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    High Engine RPM

    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.

    This entry was posted on Thursday, April 15th, 2010 at 2:18 AM and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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