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
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 Megatune. On Menu
Codebase and Outputs function set LED18 (D15) to IRQ