Innovate Motorsports DL-32 - Manual (Page 15)

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With pulsed signals the measured information is contained either in the frequency of a pulse train
or the duty cycle of a pulse trains. Examples for frequency type signals are RPM, speed and
some MAF sensors. Duty cycle signal examples are contact dwell of a contact type ignition
system or the duty cycle of fuel injectors.
For all pulse type signals the actual voltage of a pulse must rise (ON phase) above 2.5V, the
pulse must fall (OFF phase) below 2.5V to be measurable by the DL-32.
7. RPM measurement
7.1 RPM Measurement basics
Most RPM measurement methods use the ignition system of the car as a convenient source of
RPM dependent pulses. Other methods use a TDC sensor (one pulse per rotation), cam sensor,
or fuel injection pulses (number of pulses/rotation is dependent on the fuel-injection system).
Some actually measure the AC frequency created by the car's alternator.
Because the number of pulses per crank rotation is dependent on the ignition system and engine
type, a universal RPM measurement method must be adaptable to the different environments
encountered. The typical ignition system consists of an ignition coil, a coil driver that switches
current to the coil on and off, and a distributor. When current is switched on to the coil, the coil
stores energy in its magnetic field. When the current is switched off, that energy gets discharged
at a very high voltage pulse on the coil’s secondary winding, creating a spark.
A capacitive discharge ignition system (CDI) uses a capacitor to store the spark energy. The
capacitor is charged to about 400V and then rapidly discharged over the ignition coil's primary
winding. The coil thus only acts as transformer and does not store energy (and can therefore be
smaller). The advantage of a CDI system is a very high and fast rising spark voltage (less
susceptible to spark fouling). The weakness of the CDI system is the very short duration spark,
which might not be long enough to ignite the mixture. Multispark ignition systems try to overcome
the inherent weakness by creating multiple spark pulses over some degrees of crank rotation to
increase the likelihood of igniting the mixture. The distributor switches the spark voltage to the
appropriate spark plug.
7.1.1 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.