05-6
MMC2005-001_05A.FM
SECTION 05 - ENGINE PREPARATION
With all operating parameters correct, normal
combustion will take place. However, if for some
reason the temperature inside the cylinder is in-
creased during combustion, abnormal combustion
will occur and lead to serious engine damage.
DETONATION
In detonation, the spark plug initiates burning and
the air/fuel mixture starts to burn in the usual man-
ner but as combustion continues, the heat gener-
ated affects the large portion of the yet unburned
air/fuel mixture.
This unburned mixture temperature becomes so
high that it burns spontaneously creating high-velocity
pressure waves within the combustion chamber.
These shock waves can sometimes be heard as
pinging. While these shock waves can be detri-
mental to the mechanical integrity of the engine,
it is the excessive heat that causes most problems
in 2-strokes. The piston may expand excessively
causing a seizure or the piston may melt. The
melting will occur at the hottest points, which will
be right below the spark plug and around the edge
of the piston — often at a ring locating pin. If al-
lowed to continue, a hole may melt completely
through the top of the piston.
PRE-IGNITION
Pre-ignition is the ignition of the mixture inside the
combustion chamber before the timed spark. Pre-
ignition sources are generally an overheated spark
plug tip or a glowing carbon deposit on the piston
head. Since ignition occurs earlier than the timed
spark, the hot gases stay longer in the combustion
chamber, thus increasing cylinder head and piston
temperatures to a dangerous level.
Usually the piston is subject to damage. It may
seize or the aluminum on the exhaust side of the
piston dome may melt. Pre-ignition is always pre-
ceded by detonation.
CAUSES OF DETONATION:
Octane of the fuel is too low.
Air/fuel mixture is too lean.
a. Incorrect jetting.
b. Air leaks.
c. Varnish deposits in carburetor.
d. Malfunction anywhere in fuel system.
Spark plug heat range too high.
Ignition timing too far advanced.
a. Initial timing incorrect.
b. Ignition component failure.
Compression ratio too high.
a. Improperly modified engine.
b. Deposit accumulation on piston dome or head.
Exhaust system restrictions.
a. Muffler plugged/restricted.
b. Tail pipe diameter too small.
c. Incorrect design of expansion chamber.
General overheating.
a. Broken fan belt.
b. Loss of coolant.
c. Lack of snow on heat exchangers.
Coolant or water entering combustion chamber.
SQUISH AREA
Rotax cylinder heads incorporate a squish area. This
area is basically a
ledge
projecting beyond the com-
bustion chamber area. In operation, as the piston
ascends and approaches the ledge, a rapid squeez-
ing action is applied to the air/fuel mixture con-
tained in the area immediately between the piston
dome and the ledge. This squashing action forces
the entrapped mixture rapidly into the combustion
chamber area, creating a greater mixture turbu-
lence. Additionally, the small volume and large sur-
face area of the squish band allow a better cooling
of the end gases to help prevent detonation.
Содержание ski-doo MACH Z 1000 SDI
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Страница 196: ...05 72 MMC2005 001_05A FM SECTION 05 ENGINE PREPARATION FUEL TANK AND FUEL PUMP All Models except SDI 2 4 3 1 A33C2VS...
Страница 289: ...SECTION 06 DRIVE SYSTEM MMC2005 001_06A FM 06 27 422 A01D3JA 417 222 562 427 A01D3KA 417 222 677...
Страница 473: ...SECTION 12 SERVICE TOOLS MMC2005 001_12A FM 12 29 N Scratch remover P N 861 774 800 APPLICATION All models...
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