Stationary Reciprocating Internal Combustion
Engines (RICE):
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Four-Stroke Engines
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A four-stroke engine completes the power cycle in two revolutions
of the crankshaft. The first stroke is the intake stroke during
which the intake valve opens and the exhaust valve closes. The
downward motion of the piston draws air (direct  injected) or a
mixture of air and fuel (premixed) into the cylinder. During the
second stroke, the intake valve closes, and the fuel is injected
(direct injected) into the cylinder as the piston moves upward
to compress the air and fuel mixture. As the piston finishes its
upward stroke, a spark ignites the mixture, causing a sudden increase
in temperature and pressure. The increased pressure drives the
piston downward (i.e., the third stroke), delivering power to the
crankshaft. During the fourth stroke, the exhaust valve opens and
the piston moves upwards to force the exhaust gases out of the
cylinder. The regulation will affect two types of spark ignition,
four-stroke engines: 4SLB and 4SRB.
Four-Stroke Lean Burn.
Compared to the 2SLB engine, the 4SLB engine reduces the presence
of high fuel concentration and temperature gradients in the cylinder
by mixing the air and fuel during the second stroke. Compared to
a 4SRB engine, the increased A/F ratio in 4SLB engines reduces combustion
and exhaust temperatures. A/F ratios for this engine configuration
are similar to those of 2SLB engines.
Four-Stroke Rich Burn.
4SRB engines have A/F ratios near stoichiometric, meaning that in
these engines the proportion of fuel relative to air is greater than
in lean-burn engines. All turbo-charged engines that do not introduce
fresh air to sweep combustion gases out of the cylinder after ignition
are 4SRB engines (GRI, 2000). A/F ratios for these engines typically
range between 16:1 and 20:1. Exhaust temperature is higher in rich-burn
engines than in leanburn engines.
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