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SECTION 03 - ENGINE PREPARATION
IGNITION SYSTEMS
Two stroke engines in watercraft rely on an elec-
tric spark to initiate combustion of the air/fuel
charge which has been inducted into the combus-
tion chamber. For the engine to operate efficient-
ly, the spark must be delivered at precisely the
right moment in relation to the position of the pis-
ton in the cylinder and the rotation at speed of the
crankshaft. Additionally, the spark must be of suf-
ficient intensity to ignite the air/fuel mixture, even
at high compression pressure and high RPM.
It is the function of the ignition system to gener-
ate this voltage and provide it to the spark plug at
the correct time.
CDI System
The Nippondenso Capacitor Discharge Ignition
(CDI) system has 4 magnets located on the crank-
shaft flywheel. AC voltage is induced in the gen-
erating coils as the poles of the magnets rotate
past the poles of the coils. Timing is controlled by
the position of the coil poles relative to the mag-
net poles, which are directly related to piston po-
sition. The CDI module 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 volt-
age (150-300 V) of the generating coil to the
20,000 – 40,000 volts necessary to jump the spark
plug gap and initiate the burning of the air/fuel
mixture in the combustion chamber.
NOTE:
On several models, the CDI module is in-
tegrated in the ignition coil.
DC-CDI System
The Nippondenso Direct Current-Capacitor Dis-
charge Ignition system offers a more powerful
and stable ignition at low RPM’s.
The magneto is responsible for charging the bat-
tery, which is the primary source for this system.
A trigger coil is mounted outside the rotor (inside
the magneto housing of the engine) and is not ad-
justable. Its purpose is to signal the engine posi-
tion to the MPEM. The rotor has 2 protusions
(180° apart) that, when couple with the trigger
coil, accomplish the signaling.
The ignition coil steps up the voltage input from
the MPEM to high voltage current for the spark
plugs.
All Systems
Maximum power from a given engine configura-
tion is produced when peak combustion chamber
pressure (about 750 PSI) takes place at about 15°
after crankshaft rotation ATDC. Normal combus-
tion is the controlled burning of the air/fuel mix-
ture in the cylinder. The flame is initiated at the
spark plug and spreads to the unburned at the
edges of the cylinder.
The flame front travels through the cylinder at
about 30 m (100 ft) per second. In order to
achieve maximum pressure at about 15° ATDC,
the spark must occur about 15° BTDC.
Complete combustion will finish at about 35° ATDC.
The actual amount of spark advance BTDC is depen-
dent 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 ex-
ists, it is thought to be related to intake speed and
mixture turbulence. Hence, flame speed increas-
es 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 BTDC for the needs of the engine will
cause the engine to detonate.
Ignition advance on Rotax engines is measured by
a linear distance of piston travel BTDC. A dimen-
sion taken through a straight spark plug hole in the
center of the head is a direct measurement. A di-
rect measurement can be converted to degrees
of crankshaft rotation by the appropriate formulas.
Initial ignition timing procedures can be found in
the appropriate model year Shop Manual.
When working with modified engines it may be
necessary to alter ignition timing from stock spec-
ifications to achieve maximum power output or to
prevent engine damage.
Select a fuel that meets the octane requirement
for the compression ratio of your engine (REFER
TO OCTANE REQUIREMENTS FOR ROTAX EN-
GINES). Once you have chosen the fuel, you will
be tuning the engine (IGNITION TIMING) for max-
imum efficiency for that fuel. If you change to an-
other brand of fuel or a different octane it will be
necessary to re-tune the engine. Failure to do so
may cause engine damage or power loss.
Summary of Contents for Bombardier
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