MegaSquirt requires a pulse from the ignition to syncronize the fuel injection. This signal provides the position of the crank and may be used different ways to provide spark during normal running and during cranking. There are two modes of operation; Next Cylinder Mode and This Cylinder mode. The mode is determined by MegaSquirt from the entry of Trigger Angle. . If the Trigger Angle is from 0º to 20º then MegaSquirt is in Next Cylinder mode. If the Trigger Angle is from 50º to 90º then MegaSquirt is in This Cylinder mode. We will look at how these two modes work, review the advantages and disadvantages, and then show some examples of how to implement each..

To fully understand how these two modes operate we have to look at a timing diagram when normally running, and when cranking. In the below diagram the engine is running, the spark advance is 25º BTDC. Waveform 1 is in Next Cylinder mode and waveforms 2 and 3 are in This Cylinder mode. The timing of the IRQ-1 at the output of the MS opto isolator is shown for waveform 1-3. In the Next Cylinder mode IRQ goes low from 20º - 0º BTDC. In This Cylinder mode that signal goes low 90º - 50º BTDC. In waveform 1 the signal goes low at 10º BTDC, the Trigger Angle entry is 10º and therefore is in Next Cylinder mode. In waveform 2 & 3 the IRQ goes low at 60º BTDC, the Trigger Angle entry is 60º and therefore these waveforms are in This Cylinder mode.

In both modes and all three waveforms, the time to begin charging the ignition coil, and the time to fire (spark) the ignition coil is calculated. In the Next Cylinder mode that calculation determines the time to charge and fire the Next cylinder. So, even though the trigger occurs at 10º BTDC (Trigger Angle) of cylinder #1, the trigger determines the calculation and timing for the spark at #3 (4 cylinder engine with 1-3-4-2 firing sequence)

In This Cylinder mode the IRQ goes low at 60º (Trigger Angle) which is well before the spark needs to happen for This cylinder. The trigger in this case determines the timing of the spark for #1 cylinder. The spark occurs quickly after the Trigger Angle. Waveform 3 is a special case of waveform 2. In waveform 3 the time between the Trigger Angle (falling edge at 60º BTDC) and the rising edge (Trigger Return) is greater than 50º. This characteristic can be useful for cranking. Waveform 1 & 2 are representative of the Crane trigger wheel. The plastic Crane trigger wheel was modified to achieve waveform 3.

The width of the coil voltage pulse is the dwell time. This is a fixed time for charging the coil when dwell is on. When dwell is off it is a fixed angle.

You can view the trigger on the front panel of MegaSquirt. The second LED on the front panel (Normally showing warmup ) can be set to show the current status of the trigger (IRQ) to make it easier to adjust the distributor. This LED is only of real use for hall sensors and optical sensors and needs to be configured in TunerStudio. On Menu Codebase and Outputs function set LED18 (D15) to IRQ trigger.