Monthly Archives: April 2020

Secondary Ignition Current

I measured this on my workbench with a magnet attached to a drill spinning next to a spare ignition trigger. I measured the coil as 3.2 ohms and set my power supply to 14v. I used a shunt resistor at the output of the power supply to measure the current draw of a single coil/trigger combo. I expected to measure a peak current of about 14v/3.2ohms = 4.4 amps. It turns out the trigger applies only 11.4v across the coil, explaining why I only see a peak of ~3.5 amps.

The ignition spark is generated when the current drops sharply to 0. There’s a period of almost zero current lasting 2.5ms while the magnet is still close to the trigger. This period is longer at slower RPMs and I believe it gets shorter at higher RPMs. I wasn’t able to test higher RPMs with my drill setup, but I did move the magnet far enough from the trigger that it was barely triggering and saw periods of ~1ms at the same RPM.

Using this data, I simulated the expected waveform at a range of RPMs to show how the average current draw decreases as RPMs increase. It’s only about a 13% decrease in current and probably not enough to be readily noticeable.

While investigating this, I noticed the triggers de-energize the coils after about 2 seconds without a trigger. This greatly reduces their drain on the battery while the engine isn’t running.

Main Bus Voltage while Starting Engine

There’s some debate on whether avionics should be on during engine start. The concern seems to be voltage transients due to engaging and disengaging large solenoids and the starter motor. I captured this at ~40kHz and don’t see any potentially dangerous transients in this nominal engine start.

I suspect the dips until 1 second are due to higher torque during the compression strokes before engine started running. I continued to hold the start button until ~2.5 seconds. You can see some periodic noise that I believe is due to the secondary ignition coils charging.