Ignition System, Spark Plugs - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

IGNITION SYSTEMS, SPARK PLUGS
Two-stroke engines in snowmobiles rely on an electric spark to initiate combustion of the fuel/ air
charge which has been inducted into the cylinder. For the engine to operate efficiently, the spark
must be delivered at precisely the right moment in relation to the position of the piston in the
cylinder and the rotational speed of the crankshaft.
Additionally, the spark must be of sufficient intensity to fire the fuel mixture, even at high
compression pressure and high RPM.
It is the function of the ignition system to generate this voltage and provide it to the spark plug at
the correct time.
The Nippondenso capacitor discharge ignition (CDI) system has magnets located on the crankshaft
flywheel. AC voltage is induced in the generating coil(s) as the poles of the magnets rotate past the
poles of the coils. Timing is controlled by a trigger coil or the position of the coil poles relative to
the magnet poles, which are directly related to piston position. The CD (or amplifier) box contains
the electronic circuitry to store and control the initial voltage and deliver it to the ignition coil (and
then the spark plug) at the correct moment. The ignition coil is a transformer that steps up the
relatively low voltage, 150-300 V, of the generating coil to the 20,400-40,000 volts necessary to
jump the spark plug gap and initiate the burning of the fuel/ air mixture in the combustion
chamber.
Maximum power from a given engine configuration is produced when peak combustion chamber
pressure (about 750 P. S. I.) takes place at about 15° of crankshaft rotation A. T.D.C. Normal combustion
is the controlled burning of the air/fuel mixture in the cylinder. The flame is initiated at the spark
plug and spreads to the unburned mixture at the edges of the cylinder.
This flame front travels through the cylinder at about 100 feet per second. In order to achieve
maximum pressure at about 15° A. T. D. C., the spark must occur about 15° before T.D.C. Complete
combustion will finish at about 35° A. T.D.C. The actual amount of spark advance B. T.D.C. is
dependent upon bore size, combustion chamber shape, operating RPM, mixture turbulence and
the actual flame speed.
Flame speed is directly proportional to piston speed in an almost linear fashion. Though it is not
completely understood why this relationship exists, it is thought to be related to intake speed and
mixture turbulence. Hence, flame speed increases as RPM increases. It also increases as the air/
fuel ratio becomes leaner.
Because the flame speed is slower at lower RPM's, more advance at low RPM is necessary for
maximum performance. Advancing the spark too much B. T.D.C. for the needs of the engine will
cause the engine to go into detonation.
The optimum ignition would then have timing significantly advanced at lower RPM, but would
retard the timing at higher RPM to keep the engine out of detonation. Generally, as the ignition
timing is advanced, the low end mid range power will be improved and the peak power will be
moved to a lower RPM. Retarding the timing will generally reduce low and mid range power but
may allow jetting to be leaner and increase peak power. Peak power will be moved to a higher
RPM. These are generalizations and ignition timing must be optimized depending on engine
design, RPM range and operating conditions.
Ignition advance on Rotax engines is measured by a linear distance of piston travel B.T.D.C. A
dimension taken through a straight spark plug hole in the center of the head is a direct measurement.
A dimension through an angled plug hole on one side of the head is an indirect measurement. A
direct measurement can be converted to degrees of crankshaft rotation by the appropriate
formulas. Initial ignition timing procedures can be found in the Shop A4anual for the particular
model being worked on.
Section 04 ENGINE PREPARATION
04-93