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Discussion on Ignition Noise reduction and Performance Spark Plug Wiring Optimization

Discussion on Ignition Noise reduction and Performance Spark Plug Wiring Optimization

Dec 28th 2022

By: C.W. Jones

This note is an initial discussion of automotive electromagnetic noise from ignition systems and their effects on other car electronics, including radios or data systems (primarily to be sure the author is thinking about the problem correctly), and seeks to define steps for identifying the major elements of the problem, for quantifying associated performance parameters and for selecting practical means for interference mitigation (achievement of EMC ).

Premises for this note:

1.Ignition systems and associated wiring should deliver adequate pulsed voltages to assure spark initiation and sufficient follow-on currents to fully ignite the fuel-air mixture in the combustion chamber.

2.Electromagnetic interference (EMI ) caused by the ignition system and wiring should not interfere with critical communications (voice or data) to/from/on the vehicle or cause degradation of the vehicle inhabitant experience.

It is interesting that in the early days of radio, engineers argued extensively about whether it would be practical to have radios in cars because of ignition noise. That problem was solved.

The undesirable EMI (a lack of EMC) produced by the ignition system will be conducted or radiated to interact with sensitive units such as radios and data systems. The major steps in this process are:

  • The ignition system produces a voltage across the spark gap which produces a spark. This causes a large voltage change (dV/dt).
  • The available charge (current) from the ignition system flows through the spark to ignite the fuel. This causes a longer-term current through the wires.
  • The spark breakdown and subsequent current flow through the ignition wiring produce radiated fields and conducted transients
  • The radiated fields and pulsed currents in signal and/or grounding circuits can interact with radios, etc. to produce undesirable effects. These effects could be due to interactions at the RF frequencies used (including high-speed computer processing), at intermediate frequencies (e.g., IF circuits), or at lower operational frequencies used in digital or analog processing.
  • Achievement of EMC requires intervention in one or more of these steps. Because of the importance of geometric and component details, EMI effects can vary from one vehicle to another, even for very similar vehicles and radio units, for example.

    The severity of the EMI effects depends on multiple factors, including the voltage and current magnitudes and pulse shapes in the ignition secondary circuit, including the spark plug wiring; the relative geometry of the ignition secondary wiring concerning the units susceptible to interference (including the associated power, antenna, grounds, and another wiring), the operational frequencies (to include the RF, IF data and functional frequencies/rates) and signal levels, and any installed effective EMI mitigation measures (including shielding, filters, grounding, or digital signal processing such as that in cell phones).

    These are competing requirements since practical means of achieving EMC can also limit spark voltages and (primarily) follow-on spark currents which could limit combustion performance.

    The ignition systems have been designed to provide adequate voltages and currents (power/energy) to support engine operation under normal operation. Due to the large variability of engine designs (for highway use) and on-vehicle electronics (radios, cell phones, sound systems, and various electronic control systems), it was not deemed practical to define specific (tailored) EMI reduction efforts. A common approach over the past few decades for addressing the EMI issue (primarily to prevent radio and sound system interference, but also to protect other vehicle electronics) has been to use ignition wires with carbon-based (or similar) cores which have resistances of the order of 10K (or more) Ohms/foot. This permits meeting standards (e.g. ICISPR 25) which set maximum EM emission limits from ignition (and other) systems on the car, as well as EMI field and conducted transient levels which radios, etc. should be able to withstand. These limits have been demonstrated to reduce interference to acceptable levels. This approach requires manufacturers (worldwide) to conduct tests of all new designs to demonstrate compliance. Since costs can be amortized across many thousands of vehicles, detailed design and testing work can be performed. However, manufacturers and those developing the standards have included margins in values of both the emitted field levels and the hardness of electronics so that the resulting protection will be robust (i.e., plenty of margins).

    This approach becomes more complicated in the case of high-performance engines with associated tailored ignition systems and additional vibration/thermal loads. A different construction of components may be used to provide reliability, but these can limit EMI control. Also, the uniqueness of the vehicle/engine/electronics designs and the relatively small number of vehicles increases the cost burden of any specialized EMI design and testing.

    In general, the higher the resistance of the ignition wires (while still permitting reliable engine performance), the lower the EMI. The importance of the inductance of these wires will depend upon how the EMI is getting into radios, for example, and which internal circuits are being impacted (e.g., RF or IF or other frequencies, since the impedance of the wires is proportional to the frequency of the EMI times the wire inductance) and the baseline wire/circuit resistance. The radiated portion of the EMI is generally increased in amplitude as the area of the wire/ground loops is increased.

