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ROTAX TWO-STROKE
ENGINES
Engines in Ski-Doo
snowmobiles are lightweight, extremely efficient, high output,
two-stroke motors. In a two-stroke engine, the piston transmits
combustion forces to the crankshaft, like in an automobile engine,
but it also acts as the valve, opening and closing the intake
and exhaust ports. All Ski-Doo engines are designed and built
in Austria by Bombardier-Rotax, the world leader in two-stroke
engine technology. Rotax engines are the strongest, most powerful
normally aspirated engines for their displacement in the world.
Rotary
valves
A Rotax and Ski-Doo
exclusive in the snowmobile industry, rotary valve engines use
a gear-driven rotating disc to control the flow of fuel/air mixture
into the crankcase. This set-up provides superior engine efficiency
and performance at high rpm because it allows more fuel/air mixture
directly into the crankcase and because the valves open with
great precision.
Reed valves
This method of
fuel/air mixture induction is used in Rotax high performance
triple-cylinder engines, where it is more efficient and economical
than a rotary valve system. It consists of a set of flat carbon
fiber reeds covering the opening between the carburetor and crankcase.
The vacuum created in the crankcase when a piston moves upward
pulls the reeds open and allows the fuel/air mixture in. When
the piston begins its downward stroke, the compression of gases
under it closes the reeds.
Intake Resonator
Used only on three-cylinder,
reed-valved engines, this tube connects the three carburetors
to each other, equalizing the air pressure between them, so the
fuel/air mixture pressure is identical in all three sections
of the crankcase. This keeps all three cylinders firing equally.
Digital Performance
Management (DPM) system
Electronic solenoids
monitor temperature and altitude and feed the information to
a microchip, which adjusts carburetion to create an ideal fuel/air
mixture. An electronic primer and enrichener is also part of
the DPM system. DPM provides improved fuel economy, never requires
carburetor adjustment for conditions and eliminates the need
for a manual primer or choke.
HAC (High
Altitude Compensator)
The HAC counters
the carburetion richness that can occur because of the altitude
and temperature changes common in mountain riding. A diaphragm
in the HAC flexes in response to air pressure changes, mixing
more or less fuel with air in the carburetor.
R.A.V.E.
(Rotax Adjustable Variable Exhaust)
Another Rotax and
Ski-Doo exclusive. The R.A.V.E. valve provides optimal throttle
response and increased power in the low- and middle-rpm ranges
and increased fuel economy and emissions quality by changing
the height of the cylinder's exhaust ports. At low- and mid-range
rpm, where most riding is done, the closed valve increases torque
and improves fuel economy by creating a longer powerstroke and
later release of the exhaust gas shock wave. And at high rpm,
the valve opens, allowing for more freely flowing gases--more
efficient operation. Essentially, a R.A.V.E. engine has two separate,
efficient power curves.
Tuned exhaust
pipes
Ski-Doo exhaust
pipes are painstakingly crafted to enhance engine performance
as well as cool exhaust gases and quiet the engine. When exhaust
exits the combustion chamber, the pipe's precise length and shape
reflects sound waves from the exhaust to the edge of the exhaust
port and push any escaping fresh, unburnt fuel/air mixture back
into the cylinder. Fuel efficiency is increased, as is emissions
quality. Factory Ski-Doo tuned pipes are so precisely crafted,
aftermarket pipes rarely enhance an engine's power.
Cooling systems
Like all two-stroke
engines, Rotax powerplants run at high rpm and high temperatures
and must be cooled properly to achieve maximum performance. Liquid-cooled
engines are very efficient, circulating a liquid around the engine
through a reservoir and rechilling it in heat exchangers under
the sled's tunnel. On Ski-Doo air-cooled sleds, an axial fan
blows cold outside air over the cylinder heads to cool the engine.
A new fan-in air duct delivers cool air from outside the cab
to the system for enhanced system performance.
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