Stationary Reciprocating Internal Combustion
Engines (RICE):
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Two-Stroke Engines
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A two-stroke engine completes the power cycle in one revolution
of the crankshaft. The crankshaft in an internal combustion engine
is attached to the pistons. When the pistons move up and down,
the crankshaft turns and converts the reciprocating motion of the
pistons into rotary motion. The first stroke begins with the piston
at the top of the cylinder. At this time, the engine’s combustion
chamber contains a compressed mixture of fuel and air. The mixture
is ignited by a spark that causes a sudden increase in temperature
and pressure that forces the piston downward, transferring power
to the crankshaft. As the piston travels downward, air and exhaust
ports are uncovered, allowing combustion gases to exit and fresh
air to enter.
During the second stroke, the air and exhaust ports close and fuel
is injected into the cylinder. As the piston returns to its starting
position, the upward motion compresses the fuel and air mixture.
When the piston reaches the top of the cylinder, the compressed
fuel and air mixture is ignited again and the cycle begins again.
Because fresh air is used to clear combustion gases from the cylinder,
two-stroke engines operate with an A/F ratio greater than stoichiometric
and are, therefore, all of the “lean-burn” design type.
A/F ratios for 2SLB engines range between 20:1 and 60:1. Their
exhaust temperatures are normally between 550 and 800°F. All
2SLB engines are direct-injected (i.e., fuel is injected directly
into the cylinder) (GRI, 2000).
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