Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
characteristics which are dominated by the optimization of the scavenging characteristics and
also the unsteady gas dynamics within the exhaust manifold; a comprehensive debate on the
latter topic is in Chapter 5. While the decision on the number of cylinders which will best
optimize the exhaust system gas dynamics is left to the debate in Chapter 5, nevertheless the
trapping characteristics to be optimized by scavenging are found by the enhancement of the
in-cyUnder flow process at the minimum port heights allied to maximized port flow areas.
The optimization of the in-cylinder flow process can be conducted experimentally using
the single-cycle testing method [3.48] or theoretically using CFD [3.45]. Before embarking
on either of these routes, some basic principles for the design should be established.
Bear in mind that the external "blower," of whatever configuration, is going to force air
into the cylinder virtually throughout the entire scavenge flow period from the opening of the
scavenge ports to their closing. This is in marked contrast to the action of the crankcase pump
in the crankcase compression engine where the majority of the scavenge flow is concentrated
around the bdc period, and very little of it occurs in the latter third of the scavenge period as
the crankcase pressure has decayed considerably by that point. Therefore, for the "blown"
engine, at equality of air flow rate and port area by comparison with a crankcase compression
engine, the scavenging process is conducted with an air flow which has a longer duration but
at velocities which are more constant and of a lower amplitude.
To achieve optimization of external scavenging, in terms of maximized port areas and
minimized port heights, utilization of the majority of the port circumference assumes a high
priority. A sketch of just such an approach is shown in Fig. 3.40, and is easily recognized from
the history of loop-scavenged two-stroke diesel engines [3.49]. As the need for a transfer port
from the crankcase has been eliminated, it becomes possible to direct the scavenge flow from
the exhaust port side of the cylinder, and as close as possible to the same plane as the exhaust
ports. This can reduce, and potentially eliminate, the deleterious plan and elevation angles of
deviation for the flow of the first scavenge port, shown as angles El and E2 in Figs. 3.35 and
3.36. Many designs [3.49] have employed varying angles of upsweep on the ports, UPM° in
Fig. 3.35, but my view is that this complicates the liner design and manufacture, and deteriorates
the magnitude of the effective port areas. However, this advantage in gas-dynamic terms
must not result in a sacrifice of good scavenging characteristics. In short, it is recommended
that the optimization on the QUB single-cycle test apparatus is conducted logically using
various test liners machined where the focus of the port edges is positioned at values dictated
by their proportion of the cylinder radius, rcy, along the axis of the cylinder on the scavenge
side.
If the design is for a multi-cylinder unit, then the inter-cylinder disposition of the scavenge
duct as a siamesed layout serving two adjacent cylinders is a straightforward procedure.
The design process for such a cylinder is geometrically complex and tedious. This tedium
is greatly eased by the employment of the simple computer program, Prog.3.5 BLOWN PORTS.
3.5.6.1 The use of Prog.3.5, BLOWN PORTS
Fig. 3.41 shows a screen print of the display. In terms of the relationship of the data input
values for the program to the text in Sec. 3.5.6, and to Fig. 3.41, the following explanation is
offered. Let each line of input data be examined in turn.
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