Anatomy of A Spark

Anatomy of a spark

TDC

Kevin Cameron

IN AN IGNITION SYSTEM, A SPARK PULSE is produced in either of two ways: in the traditional Kettering ignitions of old (the kind you could work on and actually hope to fix), or in modern capacitive-discharge systems.

In the Kettering systems, primary current in a coil, wrapped around an iron core, creates a magnetic field. When this primary current is suddenly switched off (typically by the opening of mechanical “points”), the magnetic field collapses, using the principles of induction to transfer its energy to a secondary coil wrapped around the same iron core and having a great many more turns of wire than the primary. Each turn induces a voltage, and all these voltage-turns combine to produce a high-voltage output pulse sent to the sparkplug gap.

In capacitor-discharge systems, energy is stored differently. Primary current is sent not to a coil but to a charge-storage device called a capacitor. When a spark pulse is desired, an electronic switch releases the charge stored in the capacitor and dumps it into the primary side of a coil. Again, through magnetic induction, field energy is transferred to a many-turn secondary winding, generating a very fast, high-voltage pulse.

Electrons don’t whiz down the plug wire at the speed of light. Rather, when the coil pushes excess electrons into the wire, it is their field that propagates at high speed, rather like the wave of jostling that radiates through a crowd of people who are all trying to get away from something disagreeable. The result of this process is that opposite charges appear on the two sides of the sparkplug gap, generating an electric field across the space between them. That space is filled with fuel-air mixture at whatever high pressure has resulted from the compression stroke. The later the ignition timing, the higher this pressure will be, because the piston will have risen closer to Top Dead Center.

The electric field across the plug gap is a powerful accelerating force on any charged particle-negative electron or positive ion-that may be in reach. And there are always some charged particles available by the myriad accidents of molecular motion-especially near the hot plug electrodes, in the heated, compressed but, so far, unburned mixture. The electric field accelerates these particles, and the speed they reach depends upon how much average distance there is between their inevitable collisions with gas molecules. The farther they travel before hitting a molecule of the fuel-air charge, the more speed they will be able to gain. If all goes well, some of these collisions will be energetic enough to knock yet more electrons loose from their parent atoms. They, in turn, will also be accelerated by the electric field, and will lead to a cascade of free electrons. This process is called ionization.

A conductor is any substance that has mobile, available electrons or positive charges in it, able to move as a current. When the fuel-air mixture is ionized by an adequate voltage across it, it becomes a conductor just like a wire. When this happens, a torrent of electrons briefly rushes across the gap in the form of a spark, efficiently heating the fuel-air mixture there by hammering at its molecules in trillions of collisions.

Hydrocarbon fuel molecules are normally quite stable-after all, they have survived all those millions of years of underground storage as petroleum. But in the spark’s storm of energetic electrons, those molecules are knocked to pieces-pieces that are more attracted to the many available oxygen atoms than to their former hydrogen and carbon partners. The new arrangement results in a lower energy state for the resulting products-carbon dioxide and water. The energy difference is released as heat, and the molecular process of changing partners this way is called combustion. It will probably occur if the spark happens to pass through a reasonably mixed, properly proportioned bit of mixture. Otherwise it will not.

Combustion can occur in the spark gap without resulting in ignition and burning of the entire charge. The tiny flame kernel generates heat from its beginnings of combustion, but it is also losing heat-by radiation, by convective contact with cooler gas surrounding it, and by conduction to the massive and (relative to the little flame, at least) cooler plug electrodes. Like so many brave little start-up businesses, the expenses may exceed the income, and the little flame kernel may go out; chuff, a misfire.

Even if the struggling flame kernel does make a go of it, power can still be lost without actual misfire. The accidents of in-cylinder turbulence may swirl this embryonic flame away to an outlying area, from which it takes longer than usual to light up the entire cylinder. Or the kernel, finding itself in a region a little too lean or too rich, may need extra time in which to grow and spread. The effect in either case will be like late timing, and that particular combustion cycle will be a little weak.

So, making power in a spark-ignition engine is a statistical affair. Ever since the beginnings of engine testing early in the century, engineers have known about this phenomenon, which they call “cycle-to-cycle variation.” When things are, on average, close to correct-good fuel-air mixture, good vaporization, decent spark, fortunate turbulence-most power strokes can be reasonably strong; but there will be steady, random variation in'peak combustion pressure because conditions are never exactly the same from cycle to cycle.

A major focus of engine research has been to understand this variation, and to find ways to make all cycles as good as the very best ones.