Innovate Motorsports SSI-4 - Manual (Page 9)

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4.1.3 Rotary Engines (Wankel Engine)
A rotary engine consists of a roughly triangle shaped rotor rotating in a roughly elliptical chamber.
The three spaces left between the chamber and the rotor go through the four cycles of a four-
stroke engine for each rotation of the rotor. A single (or dual) spark plug at a fixed position in the
chamber ignites the mixture of each space in sequence. Therefore, a rotary engine requires 3
sparks for every rotation of the rotor. The mechanical power from the rotor is coupled to an
eccentric gear to the output shaft. This gear has a 3:1 gear ratio and the output shaft therefore
rotates 3 times faster than the rotor. The output shaft is the equivalent of the crankshaft on a
piston engine. Because RPMs are measured conventionally as the rotations of the crankshaft,
the rotary engine requires one spark for every 'crankshaft' rotation, the same as a two-stroke
engine.
4.2 How the SSI-4 determines RPM
4.2.1 Measurement Method on Channel 1
The SSI-4 measures RPM not by measuring the number of pulses over a time period, as a
tachometer does. That measurement would be too slow to provide adequate correlation between
input channels. Instead the SSI-4 measures the time between input pulses and from that
calculates RPM for each pulse measurement.
This measurement method has a few caveats though:
1. If the RPM pulse signal is derived from the ignition signal, a multi-spark ignition system
will trigger the measurement multiple times for each pulse. This throws the measurement
off because the SSI-4 does not know if the pulses are for each ignition event (one per
cylinder cycle) or because of multispark. This is specially problematic because the
number of multispark pulses also varies with RPM in a lot of ignition systems. Fortunately
many multispark ignition systems output a tach signal with only one pulse per engine
cycle. But some, notably Ford EDIS systems, output all pulses and therefore may require
a special tach adapter.
2. Odd fire engines, like V-Twin motorcycle engines and odd-fire V6 engines have ignition
pulses that are not evenly spaced. For example a 60 degree V-Twin running at 10
degrees ignition advance will fire cyl. 1 at 10 degrees BTDC. Then fire cyl. Two 420
degrees later at 410 degrees. Then fire cyl 1 300 degrees later at 710 degrees. This
means the ignition pulses sent to the SSI-4 are alternating between 420 and 300 degrees
apart and therefore the time between pulses alternates. The SSI-4 therefore measures
the times between ALL pulses for a complete engine cycle (2 rotations) and averages the
times between them.
4.3 Connecting an RPM signal (Channel 1)
- Connect the RPM signal to the CH1+ input screw terminal.
- Connect the CH1- signal to the ground of the RPM signal source. For example the
ground of the ignition system at the ignition box, not where it’s ground wire connects to
the frame or engine.