Innovate Motorsports DL-32 - Manual (Page 16)

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7.1.2 Two-Stroke Engines
On a 2-stroke engine there is a spark for every crank rotation, so the spark frequency doubles
compared to a 4-stroke.Very few multi-cylinder 2-strokes have distributors. For those that do, the
number of ignition pulses per crank rotation is equal to the number of cylinders. Most two-stroke
engines have one coil for every cylinder. The coil fires once for every crank revolution, the same
as on a 4-Stroke Waste Spark system.
7.1.3 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.
7.2 How the DL-32 determines RPM
The DL-32 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 DL-32 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 DL-32 does not know if the pulses are for each ignition event (one per
cylinder cycle) or because of multi-spark. This is specially problematic because the
number of multi-spark pulses also varies with RPM in a lot of ignition systems.
Fortunately many multi-spark ignition systems output a tach signal with only one pulse
per engine cycle. But some, notably Ford EDIS systems, output all pulses and therefore
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 DL-32 are alternating between 420 and 300
degrees apart and therefore the time between pulses alternates. The DL-32 therefore
measures the times between ALL pulses for a complete engine cycle (2 rotations) and
averages the times between them.
7.3 Programming the RPM input