Voltage at the Spark Plug
Voltage at the Spark Plug
First, let’s consider the the way the ignition in our engines operates. When the contact breaker points open, the voltage induced in the magneto coil’s secondary winding rises rapidly until it reaches a value high enough to exceed the breakdown voltage of the spark plug’s gap. At this point a spark forms, ionising the gas in the gap which, in turn, drastically reduces its electrical resistance. This allows current to flow until the path of ionized gas is broken or the current drops below a minimum value and the spark disappears. By this time the compressed fuel/air mixture has been ignited and the power stroke of the engine is underway.
The value of the breakdown voltage for the gap on spark plugs in engines is dependent on many factors including:
Shape of the electrodes: For the same size of gap, sharp points will require a lower voltage than will dull points. So, for the same spark plug gap, sharp edges on the electrode will discharge at a lower voltage than on old, worn out spark plugs which have rounded electrodes.
Size of the spark plug gap: The larger the gap, the higher the voltage required. Remember that as the electrodes wear, the gap widens so plug gaps need to be checked and readjusted regularly.
Cylinder pressure:
This is highest at low speed and high load situations (think accelerating from a stand still) and at high RPM operation so under these circumstances a higher voltage is required.
Compression Ratio: The higher the compression ratio, the higher the voltage required.
Air/fuel ratio: The weaker the mixture (less fuel per volume of air), the higher the voltage requirements. If the air-fuel mixture becomes weaker because of problems with the fuel system, misfire occurs more easily.
Overall engine temperature: The lower the overall engine temperature, the higher the required voltage.
Electrode temperature:
As the electrode temperature rises, the required voltage drops. Since electrode temperature rises at higher engine speeds, misfires can occur more readily at low engine speeds.
Humidity:
As the ambient humidity rises, electrode temperature decreases, so the required breakdown voltage increases slightly.
Remember that when the points open, the secondary HT voltage starts at zero and rises rapidly until it reaches a value sufficient to produce the spark. We are often asked what voltage our refurbished magnetos produce. The answer is 'It varies but - just enough to produce a spark under the current set of variables.' That voltage is not quite as high as you might think - probably less than 10,000 volts most of the time.