phenomenon in which an electric current passed through a coil will produce an electric current in an unconnected coil in close proximity to it. Duh.
Simply put, there are two coils, one, the Primary coil, contains a few hundred turns and is connected to the 12V battery current. Another winding around the primary coil is the secondary, but consists of many thousands of turns. It is a peculiarity of the electro-magnetic effect that, when the current to the primary coil is switched off, a voltage is induced in the secondary coil, which is connected to a spark plug. Because there are many hundreds more turns on the secondary coil than the primary, the voltage induced in the secondary coil is much higher, as much as 120,000V, although the current (amps) is much less.

This amount of electricity has to earth somewhere, and in fact jumps across the electrode gap on the spark plug and dissipates to earth through the engine and earthing strap, but in so doing the flash and heat generated ignites the petrol/air mixture in the cylinder.

You’ll note that it is when the primary circuit is turned off that the voltage in the secondary coil is induced. In a mechanical distributor, the dwell angle is the length of time that the points are closed and current is connected to the primary coil. At the instant that the contact breaker points open the secondary coil produces the HT voltage which then jumps across the distributor rotor to the distributor cap terminal nearest to it and thence to the spark plug through the HT lead.

The dwell angle is not exactly critical, but it is very important. If the contact breaker points are set too close to the CB cam then the points will be closed for too short a time. The primary coil will not have powered up properly and then when the contact breaker points open the resulting secondary voltage will be weak, producing a feeble spark which might not ignite the petrol/air mixture around the spark plug (‘missing’), or cause a poor combustion, (lower power and higher emissions.)

That was in the old crude mechanical days.

Now we have the EI module. The action of the contact breaker points, condenser,  rotor arm and distributor cap have all been replaced by electronic means. Now, knowing the rpm of the engine from sensor input (CMP), the EI module can optimise the time in which the primary coil is charged, and open the primary circuit at exactly the right moment, sending the spark to match the exact demands of the engine at that very nanosecond.

No ‘points’ will ever burn, pit or close, no condensor to fail, no distributor cap to crack or terminals to corrode, no maintenance to carry out. No wonder that service intervals are now 12 months or more.

The old term dwell angle has fallen out of use, since in an electronic circuit there is no angle. Instead the time the primary circuit is charging is called charging time or closed time