In the two-stroke marine engine when the crankshaft is rotating counter clockwise, the piston travels downwards, a transfer port between the crank case and the cylinder is uncovered; and the exhaust gases leave the cylinder through the exhaust port. A lightly compressed fresh fuel air charge travels from the crank case to the cylinder through the transfer port as the port opens. Since the incoming charge is under pressure, it rushes into the cylinder quickly, and removes the exhaust gases from the previous combustion. In the next phase, the upward movement of the piston closes the exhaust and transfer ports, and the air-fuel mixture in the cylinder is compressed.
A vacuum is also created. Further upward piston movement uncovers the intake port, and a fresh-fuel air charge is then drawn into the crankcase. In the third phase, as the piston approaches top dead centre, the spark plug fires igniting the compressed fuel-air mixture. The piston is then driven downward by the expanding gases. The fourth phase begins when the piston uncovers the exhaust port. The exhaust gases leave the cylinder through the exhaust port.
As the piston moves downwards, “the intake port is closed and the mixture in the crankcase is compressed in preparation for the next cycle (Clymer: 4-5). The main controls for operating the craft are on the handle bars. The start button and the stop button help to start and stop the engine respectively. On the right-hand side is a lever, which is a throttle not a brake. More advanced models also have another lever that controls a clutch and an LCD (liquid crystal display) that shows speed, fuel levels, and other data (eHow, 2008).
Jet Ski Propulsion For safety, a ski-type life vest has to be worn while operating the jet ski (Lamy: 29). Since the possibility of falling off the machine is high, the propeller which can be potentially dangerous is eliminated, and replaced with the jet drive (Armstrong: 59). In the jet drive (Figure 3), an impeller propels a large amount of water from underneath the jet ski through a steering nozzle at the rear of the craft.
The impeller (Figure 4) is a rotor-like device that sits inside a cylindrical passageway in the body of the jet ski, to the rear. When the ignition starts the engine running, the impeller is turned. Fig. 3. Jet Ski Propulsion: The Engine and Jet Drive (Wilson, 2008) Fig. 4. High Output Impeller and Steering (Yamaha FX) (Wilson, 2008) The impeller is composed of curved blades, similar to a propeller, the only difference is that they take water into the jet ski rather than push it out.
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