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    Ignition Spark Energy

    April 15th, 2010

    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.
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    Spark Timing

    April 15th, 2010

    Now, in a single-spark ignition engine (one per cylinder), the combustion flame begun at the spark plug travels comparatively slowly away from the plug. As the flame moves throughout the combustion space, heat released by the burning fuel causes burned gas expansion. This tends to increase the velocity of fame travel. Spark timing, therefore, has much to do with this increase in cylinder pressure and rate of combustion flame. For example, starting it too early (too much initial timing) can lead to problems of detonation, while starting it too late can cause cylinder pressure loss and reduced power. And believe it or not, probably the most influential factor affecting flame rate (aside from actual air/ fuel ratios) is turbulence within the combustion chamber . But if you’re into building race engines, you probably already know about this. If you don’t…. But back to our cylinder.
    Actual spark voltage requirements vary with the amount of spark plug electrode gap and pressure conditions
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    Ignition Voltage

    April 15th, 2010

    And in accordance with the engine’s particular firing order, we’re now back to the combustion process. But for purposes of a clearer understanding of how this combustion spark was developed, let’s return to the engine’s battery and see what takes place from wet cell to “fire.”
    In spark ignition engines, there are two basic methods by which ignition voltage is produced. One requires battery voltage input, the other uses permanent magnets and the disruption of magnetic fields to produce secondary voltage output. This second form of ignition voltage is called a magneto (so named because of its use of magnets for the production of ignition voltages). However, since battery ignition systems are the more common, let’s first examine how they develop ignition voltage.
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    Distributor

    April 15th, 2010

    Next comes the ignition distributor, the ignition points it houses, a capacitor and some sort of timing advance mechanism (one mechanical and one, usually, governed by intake manifold vacuum). We’ve already mentioned that a distributor’s ignition points serve as an electrical “switch” for the interruption of current (voltage) flow through the ignition points. Such points are opened and closed Dy the rotation of an eccentric cam fixed to the distributor shaft which is driven by the engine (and in speed proportion to engine rpm, normally at one-half engine speed). Also attached to the distributor shaft is a rotor which is in electrical contact with a terminal in the distributor cap and secondary voltage wires leading directly from the ignition coil. As the rotor turns (sounds like some sort of soap opera), its tip comes into alignment with terminals located around the inner circumference of the distributor cap, each of which leads to one of the engine’s spark plugs by way of a plug wire. This provides plug voltage “timed” with the engine’s piston position, since the distributor shaft (and, consequently, its rotor) is related to crankshaft position, which is related to piston position—which lives in the house that Jack built!
    Also inside the distributor, or at least an electrical part of its operation, is a condenser (or capacitor).
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    High Engine RPM

    April 15th, 2010

    As engine speed increases to a maximum, less combustion time is available, volumetric efficiency (cylinder filling efficiency) is reduced, so less fuel and air are available for combustion, flame travel is slower, and ignition timing needs to be advanced. And if you’ve heard of devices that retard timing at high engine rpm, then there was probably too much “intermediate rpm” advance in getting from low to high engine speed. High-gear ignition-retard units that improve race track speeds do work, but not for your point A to point B fuel miser.
    Probably not worthy of expanded discussion, but related to ignition systems, are engines operated by “compression ignition.” You know them as diesei engines that initiate combustion spontaneously as a result of extremely high (by comparison to spark ignition engines) compression ratios. Such ratios are often on the order of 12 to 21:1, so the precombustion temperature of air near the end of the compression stroke is very high. Just prior to the piston reaching top dead center, a measured quantity of fuel is injected into an engine’s cylinder and, in combination with the now-heated air, ignites and combusts just about as fast as it can be injected into the cylinder. But this is the diesei principle, which may become the subject of a future Shop’Series … if there is sufficient

    reader interest. All you have to do is let us know. Which brings us around to wrap-up time.
    Keep in mind that spark timing and intensity (joule level vs. time) are the ingredients of successful ignition. What happens from ignition forward depends on many conditions not related to the ignition system. The conventional system uses points, a set of coils (primary and secondary windings) placed around an iron core, and a spark distributor that times secondary voltage to each of the engine’s spark plugs. Substitutions (as in the case of types of electronic components) and additions (such as multiple spark systems) can be made to this basic method of spark timing and frequency. But the object of it ail is initiation of the combustion process. What happens from then depends on other non-ignition conditions.
    It’s like the old saying “You can lead a horse to water, but he’ll only eat the okra.” Well, maybe you heard a different version. We’re probably misteafcen . . . again.

    REVIEW QUESTIONS: True or False
    1. The combination of air and fuel, in the presence of heat and pressure, can be called sublimation.
    2. Any form of uncontrolled combustion is a form of detonation.
    3. In a typical combustion chamber, ignition of the air/fuel charge takes place directly opposite the spark plug and travels uniformly toward the exhaust valve.
    4. Other than the conditions and amount of air/fuel mixture in a given engine’s cylinders, turbulence is the most important factor in flame travel.
    5. Lean air/fuel mixtures require less ignition voltage to begin combustion than do mixtures that are richer.
    6. In a conventional battery ignition coil, the secondary windings are outnumbered by the primary windings, resulting in higher secondary voltages necessary for spark plug operation.
    7. An ignition system’s points, operated by a distributor cam, are used to interrupt primary voltage flow.
    8. Condensers act as “electrical shock absorbers,” resulting in rapid collapse of the magnetic field and increased point life.
    9. Vacuum advance mechanisms provide increases in the amount of ignition timing as intake manifold vacuum decreases.
    10. During part-throttle engine operation, the compression of air/fuel mixtures is quite high, resulting in rapid combustion flame speed and less ignition timing.
    11. Diesel engines incorporate air/fuel mixture combustion as a result of heat and pressure, with fuel being injected after the compression and heating of air within the cylinder.
    12. Multiple-spark ignition systems are excellent ways of improving part-throttle combustion efficiency when cylinder pressures are high and combustion rates are rapid.
    13. So-called high-energy ignition (HEI) systems have become commonplace with the advent of lean mixture (lean-burn) engines designed to meet exhaust emissions standards through the reduction of misfire.
    14. Reasons our school had no cheer leaders were mentioned in a previous Shop Series on rear.. . uh, differentials.

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