Exhaust System Descriptions; Exhaust Pulse Scavenge Effect - Honda Motorcycle Service Manual

EXHAUST SYSTEM
SYSTEM DESCRIPTIONS
The exhaust system serves another functions i n addition t o discharging the exhaust gas.
Since the exhaust gas discharged from the exhaust port is pressurized and very hot,
it
swells suddenly and produces a loud
noise if it is discharged from the exhaust port directly into the atmosphere. It also lowers the exhaust efficiency as the gas is
diffused from the exhaust port. To prevent the above problems, the exhaust gas is drawn from the exhaust port into the muf-
fler to be swelled and is discharged into the atmosphere after its temperature and pressure are lowered. By varying the sizes
and diameters of sections of the exhaust system, the airlfuel mixture can be drawn into the cylinder more effectively. This is
called the EXHAUST PULSE SCAVENGE EFFECT. Utilizing this effect in exhaust system design results in significant im-
provements in engine performance, especially on 2-stroke engines.
EXHAUST PULSE SCAVENGE EFFECT
When the exhaust valve (or port) opens w i t h the engine on the
exhaust stroke, the exhaust gas flows rapidly from the ex-
haust port into the muffler. A t the end o f the exhaust stroke,
the gas flow slows down, but due t o the inertia of liquid mass.
pressure i n the cylinder goes down below the atmospheric
pressure; in other words, negative pressure is applied t o the
cylinder for a short time. As the intake valve lor scavenge port)
opens, the airlfuel mixture quickly drawn into the cylinder.
INTAKE VALVE OPENS
A
STROKE
The discharged gas flows through the muffler forming a high
speed pressure wave. Due t o the inertia of liquid mass.
negative pressure is applied t o the exhaust port where the
pressure wave had passed. When the exhaust valve (or port1
opens on the next exhaust stroke, the exhaust gas is drawn
out by the negative pressure, and the exhaust efficiency is
improved.
EXHAUST VALVE OPENS
L
RESSURE WAVE
ATEXHAUSTPORT
Date o f Issue: March, 1995
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MOTOR
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