Ignition Timing and Combustion

 

Summary of Key Points on Ignition Timing and Combustion

Ignition Timing Approaches

  • Unified Timing Methods:
    • Piston position, crankshaft angle, or flywheel markings are interchangeable methods for specifying ignition timing.
    • If discrepancies arise between methods, the system or tuner may have issues.

Effects of RPM on Timing

  • Relationship Between RPM and Combustion Time:

    • Higher RPM reduces the time available for combustion.
    • Fixed timing (e.g., 30° BTDC) results in less combustion time as RPM increases (time is halved with every doubling of RPM).
    • To compensate, ignition advance is used, allowing the spark to occur earlier at higher RPMs.

  • Ignition Advance and Retard:

    • Advance: Spark occurs earlier (more degrees BTDC) to ensure proper combustion at high RPM.
    • Retard: Spark occurs later (closer to TDC), used during low RPM or specific conditions.

Combustion Factors and Their Impact on Timing

  • Factors That Shorten Burning Time:

    • Higher pressure, temperature, and air density.
    • Richer fuel-air mixtures.
    • Greater turbulence in the combustion chamber.

  • Factors That Lengthen Burning Time:

    • Lower pressure, temperature, and air density.
    • Leaner fuel-air mixtures.
    • Residual burned gases (e.g., at idle or part throttle) reduce efficiency by diluting fresh charges.
    • Less turbulence slows flame propagation.

Turbulence as a Critical Factor

  • Turbulence significantly reduces burning time by mixing the fuel-air charge more effectively.
  • In turbulent mixtures, burning occurs up to 10 times faster than in quiet mixtures.
  • Two-stroke engines rely heavily on port design to enhance turbulence (e.g., transfer and boost ports).

Typical Ignition Timing Curves

  • Advance Curve:

    • Starts at a small initial advance (e.g., 10° BTDC) and increases with RPM to compensate for reduced combustion time.
    • Turbulence increases with RPM due to faster gas and piston motion, reducing the need for further advance beyond a certain speed (e.g., ~3,000 RPM).

  • Constant Advance at High RPM:

    • At higher speeds, increased turbulence offsets reduced burning time, stabilizing the advance requirement.

Critical Takeaways

  1. Timing Graphs:

    • Represent the combined effects of RPM and turbulence but don’t account for factors like air density, residual gases, or fuel mixture.

  2. Key Adjustments:

    • At higher altitudes (lower air density), additional advance is required.
    • Richer mixtures require less advance due to faster burning.

  3. Importance of Turbulence:

    • Enhancing turbulence improves burning speed and engine efficiency, especially in two-stroke designs where porting influences flow.

By understanding how timing adapts to RPM, turbulence, and other combustion factors, tuners can optimize engine performance across a range of conditions.

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