Innovate Motorsports ST-12 - Manual (Page 16)

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5.2.3 Four-Stroke Engines
On a typical 4-stroke engine each spark plug fires once for every two crank rotations. The coil on a
distributor-equipped 4-stroke has to create sparks for every cylinder. The number of ignition pulses per
crank rotation in this case is the number of cylinders divided by 2.
Some engines have one coil for every 2 cylinders instead of a distributor. The coil fires two spark plugs at
the same time. One spark is wasted because it fires one cylinder at the end of its exhaust stroke.
Therefore, this system is called a Waste Spark System. Each coil of a Waste Spark System fires once for
every crank revolution.
Other distributor-less 4-stroke engines use one ignition coil for every spark plug. This ignition system fires
each coil once for every 2 crank revolutions.
Coil-on-Plug ignition systems actually incorporate the ignition coil in a module that plugs directly onto a
spark plug and do not have a spark plug wire.
5.2.4 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.
5.2.5 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.
5.2.6 How the ST-12 determines RPM
The ST-12 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 ST-12 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
ST-12 does not know if the pulses are for each ignition event (one per cylinder cycle) or because
of multispark. This is especially 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 require a special tach adapter.