Delta. Can you explain burn time. I see the initial voltage spike. Then a little ringing. Then the 2.? Milliseconds where the voltage remains at 26 volts. Why does the voltage do this, why is that burn time.
What you see in the graph, is what is happening in the coil's primary. But it represents also what's happening simultaneously in the secundary circuit/between the sparkplugs electrodes. In principle the voltages shown in the primary circuit are multiplied by the winding ratio. IIRC the CB750 coil's primary circuit has 380 and the secundary 15000 windings. That makes a ratio of say 1:40. As a rule of thumb the voltages shown in the primary can then be multiplied by 40 if it happened to be your coil. So this gives you some idea of the voltages at the sparkplug. But... let's not concentrate on voltages. What is delivered between the sparkplugs electrodes is
energy or better said:
heat which is normally expressed in (milli)Joules. The voltage is just a way of looking at the phenomenon and quite frankly, I don't find it very meaningful but we can use it. Anyway, the initial high peak is the voltage needed to ionize the air/fuel mix between the electrodes, to become conductive. As soon as the spark starts, voltage drops dramatically to a level where we nonetheless still have a period where arcing takes place between the electrodes. That is the socalled
burn time. If, for whatever reason, the initial peak was not sufficient to start the flamefront travelling through the combustion chamber, the burn time helps completing this. Think of a very energetic zig zag saw like activity in arcing going on. Overthink this in slow motion and you will get the picture.
A few things to bear in mind. The sparkplug has only two duties: A. To ignite just a tiny bit of mixture around its electrodes and B to stay clean, so it can continue to do A. Let's look at A. As soon as the plug has succesfully performed duty A, the created flamefront in principle will take care of the rest of the combustion. That's the idea. Again, overthink the action in slowmo and you will understand. To resume, you need a relative high peak to establish a spark and then a period of time the electrodes are still arcing to deliver all in all sufficient heat. The voltage is dependent on 1. the electrode gap's wideness and 2. the conditions in the combustion chamber. The latter varies very much with load/rpm but also with other conditions like (partly) fouled sparkplugs and the presence (or not) of carbon deposits in the combustion chamber. Please do not forget the combustion engine is a very primitive machine and in evolution only the next step after the steam engine. On a steamlocomotive you had the driver (engineer) and the stoker. From time to time the stoker would open a lid to add coal to a fire already in process. New with the arrival of the combustion engine was that we now put in the fuel first and then let the sparkplug strike a match. I agree, it is ridiculously outdated and, given that only some 20% of the energy is transformed into motion and the rest is lost in heat, such a machine should have been phased out long time ago. Yes, we all are riding old stinking furnaces.
Let's focus a bit on the rise time. The shorter that time, the less the ignition will suffer from parasitic drain. Parasitic drain is caused by old, (partly) fouled or even wet sparkplugs and/or detoriated plugcaps and/or old high tension cables. CDI systems clearly have the advantage here. Rise time with a CDI is extremely short, but the concept also has disadvantages: no burn time to mention one
.
