Design and Simulation of Two Stroke Engines

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Chapter 2 - Gas Flow through Two-Stroke Engines
gas at Te and p^V-^
(a) compression pressure wave
(b) expansion pressure wave
Fig. 2.1 Pressure wave nomenclature.
oce. It is also moving gas particles at a gas particle velocity of ce and in the same direction as
the wave is being propagated. At a point on the expansion wave in the pipe the pressure is pj,
where p; < po, and it is being propagated at a velocity, (Xj. It is also moving gas particles
at a gas particle velocity of q, but in the opposite direction to that which the wave is
being propagated.
At this point you can draw on your personal experience of sound waves to help understand
the physical nature of the statements made in the preceding paragraph. Imagine standing
several meters away from another person, Fred. Fred produces a sharp exhalation of
breath, for example, he says "boo" somewhat loudly. He does this by raising his lung pressure
above the atmospheric pressure due to a muscular reduction of his lung volume. The compression
pressure wave produced, albeit of small amplitude, leaves his mouth and is propagated
at the local acoustic velocity, or speed of sound, to your ear. The speed of sound involved
is on the order of 350 m/s. The gas particles involved with the "boo" leaving Fred's
mouth have a much lower velocity, probably on the order of 1 m/s. However, that gas particle
velocity is in the same direction as the propagation of the compression pressure wave, i.e.,
toward your ear. Contrast this simple experiment with a second test. Imagine that Fred now
produces a sharp inhalation of breath. This he accomplishes by expanding his lung volume so
that his lung pressure falls sharply below the atmospheric pressure. The resulting "u....uh"
you hear is caused by the expansion pressure wave leaving Fred's mouth and propagating
toward your ear at the local acoustic velocity. In short, the direction of propagation is the
same as before with the compression wave "boo," and the propagation velocity is, to all
intents and purposes, identical. However, as the gas particles manifestly entered Fred's mouth
with the creation of this expansion wave, the gas particle velocity is clearly opposite to the
direction of the expansion wave propagation.
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