
03-8
SECTION 03 - ENGINE PREPARATION
By doing this, the large volume of air/fuel mixture
taken into the engine during intake is wholly re-
tained inside the engine, and not partially expelled
with the exhaust. Combustion therefore, is with a
completely fresh charge.
Boost Port
The boost port or finger port as occasionally re-
ferred to, is a cavity in the wall of a cylinder. The
prime purpose of this port is to assist the clearing
of burnt gases from the combustion chamber. In
doing so, it allows a fresher charge for the next
combustion phase and at the same time, cools
the piston dome. This port comes into function af-
ter the third phase of engine operation, or during
the transfer stroke of the piston.
Unlike the transfer port, the boost port does not
connect to the crankcase via a passage in the cyl-
inder wall. On the 587 engine, the boost port com-
municates with the crankcase via a hole in the
piston skirt. On all 1994 and later Sea-Doo en-
gines (587, 657, 717, 787 and 947) the boost port
is connected with the crankcase and functions
much as a transfer port.
BASIC ENGINE THEORY
Basically, the 2-stroke engine performs the same
operation as a 4-stroke. However, instead of com-
pleting the cycle in 4-stroke or 720° crankshaft ro-
tation, the 2-stroke engine completes the cycle in
only 360° (2-stroke). In other words, the 2-stroke
engine piston travels to Top Dead Center and
back to Bottom Dead Center once, to complete
the intake, compression, combustion and exhaust
phases.
NOTE:
On all Sea-Doo engines (except the 947),
the intake is controlled by a rotary valve. On the
947 engine, the intake is controlled by reed
valves. In the following illustrations, the intake is
controlled by the piston skirt. Although these en-
gine configurations have their own characteris-
tics, the basic engine theory remains the same.
The illustrations are provided to demonstrate the
principles of operation of a 2-stroke engine.
1. Intake and Secondary Compression
Port Condition:
Intake port: Partially opened
Exhaust port: Closed
Transfer port: Closed
Boost port: Closed
When the piston starts its upward stroke, a vacu-
um is created inside the crankcase and the air/fuel
mixture is sucked in from the carburetor via the
intake port. At the same time, the piston blocks
the exhaust and transfer ports, and compresses
the fuel charge in the combustion chamber (sec-
ondary compression).
1. Exhaust
2. Compressed charge
3. Intake
4. Fresh charge
2. Ignition and Combustion
Port Condition:
Intake port: Opened
Exhaust port: Closed
Transfer port: Closed
Boost port: Closed
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2
4
3
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Summary of Contents for Bombardier
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