Ignition Spark Energy
Ignition fundamentals. we get into how the fire gets started . . . and different ways to initiate all the pyrotechnics
We’ve deliberately omitted discussion of the ignition process and the means by which it can be accomplished. Our reason was that it might be better to delay discussion on what happens both up and downstream of this part of an engine’s operation so that when we got to it— and now we’re there—it would be a more easily understood subject. Hopefully, this decision wasn’t a mistake.
Return with us now to inside an engine’s cylinder, after air and fuel have been compressed prior to combustion but just at the instant before ignition. Keep in mind also that what we have here is a chemical mix of air and hydrocarbons (fuel) which will undergo chemical and energy changes during the combustion process. Technically, this whole soggy mess is termed “oxidation” and might be called the combination of oxygen (air) and fuel.
Actually, at the risk of oversimplification, this is true even of fuels that are oxygen carriers (such as nitromethane).
For you students of chemistry, such compounds as carbon, hydrogen and oxygen are important “participants” in the combustion process, because they may be formed as so-called intermediate compounds during fuel oxidation. Of these, aldehydes are particularly interesting, since the “stink” from an engine’s exhaust is normally formed by the various aldehydes. Of course if you aren’t a student of chemistry (and we are not), you might think of an aldehyde as the outer skin of an aide. But this is a Series on ignition systems and processes. And we’ve digressed.
Back inside the cylinder, we suggested there was a compressed air/ fuel mixture ready for ignition. Assuming that a properly timed spark is delivered, three basic types of combustion can result. By way of review, these are (1) normal flame travel (propagation), (2) spontaneous combustion of unburned fuel (detonation), and (3) preignition or ignition of the air/fuel mixture by means other than controlled spark. Of these three, detonation is the most severe in terms of parts damage from sudden cylinder pressure rise. Preignition usually results in lost net cylinder pressure from early ignition and can sometimes lead to detonation. Most of this was discussed in the Shop Series on The Combustion Process.
So, on the assumption that we can avoid the problems of uncontrolled combustion, let’s examine first what happens when ignition is correct. Then we’ll discuss conventional ways of providing the proper amount of timed ignition spark, and maybe even a couple of unconventional methods. Keep in mind, at this point, that we’re examining the characteristics of spark ignition engines. The auto-ignition (diesel) stuff will follow.
A. Ignition voltage requirements are affected by the condition of air/fuel mixture ratios near the spark plug electrode at the time of ignition. An engine’s mechanical compression ratio also affects ignition voltage needed to initiate combustion, but air/fuel ratio typically requires a change in ignition voltage. As the chart indicates, leaner mixtures require higher ignition voltages (spark energy).
B. In a conventional spark-ignition system, a set of contact points is caused to open and close in accordance with the shape of a distributor points cam. The amount of time a set of points remains closed (current flowing through the primary side of the ignition coil) is called point dwell. The amount of time (distributor shaft rotation) the points remain open affects maximum point clearance (gap). For example, the earlier a point set is caused to open, the longer (and wider) will be the gap or unseated time. In effect, this is a form of advancing ignition timing (causing secondary voltage to be delivered to the spark plugs sooner than with a smaller point
gap)