Design and Simulation of Two Stroke Engines

Chapter 2
Gas Flow through Two-Stroke Engines
2.0 Introduction
The gas flow processes into, through, and out of an engine are all conducted in an unsteady
manner. The definition of unsteady gas flow is where the pressure, temperature and
gas particle velocity in a duct are variable with time. In the case of exhaust flow, the unsteady
gas flow behavior is produced because the cylinder pressure falls with the rapid opening of
the exhaust port by the piston. This gives an exhaust pipe pressure that changes with time. In
the case of induction flow into the crankcase through an intake port whose area changes with
time, the intake pipe pressure alters because the cylinder pressure, or crankcase pressure in a
simple two-stroke engine, is affected by the piston motion, causing volumetric change within
that space.
To illustrate the dramatic variations of pressure wave and particle motion caused by unsteady
flow in comparison to steady flow, the series of photographs taken by Coates [8.2] are
shown in Plates 2.1-2.4. These photographs were obtained using the Schlieren method [3.14],
which is an optical means of using the variation of the refractive index of a gas with its
density. Each photograph was taken with an electronic flash duration of 1.5 us and the view
observed is around the termination of a 28-mm-diameter exhaust pipe to the atmosphere. The
exhaust pulsations occurred at a frequency of 1000 per minute. The first photograph, Plate
2.1, shows the front of an exhaust pulse about to enter the atmosphere. Note that it is a plane
front, and its propagation within the pipe up to the pipe termination is clearly one-dimensional.
The next picture, Plate 2.2, shows the propagation of the pressure wave into the atmosphere
in a three-dimensional fashion with a spherical front being formed. The beginning of
rotational movement of the gas particles at the pipe edges is now evident. The third picture,
Plate 2.3, shows the spherical wave front fully formed and the particles being impelled into
the atmosphere in the form of a toroidal vortex, or a spinning donut of gas particles. Some of
you may be more familiar with the term "smoke ring." That propagating pressure wave front
arrives at the human eardrum, deflects it, and the nervous system reports it as "noise" to the
brain. The final picture of the series, Plate 2.4, shows that the propagation of the pressure
wave front has now passed beyond the frame of the photograph, but the toroidal vortex of gas
particles is proceeding downstream with considerable turbulence. Indeed, the flow through
the eye of the vortex is so violent that a new acoustic pressure wave front is forming in front
of that vortex. The noise that emanates from these pressure pulsations is composed of the
basic pressure front propagation and also from the turbulence of the fluid motion in the
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