    The emitted EMI can be modeled in some detail for specific cases, but there are numerous uncertainties/variabilities which would likely decrease the value of such modeling. Testing can be done to quantify the nature of the EMI versus design choices, but it may need to be extensive (and somewhat expensive) in order to yield meaningful results.

    Understanding how the EMI is getting into the radio or data systems can be more difficult. Identifying these coupling mechanisms is necessary to understand what frequencies of the produced EMI are causing the noise problems and to develop effective mitigation. If such electronics have been designed and tested to operate in EM environments and meet ISO, IEC, or other standards, the levels set in those standards (vs frequency) can be used to help set baseline thresholds for effects (if needed).

    Thus, in a funding-limited environment, for the short term, it seems most useful to work on the following tasks. It does not seem useful to take 'shots in the dark and expect any meaningful improvements.

    1. Car Team members and related experts should document their experience to date:

    a. What noise problems have been encountered? How did it impact the team/car/driver? With what types of radios, data systems, or other electronics? Manufacturers, models, and frequencies. Do the radios or data systems use any spread-spectrum technologies (like in your cell phones)? What is known about the ignition components and designs? What is known about the locations and wiring of antennas, radios, processors, headsets, etc.?

    b. What ignition system changes have seemed to reduce noise? Have any of these changes seemed to eliminate the noise problem?

    2. Maximize the wire impedance (primarily resistance, but also inductance if higher frequency EMI is found to be important) consistent with satisfactory engine operation.

    3. Minimize the ignition wire spacing from the engine metal surfaces, consistent with thermal and vibration requirements. This will likely produce smaller noise improvements than those of step 2.

    4. Assure ignition system-related grounds are as short and direct as possible to "chassis ground", and have low resistance, and low inductance connections.

    5. Eliminate ground loops to the extent possible. Use separate ground paths to "chassis ground" for radios or data systems. Add inductors and/or bypass capacitors to ground connections for such systems.

    In the longer term:

    A. Monitor which types of radio or data systems are experiencing interference and collect information on the major frequency bands used in their operation (including, RF, IF, digital processing, audio, etc.). This would help with decisions such as what series inductance in wires and/or shunt capacitances might help mitigate EMI (or not). Identify any EMI standards (e.g., CISPR 25) that the units may have had to meet (check stickers on typical units and/or technical manuals).

    B. If adequately high wire resistances are not practical (from an implementation or reliability standpoint) for high-performance engines, consider the use of series resistors (like in some older ignitions, but perhaps with resistors near the ignition box, not near the plugs to help with reliability).

    C. Compile measured data for ignition systems which should include the following:

    a. Circuit diagrams for ignition system(s) of interest, including component values/descriptions, wire lengths, etc.

    b. Voltage pulse shape out of ignition box throughout a combustion cycle (with a time resolution of the initial voltage spike and voltage drop to help estimate the frequency spectrum of the pulse)

    c. Current pulse shape after the spark forms

    d. Separation distance of ignition wires from metal engine surface, and lengths of wires

    e. Descriptions of ignition grounds and radio/data system grounds on typical cars. This would include types of connections (and terminations), wire/cable sizes and lengths, any use of shields and associated grounding, etc.

    f. RPM and cylinder firing rates of interest.

    g. Wire resistance and inductance value range of interest.

    h. Source impedances vs time for the ignition box outputs. An equivalent circuit for the unit (applicable at low-, medium- and/or high-frequencies) would be useful, if available.

    D. It would seem useful to have some simple scaling models (perhaps in an Excel or MatLab, etc., format) which could be used to evaluate some design trades. Some such models likely exist. If available, they could be good starting points. To build and use such models and make them useful, the information from Step C above (at least), would be needed

    E. Estimate the ranges of EMI levels required to produce interference on radios and data systems of interest, both in-band and out-of-band. Compare estimated EMI levels with normal signal levels. Explore potential protection measures which could be implemented for those systems (independent of the ignition system).

    There don't seem to be any 'silver bullets' for an optimal solution for this issue. But, EMI is not an unknown or unsolvable phenomenon. It should be possible to find a robust solution that can make things work reliably. Compiling some of the information listed above could provide a basis for selecting the most efficient development and mitigation approaches. Component manufacturer and car team inputs will be important for this.